Somatosensory & Motor Research

ISSN: 0899-0220 (Print) 1369-1651 (Online) Journal homepage: http://www.tandfonline.com/loi/ismr20

Characterization of the Percepts Evoked by Discontinuous Motion over the Perioral Skin G. K. Essick, M. McGuire, A. Joseph & O. Franzen To cite this article: G. K. Essick, M. McGuire, A. Joseph & O. Franzen (1992) Characterization of the Percepts Evoked by Discontinuous Motion over the Perioral Skin, Somatosensory & Motor Research, 9:2, 175-184 To link to this article: http://dx.doi.org/10.3109/08990229209144769

Published online: 10 Jul 2009.

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Characterization of the Percepts Evoked by Discontinuous Motion over the Perioral Skin G. K. Essick,**'M. McGuire,+ A. Joseph,* and 0. m e n s *Dental Research Center and ?School of Medicine, University of North Carolina, Chapel Hill, North Carolina 27514; $Departments of Anatomy and Psychology, Uppsala University, Uppsala, Sweden

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Absrract The capacity of human subjects to process information about discontinuous and continuous movement was evaluated. Constant-velocity brushing stimuliwere delivered through apertureplates that rested lightly upon the mandibular skin. Each plate consisted of either two spatially separated, slit-like openings or a single continuous, longer opening. It was discovered that percepts of smooth apparent motion were achieved with the split apertures (i.e., from discontinuous movement) for only limited ranges of stimulus velocity. Moreover, the optimal velocity supporting smooth apparent motion increased with the separation between the slit-like openings. In a second series of experiments, subjects' ability to discriminate opposing directions of discontinuousand continuous movement was evaluated. It was found that subjects could derive directional information from percepts elicited by discontinuous movement. However, the capacity to discriminateopposing directions of continuous movement cannot be explained solely in terms of the ability to process information about the change in position of a stimulus from its onset to its offset. Key words facial, sensitivity, motion, apparent, direction

Recently published studies from our laboratory have shown that human subjects possess appreciable capacity for processing information about tactile stimuli that move smoothly and continuously across the skin of the face (Essick et al., 1988, 1989, 1990, 1991). Moreover, the percept of direction of motion was shown to be highly dependent on both the velocity at which the stimuli are delivered (Essick et al., 1988) and the length of skin traversed (Essick et al., 1989,1990). The primary purpose of the present study was to obtain preliminary information regarding subjects' use of cues signaling overall change in stimulus position to distinguish opposing directions of movement across the p e r i o d skin. The fact that different types of cues (e.g., position and time vs. velocity) may be used to process information about motion was recognized early in regard to vision (Exner, 1875) and somatosensation (Hall and Donaldson, 1885, p. 572), and has received much attention in recent 1. To whom all correspondence should be addressed, at 02 Dental Research Center, CB #7455, Chapel Hill, North Carolina 275597455.

Somatosensory and Motor Research,

Vol. 9, No. 2, 1992, pp. 175-184

years from visual psychophysicists (see Perception, Vol. 14,1985; see also Kolers, 1972, and Anstis, 1980). In contrast, few somatosensory psychophysical studies have addressed this issue. Over I0 years ago, Dreyer and coworkers reported that human subjects may use information about the initial and final positions of a slowly moving tactile stimulus to distinguish opposing directions of motion (Dreyer et al., 1978). In a subsequent study, these investigators measured human performance on a direction discrimination task during which brushing stimuli were delivered to the ventral forearm at velocities ranging from 0.5 to 250 cdsec. The apertures through which the stimuli were delivered consisted of single openings 1, 2, 3, 4, 5 , and 6 cm long, as well as two 0.25-cm long slit-like openings, separated by distances such that the outer dimension of the separated openings matched the lengths of the continuous apertures. Consistent with their hypothesis, Dreyer and colleagues discovered that at slow velocities subjects' ability to identify stimulus direction was similar for the continuous and split apertures, even though the area of skin stimAccepted March 3. 1992

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ESSICK ET AL.

ulated through the split apertures was 50-90% less than that stimulated through the continuous apertures. At all other velocities, the performance achieved with each continuous aperture was higher than that attained with the split aperture of overall comparable length. Moreover, it was observed that for the split apertures, direction discrimination increased with increasing separation between the two slit-like openings. These findings, as well as preliminary observations made in our laboratory with a split aperture, have been reported in abstract form (Dreyer et al., 1979; Essick and McGuire, 1986).

