Perception, 1979, volume 8, pages 93-103
Auditory texture perception
Susan J Lederman Department of Psychology, Queen's University, Kingston, Ontario K7L 3N6, Canada Received 27 January 1977, in revised form 12 October 1978
Abstract. A set of three studies was designed to investigate the role of touch-produced sounds in the perception of surface texture. Subjects were capable of judging roughness on the basis of sounds alone. Auditory judgments were similar, but not identical to corresponding haptic touch judgments. When both sources of information were available, subjects tended to use the tactile cues. The nature of the auditory stimulus for roughness is considered. 1 Introduction Taylor et al (1973) contend that texture perception at least potentially involves the coordinated action of a number of sensory systems—cutaneous, kinaesthetic, visual, and auditory. Such a multimodal view of texture perception calls for systematic evaluation of the nature of the processing involved and of the relative contributions of the various systems. Can we use each of these sources of information, and equally important, do we? Previous experiments (e.g. Lederman 1974; Taylor and Lederman 1975) have examined the role of cutaneous factors such as skin deformation in the perception of surface roughness. It was not possible to assess the independent contribution of kinaesthesis in these early studies, as subjects actively explored the stimulus surfaces. Recent pilot work indicates, however, that subjects are also capable of evaluating the roughness of surfaces moved across the stationary fingertips. In the earlier studies mentioned, the subjects were denied both auditory and visual information about the surfaces. The present paper evaluates the role of touchproduced sounds in judging surface roughness. The sounds a person produces when touching vary considerably with the materials being touched, the mechanical state of the skin, and the various hand speeds and forces used during exploration. Katz (1925) has shown that people are highly skilled in using such sounds to identify the material of various objects. Can they also use them to differentiate the roughness of surfaces? A recent television commercial provides informal evidence that this may be so. The advertisement begins with a closeup of a young man's face. Half is shaved with one brand of shaving cream, the other half with a competing brand. To show how much closer the shave is with the cream being advertised, a credit card is drawn along the skin of first one and then the other side of the face. The sounds produced make it quite evident which side is smoother. The present set of studies provides more rigorous evidence that people are capable of making such discriminations when cutaneous, kinaesthetic, and visual cues are eliminated. However, judgments of roughness made in the presence of both tactile and auditory cues tend to approximate more closely those made by touch alone than those made by audition alone. 2 Experiments 1 and 2 Lederman and Taylor (1972) and Lederman (1974) found that subjects could not easily differentiate the roughness of regularly grooved surfaces in which the uncut portion between the grooves (the 'land') was the only aspect of the surface to be varied. It is possible that sounds produced by touching such surfaces could serve as
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an additional source of information. If the sounds provide information which conflicts with the tactile cues, and if they are attended to, then estimates of surface roughness made when both auditory and tactile cues are available might prove to be different from those made when the latter are present alone. If, on the other hand, the auditory cues simply complement the tactile cues, or if they are ignored altogether, then with or without the auditory information, estimates of roughness should be essentially the same. In this second situation, the auditory cues might simply improve the reliability of the observer's judgment without altering the actual value of the roughness percept. The first experiment was carried out to determine whether subjects could use only touch-produced sound cues to judge surface roughness. The second experiment provided comparative tactile data, and examined whether subjects would actually use the additional auditory cues to help them in a very difficult tactile roughnessestimation task. In experiment 1, subjects made magnitude estimates of the roughness of a set of grooved plates (varying in land width) by touch-produced sounds alone (condition A). Sounds were generated by the experimenter. In experiment 2, subjects judged the roughness of the same plates used in experiment 1, but under two different conditions. In the first, roughness estimates were obtained when the accompanying sounds were eliminated (condition T); in the second, subjects could use both the tactile and the auditory information they themselves produced in making their estimates of tactile roughness (condition T+A). 2.1 Method 2.1.1 Subjects. Experiment 1. Five students, four females and one male, were paid to participate. Their ages ranged from twenty-three to thirty-five years; all subjects were righthanded, as defined by preferred writing hand, claimed to have normal hearing, and were experimentally naive regarding the present kind of psychophysical experiment. Experiment 2. Six subjects, three females and three males, were paid for their participation. Their ages ranged from nineteen to twenty-seven years. Four of the subjects were students at Queen's University, and two were employed in the psychology department as research assistants. Subjects were right-handed and claimed they had normal hearing. Four of the subjects were experimentally naive; the remaining two had some experience in unrelated tactile experiments. 2.1.2 Apparatus and stimuli. The touch apparatus has been described in detail elsewhere (e.g. Lederman and Taylor 1972). It functions like a classical balance scale, its purpose being to control the force a person exerts when touching a surface. It is possible to remove weights from one end of the balance arm so that the individual has to exert an equal counterbalancing force as s(he) moves her(his) finger across the stimulus surface placed in a tray at the other end. The stimuli used in experiments 1 and 2 were plates of aluminium alloy, 14 cm x 11 -4 cm x 0-5 cm, each with a set of linear grooves cut parallel to the line of the balance arm and extending across the width of the central third of the plate. The groove width and depth of the plates were both 0* 125 mm. The land width varied in 0-125 mm increments from 0-25 to 1-00 mm and there was one additional plate of 0-18 mm land width. There were thus eight stimulus plates in all. Subjects were blindfolded in both experiments, and wore cotton plugs and headphones to eliminate the touch-produced sounds in condition T of experiment 2. All three experiments were run in a sound-proof booth to eliminate extraneous noise.
