Task specificity and anatomical independence in perception of properties by means of a wielded object.

Journal of Experimental Psychology: Human Perception and Performance 2014, Vol. 40, No. 6, 2372-2391

© 2014 American Psychological Association 0096-1523/14/$ 12.00 http://dx.doi.org/!0.1037/xhp0000014

Task Specificity and Anatomical Independence in Perception of Properties by Means of a Wielded Object Jeffrey B. Wagman

Alen Hajnal

Illinois State University

University of Southern Mississippi

Behavior is typically organized with respect to a goal to be achieved rather than the anatomical components used in doing so. Similarly, perception is typically organized with respect to a property to be perceived rather than the anatomical components used in doing so. Such task specificity and anatomical independence is manifest in perception of properties of a wielded object. In 6 experiments, we investigated whether these properties might also be manifest in perception of properties by means of a wielded object. In particular, we investigated perception of whether a surface could be stood on when the object used to explore that surface is wielded by the preferred and nonpreferred hands (Experiment 1), by 1 or both hands (Experiment 2), by different 2-handed grips (Experiment 3), and by entirely different limbs (i.e., the hand and the foot, Experiments 4 -6 ). In general, the results show that perception reflected the action capabilities of the perceiver but was largely unaffected by the (configurations of) anatomical components used to wield the object. The results highlight the haptic system as a smart perceptual device and as a multifractal biotensegrity structure. Keywords: haptic perception, affordances, tensegrity, tool use

example, locomotion over a ground surface can be performed by means of walking, running, crawling, hopping, or slithering. De spite differences in the required anatomical components or coor dination patterns, each of these means of locomotion can result in the same or a functionally equivalent end. Performing a given behavior requires flexibly and temporarily recruiting potentially independent anatomical components into a task-specific device capable of achieving a behavioral goal (Bingham, 1988; Kugler & Turvey, 1987). It requires assembling task-specific control units from potentially independent anatomical units (Turvey, 2007). It is important to note that such flexibility is manifest in per ception as well as in behavior. Perception of most (if not all) properties also exhibits both task specificity and anatomical inde pendence. Perception is organized with respect to a property to be perceived rather than with respect to the anatomical components used to do so (Gibson, 1966, 1979). Accordingly, a given property can be perceived by means of different anatomical components, and such means can be functionally equivalent in accomplishing the intended perceptual goal. For example, different perceptual modalities generally consist of different anatomical components, yet properties of a given object or surface can be perceived with a comparable degree of accuracy by vision, by touch, and by audi tion (see Carello, Wagman, & Turvey, 2005; Carello & Wagman, 2009 for reviews). Perceiving a given property requires flexibly and temporarily recruiting potentially independent anatomical components into a task-specific device capable of achieving a perceptual goal— a “smart perceptual device” (Runeson, 1977). In short, it requires assembling task-specific detection units from potentially independent anatomical units. It is important to note that task specificity and anatomical independence are manifest within as well as between perceptual modalities. People can perceive many properties of an unseen object by means of effortful or dynamic touch— by hefting, wield-

One of the fundamental hallmarks of behavior across the animal kingdom is flexibility—the interchangeability of means to achieve an end (Gibson, 1994; Turvey, 2013). In particular, the perfor mance of most (if not all) behaviors exhibits both task specificity and anatomical independence. Task specificity refers to the fact that, in general, behavior is organized with respect to a goal to be achieved, despite changes in context or circumstance. Anatomical independence is a particular kind of task specificity in which the change in context or circumstance includes the recruitment of a different set of anatomical components to perform a given behav ior. Behavior typically exhibits both properties in that it is orga nized with respect to a goal to be achieved rather than with respect to the anatomical components used to do so (Reed, 1982). Accord ingly, a given behavior can be performed by means of different anatomical components, and such means can be functionally equivalent in accomplishing the intended behavioral goal. For

This article was published Online First November 3, 2014. Jeffery B. Wagman, Department of Psychology, Illinois State Univer sity; Alen Hajnal, Department of Psychology, University of Southern Mississippi. Pordons of these data were presented at the Seventeenth International Conference on Perception and Action (July 2013, Estoril, Portugal), the 53rd Meeting of the Psychonomic Society (November 2013, Toronto, Canada), and the North American Meeting of the International Society for Ecological Psychology (June 2014, Oxford, OH, USA). We thank Dan Jackson and Collin Klabisch for help with data collection, Brian Day for serving as the model in the figures, and Dawn McBride for help with data analysis. Correspondence concerning this article should be addressed to Jeffrey B. Wagman, Department of Psychology, Illinois State University, Campus Box 4620, Normal, IL 61790-4620. E-mail: [email protected] 2372

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ing, and maneuvering that object with muscular effort (see Turvey & Carello, 2011, for a review). A given object property can be perceived in this manner with different configurations of the touch system. For example, object length can be perceived when an object is wielded by a given effector (the hand) about different joints (the wrist, the elbow, or the shoulder; Pagano, Fitzpatrick, & Turvey, 1993), with different organizations of the same effector (i.e., with different grips; Pagano, Kinsella-Shaw, Cassidy, & Turvey, 1994), with different numbers of comparable effectors (i.e., with one or both hands; Carello, Fitzpatrick, Domaniewicz, Chan, & Turvey, 1992), and by entirely different effectors (hand, foot, or torso; Hajnal, Fonseca, Flarrison, Kinsella-Shaw, & Carello, 2007a,2007b; Palatinus, Carello, & Turvey, 2011). Length of a wielded object can also be perceived by the preferred or nonpreferred hand—with no differences in accuracy, reliability, or in the scaling of perceived length to mass distribution between the two hands (Carello, Kinsella-Shaw, Amazeen, & Turvey, 2006). Such results are not only consistent with the task-specificity and anatomical independence of perception but also with the charac terization of the haptic system as a smart perceptual device (Carello et al., 1992; Runeson, 1977). The dual role of the touch system as an organ of perception and an organ of behavior make it an ideal system in which to investi gate both task specificity and anatomical independence. These properties are also likely to be manifest in perception by means of wielded objects. Just as people can perceive properties of unseen objects by hefting, wielding, and maneuvering those objects, they can perceive functional properties (i.e., affordances-, Gibson, 1979) of unseen surfaces by probing, poking, tapping, or striking those surfaces with a wielded object, such as whether an obstacle or gap can be stepped over or a surface can be stood on (e.g., Burton, 1992; Fitzpatrick, Carello, Schmidt, & Corey, 1994). This ability is relatively ubiquitous, though often unrecognized in everyday life. People perceive properties of objects and surfaces by means of objects such as toothbrushes, eating or writing utensils, scissors, and even footwear. Less mundanely, visually impaired individuals perceive properties of objects in their path by means of long canes, amputees perceive properties of objects by means of prosthetic limbs, and surgeons perceive properties of internal organs by means of laparoscopic tools. The ability to temporarily incorporate an inert object into the haptic system is, itself, preliminary evidence of task specificity and anatomical independence in the haptic system. However, demon strating that perception of affordances of a surface by means of a wielded object reflects the action capabilities of the perceiver but not the (configuration of) anatomical components used to wield the object would provide even stronger evidence toward this end in two ways. First, such a pattern of results would demonstrate that perception of properties by means of a wielded object reflects the property to be perceived despite material differences between the limb and the wielded object (e.g., organic vs. inert, deformable vs. rigid, heterogeneous vs. homogeneous) and anatomical, physiolog ical, and behavioral differences between the (configuration of) limbs. Second, in this task, the wielded object is temporarily attached to the body, and participants are unlikely to have expe rience performing this task. Therefore, performing this task would seem to require the spontaneous recruitment of a novel, smart perceptual device.

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Demonstrating anatomical independence in perception necessar ily requires demonstrating that equivalent measurement capabili ties are instantiated over measurement devices comprising differ ent (configurations of) anatomical components. The strategy is similar to that used in demonstrating anatomical independence in perception of wielded objects (e.g., Carello et al., 1992; Hajnal et al., 2007a; Palatinus et al., 2011) and in demonstrating modality independence in perception of properties by touch, vision, and audition (e.g., Fitzpatrick et al., 1994; Regia-Corte & Wagman, 1998). Such a strategy is especially appropriate if the stimulation patterns that support perceptual constancies of this type are invari ant across energy media and anatomical structures (see Carello, Wagman, & Turvey, 2005; Carello & Wagman, 2009; Turvey & Fonseca, 2014). In six experiments, we investigated whether perception of af fordances for standing on an inclined surface by means of a wielded object is task specific and anatomically independent. Specifically, we investigated perception of this property when the object used to explore the surface was wielded by the preferred and nonpreferred hands (Experiment 1), by one or both hands (Exper iment 2), by different two-handed grasps (Experiment 3), and by two disparate limbs (i.e., the hand and the foot, Experiments 4 -6 ). In each of the six experiments, we expected that despite anatom ical, physiological, and behavioral differences between the (con figurations of) anatomical components used to wield the object and explore the surface, perception would reflect the action capabilities of the perceiver but would be unaffected by the (configurations of) anatomical components used in doing so.