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MATERIALS AND METHODS Two experiments were conducted with 13 neurologically normal human volunteers (not all subjects participated in both experiments; see below). In all testing sessions, brushing stimuli were applied to the same region of facial skin innervated by the mental branch of the mandibular nerve, and were controlled for velocity, direction, the length of skin traversed, and the width of skin contacted. This mode of stimulation has been successfully employed and is described in detail in our previous human psychophysical and primate neurophysiological experiments (Essick and W t s e l , 1985a,b). To summarize briefly, an aperture was cut into a Teflon plate, which was placed lightly on the skin to define the area to be stimulated and to minimize skin stretch by the stimuli. The Teflon around the aperture was beveled so that the brush bristles maintained a constant degree of deflection as they moved onto and off the skin. The computer-interfaced servomotor was equipped with a soft artist’s brush that applied no more than 5 g of force to the skin, independent of the velocity of brush motion (Franzen et al., 1984). The width of the brush exceeded the width of the aperture, so that the total area of exposed skin was contacted by the moving bristles. The velocities and aperture dimensions were selected on the basis of results obtained from preliminary studies in which tactile acuity within the distribution of the mental nerve was assessed. In Experiment 1, the apparent velocities supporting the percept of smooth apparent motion from delivery of discontinuous movement were determined. In Experiment 2, subjects’ ability to distinguish opposing directions of discontinuously and continuously moving tactile stimuli was evaluated. Experiment I

Each of six subjects participated in four sessions during which brushing stimuli were delivered across a split aperture positioned on the face, directly over the mental foramen and parallel to the inferior border of the man176

dible. A split aperture consisting of two parallel slitlike openings (0.25 cm long and 1.0 cm wide, separated by 0.25 cm), providing a “traverse length” of 0.75 cm, was used in two of the sessions. A split aperture providing a traverse length of 1.5 cm (i.e., the two slitlike openings were separated by 1.0 cm) was used in the other two sessions. Twenty-five brush strokes moving from lateral to medial (i-e., toward the chin) at each of 10 velocities were delivered during each session in random order. The velocity range used differed slightly for each aperture: 0.5-64 c d s e c for the 0.75cm-long split aperture, and 8-128 c d s e c for the 1.5cm-long split aperture. The subject was cued approximately 2 sec before each stimulus as to trial initiation, and signaled his or her response by pressing one of three push buttons denoting the percept of choppy succession, smooth continuous motion, or simultaneity (hereafter denoted as SUCCESSION, SMOOTH MOTION, and SIMULTANEITY,respectively). Before any data were collected, each subject participated in at least one training session with each aperture, thus learning to establish and maintain criteria for these responses. After completion of the 24 data collection sessions, the relative frequency with which each response was reported was calculated for each subject, velocity, and aperture. Experiment 2

Nine subjects contributed to the second experiment, which sought to determine the relative importance to directional sensitivity of cues made available at the times of stimulus onset and offset. After a minimum of 2 hr of training on the task, each subject participated in eight experimental sessions during which directional sensitivity was measured for stimuli delivered at one of two sets of velocities (0.5, 2, 6, 12, and 32 c d s e c , or 1, 4, 8, 16, and 64 cdsec) and over one of four apertures that differed in length, width, and continuity (i.e., sizekhape; see Fig. 2, below). The combinations of velocity set and aperture type were randomized for each subject. Each session consisted of 240 trials, 48 trials for each of the five test velocities. A temporal two-alternative forced-choice paradigm was employed to minimize response bias (Gescheider, 1985; see also Essick et al., 1990). Specifically, during each trial the brush moved toward the subject’s chin during one stimulus interval, and away from the subject’s chin (toward the ear)during the other stimulus interval. The stimuli moved at the same velocity during both intervals. After each trial the subject attempted to identify the interval (first or second) in which the brush moved toward his or her chin. The 240 trials were randomized as to (1) the velocity of movement and (2) the interval in which the brush