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2.1.3 Procedure. The experimental procedure was very similar to that described by Lederman and Taylor (1972), the major difference being that the touching in experiment 1 was performed by the experimenter rather than the subject. In the first experiment, the subject sat with her/his back to the apparatus, but directly in front of it so as to hear the touch-produced sounds most clearly. In experiment 2, s(he) sat beside the apparatus and placed her/his right elbow in a padded, swivelled arm support mounted directly above the fulcrum of the balance arm. S(he) was told to move the fingertip of the middle finger back and forth across the stimulus surface (at right angles to the balance arm) at any rate s(he) wished, provided only that the same speed was maintained throughout the experiment. (Subjects generally chose a speed of approximately 5 cm s"1.) There seemed to be little trouble in complying with these instructions. Three forces were used in experiments 1 and 2—28, 112, and 448 g. The experimenter (experiment 1) or subject (experiment 2) pressed with sufficient force to maintain the balance arm steady and level at all times. A modified magnitude-estimation procedure was used with neither standard nor modulus (Lederman and Taylor 1972). Subjects assigned any positive (nonzero) number in proportion to the roughness of the surface. Comparison was made with the immediately preceding plate. Subjects were told they could change their number system across but not within days. Provided the subject responded in terms of ratios, logarithmic transformation of the scores would maintain the same pattern of response in the data regardless of the numbers used across days. Subjects were asked how rough the surfaces 'seemed' in experiment 1 and 'felt' in experiment 2. 2.1.4 Experimental design. Experiment 1. Using only the experimenter-generated sounds, five subjects judged the roughness of eight plates varying in land width, with each plate presented at three forces; these twenty-four conditions were repeated within a day, for six days. The presentation order of the twenty-four trials within replication was selected according to a controlled randomization procedure. Before the experiment began, subjects were given one day's practice in order to accustom themselves to the experiment and to the task of assigning numbers appropriately. The data from this practice session were not used. Experiment 2. A five-factor, completely crossed, factorial design was used in which the factors were subjects (6), days (8), mode of judgement, i.e. condition T or condition T + A (2), force (3), and land width (8). Presentation order of the twentyfour land width x force conditions was again chosen by a controlled randomization procedure. Conditions T and T + A were counterbalanced within each subject across days. As in experiment 1, a practice day was given at the beginning. 2.2 Results The magnitude estimates were logarithmically transformed and subsequently analyzed by analysis of variance. In experiment 1 the land width, force, and land width x force effects were all highly significant (p < 0-0001, p < 0-0005, and p < 0-005, respectively). The land width x force x replication x day effect was also significant (p < 0-05), but was extremely small relative to the other factors and will therefore be ignored. The pattern of responses may be seen in figure la. Lg magnitude estimates of perceived roughness are plotted as a function of lg land width for the three force conditions. The data points represent lg geometric means and are based on fifty responses (five subjects, with two replications and five days per subject). Perceived roughness decreases slightly with increasing land width; however, it is positively related to finger force, and this second effect is quite substantial. The
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differences in roughness due to finger force are somewhat larger for the wide-land plates than they are for the narrow-land surfaces, as indicated by the significant land width x force interaction. The results of experiment 2 are shown in figure lb. As there were no significant differences between conditions T and T + A of touching, the average roughness values have been plotted as a function of lg land width for each of the three force conditions. Land width, force, and land width x force effects are again all highly significant (p < 0-001). The directions of the effects are similar to those found in experiment 1. However, land width appears to have a greater effect on perceived roughness, particularly for the wide-land plates, than in experiment 1, while force seems to affect roughness to a lesser extent. An additional analysis was performed to determine whether subjects' reliability was different when judgments were made by touch alone as compared to touch and audition. The analysis is called ANOVAVA, and uses an estimate of variability as the raw data for an analysis of variance. Details may be found in the appendix of the paper by Lederman and Taylor (1972). Subjects were equally reliable in their judgments whether sound cues supplemented the tactile information or not—none of the effects involving the mode-of-judgment factor was significant. It is not possible to make meaningful comparisons with the reliability of judgments based on sounds alone (experiment 1) because the subjects were different.
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A T+A T A T+A T A T+A T A T+A T low-speed high-speed low-speed high-speed low-force high-force Experimental condition
Figure 3. Experiment 3. (a) Perceived roughness estimates (lg geometric means) for the twelve experimental conditions involving mode of judgment, force and speed, (b) Variance of the lg roughness estimates for the twelve experimental conditions shown in figure 3a. The variances were first calculated within each subject and then pooled. For explanation, see text.