Experiment 1: Preferred Versus Nonpreferred Hand In Experiment 1, we investigated the ability to perceive affor dances for standing on an inclined surface when that surface is explored with a dowel held in the preferred hand and in the nonpreferred hand. There are numerous anatomical, physiological, and behavioral differences between the preferred and nonpreferred hands (Healey, Liederman, & Geschwind, 1986; Porae, & Coren, 1981; Triggs, Calvanio, Macdonell, Cros, & Chiappa, 1994). Ac cordingly, many unimanual tasks are more quickly, accurately, or powerfully performed with the preferred hand than with the non preferred hand (e.g., throwing a ball, drawing, strumming a guitar; see Brown, Roy, Rohr, & Bryden, 2006; Corey, Hurley, & Foundas, 2001). Moreover, the two hands exhibit behavioral special ization in many bimanual tasks; whereas the preferred hand typi cally provides fine-grained stabilization, the nonpreferred hand typically provides coarse-grained stabilization (Guiard, 1987). However, these anatomical, physiological, and behavioral differ ences between the two hands ought not to matter in this task, if perception of affordances of surfaces by means of wielded objects exhibits anatomical independence and task specificity. Consequently, we expected that perception of whether the sur face could be stood on would reflect the action capabilities of the perceiver but would be unaffected by which hand was used to wield the object and explore the surface. We expected a specific pattern of results across a number of dependent measures. First, the likelihood that the surface is perceived to afford standing on would decrease as surface angle increases but would be unaffected by the hand used. Second, the perceptual boundary for this behavior would be comparable to the behavioral boundary and would not

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differ for the two hands. Third, participants would be able to successfully differentiate surfaces that could be stood on from those that could not, and this ability would not differ for the two hands. Fourth, confidence would be lowest (and response latency would be longest) within a range of surface angles that includes the perceptual boundary (cf. Fitzpatrick et al., 1994), but neither variable would be affected by hand used to explore the surface.

Method Participants Twenty undergraduate students (one man, 19 women) at Illinois State University participated in this experiment in fulfillment of an extra credit option in their psychology courses. In this and all experiments, written informed consent was obtained from each participant prior to the experiment, and the protocol was approved by the Institutional Review Board at Illinois State University in accordance with the Declaration of Helsinki. The procedure re quired that participants attempt to stand on an inclined surface. Therefore, in the interest of participant safety, participants in this and all other experiments were deemed ineligible for the study (prior to data collection) if they weighed more than 91 kg (200 lbs) or if they wore inappropriate footwear (e.g., sandals, shoes with heels, or those without rubber soles). The average height of par ticipants was 167.0 cm (SD = 6.9 cm). The average weight was 64.3 kg (SD = 8.8 kg). All participants were classified as righthanded on the basis of their responses to the Lateral Preference Inventory (Coren, 1993).

Materials and Apparatus The main apparatus consisted of a wooden surface (152 cm X 76 cm), reinforced with metal braces so that it could support a participant up to 105 kg. One end of the surface was hinged to a wooden frame (91 cm X 76 cm) so that the angle of inclination could be adjusted. The other end of the surface was supported by a metal dowel resting on stationary pairs of hooks that were screwed into the back of wooden studs (122 cm tall, one on each side of the platform). Pairs of hooks were located at seven different heights (33.50 cm, 40.75 cm, 50.25 cm, 58.75 cm, 69.00 cm, 82.75 cm, and 92.50 cm from the laboratory floor) so that the surface could be set at seven different angles of inclination, ranging from 15° to 45° in increments of 5° (see Figure 1). The sides of the wooden studs facing the participant were covered with cardboard so that the hooks were not visible to the participant. Participants wore a pair of blackened safety goggles and used a wooden dowel (1 m in length, 1 cm in diameter) to explore the surface. A digital stopwatch was used to record response latency. A support railing was available for participant safety during both the perceptual and behavioral tasks in case of a loss of balance. A digital scale was used to measure participant body weight. Portions of the Lateral Preference Inventory (Coren, 1993) were used to measure handedness and footedness.

Procedure Perceptual task. Each participant stood on the scale, and his or her body weight was recorded. Then the participant completed

Figure 1. The experimental tasks in the preferred hand and nonpreferred hand conditions in Experiment 1. See the online article for the color version of this figure.

the handedness and footedness portions of the Lateral Preference Inventory. The participant stood approximately 1 m from the surface (set at 45° before the participant arrived), put on the blackened goggles, and was handed the dowel. He or she grasped the dowel with either the preferred or nonpreferred hand (depend ing on the condition) and placed the distal tip of the dowel on the floor. The experimenter adjusted the angle of the surface and indicated to the participant to begin the trial. The participant explored the surface with the dowel (by probing, scraping, tapping; see Figure 1) and provided two responses. First, the participant reported (yes or no) whether he or she would be able to stand on the surface with both feet without bending at the knees or waist or going up on the toes (cf. Fitzpatrick et al., 1994; Malek & Wagman, 2008). Second, the participant rated his or her confidence in the yes or no response on a scale ranging from 1 to 7, where 1 indicated “not at all confident” and 7 indicated “extremely confi dent.” The experimenter recorded the latency between the instruc tion to begin the trial and the yes or no response (i.e., the time taken to explore the surface with the dowel). After both responses


were provided, the participant placed the distal tip of the dowel on the floor, and the angle of the surface was adjusted for the next trial. No restrictions were placed on how the participant explored the surface with the dowel or the time required to provide both responses. No explicit measures were taken to prevent the partic ipant from hearing the contact between dowel and surface. Each participant performed this task while grasping the dowel in the preferred and nonpreferred hand (during separate blocks of trials). The order of these conditions was counterbalanced across participants. Each angle was presented three times for each hand, and the order in which angles were presented was randomized within that block. Therefore, each participant completed a total of 42 trials. Participants did not approach or attempt to stand on the surface until all trials of the perceptual task were completed. Behavioral task. After the perceptual task was completed, the participant removed the blindfold, and the surface was initially set to the smallest angle of inclination (15°). Each participant then attempted to stand with both feet on the lower edge of the surface for 5 s without bending at the knees or waist, going up on the toes, or grasping the support railing. If the participant was able to do so successfully, then he or she stepped down, the surface was raised to the next steepest angle, and the participant again attempted to perform this task. This procedure was repeated until the participant was unable to perform the task. The steepest surface angle that could be stood on in this manner was recorded as the behavioral boundary for that participant.

Results Perceptual Task Probability data. The percentage of yes (i.e., could be stood on) responses in each condition was compared in a 2 (Hand: Preferred vs. Nonpreferred) X 7 (Surface Angle) repeatedmeasures analysis of variance (ANOVA). A main effect of surface angle, F(6, 114) = 172.85, p < .001, Tip = .90, showed that the percentage of yes responses decreased as angle increased. Neither the main effect of hand, F (l, 19) = 0.92, p = .35, tip = .05, nor the Hand X Angle interaction, F(6, 114) = 0.39, p = .88, T|p = .88, was significant1 (see Figure 2, top panel). Perceptual boundaries (individual participant data). The perceptual boundary for each participant in a given condition was the steepest angle that received a yes response on at least half of the trials in that condition (i.e., on at least two of the three trials; cf. Malek & Wagman, 2008). Across participants, the values ranged from 15° to 40° for the preferred hand and from 20° to 40° for the nonpreferred hand. A t test found no significant difference between the mean perceptual boundaries for the preferred hand (M = 31.3°, SD = 3.2°) and the nonpreferred hand (M = 32.3°, SD = 5.5°), t(l9) = 0.75, p = .46. Perceptual boundaries (aggregate data). Probit analysis (Finney, 1971) was used to determine the angle that would have resulted in a yes response on 50% of the trials in each condition (cf. Fitzpatrick et al., 1994; Malek & Wagman, 2008). This analysis revealed that the perceptual boundary for the preferred hand was 33.4° (with lower and upper bounds on a 95% confidence interval of 32.2° and 35.5°, respectively), and for the nonpreferred hand it was 34.4° (with lower and upper

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bounds of 32.2° and 35.5°, respectively). The overlapping con fidence intervals suggest that these values do not differ (see Figure 2, top panel). Both at the level of the individual partic ipants and at the level of the aggregate data, perceptual bound aries were comparable to the behavioral boundary (M = 29.0°, SD = 3.5°), which ranged from 25° to 35° across participants.

Signal Detection Data The mean proportion of hits (yes responses to angles less than or equal to the perceptual boundary) and false alarms (yes responses to angles greater than the perceptual boundary) were compared in each condition. For each condition and for each participant, proportion of hits and false alarms, respectively, were calculated by (a) dividing the total number of hits by the number of trials for which the angle was less than or equal to the perceptual boundary and (b) dividing the total number of false alarms by the number of trials for which the angle was greater than the perceptual boundary. A 2 (Hand) X 2 (Response Type: Hits vs. False Alarm) ANOVA on these values revealed a main effect of response type, F (l, 19) = 703.8, p < .001, rip = 0.97. The proportion of hits (M = 0.92, SD = 0.09) was greater than the proportion of false alarms (M = 0.24, SD = 0.18). Neither the main effect of hand, F (l, 19) = 1.08, p = .32, tip = 0.05, nor the Hand X Response Type interaction, F (l, 19) = 0.97 p = .34, Tip = 0.05, was significant. A t test on (corrected) d' values showed no difference in how well participants were able to differentiate surfaces that afford standing on from those that do not, when exploring those surfaces with the preferred (M = 2.16, SD = 0.52) or nonpreferred hand (M = 2.03, SD = 0.45), r(19) = 0.84, p = .41.