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moved toward the chin. Feedback as to correctness was offered after each trial, and a minimum 3.5-sec intertrial interval was observed. After each session, the subject’s capacity to distinguish opposing directions of stimulus motion at each velocity was evaluated. Specifically, estimates of d’ were calculated to reflect the distinctness of the percepts evoked by the two opposing directions of movement (see Essick et al., 1988, for computational details). Accordingly, a d’ value of 0.0 implies that the subject exhibited no capacity to distinguish the two directions. In contrast, a d‘ value of 4.0 indicates perfect (or nearperfect) performance on the discrimination task (i.e., direction of motion was unambiguous). After all data were collected, the d‘ values were subjected to repeatedmeasures analysis of variance to determine the effect of aperture sizehhape, the velocity of movement, and the aperture sizehhape x velocity interaction on directional sensitivity. RESULTS AND DISCUSSION OF EACH EXPERIMENT Experiment 1

We hypothesized that the percepts elicited by brushing stimuli, applied over the 0.75-cm long split aperture and the 1.5-cm-long split aperture, would reflect the operation of a mechanism sensitive primarily to cues made available at the times of stimulus onset and offset. This is based on our previous finding that very little information about movement is made available whenever the length of skin traversed is limited to 0.25 cm, the length of each slit-like opening (Essick and Whitsel, 1985b;Essick et al., 1989,1990). Moreover, it is readily appamnt even to naive subjects upon their first exposure to stimulation through the split apertures that three percepts are elicited. At very slow velocities, one invariably experiences a percept of SUCCESSION or sequential contact of the two skin sites by the brush. At very high velocities, only a single, brief stimulus interval, devoid of the percept of motion, is felt. In accord with previous studies of haptic apparent motion, we have adopted the term SIMULTANEITYto denote this percept. Over some intermediate range of velocities, SMOOTH MOTION is perceived; this, in the opinion of G. K. Essick as a subject in the experiment, does not differ significantly from the motion percepts generated by brushing stimuli moving continuously across the same skin field. Figure 1 depicts the mean ? 1 standard deviation (n = 6 subjects) proportion of responses (SUCCESSION, SMOOTH MOTION, and SIMULTANEITY) obtained at each velocity of movement from each of the two apertures.

Note that for the 0.75-cm-long split aperture, an apparent velocity of 8 cdsec supported the percept of SMOOTH MOTION for the greatest percentage of time (96% on the average). This was clearly not the case for the 1.5cm-long split aperture. For this aperture, an apparent velocity of 32 CdSeC supported the percept of SMOOTH MOTION for the greatest percentage of time (93% on the average). At least two laboratories have shown that cutaneous apparent motion is evoked from the successive presentation of two discrete tactile stimuli if the duration of each stimulus and the time interval between the onsets of the two stimuli fall within limited ranges of values (Shemck and Rogers, 1966; Shemck, 1968; Kirman, 1974). Furthermore, the nature of the tactile stimuli (i.e., vibrotactile or electrocutaneous), their spatial separation up to 40 cm, and the body region stimulated were shown to have relatively minor effects on the percepts or on the temporal conditions required for their production. In order to determine whether the temporal conditions for apparent motion found in our laboratory would be comparable to those determined by Shemck and Rogers (see Shemck and Rogers, 1966, Figs. 5 and 6, pp. 178-179; Shenick, 1968, Fig. 15-3, p. 339) and by Kirman (see Kirman, 1974, Fig. 2, p. 3), the following analysis was undertaken. For each velocity, (1) the duration the brush remained in each slit-like opening (i.e., the exposure, or EXPO)and (2) the time between its onset (first contact) with each of the slit-like openings (i.e., the stimulus onset asynchrony, or SOA) were calculated (see Table 1). On the basis of Figure 1, a thin solid line was then drawn to enclose those temporal conditions that supported the response SMOOTH MOTION on at least 75% of the trials in the present study. For comparison, a thick dashed line was drawn to indicate the predicted boundary, based on the findings of Shemck and Rogers and of Kirman, between the percepts of SUCCESSION and SMOOTH MOTION;a thick solid line was drawn to indicate the predicted boundary between the percepts of SMOOTH MOTION and SIMULTANEITY. As illustrated by Table 1, there was very good agreement between the empirically determined and predicted velocity ranges for the 0.75-cm-long and the 1.5-cm-long split apertures. Moreover, although not shown in Table 1, the data from Shemck and Rogers (or from Kirman) predict that smooth apparent motion would have been achieved most 6equently for velocities 6-8 cdsec (or 12 cdsec) for the 0.75-cm-long split aperture and for velocities 16-32 c d s e c (or 32 c d sec) for the 1.5-cm-long split aperture. These predicted “optimal” velocities compare favorably with those empirically observed (Le., 8 c d s e c and 32 c d s e c for the 0.75-cm-long split aperture and the 1.5-cm-long split 177