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differentiate between the two force conditions best when audition is used; there are no clear-cut differences among, A, T4-A, and T force-difference scores when high speed is used. When analyzing the effects of speed on perceived roughness, it is imperative that the pattern found in the overall analyses (figure 3a) be checked against the corresponding analyses for individual subjects; initial pilot work indicated that subjects showed marked differences in their use of auditory cues resulting from variation in hand speed. Across subjects (figure 3a), there is a tendency, when pressing with low force, to judge auditory roughness as slightly greater in the highspeed than in the low-speed condition. This finding opposes the effect of speed when touch and audition or touch alone is used; with the latter two modes of judgment, there is a slight tendency for roughness estimates to decrease in the highspeed as compared to the low-speed condition. The overall pattern just described for low force also holds quite well within subjects: seven of eight subjects showed the A pattern (one clearly showed the reverse); all of the subjects showed the corresponding T + A and T patterns just described. The effects of speed when subjects press with the high force can be examined in a similar manner. When data are averaged across subjects, there is a tendency for judgments of roughness to decline with increasing hand speed, regardless of sensory mode. The pattern in conditions T and T + A is once again true of each subject. However, the same cannot be said of the condition A data. Three of the same seven subjects mentioned in the previous paragraph made greater estimates of roughness in the high-speed-high-force condition than in the low-speed-high-force condition, i.e. they performed as they had in the corresponding low-force conditions. But the other four (of seven subjects) now switched to making lower estimates in the highspeed condition. The eighth subject continued (as in the low-force condition) to make lower estimates in the high-speed than in the low-speed condition. Thus, when using the high force, only five of eight subjects showed the effect of speed evident in figure 3 a. If the absolute speed-difference scores for each mode are now examined (figure 3a), it is apparent that the largest differentiation occurs when audition is used (in both force conditions). This pattern is true for six of eight subjects in the low-force condition, but for only four of eight subjects in the high-force trials. Summarizing the effects of hand speed, subjects tend to vary in the way they use the speed-dependent, auditory cues to evaluate roughness. Some consistently tended to increase their estimate as the experimenter moved her hand faster across the surfaces (three of eight subjects), one consistently lowered her estimates, and four subjects tended to do one or the other, depending upon which force was used. In contrast, all subjects consistently lowered their tactile and tactile plus auditory estimates of roughness slightly with increasing hand speed. The absolute condition A speed-difference scores were clearly and consistently larger than those for conditions T + A and T, but only in the low-force trials. The variances of the audition, touch plus audition, and touch estimates for the same twelve experimental conditions shown in figure 3a were also examined. Variances were calculated within subjects and then pooled. Otherwise, the variances of the auditory judgments would be artificially inflated because in condition A, subjects consistently, but differently, judged the effects of hand speed. The means of the variances are shown in figure 3b. There are no differences among the variances for the three modes of judging roughness, except for the low-force-lowspeed condition. In the latter case, auditory judgments are more than twice as variable as touch plus audit on or touch estimates. Nonetheless, it is clear (figure 3 a)
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that the touch plus audition roughness estimates are closer to those for touch than for audition in all four force x speed conditions. The latter pattern is consistent within as well as across all subjects. 4 General discussion In all three modes of perceiving, roughness is a monotonically increasing function of groove width. The less steep arm of the tactile curve occurs when narrow-grooved surfaces are presented. This finding replicates earlier studies, e.g. Lederman (1974). I suggested at that time that the flatter portion of the psychophysical roughness curve is due to masking of the narrow-grooved surfaces (involving only the narrowest groove width here in experiment 3) by surface irregularities on the lands (caused by the stimulus production technique). The remaining portion of the curve, it will be observed, is essentially linear, the usual power function obtained with prothetic continua. The slope, m, of the straight line which best fits this portion is 1-21 (r = 0-99); when all data points are included, m = 0-94 for condition T and r = 0-98. Such roughness functions are less steep than that reported by Stevens and Harris (1962; m = 1-5); however, a difference is not surprising since it is highly unlikely that roughness is a function of a single physical dimension. There are numerous differences between the sandpaper stimuli used by Stevens and Harris, and the regularly grooved metal surfaces used here, e.g. height, regularity, and contour of the surface bumps and valleys, heat capacity and conductivity, etc. Steepness of the tactile roughness functions could well change depending upon which stimulus dimension(s) was employed. As for the auditory roughness data, they too fail to be described overall by a power function, although the reason(s) may be different. As will be discussed below, auditory-roughness judgments may combine both prothetic and metathetic characteristics to the extent that both intensity and frequency cues are used, respectively. However, to permit gross comparisons with the tactile function, straight lines were fitted to the auditory curve, once with the data point for the narrowest groove width excluded, and once with it retained irn = 0-63, r = 0-97 and m = 0-40, r = 0-91, respectively). The 99% confidence interval for a mean slope of 0-40 (calculated from the slopes of the individual subject functions) does not include zero. This result indicates that auditory roughness discrimination of surfaces varying in groove width is possible; however, it is poorer than the corresponding tactile or tactile plus auditory discriminations. The T + A function is fitted by straight lines with slopes and correlation coefficients very similar to those for touch (i.e. m = 0-91, r = 0-97, when all data points are included; m = 1-24, r = 0-99 for the reduced analysis). Considering the results of all three experiments, it is evident that observers' judgments of auditory roughness reflect their ability to recognize changes due to alterations in groove and land width, as well as in finger force and hand speed. Subjects might be using at least two aspects of the changing auditory cues to judge roughness. The first is the pitch of the sounds, and the second, loudness. What changes in these percepts can be noted as the major parameters of the present experiments are altered? We have casually observed in the lab that pitch tends to decrease with increasing groove width. Might subjects have judged auditory roughness by increasing their estimates as the pitch decreased? It is not possible to consider the effects of land width in a similar manner because casual judgments of pitch are too difficult to make. Relative loudness of sounds produced by touching these different surfaces is also difficult to evaluate without the aid of controlled psychophysical analysis.