Confidence Data Across participants, mean confidence ratings ranged from 2.9 to 6.5 for the preferred hand and from 3.4 to 6.5 for the nonpreferred hand. These values were compared in a 2 (Hand: Preferred vs. Nonpreferred) X 7 (Surface Angle) ANOVA. There was a main effect of surface angle, F(6, 114) = 12.71, p < .001, T)p = .40. Consistent with previous research (e.g., Fitzpatrick et al., 1994), confidence ratings were generally lowest when the surface angle was in a range that included the perceptual boundary (approximately 30-35°). Neither the main effect of hand, F (l, 19) = 0.60, p = .45, tip = 0.45, nor the Hand X Surface Angle interaction, F(6, 114) = 1.79, p = .11, Tip = 0.11, was significant (see Figure 2, middle panel).

1 Power analyses were conducted for all nonsignificant main effects and interactions in all experiments using the G'Power program (Faul, Erdfelder, Lang, & Buchner, 2007). Effect sizes were estimated using significant main effects or interactions reported by Wagman and Hajnal (2014). As in the current experiment, these authors investigated perception of affordances for standing on an inclined surface by means of a rod wielded by different (configurations of) anatomical components and used the same set of dependent measures. For all tests, Cohen’s/w a s estimated to be between 0.37 and 0.58 (a “large” effect size, see Keppel and Wickens, 2004). Using this effect size, G*Power estimated power for the nonsignificant main effects and interactions reported in this article were estimated to be greater than .95.

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Discussion The pattern of results was completely consistent with our hy potheses. Specifically, despite putative anatomical, physiological, and behavioral differences between the preferred and nonpreferred hands, none of the dependent measures were affected by the hand used. Unlike many manual motor tasks, performing this unimanual perceptual task does not seem to require exploiting the differential dexterity and specialization of the two hands (cf. Guiard, 1987). Such results provide preliminary evidence o f the task specificity and anatomical independence of perception of affordances of surfaces by means of wielded objects.

Experiment 2: One Hand Versus Two Hands In Experim ent 2, we investigated the ability to perceive affor dances for standing on inclined surface when that surface is explored with a dowel held with one hand (i.e., the preferred hand) or with both hands. Using both hands to perform a given manual task increases the number of muscles, joints, and points of contact brought to bear in performing that task. This might increase the forces that can be applied with that object but may simultaneously decrease the dexterity with which the object can be controlled. Accordingly, some manual tasks are more quickly, accurately, or powerfully performed as bimanual tasks than as unimanual tasks (e.g., swinging a baseball bat or using a snow shovel), and vice versa (e.g., drawing with a pencil or using a toothbrush). However, these anatomical, physiological, and behavioral differences ought not to matter in this task if perception o f affordances of surfaces by means of wielded objects exhibits anatomical independence and task specificity. Consequently, we expected an analogous pattern o f results to Experim ent 1.

Method Participants

Figure 2. Percent of yes responses (top panel), confidence ratings (mid dle panel), and response latency (bottom panel) for the preferred hand (PH) and nonpreferred hand (NPH) conditions in Experiment 1. Error bars represent standard error.

Twenty undergraduate students (four men, 16 women) at Illinois State University participated in this experiment in fulfillment of an extra credit option in their psychology courses. The average height of participants was 167.3 cm (SD = 1 1 .6 cm). The average weight was 65.6 kg (SD = 11.3 kg). Eighteen participants were righthanded, and two participants were left-handed.

Materials and Apparatus The materials and apparatus were the same as in Experiment 1.

Response Latency Across participants, mean response latencies ranged from 2.4 s to 21.7 s for the preferred hand and from 2.2 s to 21.0 s for the nonpreferred hand. A 2 (Hand: Preferred vs. Nonpreferred) X 7 (Surface Angle) ANOVA revealed a main effect of surface angle, F(6, 114) = 5.53, p < .001, T)p = .23. Consistent with previous research (e.g., Fitzpatrick et al., 1994), response latencies were generally longest when the surface angle was in a range that included the perceptual boundary. Neither the main effect of hand, F (l, 19) = 0.92, p = .35, T]p = .05, nor the Hand X Surface Angle interaction, F(6, 114) = 2.02, p = .07, rjp = .10, was significant (see Figure 2, bottom panel).

Procedure Perceptual task. The task was the same as in Experiment 1, except each participant grasped the dowel with one hand (the preferred hand) and with two hands (during separate blocks of trials). In the one-hand condition the participant grasped the dowel in his or her preferred hand. In the two-hand condition, the par ticipant grasped the dowel in his or her nonpreferred hand and then placed the bottom of their preferred hand flush with the top of the nonpreferred hand (see Figure 3, bottom panel). Restrictions (or lack thereof), number o f trials, randomization, and counterbalanc ing were the same as in Experim ent 1.


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20° to 40° for two hands. A t test found no difference between the mean perceptual boundary for one hand (M = 29.5°, SD = 6.0°) and two hands (M = 30.5°, SD = 5.4°), t(19) = 1.00, p = .33. Perceptual boundaries (aggregate data). Probit analysis re vealed that the perceptual boundary for one hand occurred at 32.3° (with lower and upper bounds on a 95% confidence interval of 31.1° and 33.4°, respectively), and that the perceptual boundary for two hands occurred at 32.5° (with lower and upper bounds of 31.5° and 33.6°, respectively), suggesting that these values do not differ (see Figure 4, top panel). Both at the level of the individual participants and at the level of the aggregate data, perceptual boundaries were comparable to the

Figure 3. The experimental tasks in the one-hand and two-hand condi tions in Experiment 2. See the online article for the color version of this figure.

Behavioral task. Experiment 1.

The behavioral task was the same as in

Results Perceptual Task Probability data. A 2 (Hands: One vs. Two) X 7 (Surface Angle) repeated-measures ANOVA on percentage of yes re sponses revealed a main effect of surface angle, F(6, 114) = 106.3, p < .001, r\l = 0.84. The percentage of yes responses decreased as the angle increased (see Figure 4). The main effect of number of hands, F (l, 19) = 0.31, p = .59, T)p = .02, was not significant. There was a Hands X Surface Angle interaction, F(6, 114) = 2.67, p < .05, rip = 0.12. However, follow-up t tests failed to reveal any significant differences in the percentage of yes responses for one or two hands at any angle (all ps > .05; see Figure 4, top panel). Perceptual boundaries (individual participant data). Perceptual boundaries ranged from 20° to 45° for one hand and from

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Angle (deg) Figure 4. Percent of yes responses (top panel), confidence ratings (mid dle panel), and response latency (bottom panel) for the one-hand (1H) and two-hand (2H) conditions in Experiment 2. Error bars represent standard error.

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behavioral boundary (M = 28.5°, SD = 3.7°), which ranged from 20° to 35° across participants.

Signal Detection Data A 2 (Hands: One vs. Two) X 2 (Response Type: Hits vs. False Alarm) ANOVA on proportion of hits and false alarms revealed a main effect of response type, F (l, 19) = 323.5, p < .001, -qp = .95. The proportion of hits (M = .90, SD = 0.11) was greater than the proportion of false alarms (M = 0.19, SD = 0.22). Neither the main effect of number of hands, F (l, 19) = .35, p = .56, r\l = .02, nor the Hands X Response Type interaction of number, F( 1,19) = 1.86, p = .19, rip = 0.09, was significant. There was no difference in (corrected) d! values when participants explored the surfaces with one hand (M = 2.15, SD = 0.71) or two hands (M = 2.31, SD = 0.52), f(19) = 1.29, p = .21.

Confidence Data Across participants, mean confidence ratings ranged from 3.9 to 6.5 for one hand and from 3.9 to 6.6 for two hands. A 2 (Hands: One vs. Two) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F(6, 114) = 19.79, p < .001, rip = .51, and a main effect of number of hands, F (l, 19) = 6.39, p < .05, T|p = .25. Confidence ratings were generally lowest when the angle was in a range that included the perceptual bound ary, and participants were more confident when using one hand (M = 5.8, SD = 1.1) than when using two hands (M = 5.6, SD = 1.2). The Hand X Surface Angle interaction of was not significant, F(6, 114) = .1.54, p = .17, rip = .08 (see Figure 4, middle panel).

Response Latency Across participants, mean response latencies ranged from 2.7 s to 13.6 s for one hand and from 3.2 s to 12.6 s for two hands. A 2 (Hands: One vs. Two) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F(6, 114) = 12.6, p < .001, rip = 0.39. Response latencies were generally longest when the angle was in a range that included the perceptual bound ary. Neither the main effect of number of hands, F (l, 19) = 0.92, p = .76, rip = .005, nor the Hands X Surface Angle interaction, F(6, 114) = .836, p = .55, rip = .042, was significant (see Figure 4, bottom panel).

Discussion The pattern of results was generally consistent with our hypoth eses. Somewhat unexpectedly, the likelihood that the surface was perceived to afford standing on was affected by the number of hands used, but the nature of this effect was unclear. In addition, participants were more confident when using one hand than when using two hands. It is possible that such differences were due to differences in how dexterously the object could be controlled with one or both hands. However, despite such differences, the remain ing (performance-based) dependent measures (perceptual bound ary, signal detection variables, and response latency) were unaf fected by number of hands used. Such results provide additional evidence of the task specificity and anatomical independence of the perception of affordances of surfaces by means of wielded objects.