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.75 cm long SPLIT APERTURE

i

10

loo

1

10

100

1

10

100

1.5 cm long SPLIT APERTURE

1

Velocity (cmlsec)

10

100

Velocity (cmlsec)

FIGURE 1. Mean frequency of the responses SUCCESSION, SMOOTH MOTION, and SIMULTANEITY reported by six subjects to brushing stimuli delivered at different velocities through two split apertures. The slit-like openings of the 0.75-cm-long split aperture were separated by a distance of 0.25 cm; those of the 1.5-cm-long split aperture were separated by a distance of 1.0 cm. Curves have been constructed through the mean data. The vertical bars denote f 1 standard deviation. Note that the ordinate is linear and the abscissa logarithmic.

aperture, respectively). The overall agreement among our data and those of Shemck and Rogers and of Kirman is interpreted to imply that the motion percepts evoked in all three laboratories may have been processed by the same perceptual mechanism. It is of interest to note that Korte’s laws (i.e., those spatial and temporal relationships hypothesized to result in visual apparent motion; see Neuhaus, 1930; Kolers, 1972) predict that the optimal interstimulus interval (IS1 = SOA - EXPO) supporting apparent motion should increase with increasing spatial separation. In contrast to this prediction, the optimal IS1 in the present study remained constant at appmximately 32 msec with the increase in the separation of the slit-like openings h m 0.25 to 1.O cm. This was because a fourfold increase 178

in the optimal velocity accompanied the fourfold increase in spatial separation. The increase in the optimal velocity is, moreover, inconsistent with the hypothesis of “velocity constancy’’ purported by Korte’s first and third laws (see Kolers, 1972). Taken together, it seems that the stimulus conditions supporting tactile apparent motion over the perioral region are not, in general, in accord with the spatial and temporal relationships predicted by Korte’s laws for visual apparent motion. Experiment 2

..

Since perception of direction of motion necessarily require$bprocessingof information about change in position over time, we hypothesized that insight into the lengths

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of the information-beii spatial intervals could be gained by comparing the directional sensitivities observed with the four apertures, which vary in length, continuity, and width (see Fig. 2). We also hypothesized that data obtained over a wide range of stimulus velocities would be required to assure that the optimal temporal conditions of stimulation were attained for each aperture. The data are presented in Figure 2, which illustrates the relationships among mean directional sensitivity, d‘, and velocity (VEL). Repeated-measures analysis of variance confirmed that the aperture sizekhape, F (3, 24) = 40.9, p < O.OOO1; the velocity of movement, F (9, 72) = 50.7, p < O.OOO1; and the aperture size/ shape x velocity interaction, F (27, 216) = 5.0, p < O.OOO1, were all significant determinants of d‘. Directional sensitivity was clearly highest whenever the 0.75cm-long continuous aperture was employed and lowest whenever the 0.35-cm-long continuous aperture was employed. Moreover, subjects’ capacity to discriminate

opposing directions of continuous movement was velocity-tuned, being maximal whenever the brushing stimuli were delivered at approximately 6 cdsec. Thus, the effects of traverse length and stimulus velocity observed in this experiment are wholly consistent with those reported in other published studies (Essick et al., 1988, 1989, 1990). Of particular importance to the present study is

the hding that the velocity-tuning curves obtained with the 0.75-cm-long continuous aperture and the split apertures were not comparable. Specifically, the velocitytuning curve obtained with the continuous “long” aperture significantly exceeded the curve obtained with the “split” aperture over the velocity range of 4-8 cm/ sec ( p 0.03, paired f tests, Bonferroni-corrected;see also Fig. 2). In addition, the directional sensitivities attained with the two split apertures were not significantly different, even though the “split” aperture was twice as wide as the “split narrow” aperture: F (1, 8) =

