The cyclopean ternus display and the perception of element versus group movement

Vision Res. Vol. 31, No. 12, pp. 2085-2092, 1991 0042-6989/91 53.00 + 0.00

Printed in Great Britain. All rights reserved Copyright 0 1991 Pergamon Press plc

THE CYCLOPEAN TERNUS DISPLAY AND THE PERCEPTION OF ELEMENT VERSUS GROUP MOVEMENT

ROBERT PATTERSON,’ PATTI HART* and DIANE NOWAK’

‘Department of Psychology, Washington State University, Pullman, WA 99164 and ‘Department of Psychology, Montana State University, Bozeman, MT 59717, U.S.A.

(Received 18 September 1990; in revised form 15 April 1991)

Abstract-This study investigated the perception of bistable stroboscopic motion (Iemus display) with cyclopean stimuli created from retinal disparity embedded in dynamic random-element stereograms, the responses to which arise at binocular-integration levels of the visual system. To provide comparison data, observers were also tested with luminance-domain stimuli matched as closely as possible to their cyclopean counterparts. The results showed that the perception of element vs group movement was similar for both stimulus domains: element movement predominated at short interstimulus intervals (ISIS) while group movement predominated at long ISIS, and there was a tendency for a greater percentage of group movement to occur with a longer frame duration. These results cast suspicion on the interpretation of bistable motion that assumes element movement is a signature of a lower-level, short-range motion system whereas group movement is a signature of a higher-level, long-range system; both percepts are engendered at binocular-integration levels of vision.

Cyclopean Stereopsis Motion perception

INTRODUCTION

Bistable stroboscopic motion (Ternus, 1926, 1938; Pantle & Picciano, 1976; Petersik & Pantle, 1979; Petersik, 1989) refers to motion induced with two alternating frames of a motion display, each frame containing a group of three elements such as circles. In the second frame the circles are displaced horizontally relative to the first frame by an amount equal to the distance between the centers of the circles (see Fig. 1). When the frames are alternated over time, one of two motion percepts is seen, depending upon the interstimulus interval (ISI) between frames. At short ISIS (e.g. 10 msec), the middle two circles appear stationary with the outer circle moving from end to end; this is called “element” movement. At longer ISIS (e.g. 90msec), the circles appear grouped together and moving from side to side; this is called “group” movement. The motion is stroboscopic or apparent because it is seen with stationary stimuli presented intermittently.

The theoretical importance of bistable motion comes, in part, from the idea that the two competing percepts may reflect the operation of two different motion systems (e.g. Pantle & Picciano, 1976; Braddick, 1980; Petersik, 1989). On this idea, contemporary theories of motion

Stroboscopic motion

perception (Anstis, 1980; Braddick, 1974, 1980; Nakayama, 1985; Chubb & Sperling, 1989; Petersik, 1989) posit the existence of two processes. A lower-level process (“short-range” or SR process) presumably involves the perception of motion across small spatial displace- ments (e.g. 15 min arc) and brief temporal intervals (e.g. 100 msec). Input to this system is thought to be spatiotemporal variations of contours defined by luminance differences. Additional characteristics include a sensory mode of processing and a lack of binocular- integration properties (i.e. short-range motion is not perceived with dichoptic stimulation). A higher-level process (“long-range” or LR process) presumably involves the perception of motion across relatively large spatial displace- ments (several deg arc) and long temporal intervals (hundreds of msec). Input to this system is believed to be spatiotemporal variations of luminance contours that fall outside the spatiotemporal boundaries of the SR process, and higher-level shapes or features such as contours defined by texture, retinal disparity, or contrast differences. Additional characteristics include a cognitive mode of processing and the existence of binocular-integration properties. Although certain research has led to modifications of this scheme (e.g. the spatial

VR 31/L--E 2085

2086 ROBERT PATTERSON et al.

FIRST

FRAME

rl

0.0

-l ISi

SECOND i

FRAME

1

0.0

Fig. 1. Illustration depicting the Temus (1926, 1938) stimuli. Each of two frames of the motion display contains a group of three circles. In the second frame the circles are displaced horizontally relative to the first frame by an amount equal to the distance between the centers of the circles. When the frames are alternated over time, one of two motion percepts is seen depending upon the interstimulus interval (ISI) between frames. At short ISIS, the two middle circles appear stationary with the outer circle moving from end to end (“element movement”). At longer ISIS, the circles appear grouped together and moving from side to side (“group

movement”).

limit of the short-range process varies with retinal eccentricity), its basic framework-that there exists two qualitatively different processes for the perception of motion-has been retained by many authors up to the present time (e.g. Georgeson & Shackleton, 1989; Petersik, 1989). (For recent criticism of this two-process model, see Cavanagh 8z Mather, 1989 Bischof & DiLollo, 1990.)

According to Pantle and Picciano (1976). Braddick (1980), and Petersik (1989), among others, element motion in the Ternus display is a perceptual correlate or signature of the SR process. This conviction comes from the observation that the perception of such motion demonstrates properties similar to the SR system: element movement is perceived predominately at short ISIS and only when the frames of the motion display are presented to the same eye. The short-range mechanism is thought to operate to signal that the two inner, overlapping elements of the Ternus display are stationary, with the movement of the outer element mediated by the LR process (e.g. Braddick, 1980). Group motion in the Ternus display is believed to be a perceptual correlate of the LR process. This idea comes from the observation that the perception of such motion demonstrates properties similar to the LR system: group movement is perceived at long ISIS and when the frames of the motion display are presented to the same or different eyes.