Experiment 3: Different Two-Handed Grasps In Experiment 2, participants were less confident when explor ing the surface by means of a rod wielded with two hands than with one hand, perhaps because of differences in dexterity between the two conditions. In Experiment 3, we attempted to attenuate this possible difference by investigating the ability to perceive affor dances for standing on an inclined surface when that surface is explored with a dowel held with two different, two-handed grasps (i.e., with the bottom of preferred hand flush with the top of the nonpreferred hand and vice versa). Changing the spatial relation ships of the two hands in this manner keeps the number of muscles, joints, and points of contacts constant in each task. However, doing so changes how these anatomical components are organized and may change the forces that can be applied and the dexterity with which movements can be made. Accordingly, many bimanual tasks are more quickly, more accurately, or more powerfully performed when the object is grasped with the preferred hand closer to the distal end of the object (e.g., swinging a sledgeham mer or shoveling snow). However, these anatomical, physiologi cal, and behavioral differences ought not to matter in this task if perception of affordances of surfaces by means of wielded objects exhibits anatomical independence and task specificity. Therefore, we expected an analogous pattern of results to Experiments 1 and 2.

Method Participants Twenty undergraduate students (three men, 17 women) at Illi nois State University participated in this experiment in fulfillment of an extra credit option in their psychology courses. The average height of participants was 170.8 cm (SD = 7.3 cm). The average weight was 64.7 kg (SD = 11.6 kg). Nineteen participants were right-handed, and one participant was left-handed.

Materials and Apparatus The materials and apparatus were the same as in Experiments 1 and 2.

Procedure Perceptual task. The task was the same as in previous exper iments, except each participant grasped the dowel with two dif ferent bimanual grasps (during separate blocks of trials). The participant grasped the dowel with either the preferred or nonpre ferred hand as the “top hand” (i.e., the hand closer to the distal end of the dowel). When the preferred hand was the top hand, the participant grasped the dowel in his or her nonpreferred hand and then placed the bottom of his or her preferred hand flush with the top of the nonpreferred hand. When the nonpreferred hand was the top hand, the participant grasped the dowel in his or her preferred hand and then placed the bottom of his or her nonpreferred hand flush with the top of the preferred hand (see Figure 5). Restrictions (or lack thereof), number of trials, randomization, and counterbal ancing were the same as in Experiments 1 and 2. Behavioral task. The behavioral task was the same as in Experiments 1 and 2.


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Figure 5. The experimental tasks in the preferred hand top and nonpreferred hand top conditions in Experiment 3. See the online article for the color version of this figure.

Results Perceptual Task Probability data. A 2 (Top Hand: Preferred vs. Nonpre ferred) X 7 (Surface Angle) repeated-measures ANOVA on per centage of yes responses revealed a main effect of surface angle, F(6, 114) = 194.6, p < .001, rip = 0.91. The percentage of yes responses decreased as angle increased. Neither the main effect of top hand,F(l, 19) = 0.85,p = .37, rip = 0.04, northeTop Hand X Surface Angle interaction, F(6, 114) = 1.07, p = .38, r|p = 0.05, was significant (see Figure 6, top panel). Perceptual boundaries (individual participant data). Across participants, perceptual boundaries ranged from 20° to 35° in each condition. A t test found no significant difference in perceptual boundary when the top hand was the preferred hand (M = 30.3°, SD = 3.8°) or the nonpreferred hand (M = 28.5° SD = 4.3°), t( 19) = 1.51, p = .15. Perceptual boundaries (aggregate data). Probit analysis re vealed that the perceptual boundary was 31.8° (with lower and upper bounds on a 95% confidence interval of 29.0° and 34.9°, respectively) when the preferred hand was the top hand and was 31.0° (with lower and upper bounds of 30.0° and 32.0°, respec tively) when the nonpreferred hand was the top hand, suggesting that these values do not differ (see Figure 6, top panel). Both at the level of the individual participants and at the level of the aggregate

data, perceptual boundaries were comparable to the behavioral boundary (M = 29.5°, SD = 3.2°), which ranged from 25° to 35° across participants.

Signal Detection Data A 2 (Top Hand: Preferred vs. Nonpreferred) X 2 (Response Type: Hits vs. False Alarm) ANOVA on proportion of hits and false alarms revealed a main effect of response type. The propor tion of hits (M = 0.89, SD = 0.14) was greater than the proportion of false alarms (AT = 0.12, SD = 0.13), F (l, 19) = 1700.65, p < .001, tip = .99. Neither the main effect of top hand, F (l, 19) = .85, p = .37, Tip = .04, nor the Top Hand X Response Type interaction, F (l, 19) = 0.87, p = .77, T|p = .005, was significant. There was no difference in (corrected) d' values when the top hand was the preferred hand (M = 2.37, SD = 4.6) or the nonpreferred hand (M = 2.40, SD = 4.5), t(19) = 0.16, p = .88.

Confidence Data Across participants, mean confidence ratings ranged from 3.7 to 6.5 when the top hand was the preferred hand and from 3.5 to 6.7 when the top hand was the nonpreferred hand. A 2 (Top Hand: Preferred vs. Nonpreferred) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F (6 ,114) = 11.51, p < .001, T)p = .38. Confidence ratings were generally lowest when the

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on these values revealed a main effect of surface angle, F(6, 114) = 9.70, p < .001, rip = .34. Response latencies were generally longest when the angle was in a range that included the perceptual boundary. Neither the main effect of top hand F(l, 19) = .162, p = .69, rip = .008, nor the interaction of top hand and surface angle, F(6, 114) = 0.82, p = .56, Tip = .041, was signif icant (see Figure 6, bottom panel).

Discussion Again, the pattern of results across these dependent measures was completely consistent with our hypotheses. Specifically, none of the dependent measures was affected by grasp used (i.e., which hand was the top hand). Such results provide additional evidence of the task specificity and anatomical independence of perception of affordances of surfaces by means of wielded objects. However, in Experiments 1-3 the perceptual task was performed either manually or bimanually. Therefore, this evidence is somewhat narrow in scope. In Experiment 4, we attempted to broaden this scope by comparing perception by more disparate limbs— specif ically, the foot and the hand.

Experiment 4: Preferred Hand Versus Preferred Foot In Experiment 4, we investigated the ability to perceive affor dances for standing on an inclined surface when that surface is explored with a dowel held in the hand and with a dowel attached to the foot. There are numerous anatomical, physiological, and behavioral differences between the hand and the foot (for a brief review, see Hajnal et al., 2007a). Accordingly, many tasks are more quickly, more accurately, or more powerfully performed with the hands than with the feet (e.g., cutting with a knife or picking up objects) and vice versa (e.g., supporting the body or moving the body from place to place). As a result, the hand and the foot assume vastly different roles in a variety of everyday tasks. Despite such differences, however, previous research has shown that perception of properties of objects wielded by hand and by foot exhibits task specificity and anatomical independence (Hajnal et al., 2007a,b; Stephen & Hajnal, 2011). Experiment 4 investi gates whether this is also the case for perception of affordances of surfaces by means of objects wielded by hand and by foot. We expected an analogous pattern of results to Experiments 1-3. Figure 6. Percent of yes responses (top panel), confidence ratings (mid dle panel), and response latency (bottom panel) for the preferred hand top (PHT) and the nonpreferred hand top (NPHT) conditions in Experiment 3. Error bars represent standard error.

angle was in a range that included the perceptual boundary. Neither the main effect of top hand, F (l, 19) = 0.07, p = .78, trip = .004, nor the Top Hand X Surface Angle interaction, F(6, 114) = 0.80, p = .57, r|p = .04, was significant (see Figure 6, middle panel).

Response Latency Across participants, mean response latencies ranged from 2.0 s to 13.8 s when the top hand was the preferred hand and from 2.4 s to 11.7 s when the top hand was the nonpreferred hand. A 2 (Top Hand: Preferred vs. Nonpreferred) X 7 (Surface Angle) ANOVA

Method Participants Twenty undergraduate students (four men, 16 women) at Illinois State University participated in this experiment in fulfillment of an extra credit option in their psychology courses. The average height of participants was 170.2 cm (SD = 7.1 cm). The average weight was 63.7 kg (SD = 10.0 kg). Eighteen participants were righthanded and right-footed, and two participants were left-handed and left-footed.