-=

TABLE1. The Duration of Stimulation (EXPO) and Time between the First Appearance of the Brush at Each Opening (SOA) of the 0.75- and 1.5-cm-long Split Apertures

Velocity (cdsec) 0.5 1 .o

Sdit aperture 0.75 cm long 1.5 cm long EXPO (msec) SOA (msec) EXPO (msec) SOA (msec) 500 loo0 250 500 SUCCESSION

-------2.0 4.0

24.0 32.0 48.0

125 63

250 1 2 5 7

10 8 5

8

%.O

52 40 26

-

13 SIMULTANEITY

128.0

10

Note. Brushing stimuli were delivered over each aperture at each of 10 different velocities. The bold dashed and bold solid lines delimit the range of temporal conditions predicted to support apparent motion by Shenick and Rogers and by Kirman (see text). The thin solid line encloses those conditions that supported the response SMOOTH MoTtoN on at least 75% of the trials in the present study. Moreover, apparent motion was most frequently reported at 8 c d s e c for the 0.75cm-long split aperture and 32 cdsec for the 1.S-cm-longsplit aperture.

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MEAN d‘ VS. VELOCITY (N = 9 Subjects)

4.0

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-1.0

Aprrtum

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(.25) ( 2 5 ) (25) (.25) t.75-1 t.75-1 Split Split N a m I

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5.0 10.0

1.0

.35

I

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50.0 100.0

VELOCITY (crnlsec)

FIGURE2. Mean directional sensitivity d’

& 1 standard error ( n = 9 subjects) plotted as a function of the velocity of movement, logarithmically scaled, with the length, width, and continuity of skin stimulated as parameters. Opposing directions of stimulus motion were applied over each of the four apertures illustrated. The total area of skin exposed to the moving stimuli was as follows: 0.75 cm2 for the “long” aperture; 0.50 cm2 for the “split” aperture; 0.25 cm2 for the “split narrow” aperture; and 0.35 cm2 for the “short” aperture.

2.7, p > 0.13 for a main effect of aperture; F (9, 72) = 0.2, p > 0.99 for an interaction effect of aperture x velocity. The data permitted the investigation of two predictions concerning subjects’ utilization of information made available across spatial and temporal intervals of relatively different lengths. Since the width of the bar separating the slit-like openings of the split aperture was 0.25 cm, we defined “change-of-position” cues as information about stimulus direction provided by spatial and temporal intervals longer than 0.25 cm and 0.25/ VEL sec, respectively. Similarly, we defined “local” cues as information about stimulus direction provided by spatial and temporal intervals shorter than 0.25 cm (the width of each slit-like opening) and 0.25NEL sec, respectively. Our first prediction held that, if the subjects u s d o n l y local cues to judge direction of motion, then the directional sensitivity observed with the split aperture would not exceed that observed with a single 0.35 X 1 cm aperture (i.e., the “short” aperture; see Fig. 2) at any velocity of movement. This prediction was based on the following three-part rationale. First, we have 180

previously shown that d‘ is directly proportional to the length of skin traversed after a short minimum length of skin, TLo,is exceeded (Essick et al., 1990). Given the test site and range of stimulus velocities employed in the present study, TLowas predicted to be 0.0-0.43 cm (Essick et al., 1990). Consequently, directional information was expected to be made available over the entire length of each slit-like opening (i.e., 0.25 cm) at best, but it might not be made available at all (if TLo b 0.25 cm). Second, the prediction assumed the unimportance of positional cues available across the bar separating the slit-like openings of the split aperture. Thus, subjects received two spatially and temporally separated observations of the movement (i.e., one through each opening) during each brush stroke. Third, d‘ for two observations, separated sufficiently in time to assure independent processing of each observation, was expected to approximate the vectorial sum of the d’ values observed for each observation (Tanner, 1956). Therefore, whenever the split aperture was employed, directional sensitivity was predicted to be no greater than the upper limit defined by