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The very noticeable increase in loudness produced by increasing finger force does suggest at least one condition in which subjects might use increasing loudness as an index of increased auditory roughness. Some support for the use of loudness cues is offered by the casual observation that pitch tends to decrease with increasing finger force. Presumably, subjects' judgments of roughness would tend to decrease, not increase, as a function of force if pitch cues were used. Finally, one distinctly observes that as hand speed is increased, both pitch and loudness tend to increase. This state of affairs poses an interesting conflict for the subject. If s(he) uses pitch cues, s(he) should estimate that perceived roughness decreases with increasing speed. However, if s(he) prefers to attend to changes in loudness, then s(he) should judge the roughness as increasing. The results from pilot studies and experiment 3 indicate that while some subjects consistently use either pitch or loudness cues, several other individuals would appear to attend to pitch when using one finger force, and to loudness in the other force condition. From the discussion above, subjects might well be using both pitch and loudness in their evaluation of auditory roughness. However, such discussion remains necessarily speculative until direct measurements of auditory pitch and loudness of the touch-produced sounds are obtained. Spectrographic analyses would also provide important information regarding corresponding changes in frequency and intensity. Although subjects are able to evaluate roughness by sounds alone, when presented in the company of the tactile cues which produce them, the auditory cues seem to be ignored. It seems reasonable that subjects would so do when judging the roughness of the various groove-varying and land-varying surfaces, because the data suggest that tactile cues are more informative. Perhaps the shallowness of the auditory functions is due in part to the relative differences in auditory pitch discrimination. The fundamental frequency produced when the hand touches a surface with a fixed speed increases as groove and land widths narrow. But the jnd for pitch discrimination, A/, grows with increasing frequency. Poor auditory resolution, particularly of the narrow grooves and lands, might therefore result in correspondingly shallower psychophysical functions (see figures la and 2). But why did subjects tend to ignore touch-produced sounds in situations where audition seemed to do a better job than touch, i.e. when differentiating the effects of finger force (at low speed) or of hand speed (at low force)? Figure 3b suggests that there was relatively more confusion in the auditory-texture judgments made in the low-force-low-speed condition than in any of the other A or T conditions. Perhaps, then, subjects chose to ignore sounds altogether when the more reliable tactile alternative was simultaneously available (T + A). One can also use a more general version of the argument above. Outside the artificially quiet booth, extraneous sounds often mask those produced by haptic examination of something textured, such as a piece of fabric or wood. Under everyday circumstances, therefore, it would seem reasonable to attend primarily to the tactile cues—they provide good information about texture, and are available at all times. Perhaps people behave similarly in the soundproof environment because they are accustomed to judging texture on the basis of tactile information. Thus, touch-produced sounds, though usable, may occur too irregularly or infrequently to play as significant a role in roughness perception as do tactile cues. 5 Summary Subjects could judge the roughness of plates varying in groove and land width, using only the sounds produced by touching the surfaces. The auditory judgments were similar to, but not as disciminating, as those made by touch alone or by touch plus audition. When the finger force was increased, subjects tended to increase all
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auditory estimates of roughness, as was the case with the other two modes of judgment. However, in the low-speed condition, subjects tended to differentiate the effects of finger force better by audition than by touch or by touch plus audition; the three modes of judging the effects of force were about equal when the high hand speed was used. The effect of speed on auditory roughness was not so clear-cut. Some subjects consistently increased their estimates as the experimenter's hand speed increased, one subject consistently lowered her estimates, and the remaining subjects chose to do one or the other depending upon which force was used. But all subjects consistently lowered their tactile and tactile plus auditory judgments slightly as speed increased. Audition tended to be better than touch or touch plus audition, though only in the low-force condition, at differentiating the effects of hand speed on roughness. The three modes of judgment were about equally variable, except when touchproduced sounds were used in the low-force-low-speed condition. In the latter case, the variance was about twice as large as for the T or T + A judgments. Nevertheless, whenever both haptic and auditory sources of information were available, subjects tended to use the tactile cues to judge surface roughness. Acknowledgements. This research was supported by NRC grant A9854. I would like to thank Dr Lola Cuddy for the use of the soundproof booth. I would also like to thank Lynne Baxter and Denise Kinch. References Katz D, 1925 Der Aufbau der Tastwelt {The World of Touch) (Leipzig: Barth) Lederman S J, 1974 "Tactile roughness of grooved surfaces: the touching process and effects of macro- and microsurface structure"'Perception and Psychophysics 16(2) 385-395 Lederman S J, Taylor M M, 1972 "Fingertip force, surface geometry, and the perception of roughness by active touch" Perception and Psychophysics 12 401-408 Stevens S S, Harris J, 1962 "The scaling of subjective roughness and smoothness" Journal of Experimental Psychology 64(5) 498-494 Taylor M M, Lederman S J, 1975 "Tactile roughness of grooved surfaces: a model and the effect of friction" Perception and Psychophysics 17(1) 23-36 Taylor M M, Lederman S J, Gibson R H, 1973 "Tactile perception of texture" in Handbook of Perception Volume III Eds E Carterette, M Friedman (New York: Academic Press)
Auditory texture perception
Susan J Lederman Department of Psychology, Queen's University, Kingston, Ontario K7L 3N6, Canada Received 27 January 1977, in revised form 12 October 1978