Materials and Apparatus The materials and apparatus were the same as in previous experiments except that a modified sandal apparatus was used in


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the preferred foot condition (cf. Wagman & Hajnal, 2014). This apparatus consisted of the foam sole of a beach sandal (approxi mately 30 cm long X 10 cm wide). A wooden dowel (75 cm long) was affixed to midline of bottom of the sandal with Velcro such that approximately 45 cm of the dowel extended out from the toe portion of the sandal. Two dowels (30 cm) were affixed on each side of the long dowel so that, when attached to the foot, the sandal could be placed flat on the floor. Three elastic Velcro straps were used to snugly secure the sandal apparatus to the footwear of the participant (see Figure 7). Procedure Perceptual task. The task was the same as in Experiments 1-3, except each participant explored the surface with a dowel grasped in their preferred hand and attached to the footwear of their preferred foot (on separate blocks of trials; see Figure 7, top and middle panels). In the preferred foot condition, the experimenter strapped the sandal to the footwear of participant’s preferred foot using the three Velcro straps. To begin each trial, the participant stood with his or her foot angled such that the dowel was not in contact with the surface. The experimenter adjusted the angle of the surface and indicated to the participant to begin the trial. The participant lifted the foot and attached sandal apparatus off of the ground, explored the surface with the dowel attached to the sandal, and provided the yes or no response and the confidence rating. After both responses were provided, the participant placed his or her foot on the floor and (again) stood with the foot angled so that the dowel was not in contact with the surface. The angle of the surface was then ad justed for the next trial. Restrictions (or lack thereof), number of trials, randomization, and counterbalancing were the same as in Experiments 1-3. Behavioral task. The behavioral task was the same as in Experiments 1-3. R esults Perceptual Task Probability data. A 2 (Limb: Preferred Hand vs. Preferred Foot) X 7 (Surface Angle) repeated-measures ANOVA on per centage of yes responses revealed a main effect of surface angle, F(6, 114) = 117.45, p < .001, tip = 0.86. The percentage of yes responses decreased as the angle of the surface increased. There was a marginal main effect of limb, F (l, 19) = 4.13,p = .06, t]j = 0.06. However, a Limb X Surface Angle interaction, F(6, 114) = 4.20, p < .01, T|p = .18, showed that the difference between limbs occurred only at intermediate surface angles. In particular, there was a larger percentage of yes responses for the preferred hand than for the preferred foot at 30°, preferred hand (M = 0.75; SD = 0.37), preferred foot (M = 0.50; SD = 0.38), r(19) = 2.68, p = .015, and at 35°, preferred hand (M = 0.33; SD = 0.37), preferred foot (M = 0.16; SD = 0.27), r(19) = 2.70, p = .014, but at no other surface angles (see Figure 8, top panel). Perceptual boundaries (individual participant data). Across participants, perceptual boundaries ranged from 20° to 35° for the preferred hand and from 15° to 35° for the preferred foot. A t test found that perceptual boundaries occurred at a steeper

Figure 7. The experimental tasks in the preferred hand (top panel) and preferred foot conditions (middle panel) in Experiment 4. The sandal apparatus used in the preferred foot condition of Experiment 4 (bottom panel). See the online article for the color version of this figure.

angle for the preferred hand (M = 30.0°, SD = 4.5°) than for the preferred foot (M = 26.7°, SD = 5.6°), /(19) = 2.94, p < .01 (see Figure 8, top panel). Perceptual boundaries (aggregate data). Probit analysis re vealed that the perceptual boundary for the preferred hand oc curred at 32.2° (with lower and upper bounds on a 95% confidence interval of 30.2° and 34.2°, respectively), and that the perceptual

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false alarms revealed a main effect of response type, F (l, 19) = 472.00, p < .001, rip = 0.96. The proportion of hits (A/ = 0.81, SD = 0.22) was greater than the proportion of false alarms (M = 0.12, SD = 0.16). Neither the main effect of limb, F (l, 19) = 3.07, p = .09, tip = 0.14, nor the Limb XResponse Type interaction, F (l, 19) = 1.95, p = .18 T)p = 0.09, was significant. There was no difference in (corrected) d' values when exploring the surfaces with the preferred hand (M = 2.64, SD = 0.59) and preferred foot (M = 2.63, SD = 0.67), t(19) = 0.03, p = .97.

Confidence Data Across participants, mean confidence ratings ranged from 5.0 to 6.9 for the preferred hand and from 4.9 to 6.7 for the preferred foot. A 2 (Limb: Preferred Hand vs. Preferred Foot) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F(6, 114) = 17.29, p < .001, Bp = .47. Confidence ratings were generally lowest when the surface angle was in a range that included the perceptual boundary. Neither the main effect of limb, F (l, 19) = 1.02, p = .33, Bp = -05, nor the Limb X Surface Angle interaction, F(6, 114) = 1.73, p = .13, Bp = -08, was significant (see Figure 8, middle panel).

Response Latency Across participants, mean response latencies ranged from 2.2 s to 9.0 s for the preferred hand and from 2.2 s to 12.9 s for the preferred foot. A 2 (Limb: Preferred Hand vs. Preferred Foot) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F(6, 114) = 5.46, p < .001, Tip = 0.22. Response latencies were generally longest when the angle was in a range that included the perceptual boundary. Neither the main effect of limb, F (l, 19) = 0.04, p = .84 Bp = -002, nor the Limb X Surface Angle interaction, F(6, 114) = 1.55, p = .17, Tip = .076, was significant (see Figure 8 bottom panel).

Discussion

Figure 8. Percent of yes responses (top panel), confidence ratings (mid dle panel), and responses latency (bottom panel) for the preferred hand (PH) and preferred foot (PF) conditions in Experiment 4. Error bars represent standard error.

boundary for the preferred foot occurred at 29.8° (with lower and upper bounds of 27.3° and 32.2°, respectively), suggesting that these values do not differ (see Figure 8, top panel). Both at the level of the individual participants and at the level of the aggregate data, perceptual boundaries were comparable to the behavioral boundary on this task (M = 30.8°, SD = 2.9°), which ranged from 25° to 35° across participants.

Signal Detection Data A 2 (Limb: Preferred Hand vs. Preferred Foot) X 2 (Response Type: Hits vs. False Alarm) ANOVA on proportion of hits and

To a large extent, the pattern of results across the dependent measures was inconsistent with our hypotheses. As expected, signal detection variables, confidence, and response latency were unaffected by limb used. However, contrary to our expectations, both the likelihood that surface was perceived to afford standing on and the perceptual boundary on this behavior were affected by limb used. In particular, when the surface angle was in a range that included the perceptual and behavioral boundary, participants were more conservative when exploring the surface with a dowel at tached to the foot than with a dowel held in the hand. Conse quently, the perceptual boundary (derived at the level of individual participants) occurred at a steeper angle when the dowel was held in the hand than when it was attached to the foot (see Figure 8, top panel). It is possible that these observed differences in performance were due to differences in the ability to perceive this affordance by means of the hand and the foot. Such anatomical differences would be inconsistent with our hypotheses for this experiment, with the findings of Experiments 1-3, and with those of previous research (Hajnal et al., 2007a,b; Stephen & Hajnal, 2011). Alternatively, it is possible that these observed differences were due to differences


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in the balance constraints of each task. In particular, when the dowel is attached to preferred foot, the participant must balance on one foot while exploring the surface, and when the dowel is held in the hand, no such additional balance task is necessary (see Figure 7). Previous research has shown that performing a balance task inhib its the ability to perceive affordances o f a given surface (Mark, Balliet, Craver, Douglas, & Fox, 1990). Therefore, the observed differences across the two limb conditions could be due to differences in balance constraints of the two tasks rather than to differences in the ability to perceive affordances by the hand and the foot. Experiments 5 and 6 explore this possibility.

Experiment 5: Preferred Hand While Seated Versus Preferred Foot While Seated In Experiment 5, we investigated the ability o f a seated partic ipant to perceive affordances for standing on an inclined surface with a dowel held in the preferred hand and attached to the preferred foot. Given that both tasks are performed while seated, neither task requires the participant to perform an additional bal ance task (i.e., standing on one foot) while exploring the surface. Therefore, these two tasks differ in the anatomical components used to explore the surface but not in balance requirements (see Figure 9). If the differences observed across the two limb conditions in Experiment 4 were due to differences in the ability to perceive this affordance by the hand and the foot, then the same pattern of differences across limb conditions should occur in this experiment. Alternatively, if the differences observed across the limb condi tions in Experiment 4 were due to differences in the balance requirements of the two tasks (as expected), then there should be no such differences across limb conditions in this experiment. Consequently, we expected an analogous pattern of results to Experiments 1-3. First, the likelihood that the surface was per ceived to afford standing on would decrease as the angle increases but would be unaffected by limb used. Second, the perceptual boundary would be comparable to the behavioral boundary and would not differ for the two limbs. Third, participants would be able to successfully differentiate surfaces that could be stood on from those that could not, and this ability would not differ for the two limbs. Fourth, confidence would be lowest (and response latency would be longest) within a range of angles that includes the perceptual boundary, but neither variable would be affected by limb used.

Participants Twenty undergraduate students (seven men, 13 women) at Illi nois State University participated in this experiment in fulfillment o f an extra credit option in their psychology courses. The average height of participants was 174.5 cm (SD = 9.3 cm). The average weight was 69.0 kg (SD = 10.0 kg). All participants were both right-handed and right-footed.

Figure 9. The experimental tasks in the preferred hand seated (top panel) and preferred foot seated (bottom panel) conditions in Experiment 5. See the online article for the color version of this figure.

Procedure Perceptual task. The task was similar to that of Experiment 4, except in both conditions participants explored the surface while seated. The chair was placed 1 m from the inclined surface. In the preferred hand seated condition, the participant sat in the chair and explored the surface with the dowel held in the preferred hand. In the preferred foot seated condition, the participant sat in the chair and explored the surface with the sandal apparatus attached to the footwear of the preferred foot (see Figure 9). Restrictions (or lack thereof), number o f trials, randomization, and counterbalancing were the same as in Experiments 1-4. Behavioral task. The behavioral task was the same as in Experiments 1 -4 .

Results Materials and Apparatus The materials and apparatus were the same as in Experiment 4, except a cushioned metal desk chair (with no armrests) was also used (see Figure 9).