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d’ = SQRT(m2 x 0.252 + m 2 X 0.252) = m X SQRT(2 x 0.252)

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= m X 0.35

where m is the constant of proportionality, given that subjects used only local cues. In contrast to this prediction, the d‘ values obtained from use of the split aperture greatly exceeded those obtained from use of the short aperture (see Fig. 2). This difference suggests that information about the change in stimulus position across the bar separating the slit-like openings (i.e., across spatial intervals longer than 0.25 cm) was employed to discriminate direction whenever the split aperture was used. Moreover, the magnitude of the difference suggests that spatial intervals appreciably longer than 0.25 cm were employed. Further support for this interpretation is provided by the similarity of the velocity-tuning curves obtained from the split and split narrow apertures. This similarity most likely reflects comparable change-of-position cues made available by these apertures and, in turn, utilized by subjects. Clearly, cues dependent on the total skin area stimulated were not a major determinant of d’, since the tuning curves obtained with the split and split narrow apertures did not differ significantly, yet the total area of skin stimulated differed by a factor of 2. Our second prediction held that, if subjects used only change-of-position cues to infer the direction of stimulus movement, then the velocity-tuning curves obtained with the continuous 0.75-cm-long aperture and with the split aperture would be similar. This prediction followed from the fact that both apertures provided change-of-position cues over the range >0.250.75 cm and that these cues provided the predominant directional information whenever the split aperture was employed. Empirically, the velocity-tuning curve obtained with the 0.75-cm-long continuous aperture statistically exceeded the curve obtained with the 0.75cm-long split aperture over the velocity range of 4-8 c d s e c . This suggests that information other than that accounfed for by the change-of-position cues contributed to subjects’ capacity to discriminate direction of continuous motion when stimulus velocity was between 4 and 8 cdsec. In summary, we interpret the results of Experiment 2 to suggest that both cues signaling the overall change in stimulus position and cues providing information over shorter spatial and temporal intervals are employed to obtain directional information about movement across the skin. They also suggest that stimulus velocity and aperture sizekhape are determinants of the relative importance of each kind of cue. Moreover, our findings strongly imply that subjects use cues signaling the overall

change in stimulus position to obtain information about stimulus direction whenever a split aperture is employed, and that cues over shorter spatial and temporal intervals provide directional information whenever continuous movement occurs within the velocity range of 4-8 c d sec. Unfortunately, it cannot be determined from the data of this experiment (1) whether subjects use these latter cues to obtain directional information about continuous movement outside of this velocity range and (2) whether subjects use cues signaling the o v e d change in stimulus position to obtain directional information about continuous movement at any stimulus velocity. The results from Experiment 1, taken together with previously published findings from our laboratory and others, suggest that cues signaling the overall change in stimulus position may be relatively unimportant to the percept of direction of continuous movement under most circumstances. In particular, if subjects use only cues signaling change in stimulus position to infer the direction of continuous movement, the velocity tuning of d‘ may be predicted to resemble a “low-pass filter” whose “cutoff velocity” increases with the length of skin traversed. This prediction is based on the following two-part rationale. First, the apparent velocity range for which the percept of SUCCESSION is achieved broadens as the separation between the two slit-like openings is increased (see top two plots of Figure 1). Second, accurate temporal ordering of sequentially delivered positional information is accomplished at the same or shorter temporal intervals (and thus, at higher apparent velocities) as the spatial separation between the skin sites (at which the information is made available) is increased (see Hirsh and Shemck, 1%1; Shemck, 1970). In opposition to the prediction, we have shown that the capacity of human subjects to recognize direction of continuous stimulus motion over the hairy period skin is maximal at velocities within the vicinity of 6 cdsec (Essick et al., 1988, 1990) and that this relatively low optimal velocity is invariant with respect to the length of skin traversed (Essick et al., 1989). Specifically, this optimal velocity remains constant for data obtained with continuous apertures ranging in length from 0.35 cm to 1 cm for normal subjects. It is likely, however, that subjects may use cues signaling overall change in stimulus position to discriminate direction (1) for low velocities of movement, over which motion percepts are weak (see the introductory paragraphs to this paper and Langford et al., 1973; Dreyer et al., 1978), and (2) for very fast velocities of movement over long chords of skin (see Whitsel et al., 1979). Whitsel et al. (1979) showed that subjects can discriminate direction of motion at even very high velocities of movement if the length of skin traversed is long enough. Moreover, under these conditions the upper limiting velocity (i.e., the 181

ESSICK ET AL.