Abstract. A set of three studies was designed to investigate the role of touch-produced sounds in the perception of surface texture. Subjects were capable of judging roughness on the basis of sounds alone. Auditory judgments were similar, but not identical to corresponding haptic touch judgments. When both sources of information were available, subjects tended to use the tactile cues. The nature of the auditory stimulus for roughness is considered. 1 Introduction Taylor et al (1973) contend that texture perception at least potentially involves the coordinated action of a number of sensory systems—cutaneous, kinaesthetic, visual, and auditory. Such a multimodal view of texture perception calls for systematic evaluation of the nature of the processing involved and of the relative contributions of the various systems. Can we use each of these sources of information, and equally important, do we? Previous experiments (e.g. Lederman 1974; Taylor and Lederman 1975) have examined the role of cutaneous factors such as skin deformation in the perception of surface roughness. It was not possible to assess the independent contribution of kinaesthesis in these early studies, as subjects actively explored the stimulus surfaces. Recent pilot work indicates, however, that subjects are also capable of evaluating the roughness of surfaces moved across the stationary fingertips. In the earlier studies mentioned, the subjects were denied both auditory and visual information about the surfaces. The present paper evaluates the role of touchproduced sounds in judging surface roughness. The sounds a person produces when touching vary considerably with the materials being touched, the mechanical state of the skin, and the various hand speeds and forces used during exploration. Katz (1925) has shown that people are highly skilled in using such sounds to identify the material of various objects. Can they also use them to differentiate the roughness of surfaces? A recent television commercial provides informal evidence that this may be so. The advertisement begins with a closeup of a young man's face. Half is shaved with one brand of shaving cream, the other half with a competing brand. To show how much closer the shave is with the cream being advertised, a credit card is drawn along the skin of first one and then the other side of the face. The sounds produced make it quite evident which side is smoother. The present set of studies provides more rigorous evidence that people are capable of making such discriminations when cutaneous, kinaesthetic, and visual cues are eliminated. However, judgments of roughness made in the presence of both tactile and auditory cues tend to approximate more closely those made by touch alone than those made by audition alone. 2 Experiments 1 and 2 Lederman and Taylor (1972) and Lederman (1974) found that subjects could not easily differentiate the roughness of regularly grooved surfaces in which the uncut portion between the grooves (the 'land') was the only aspect of the surface to be varied. It is possible that sounds produced by touching such surfaces could serve as
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an additional source of information. If the sounds provide information which conflicts with the tactile cues, and if they are attended to, then estimates of surface roughness made when both auditory and tactile cues are available might prove to be different from those made when the latter are present alone. If, on the other hand, the auditory cues simply complement the tactile cues, or if they are ignored altogether, then with or without the auditory information, estimates of roughness should be essentially the same. In this second situation, the auditory cues might simply improve the reliability of the observer's judgment without altering the actual value of the roughness percept. The first experiment was carried out to determine whether subjects could use only touch-produced sound cues to judge surface roughness. The second experiment provided comparative tactile data, and examined whether subjects would actually use the additional auditory cues to help them in a very difficult tactile roughnessestimation task. In experiment 1, subjects made magnitude estimates of the roughness of a set of grooved plates (varying in land width) by touch-produced sounds alone (condition A). Sounds were generated by the experimenter. In experiment 2, subjects judged the roughness of the same plates used in experiment 1, but under two different conditions. In the first, roughness estimates were obtained when the accompanying sounds were eliminated (condition T); in the second, subjects could use both the tactile and the auditory information they themselves produced in making their estimates of tactile roughness (condition T+A). 2.1 Method 2.1.1 Subjects. Experiment 1. Five students, four females and one male, were paid to participate. Their ages ranged from twenty-three to thirty-five years; all subjects were righthanded, as defined by preferred writing hand, claimed to have normal hearing, and were experimentally naive regarding the present kind of psychophysical experiment. Experiment 2. Six subjects, three females and three males, were paid for their participation. Their ages ranged from nineteen to twenty-seven years. Four of the subjects were students at Queen's University, and two were employed in the psychology department as research assistants. Subjects were right-handed and claimed they had normal hearing. Four of the subjects were experimentally naive; the remaining two had some experience in unrelated tactile experiments. 2.1.2 Apparatus and stimuli. The touch apparatus has been described in detail elsewhere (e.g. Lederman and Taylor 1972). It functions like a classical balance scale, its purpose being to control the force a person exerts when touching a surface. It is possible to remove weights from one end of the balance arm so that the individual has to exert an equal counterbalancing force as s(he) moves her(his) finger across the stimulus surface placed in a tray at the other end. The stimuli used in experiments 1 and 2 were plates of aluminium alloy, 14 cm x 11 -4 cm x 0-5 cm, each with a set of linear grooves cut parallel to the line of the balance arm and extending across the width of the central third of the plate. The groove width and depth of the plates were both 0* 125 mm. The land width varied in 0-125 mm increments from 0-25 to 1-00 mm and there was one additional plate of 0-18 mm land width. There were thus eight stimulus plates in all. Subjects were blindfolded in both experiments, and wore cotton plugs and headphones to eliminate the touch-produced sounds in condition T of experiment 2. All three experiments were run in a sound-proof booth to eliminate extraneous noise.