Perceptual Task Probability data. A 2 (Limb: Preferred Hand Seated vs. Pre ferred Foot Seated) X 7 (Surface Angle) repeated-measures

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ANOVA on percentage of yes responses revealed a main effect of surface angle, F(6, 114) = 65.34, p < .001, r\l = .78. The percentage of yes responses decreased as the angle of the surface increased. Neither the main effect of limb, F{ 1, 19) = 1.04, p = .32, rip = 0.05, nor the Limb X Surface Angle interaction, F(6, 114) = 1.87, p = .09, Tip = 0.09, was significant (see Figure 10, top panel).

Perceptual

boundaries

(individual

participant

data).

Across participants, perceptual boundaries ranged from 20° to 45° in each condition. It was not possible to determine the perceptual

boundary for one participant in the preferred hand seated condition because they did not provide a response of yes on more than half the trials for any of the surface angles in this condition. Therefore, a perceptual boundary for this participant in this condition was not included in the analysis. A t test on the remaining values found no significant difference between the mean perceptual boundary in preferred hand seated (M = 29.2°, SD = 5.8 °) and preferred foot seated (M = 28.9°, SD = 9.7 °), r(19) = 0.17, p = .87. Perceptual boundaries (aggregate data). Probit analysis re vealed that the perceptual boundary for preferred hand seated occurred at 30.4° (with lower and upper bounds on a 95% confidence interval of 29.1° and 31.7°, respectively) and that the perceptual boundary for preferred foot seated occurred at 32.0° (with lower and upper bounds of 30.7° and 33.4°, respec tively), suggesting that the perceptual boundaries do not differ (see Figure 10, top panel). Both at the level of the individual participants and at the level of the aggregate data, perceptual boundaries were comparable to the behavioral boundary (M = 33.3°, SD = 6.3°), which ranged from 25° to 45° across participants.

Signal Detection Data A 2 (Limb: Preferred Hand Seated vs. Preferred Foot Seated) X 2 (Response Type: Hits vs. False Alarm) ANOVA on proportion of hits and false alarms revealed a main effect of response type, F (l, 19) = 333.26, p < .001, Tip = .95. The proportion of hits (M = 0.75, SD = 0.20) was greater than the proportion of false alarms (M = 0.09, SD = 0.18). Neither the main effect of limb, F (l, 19) = .34, p = .56, Tip = 0.02, nor the Limb X Response Type interaction, F (l, 19) = 0.90, p = .36, T)p = .05, was significant. There was no difference in (corrected) d! values in the preferred hand seated condition (M = 2.24, SD = 0.77) and in the preferred foot seated condition (M = 2.32, SD = .81), f(19) = 0.42, p = .67.

Confidence Data Across participants, mean confidence ratings ranged from 2.6 to 7.0 for preferred hand seated and from 1.7 to 6.5 for preferred foot seated. A 2 (Limb: Preferred Hand Seated vs. Preferred Foot Seated) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F(6, 114) = 5.91, p < .001, hp = 0.24. Confidence ratings were generally lowest when the angle was in a range that included the perceptual boundary. Neither the main effect of limb, F (l, 19) = 1.86, p = .19, tij = 0.09, nor the Limb X Surface Angle interaction, F(6, 114) = 1.33, p = .24, T]p = 0.07, was significant (see Figure 10, middle panel).

Response Latency

Figure 10. Percent of yes responses (top panel), confidence ratings (middle panel), and response latency (bottom panel) for the preferred hand seated (PH_seated) and preferred foot seated (PF_seated) conditions in Experiment 5. Error bars represent standard error.

Across participants, mean response latencies ranged from 3.4 s to 12.1 s for preferred hand seated and from 4.3 s to 12.6 s for preferred foot seated. A 2 (Limb: Preferred Hand Seated vs. Preferred Foot Seated) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F(6, 114) = 5.99, p < .001, T]p = .24. Response latencies were generally longest when the angle was in a range that included the perceptual bound ary. Neither the main effect of limb, F( 1, 19) = 0.07, p = .80 Tip = .004, nor the Limb X Surface Angle interaction, F(6, 114) = .51, p = .80, T)p = 0.03], was significant (see Figure 10, bottom panel).


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Discussion As expected, Experiment 5 found that none of the dependent measures were affected by limb used. This pattern of results is consistent with the hypothesis that the differences observed in Experiment 4 were due to differences in the balance requirements of the task and not to anatomical differences in the ability to perceive affordance of a surface by the hand and foot. The goal of Experiment 6 is to provide further support for this hypothesis.

Experiment 6: Preferred Foot, Standing Versus Seated In Experiment 6, we investigated the ability to perceive affordances for standing on an inclined surface with a dowel attached to the foot when the participant performed this task while standing and while seated. Doing so while standing requires the participant to perform an additional balance task (i.e., standing on one foot), and doing so while seated does not. Therefore, the two tasks differ in balance requirements but not in the anatomical components used to explore the surface (see Figure 11). If the differences in dependent measures across conditions in Experiment 4 were due to differences in balance requirements (as expected), then the same pattern of differences across the two limb conditions should occur in this experiment. Consequently, we expected that participants would be more conservative when ex ploring the surface while standing than while seated (especially when the surface angle is in a range that includes the perceptual boundary). In addition, we expected that perceptual boundaries would occur at steeper angles while seated than while standing.

Participants Twenty undergraduate students (6 men, 14 women) at Illinois State University participated in this experiment in fulfillment of an extra credit option in their psychology courses. The average height of participants was 171.5 cm (SD = 10.1 cm). The average weight was 64.2 kg (SD = 9.0 kg). All participants were right-footed. Figure 11. The experimental tasks in the preferred foot seated (top panel)

Materials and Apparatus

and preferred foot standing (bottom panel) conditions of Experiment 6. See the online article for the color version of this figure.

The materials and apparatus were the same as in Experiment 5.

Procedure Perceptual task. In the preferred foot standing condition, the blindfolded participant stood 1 m from the inclined surface and explored the surface with the sandal apparatus attached to the footwear of the preferred foot. The procedure was identical in the preferred foot seated condition, except the participant sat in a chair placed 1 m from the inclined surface and explored the surface with the dowel attached to footwear of the preferred foot (see Figure 11). Restrictions (or lack thereof), number of trials, randomization, and counterbalancing were the same as in previous experiments. Behavioral task. The behavioral task was the same as in Experiments 1-5.

Results Perceptual Task Probability data. A 2 (Posture: Preferred Foot Seated vs. Preferred Foot Standing) X 7 (Surface Angle) repeated-measures

ANOVA on percentage of yes responses revealed a main effect of surface angle, F(6, 114) = 74.83, p < .001, rip = 0.80. The percentage of yes responses decreased as the angle of the surface increased. There was also a main effect of posture, F( 1, 19) = 6.45, p < .05, T|p = 0.25, but this effect was superseded by a Posture X Surface Angle interaction, F(6, 114) = 3.92, p < .01, rip = .17 (see Figure 12, top panel). Follow-up t tests showed that the percentage of yes responses was greater in the preferred foot seated condition than in the preferred foot standing at a surface angle of 30° (preferred foot seated: M = 63.3%; SD = 37.3%; preferred foot standing: M = 33.3%; SD = 37.4%, t[19] = 3.11, p < .01) and at no other surface angles (see Figure 12, top panel). Perceptual boundaries (individual participant data). Across participants, perceptual boundaries ranged from 20° to 45° for the preferred foot seated condition and from 15° to 45° for the preferred foot standing condition. It was not possible to determine the perceptual boundary for one participant in the preferred foot standing condition because they did not provide a response of yes

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condition occurred at 31.3° (with lower and upper bounds on a 95% confidence interval of 30.1° and 32.5°, respectively), and in the preferred foot standing condition it occurred at 27.7° (with lower and upper bounds of 26.2° and 29.2°, respectively), suggest ing that these values differ (see Figure 12, top panel). Both at the level of the individual participants and at the level of the aggregate data, perceptual boundaries were comparable to the behavioral boundary (M = 33.5°, SD = 5.4 °), which ranged from 25° to 45° across participants.

Signal Detection Data A 2 (Posture: Preferred Foot Seated vs. Preferred Foot Stand ing) X 2 (Response Type: Hits vs. False Alarm) ANOVA on proportion of hits and false alarms revealed a main effect of response type, F (l, 19) = 499.5, p < .001, -rip = .96], but this effect was superseded by a Posture X Response Type interaction, F (l, 19) = 10.33,p < .01, Trip = -35. A follow-up t test showed that the proportion of hits was greater than the proportion of false alarms when seated (M = 0.71, SD = .15) than when standing (M = .51, SD = .21), t(19) = 3.21, p < .01. The main effect of posture, F (l, 19) = 2.98, p = .10, tip = .14, was not significant. There was no difference in (corrected) d' values while seated (M = 2.40, SD = 0.60) and while standing (M = 2.26, SD = 0.73), t(19) = .93, p = .36.

Confidence Data Across participants, mean confidence ratings ranged from 2.8 to 7.0 for the preferred foot seated condition and from 2.5 to 7.0 for the preferred foot standing condition. A 2 (Posture: Preferred Foot Seated vs. Preferred Foot Standing) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F(6, 114) = 8.24, p < .001, rip = .30. Confidence ratings were generally lowest when the surface angle was in a range that included the perceptual boundary. Neither the main effect of posture, F (l, 19) = 2.20, p = .16, r\j = 0.10, nor the Posture X Surface Angle interaction, F(6, 114) = 1.19, p = .032, rip = 0.06, was significant (see Figure 12, middle panel).