“cutoff velocity”) increases with the length of skin traversed, suggesting that subjects employ, in part, cues based on the overall change in position of the moving tactile stimuli.

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GENERAL DISCUSSION A number of important observations can be made from the experimental data presented in this paper. First, percepts of apparent motion can be elicited over the perioral region by brushing stimuli delivered through a split aperture; moreover, the percepts are only achieved for temporal conditions known to result in apparent motion from the delivery of discrete vibrotactile or electrocutaneous pulses to other body regions (Sherrick and Rogers, 1966; Shemck, 1968; Kirman, 1974). Second, subjects can derive directional information h m percepts of apparent motion. And third, the capacity to discriminate opposing directions of continuous movement cannot be explained solely by the ability to process information from the sequential positions of stimulus onset and offset. This observation is particularly important, since it suggests that perceptually useful information about the direction of an object moving continuously across the skin is extracted from spatial and temporal intervals shorter than those defined by the onset and offset of stimulation. Unfortunately, with conventional modes of stimulus delivery, it is difficult if not impossible to determine the relative importance of spatial and temporal intervals of different lengths to direction discrimination. However, recent technological advances should enable somatosensory researchers to address this important issue in the future (e.g., see Van Doren et al., 1987; Shenick and Cholewiak, 1990; Szaniszlo et al., 1992). Specifically, with a dense array of independently controlled contactors, investigators can study the motion percepts evoked by the sequential stimulation of two or more skin sites. The spatial separation between the sites and the onset and duration of stimulation provided at each site can be systematically varied and controlled with a high degree of precision. The findings that spatial and temporal intervals of different lengths contribute to direction discrimination led us to revisit a question of long-standing interest to this laboratory-that is, the issue as to whether more than one mechanism subserves tactile motion perception. Briefly, in a previous study the capacity of human subjects to discriminate direction of tactile motion on the dorsum of the hand was evaluated (Essick and Whitsel, 1988).A second moving stimuluswas delivered to a spatially separated skin site on the ipsilateral forelimb. Either the two stimuli were delivered simultaneously, or the onset of the stimulus at the hand site was specified to precede or follow the onset of the 182

stimulus at the other site by 150 msec. It was discovered that when the two skin sites were stimulated nonsimultaneously, subjects experienced a sensation of “translocation” or apparent motion, which they interpreted as meaningful information about movement across the skin. Importantly, this information was often indistinguishable from that provided by each moving stimulus, and consequently influenced the report of the direction of motion at the hand site. In neurophysiological experiments, two moving stimuli were delivered to similarly positioned sites within the receptive field of macaques’ directionally selective neurons in primary somatosensory cortex (Essick and Whitsel, 1992). In contrast to its effect on human perception, the order in which the two sites were stimulated did not reliably alter cortical neuron directional sensitivity. The observations were interpreted to imply that the directional information provided by the spatiotemporal order of dual-site stimulation was processed by a central neural mechanism different from that which processed information provided at each stimulus site. The concept that more than one mechanism may subserve motion perception is well established in regard to vision. Specifically, it is generally accepted that information about real continuous visual movement is processed by a mechanism (i.e., a “short-range”process) different from that which processes information about discontinuous (viz., “long-range” apparent) motion (Anstis, 1980; Braddick, 1980; Van Doorn and Koenderink, 1984; Baker and Braddick, 1985; Nakayama, 1985). The short-range mechanism processes movement only over short spatial (i.e., up to 0.25” at the fovea and to several degrees peripherally) and temporal (less than 100 msec) intervals, and is sensitive to changes in the total retinal area stimulated by moving images. In contrast, the long-range mechanism processes movement over long spatial (i.e., up to tens of degrees) and temporal (hundreds of milliseconds in length) intervals, and the stimuli whose locations and onsets define these intervals need not project to the same eye. It is tempting to speculate that the motion percepts evoked over the 0.75-cm-long continuous aperture and the 0.75-cm-long split aperture in the present study were processed by analogous short-range and longrange tactile processes, respectively. For example, the temporal conditions supporting apparent motion over the 0.75-cm-long split aperture and the 1.5-cm-longsplit aperture were found to be comparable to those observed by Shenick and Rogers and by Kirman. These investigators postulated that a tactile process analogous to the long-range process in vision was responsible for the percepts of motion in their experiments (Shemck and Rogers, 1966; Shemck, 1968; Kirman, 1974). Moreover, whereas direction discrimination of contin-