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2.1.3 Procedure. The experimental procedure was very similar to that described by Lederman and Taylor (1972), the major difference being that the touching in experiment 1 was performed by the experimenter rather than the subject. In the first experiment, the subject sat with her/his back to the apparatus, but directly in front of it so as to hear the touch-produced sounds most clearly. In experiment 2, s(he) sat beside the apparatus and placed her/his right elbow in a padded, swivelled arm support mounted directly above the fulcrum of the balance arm. S(he) was told to move the fingertip of the middle finger back and forth across the stimulus surface (at right angles to the balance arm) at any rate s(he) wished, provided only that the same speed was maintained throughout the experiment. (Subjects generally chose a speed of approximately 5 cm s"1.) There seemed to be little trouble in complying with these instructions. Three forces were used in experiments 1 and 2—28, 112, and 448 g. The experimenter (experiment 1) or subject (experiment 2) pressed with sufficient force to maintain the balance arm steady and level at all times. A modified magnitude-estimation procedure was used with neither standard nor modulus (Lederman and Taylor 1972). Subjects assigned any positive (nonzero) number in proportion to the roughness of the surface. Comparison was made with the immediately preceding plate. Subjects were told they could change their number system across but not within days. Provided the subject responded in terms of ratios, logarithmic transformation of the scores would maintain the same pattern of response in the data regardless of the numbers used across days. Subjects were asked how rough the surfaces 'seemed' in experiment 1 and 'felt' in experiment 2. 2.1.4 Experimental design. Experiment 1. Using only the experimenter-generated sounds, five subjects judged the roughness of eight plates varying in land width, with each plate presented at three forces; these twenty-four conditions were repeated within a day, for six days. The presentation order of the twenty-four trials within replication was selected according to a controlled randomization procedure. Before the experiment began, subjects were given one day's practice in order to accustom themselves to the experiment and to the task of assigning numbers appropriately. The data from this practice session were not used. Experiment 2. A five-factor, completely crossed, factorial design was used in which the factors were subjects (6), days (8), mode of judgement, i.e. condition T or condition T + A (2), force (3), and land width (8). Presentation order of the twentyfour land width x force conditions was again chosen by a controlled randomization procedure. Conditions T and T + A were counterbalanced within each subject across days. As in experiment 1, a practice day was given at the beginning. 2.2 Results The magnitude estimates were logarithmically transformed and subsequently analyzed by analysis of variance. In experiment 1 the land width, force, and land width x force effects were all highly significant (p < 0-0001, p < 0-0005, and p < 0-005, respectively). The land width x force x replication x day effect was also significant (p < 0-05), but was extremely small relative to the other factors and will therefore be ignored. The pattern of responses may be seen in figure la. Lg magnitude estimates of perceived roughness are plotted as a function of lg land width for the three force conditions. The data points represent lg geometric means and are based on fifty responses (five subjects, with two replications and five days per subject). Perceived roughness decreases slightly with increasing land width; however, it is positively related to finger force, and this second effect is quite substantial. The
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differences in roughness due to finger force are somewhat larger for the wide-land plates than they are for the narrow-land surfaces, as indicated by the significant land width x force interaction. The results of experiment 2 are shown in figure lb. As there were no significant differences between conditions T and T + A of touching, the average roughness values have been plotted as a function of lg land width for each of the three force conditions. Land width, force, and land width x force effects are again all highly significant (p < 0-001). The directions of the effects are similar to those found in experiment 1. However, land width appears to have a greater effect on perceived roughness, particularly for the wide-land plates, than in experiment 1, while force seems to affect roughness to a lesser extent. An additional analysis was performed to determine whether subjects' reliability was different when judgments were made by touch alone as compared to touch and audition. The analysis is called ANOVAVA, and uses an estimate of variability as the raw data for an analysis of variance. Details may be found in the appendix of the paper by Lederman and Taylor (1972). Subjects were equally reliable in their judgments whether sound cues supplemented the tactile information or not—none of the effects involving the mode-of-judgment factor was significant. It is not possible to make meaningful comparisons with the reliability of judgments based on sounds alone (experiment 1) because the subjects were different.
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A T+A T A T+A T A T+A T A T+A T low-speed high-speed low-speed high-speed low-force high-force Experimental condition
Figure 3. Experiment 3. (a) Perceived roughness estimates (lg geometric means) for the twelve experimental conditions involving mode of judgment, force and speed, (b) Variance of the lg roughness estimates for the twelve experimental conditions shown in figure 3a. The variances were first calculated within each subject and then pooled. For explanation, see text.