Response Latency

Figure 12. Percent of yes responses (top panel), confidence ratings (middle panel), and response latency (bottom panel) for the preferred foot seated (PF_Seated) and preferred foot standing (PF_Standing) conditions in Experiment 6. Error bars represent standard error.

on more than half the trials for any of the surface angles in this condition. Therefore, a perceptual boundary for this participant in this condition was not included in the analysis. A t test on the remaining values found no significant difference between the perceptual boundary in the preferred foot seated (M = 29.5°, SD = 6.2°) and preferred foot standing conditions (M = 27.1°, SD = 6.9°), t(18) = 1.49, p = .16. Perceptual boundaries (aggregate data). Probit analysis re vealed that the perceptual boundary in the preferred foot seated

Across participants, mean response latencies ranged from 3.3 s to 11.1 s for the preferred foot seated condition and from 2.7 s to 11.4 s for the preferred foot standing condition. A 2 (Posture: Preferred Foot Seated vs. Preferred Foot Standing) X 7 (Surface Angle) ANOVA on these values revealed a main effect of surface angle, F(6, 114) = 6.17, p < .001, T|p = 0.25. Response latencies were generally longest when the angle was in a range that included the perceptual boundary. The main effect of posture was not significant, F (l, 19) = 0.03, p = .86 r|p = 0.002. There was a marginally significant Posture X Surface Angle interaction, F(6, 114) = 2.14, p = .054, r]p = 0.10 (see Figure 12, bottom panel).

Discussion As expected, Experiment 6 found that participants were more conservative when exploring the surface with a dowel attached to their foot while standing than while seated, especially when the surface angle was close to the perceptual and behavioral boundary.


Also as expected, perceptual boundaries (derived at the level of the aggregate data) occurred at a smaller surface angle in the former condition than in the latter condition. There was also an unex pected difference between proportion of hits and false alarms while seated and while standing. Together, the pattern of results of Experiments 5 and 6 is consistent with the hypothesis that the differences between the hand and the foot conditions observed in Experiment 4 were due to differences in the balance requirements of the task and not to (anatomical) differences in the ability to perceive affordance of a surface by means of the two limbs.

General Discussion In six experiments, we investigated whether the task specificity and anatomical independence exhibited in perception of properties of a wielded object (e.g., Carello et al., 1992) is also exhibited in perception of properties by means of a wielded object. Specifi cally, we compared the ability to perceive whether an inclined surface could be stood on by means of an object held in the preferred or nonpreferred hand (Experiment 1), held in one or both hands (Experiment 2), held with two different tworhanded grasps (Experiment 3), and held in the hand or attached to the foot (Experiments 4 -6 ). We expected that perception of whether the surface could be stood on would reflect the action capabilities of the perceiver but would be unaffected by the (number or configuration of) anatom ical component(s) used to explore the surface. Specifically, we expected that (a) the likelihood that the surface is perceived to afford standing on would decrease as surface angle increases, (b) the perceptual boundary would be comparable to the behavioral boundary, (c) participants would successfully differentiate surfaces that could be stood on from those that could not, and (d) confi dence would be lowest (and response latency would be longest) within a range of surface angles that included the perceptual boundary. However, we did not expect any of these dependent measures to be affected by the (number or configuration of) anatomical component(s) used to perform the task. In Experiments 1-3, the pattern of results across the dependent measures was largely (if not completely) consistent with these hypotheses. There were few, if any, differences in the dependent measures for the different (configurations of) hand(s) used to wield the object (see Figures 2, 4, and 6). Such a pattern suggests that under the conditions investigated in these experiments, perception by means of a wielded object exhibits anatomical independence, a particular kind of task specificity. In Experiment 4, the pattern of results was inconsistent with these hypotheses. In particular, at surface angles in the range of the behavioral boundary (30°-35°), participants were more conservative in reporting that they could stand on the surface when they explored that surface with a dowel attached to the foot rather than with one held in the hand. Subse quently, the perceptual boundary (derived at the level of individual participants) occurred at a steeper angle for the hand than for the foot (see Figure 8). Experiments 5 and 6 showed that such differ ences were most likely the result of differences in balance require ments of the two tasks and not differences in ability of these limbs to perceive affordances of surfaces by means of a wielded object. In particular, Experiment 5 found no differences in any of the dependent measures across conditions that differed in anatomical

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components (dowel held in hand vs. attached to foot) but not in balance requirements (both tasks performed while sitting; see Figure 10). Conversely, Experiment 6 found differences in both the likelihood of a yes responses and perceptual boundaries across conditions that differed in balance requirements (sitting vs. stand ing) but not in anatomical components (both tasks performed with dowel attached to foot; see Figure 12). Presumably, performing a concurrent balance task inhibited the ability to perceive affor dances of the surface because it interfered with the ability to engage in exploratory behaviors (see Mark et al., 1990). In par ticular, the limitations on exploration provided by the balance task likely interfered with the ability to make subtle distinctions in the range of surface angles that included perceptual boundary but not with the ability to make less subtle distinctions outside of this range. This might be why participants in Experiments 4 and 6 were more conservative when they performed a concurrent balance task but only in a range that included the behavioral boundary (see Figures 8 and 12). Overall, the results show that despite putative anatomical, phys iological, and behavioral differences between (configurations of) anatomical components used to explore the surface, perception reflected the action capabilities of the perceiver but was largely, if not completely, unaffected by (configurations of) anatomical com ponents used to do so. Thus, the task specificity and anatomical independence of perception demonstrated for perception of prop erties of wielded objects is also manifest in perception of proper ties by means of wielded objects. Such results are consistent with the characterization of the haptic system as a smart perceptual device (Carello et al., 1992; Runeson, 1977) and with more recent characterizations of the musculoskeletal system as a biotensegrity structure (Turvey, 2007; Turvey & Carello, 2011; Turvey & Fon seca, 2014). One limitation of the current experiments is that these conclu sions rely, in part, on null effects. Specifically, the conclusions are based on a pattern of significant and null effects across a number of dependent measures across the six experiments. As stated in the introduction, we used this methodology because demonstrating anatomical independence in perception necessarily requires dem onstrating that equivalent measurement capabilities are instanti ated over measurement devices comprising different (configura tions of) anatomical components. Moreover, it is quite likely that the patterns of mechanical energy of relevance to perception of properties of (or by means of) wielded objects are invariant across anatomical structures (Pagano & Turvey, 1998; Turvey & Carello, 2011; Turvey & Fonseca, 2014). In Experiments 1, 2, 3, and 5, neither the likelihood that surface was perceived to afford standing on nor perceptual boundary were affected by the (configuration of) anatomical components used to wield the object. Of course, nonsignificant findings do not provide evidence for the absence of an effect. To minimize the possibility of Type II errors, we conducted experiments with sufficient power to detect an effect if one exists. As stated in Footnote 1, we estimated power for all statistical tests on the basis of a large effect size (i.e., Cohen’s /w a s estimated to be between 0.37 and 0.58) and still found power to exceed 95% in all statistical tests. Thus, we feel confident that if an effect of anatomical component does exist, likely we would have detected it in these experiments. Further, we did, in fact, find differences in both the likelihood that the surface was perceived to afford standing on and in perceptual

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boundary across conditions in Experiments 4 and 6. Therefore, we believe that the pattern of significant and null findings across the dependent measures in the six experiments support our conclu sions.

Smart Perceptual Devices and Perception by Means of a Wielded Object A smart perceptual device is a collection of potentially indepen dent anatomical components recruited for the purposes of solving a task-specific perceptual problem (Runeson, 1977). Among other features, smart perceptual devices are softly assembled (Carello et al., 1992; Kugler & Turvey, 1987). They are flexibly and tempo rarily assembled for the purposes of a particular perceptual task, and they are assembled from dynamic properties not anatomical components. The soft assembly of smart perceptual devices is one of the primary reasons that such systems can exhibit task speci ficity and anatomical independence. Both task specificity and anatomical independence are exhibited in perception of properties of wielded objects. Perception by dynamic or effortful touch generalizes over anatomical compo nents that differ in physiology, psychophysical achievements, and specialization or expertise for particular tasks (e.g., different grasps, rotation points, tissue contacts, and limbs; see Turvey & Carello, 2011, for a review). Perhaps more impressively, percep tion by dynamic or effortful touch also generalizes across popu lations of people who differ in physiology, psychophysical achievements, and specialization or expertise for particular tasks. Specifically, perception of properties of a wielded (or worn) object generalizes across experts and novice athletes as well as across individuals who have reduced sensory-motor capabilities due to aging, peripheral neuropathy, stroke, and spinal injury (see Carello, Silva, Kinsella-Shaw, & Turvey, 2008, for a review). Overall, such results indicate that in the context of perceiving properties of wielded objects, the meaningful division of labor is with respect to particular tasks, not anatomical units, tools, ages, or levels of expertise (Hajnal et al., 2007a). The results of the current experiments build on these findings by showing that a similar task-specific division of labor is also man ifest in perception of properties by means of a wielded object. In particular, task specificity and anatomical independence is exhib ited at two different levels. First, the success with which partici pants performed the task across the six experiments suggests that despite important material differences between the dowel and limb(s) to which it was attached (e.g., inert substance vs. organic tissue, rigid vs. deformable, homogenous vs. heterogeneous), the dowel itself did not interfere with the perception of the intended property. Rather, participants were able to quickly and effectively use the dowel as a perceptual tool. The dowel was (or quickly became) functionally transparent, serving as a functional extension of the receptor surface. In this way, the ability to perceive prop erties by means of a wielded object is continuous with the ability of humans and animals to perceive properties of surfaces by means of noninnervated appendages such as fingernails, claws, whiskers, antennae, horns, and quills (Burton, 1993). However, success in perceiving by means of a wooden dowel is perhaps more impres sive than these examples because the attachment of the dowel to the body is temporary. The functional transparency of artificial haptic perceptual tools (e.g., canes, prostheses, sports equipment,