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DISCONTINUOUS PERIORAL MOTION

uous tactile movement is clearly dependent on the total area of skin contacted (Essick and McGuire, 1986), a twofold change in the area of skin exposed by the split apertures did not significantly al€ect subjects’ directional sensitivity. The hypothesis that the percepts of motion attained with the split apertures would be processed by a longrange process analogous to that in vision is problematic, however. Specifically, the percepts were evoked over short spatial (0.25 cm and 1.0 cm) and temporal (32 msec) intervals. Assuming that the upper limiting spatial interval for the short-range process in vision is determined by receptor density and spacing, it (viz., 0.25’) corresponds to roughly 2.5 cm on the finger pad (see Phillips et al., 1983). Accordingly, the spatial intervals provided by the split apertures employed in the present study (0.25 cm and 1.0 cm) are shorter than the upper limiting spatial interval predicted for an analogous shortrange process in somatosensation. Moreover, the optimal IS1 for smooth apparent motion in the present study (32 msec) is comparable to the optimal IS1 for the short-range process in vision (40msec; see Baker and Braddick, 1985). Thus, it is unclear whether the motion percepts evoked from use of the split apertures are more consistent with the operation of a long-range or a short-range process. This issue and the question as to whether analogous short- and long-range processes underlie visual and tactile motion perception can only be resolved by additional experiments. Recently, Gardner and colleagues examined the responses of somatosensory cortical neurons to linear sequences of punctate (Warren et al., 1986a,b) and vibrotactile stimuli (Gardner et al., 1988; Gardner, 1988). These studies provide important insights into our understanding of the neural mechanism that processes information from sequences of multiple, closely spaced discrete stimuli. However, our understanding of the neural mechanism subserving the percept of motion elicited by only two discrete stimuli is much less complete. Specifically, no published investigation to date has demonstrated that the degree of central (or peripheral) neural responsivity elicited by continuously moving stimuli can be approximated by the presentation of two spatially and temporally separated punctate stimuli. Such discrete tactile stimuli are thought to elicit activity in different, albeit overlapping, populations of somatosensory cortical neurons (see Costanzo and Gardner, 1980; and Essick and Whitsel, 1988). Moreover, it has been suggested that a central mechanism sensitive to the rate and direction that peak activity “shifts” across the primary somatosensory cortex, in response to the sequential delivery of two discrete stimuli, could specify apparent velocity and direction. At present, however, such a central neural mechanism has not been

identified. Since form processing has been shown to precede long-range motion processing in the visual system (Anstis, 1980), the edge and form detectors reported in area 2 and in the boundary region between areas 2 and 5 may contribute to an early stage of processing (Sakata and Iwamura, 1978). Moreover, since the SII cortex has been suggested to serve a pattern recognition role analogous to that of the inferotemporal cortex in vision (Murray and Mishkin, 1984),it may also contribute to the percepts elicited by discontinuous movement (i.e., to apparent motion). ACKNOWLEDGMENTS

This research was supported by National Institutes of Health Grant No. DE07509. We would like to thank Dr. Roger Cholewiak, Dr. Mark Hollins, Dr.Mike Hairfield, Dr. Erick Rath, Ms. Kathy Bredehoeft, and Mr. Jim Shores for reviewing different versions of this paper. REFERENCES

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