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differentiate between the two force conditions best when audition is used; there are no clear-cut differences among, A, T4-A, and T force-difference scores when high speed is used. When analyzing the effects of speed on perceived roughness, it is imperative that the pattern found in the overall analyses (figure 3a) be checked against the corresponding analyses for individual subjects; initial pilot work indicated that subjects showed marked differences in their use of auditory cues resulting from variation in hand speed. Across subjects (figure 3a), there is a tendency, when pressing with low force, to judge auditory roughness as slightly greater in the highspeed than in the low-speed condition. This finding opposes the effect of speed when touch and audition or touch alone is used; with the latter two modes of judgment, there is a slight tendency for roughness estimates to decrease in the highspeed as compared to the low-speed condition. The overall pattern just described for low force also holds quite well within subjects: seven of eight subjects showed the A pattern (one clearly showed the reverse); all of the subjects showed the corresponding T + A and T patterns just described. The effects of speed when subjects press with the high force can be examined in a similar manner. When data are averaged across subjects, there is a tendency for judgments of roughness to decline with increasing hand speed, regardless of sensory mode. The pattern in conditions T and T + A is once again true of each subject. However, the same cannot be said of the condition A data. Three of the same seven subjects mentioned in the previous paragraph made greater estimates of roughness in the high-speed-high-force condition than in the low-speed-high-force condition, i.e. they performed as they had in the corresponding low-force conditions. But the other four (of seven subjects) now switched to making lower estimates in the highspeed condition. The eighth subject continued (as in the low-force condition) to make lower estimates in the high-speed than in the low-speed condition. Thus, when using the high force, only five of eight subjects showed the effect of speed evident in figure 3 a. If the absolute speed-difference scores for each mode are now examined (figure 3a), it is apparent that the largest differentiation occurs when audition is used (in both force conditions). This pattern is true for six of eight subjects in the low-force condition, but for only four of eight subjects in the high-force trials. Summarizing the effects of hand speed, subjects tend to vary in the way they use the speed-dependent, auditory cues to evaluate roughness. Some consistently tended to increase their estimate as the experimenter moved her hand faster across the surfaces (three of eight subjects), one consistently lowered her estimates, and four subjects tended to do one or the other, depending upon which force was used. In contrast, all subjects consistently lowered their tactile and tactile plus auditory estimates of roughness slightly with increasing hand speed. The absolute condition A speed-difference scores were clearly and consistently larger than those for conditions T + A and T, but only in the low-force trials. The variances of the audition, touch plus audition, and touch estimates for the same twelve experimental conditions shown in figure 3a were also examined. Variances were calculated within subjects and then pooled. Otherwise, the variances of the auditory judgments would be artificially inflated because in condition A, subjects consistently, but differently, judged the effects of hand speed. The means of the variances are shown in figure 3b. There are no differences among the variances for the three modes of judging roughness, except for the low-force-lowspeed condition. In the latter case, auditory judgments are more than twice as variable as touch plus audit on or touch estimates. Nonetheless, it is clear (figure 3 a)
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that the touch plus audition roughness estimates are closer to those for touch than for audition in all four force x speed conditions. The latter pattern is consistent within as well as across all subjects. 4 General discussion In all three modes of perceiving, roughness is a monotonically increasing function of groove width. The less steep arm of the tactile curve occurs when narrow-grooved surfaces are presented. This finding replicates earlier studies, e.g. Lederman (1974). I suggested at that time that the flatter portion of the psychophysical roughness curve is due to masking of the narrow-grooved surfaces (involving only the narrowest groove width here in experiment 3) by surface irregularities on the lands (caused by the stimulus production technique). The remaining portion of the curve, it will be observed, is essentially linear, the usual power function obtained with prothetic continua. The slope, m, of the straight line which best fits this portion is 1-21 (r = 0-99); when all data points are included, m = 0-94 for condition T and r = 0-98. Such roughness functions are less steep than that reported by Stevens and Harris (1962; m = 1-5); however, a difference is not surprising since it is highly unlikely that roughness is a function of a single physical dimension. There are numerous differences between the sandpaper stimuli used by Stevens and Harris, and the regularly grooved metal surfaces used here, e.g. height, regularity, and contour of the surface bumps and valleys, heat capacity and conductivity, etc. Steepness of the tactile roughness functions could well change depending upon which stimulus dimension(s) was employed. As for the auditory roughness data, they too fail to be described overall by a power function, although the reason(s) may be different. As will be discussed below, auditory-roughness judgments may combine both prothetic and metathetic characteristics to the extent that both intensity and frequency cues are used, respectively. However, to permit gross comparisons with the tactile function, straight lines were fitted to the auditory curve, once with the data point for the narrowest groove width excluded, and once with it retained irn = 0-63, r = 0-97 and m = 0-40, r = 0-91, respectively). The 99% confidence interval for a mean slope of 0-40 (calculated from the slopes of the individual subject functions) does not include zero. This result indicates that auditory roughness discrimination of surfaces varying in groove width is possible; however, it is poorer than the corresponding tactile or tactile plus auditory discriminations. The T + A function is fitted by straight lines with slopes and correlation coefficients very similar to those for touch (i.e. m = 0-91, r = 0-97, when all data points are included; m = 1-24, r = 0-99 for the reduced analysis). Considering the results of all three experiments, it is evident that observers' judgments of auditory roughness reflect their ability to recognize changes due to alterations in groove and land width, as well as in finger force and hand speed. Subjects might be using at least two aspects of the changing auditory cues to judge roughness. The first is the pitch of the sounds, and the second, loudness. What changes in these percepts can be noted as the major parameters of the present experiments are altered? We have casually observed in the lab that pitch tends to decrease with increasing groove width. Might subjects have judged auditory roughness by increasing their estimates as the pitch decreased? It is not possible to consider the effects of land width in a similar manner because casual judgments of pitch are too difficult to make. Relative loudness of sounds produced by touching these different surfaces is also difficult to evaluate without the aid of controlled psychophysical analysis.