hand-held tools) is consistent with the proposal that objects at tached to the body are perceived as part of the body because they are perceived in the same way as the body (Pagano & Turvey, 1998). Second, comparable (indeed, often identical) performance across conditions within a given experiment suggests that despite important anatomical, physiological, and behavioral differences between the (configuration of) anatomical components compared in each experiment, such differences also did not interfere with perception of the intended property. Rather, the (configuration of an) anatomical component was functionally transparent as well. Perception of a given object or surface property generalizes across (configurations of) anatomical components and objects used as perceptual tools, and smart perceptual devices can be assembled over different (configurations of) anatomical components as well as over organic and inert components. By exploiting regularities in structured stimulation patterns, smart perceptual devices ensure that perception is lawfully con strained (Carello et al., 1992). This guarantees that perception of affordances for a given behavior by means of a given anatomical component will reflect the perceiver’s action capabilities so long as (a) the structure in the stimulation pattern of relevance is sufficient to support perception of the affordance and (b) the anatomical component recruited for the task is capable of detecting this structure (see Turvey & Shaw, 1999). However, it is important to note that this does necessarily guarantee that perception by means of different (configurations of) anatomical components will necessarily be identically scaled to action capabilities. Practice perceiving a given property can lead to changes in what stimula tion variable is used to perceive that property or how that stimu lation variable is used to do so (see Stephen & Arzamarski, 2009; Withagen & Michaels, 2005). The experiments reported were not designed to investigate such changes in perception. Doing so in the context of perception of properties by means of objects attached to the body might be the topic of future research. Anatomical independence occurs when, before, during, or after practice perceiving a given property, two different (configurations of) anatomical components exploit analogous stimulation variables in perceiving that property. It is the case that through continued practice, particular anatomical components might be (or become) better suited for perceiving different kinds of stimulation patterns. As a result, some anatomical components (e.g., the hand or the foot) might be better suited than others (e.g., the back or the head) for the task of perceiving whether a surface affords standing on. This might be a topic of future research.

How Is Soft Assembly of the Haptic System Achieved? A Biotensegrity Hypothesis Understanding the remarkable task specificity and anatomical independence of the haptic system requires a brief description of the haptic system’s organs, scope, span and sensitivity. The haptic system encompasses the entire body and its dual purposes are to (a) orient the parts of the body with respect to each other and (b) establish and maintain contact between the (parts of) the body and objects and surfaces, each by means of transactions of mechanical energy (Gibson, 1966). Examples include locomotion, upright stance, probing, grasping, crawling, reaching, hitting, throwing, hefting, holding, pushing, pulling, among many others. All of these


activities can be performed successfully with or without tools. When used skillfully, a tool becomes an integrated part of the haptic system, allowing for the required transactions of mechanical energy and facilitating the subsequent registering of information. With or without a tool, the charge of the haptic system is to register invariant patterns inherent in a multitude of pressure and force differences in different directions. These invariant patterns specify (affordances for) the behaviors listed above as well as many others. Detection of such patterns is tantamount to percep tion of the body and objects attached to it. The results of the six experiments reported in this article demonstrate that the detection of such specifying invariant patterns occurs across different ana tomical components and across a given anatomical components and an inert object used as a perceptual tool. The results of the experiments reported in this article and those of previously reported experiments suggest that the mechanical stimulation patterns of relevance to the haptic system occur not at the level of individual receptors or at the point of contact with the object but rather at a much larger scale. In particular, the stimu lation patterns of relevance might occur at the level of substrate of haptic perception as a whole—the level at which tissue deforma tions occur and force balance is established between the body and objects attached to it. In such cases, mechanical forces exerted locally are registered globally as a result of the highly intercon nected nature of the haptic system. In that context, it has been argued that wielding an object with the hand (or foot) produces tissue deformation in the whole body, not just in the wielding limb, thereby allowing the individual to exploit the field-like structure of the mechanoreceptors (see Pagano & Turvey, 1998). Moreover, the registration of deformation patterns at such a global scale is consistent not only with the results of the current experiments, but also with demonstrations of the nonlocal perceptual achievements of dynamic touch such as the perception of object properties by means of prosthetic devices (e.g., Carrozza, Cappiello, Micera, Edin, Baccai, & Cipriani, 2006) and by individuals with compro mised somatosensory abilities (see Carello et al., 2008). What is the nature of such a highly sensitive, integrated, and interconnected system? Specifically, what is the geometric and spatial structure of the musculoskeletal medium in which mechan ical contact with the world takes place? Turvey and Fonseca (2014) proposed that the haptic system is a multifractal biotensegrity structure that allows for immense flexibility and adaptability at multiple scales for a multitude of tasks and behavioral functions. Tensegrity structures are geometric configurations that exhibit an interconnection of all parts and a balance of compression and tension forces across the entire system. As such, a mechanical disturbance at any part of the system immediately brings about realignment of the stresses and tensions at each connecting com ponent. In other words, forces applied at one part of the system retune the balance of compression and tension forces in the entire structure. In the case of the haptic system, such an organization is hypothesized to occur in all of the components and at all of the nested levels of the musculoskeletal system, from individual cell to the body as a whole (Turvey & Fonseca, 2014). In light of this proposed architecture of the haptic system, it is easier to appreciate the haptic system (and objects attached to it) as a smart perceptual device. The behavioral expression of a multi fractal biotensegrity architecture is task specificity and anatomical independence. Because of the vast interconnected nature of tenseg

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rity systems and the balance of forces that holds the medium together, all components of the system share equivalent roles in registering, detecting, and acting on the mechanical stimulation patterns that occur as a result of moving the body, bringing the body into contact with surfaces, and manipulating objects. Such equivalence forms the basis of functional specificity in the face of anatomical heterogeneity of components.

A Perceptual Constancy of Perception of and by Means of Wielded Objects Successfully performing everyday behaviors requires not only perception of affordances, but also a perceptual constancy of affordances. That is, perception of affordances for a given behav ior ought to reflect the person’s action capabilities over the variety of circumstances in which that affordance is encountered and over the variety of means by which that affordance might be perceived (Turvey, 1992; Wagman & Malek, 2008; Wagman & Day, 2014). Exploitation of regularities in stimulation patterns by smart per ceptual devices is perhaps the most promising means by which to explain perceptual constancies such as the ones discussed in this article (Gibson, 1979). Specifically, perception of affordances for standing on an inclined surface by means of a wielded object was shown to be invariant across changes in hand, number of hands, configuration of hands, and limbs used to wield the object. Such results are consistent with research showing similar perceptual constancies in both perception of properties of wielded objects and in perception of affordances, more generally. We expect that a similar pattern of results would occur in perception of whether a surface affords walking on (see Kinsella-Shaw, Shaw, & Turvey, 1992). This might be a topic of future research. In general, per ception of properties of wielded objects tend to remain invariant despite changes in the limb (or limbs) used to wield the object (Carello et al., 1992, 2006; Hajnal et al. 2007a,2007b; Palatinus et al., 2011), the dynamics of wielding (Solomon & Turvey, 1988), and the media in which the wielding occurs (Pagano & Cabe, 2003). In all likelihood, such constancy in perception of a wielded object is due to detection of an invariant pattern of resistance to rotational acceleration in different directions about a joint (see Turvey & Carello, 2011). It is likely that detection of an invariant stimulation pattern also underlies the perceptual constancy of affordances for standing on an inclined surface by means of a wielded object. One possibility for such an invariant is texture density gradient of the surface. As the perceiver-actor explores the surface with the rod, the angle between the surface and limb changes at a different rate depending on the steepness of the surface. The accompanying pattern of tissue deformations provides a haptic gradient that may similarly be informative about the steepness of that surface (Fitzpatrick et al., 1994; Regia-Corte & Wagman, 2008). Presumably, such a stimulation pattern would provide information about affordances for standing on an inclined surface despite changes in the means by which the surface is explored. A rigorous testing of this hypothesis might be the topic of future research.

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Received June 18, 2014 Revision received August 21, 2014 Accepted September 16, 2014 ■

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GRAPES-Grounding representations in action, perception, and emotion systems: How object properties and categories are represented in the human brain.

Improvement in specificity of ultrasonography for diagnosis of breast tumors by means of artificial intelligence.

A magnocellular contribution to conscious perception via temporal object segmentation.

Building Tool Use From Object Manipulation: A Perception-Action Perspective.

Investigating the phenomenon of "cognitive-motor interference" in multiple sclerosis by means of dual-task posturography.

Dynamic object perception by pigeons: discrimination of action in video presentations.

The effect of object familiarity on the perception of motion.

Perception of size of one object among many.