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The very noticeable increase in loudness produced by increasing finger force does suggest at least one condition in which subjects might use increasing loudness as an index of increased auditory roughness. Some support for the use of loudness cues is offered by the casual observation that pitch tends to decrease with increasing finger force. Presumably, subjects' judgments of roughness would tend to decrease, not increase, as a function of force if pitch cues were used. Finally, one distinctly observes that as hand speed is increased, both pitch and loudness tend to increase. This state of affairs poses an interesting conflict for the subject. If s(he) uses pitch cues, s(he) should estimate that perceived roughness decreases with increasing speed. However, if s(he) prefers to attend to changes in loudness, then s(he) should judge the roughness as increasing. The results from pilot studies and experiment 3 indicate that while some subjects consistently use either pitch or loudness cues, several other individuals would appear to attend to pitch when using one finger force, and to loudness in the other force condition. From the discussion above, subjects might well be using both pitch and loudness in their evaluation of auditory roughness. However, such discussion remains necessarily speculative until direct measurements of auditory pitch and loudness of the touch-produced sounds are obtained. Spectrographic analyses would also provide important information regarding corresponding changes in frequency and intensity. Although subjects are able to evaluate roughness by sounds alone, when presented in the company of the tactile cues which produce them, the auditory cues seem to be ignored. It seems reasonable that subjects would so do when judging the roughness of the various groove-varying and land-varying surfaces, because the data suggest that tactile cues are more informative. Perhaps the shallowness of the auditory functions is due in part to the relative differences in auditory pitch discrimination. The fundamental frequency produced when the hand touches a surface with a fixed speed increases as groove and land widths narrow. But the jnd for pitch discrimination, A/, grows with increasing frequency. Poor auditory resolution, particularly of the narrow grooves and lands, might therefore result in correspondingly shallower psychophysical functions (see figures la and 2). But why did subjects tend to ignore touch-produced sounds in situations where audition seemed to do a better job than touch, i.e. when differentiating the effects of finger force (at low speed) or of hand speed (at low force)? Figure 3b suggests that there was relatively more confusion in the auditory-texture judgments made in the low-force-low-speed condition than in any of the other A or T conditions. Perhaps, then, subjects chose to ignore sounds altogether when the more reliable tactile alternative was simultaneously available (T + A). One can also use a more general version of the argument above. Outside the artificially quiet booth, extraneous sounds often mask those produced by haptic examination of something textured, such as a piece of fabric or wood. Under everyday circumstances, therefore, it would seem reasonable to attend primarily to the tactile cues—they provide good information about texture, and are available at all times. Perhaps people behave similarly in the soundproof environment because they are accustomed to judging texture on the basis of tactile information. Thus, touch-produced sounds, though usable, may occur too irregularly or infrequently to play as significant a role in roughness perception as do tactile cues. 5 Summary Subjects could judge the roughness of plates varying in groove and land width, using only the sounds produced by touching the surfaces. The auditory judgments were similar to, but not as disciminating, as those made by touch alone or by touch plus audition. When the finger force was increased, subjects tended to increase all
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auditory estimates of roughness, as was the case with the other two modes of judgment. However, in the low-speed condition, subjects tended to differentiate the effects of finger force better by audition than by touch or by touch plus audition; the three modes of judging the effects of force were about equal when the high hand speed was used. The effect of speed on auditory roughness was not so clear-cut. Some subjects consistently increased their estimates as the experimenter's hand speed increased, one subject consistently lowered her estimates, and the remaining subjects chose to do one or the other depending upon which force was used. But all subjects consistently lowered their tactile and tactile plus auditory judgments slightly as speed increased. Audition tended to be better than touch or touch plus audition, though only in the low-force condition, at differentiating the effects of hand speed on roughness. The three modes of judgment were about equally variable, except when touchproduced sounds were used in the low-force-low-speed condition. In the latter case, the variance was about twice as large as for the T or T + A judgments. Nevertheless, whenever both haptic and auditory sources of information were available, subjects tended to use the tactile cues to judge surface roughness. Acknowledgements. This research was supported by NRC grant A9854. I would like to thank Dr Lola Cuddy for the use of the soundproof booth. I would also like to thank Lynne Baxter and Denise Kinch. References Katz D, 1925 Der Aufbau der Tastwelt {The World of Touch) (Leipzig: Barth) Lederman S J, 1974 "Tactile roughness of grooved surfaces: the touching process and effects of macro- and microsurface structure"'Perception and Psychophysics 16(2) 385-395 Lederman S J, Taylor M M, 1972 "Fingertip force, surface geometry, and the perception of roughness by active touch" Perception and Psychophysics 12 401-408 Stevens S S, Harris J, 1962 "The scaling of subjective roughness and smoothness" Journal of Experimental Psychology 64(5) 498-494 Taylor M M, Lederman S J, 1975 "Tactile roughness of grooved surfaces: a model and the effect of friction" Perception and Psychophysics 17(1) 23-36 Taylor M M, Lederman S J, Gibson R H, 1973 "Tactile perception of texture" in Handbook of Perception Volume III Eds E Carterette, M Friedman (New York: Academic Press)