Note, however, that the theoretical connec- tion between element movement and the SR process on the one hand, and group movement

and the LR process on the other hand, is not as clearcut as these authors would suggest. For instance, increases in element size are known to decrease the perception of element motion in the Ternus display (Breitmeyer & Ritter, 1986), but increases in element size in- crease the maximum short-range displacement in a Braddick-type display (e.g. Bell & Lappin, 1973). Moreover, Petersik (1986) has shown that group movement is perceived at short ISIS. which favor the SR process, when all of the Ternus-display elements fall within a small spatial region (e.g. 15 min arc). On the basis of that observation, he concluded that there is no necessary one-to-one relation between the operation of a given motion process and phenomenal experience.

In order to learn more about the perception of bistable stroboscopic motion, we investigated the phenomenon using cyclopean stimuli created from dynamic random-element stereograms (Julesz, 1960, 1971). Such stereograms are composed of dichoptic arrays of thousands of randomly-ordered dots. Disparity is created by laterally shifting a subset of dots in one eye’s view and leaving unshifted corresponding dots in the other eye’s view (the shift is camouflaged by background dots). The stimulus defined by disparity is cyclopean, which refers to information existing at binocular-integration levels of the visual system. Such stimuli would bypass any kind of motion detector whose input is luminance-domain stimuli. Thus, the stimulus can be seen neither monocularly nor by someone who lacks stereopsis; an indi- vidual with stereopsis perceives the stimulus in a depth plane different from that of the background. Briefly presenting two stationary, adja- cent cyclopean stimuli in rapid succession produces cyclopean apparent motion: a single stimulus is perceived floating in depth either in front of or behind background and moving laterally (Julesz & Payne, 1968). In our study, two sets of three cyclopean circles were presented in succession creating a cyclopean Ternus display.

METHODS

Observers

Seven individuals (4 males, 3 females) served as observers, six of whom were naive with respect to the purpose of this study. All observers had normal or corrected-to-normal visual acuity and good binocular vision

Cyclopean Temus display 2087

Apparatus and stimuli

The system used to create the cyclopean stimuli has been described by Shetty, Brodersen and Fox (1979) and Fox and Patterson (1981). The observer viewed matrices of red and green dots displayed on a Sharp high-resolution color monitor (model XM-1900). The size of each pixel was 5.7 min arc at a viewing distance of 150 cm; the luminance of the random-dot arrays was 25 cd/m2. Stereoscopic viewing was accom- plished by the anaglyph method, wherein red (Wratten # 29) and green (Wratten # 58) filters were placed before the eyes of the observer. A stereogram generator (hardwired device) electronically controlled and red and green guns of the monitor. The generator produced the random dots, created disparity (which produced cyclopean stimuli), and specified the X/Y coordinates of the stimuli (the background dots were correlated between the eyes). All dots were replaced dynamically, with positions as- signed randomly, at 60 Hz, which allowed the stimuli to be briefly exposed without monocular cues (Julesz & Payne, 1968). The duration of exposure of the stimuli was precisely controlled in integer-multiples of the frame duration (16.7 msec) of the monitor. Two optical programmers (modified black and white video cam- eras) were also employed. The voltage emitted by each programmer (whose scan rate was synchronized with that of the monitor) was digitized and used as code for determining where disparity was to be embedded in the stereogram.

Each optical programmer scanned three high- contrast white circles on a black background. The programmers transformed the white circles into cyclopean circles on the monitor. The di- ameter of each cyclopean circle was 1.3 deg arc, and the center-to-center distance between circles was 1.9 deg arc. The disparity of the cyclopean circles was 22.8 min arc in the crossed direction relative to the background dots of the stereogram (as indicated below, the observer was free to vary fixation from the background to the circles, which would cause them to be imaged onto corresponding retinal points and the background would then carry an uncrossed disparity of 22.8 min arc).

We thought that it was important to obtain data with stimuli defined by luminance contours in order to provide some kind of baseline per- formance, even though we could not create luminance stimuli that matched exactly the

physical characteristics of the cyclopean circles. To that end, this study also employed luminance-domain circles presented with a two- channel Gerbrands Harvard tachistoscope. Because the cyclopean circles were defined as a disparity increment in the crossed direction relative to a background of zero disparity, we defined the luminance circles in the tachistoscope as a luminance increment relative to a background of near-zero luminance (white circles on a dark background). The luminance of each circle was 35.0cd/m2 and that of the background was 0.4 cd/m2. Viewing distance from the observer’s eyepiece to the stimuli posi- tioned in each channel was 55 cm. The size and spacing of these circles were the same as for the cyclopean case, 1.3 deg arc dia and center-to- center spacing of 1.9 deg arc.

Design and procedure

The observers were tested on their perception of stroboscopic motion with two values of frame duration (184 or 284 msec for cyclopean stimuli; 180 or 280 msec for luminance stimuli) and 6 values of ISI (0, 17, 33, 50,67 or 84 msec). (The slight differences in frame duration between the cyclopean and luminance stimuli were due to equipment limitations). These parameters made this a 2 (frame duration) x 6 (ISI) x 2 (type of stimulus) factorial design. Only two frame durations were used because we discovered during pilot testing that values longer than 284msec produced responses classified mostly as “group”

lNTWJlMULUS INTERVAL (MS)

Fig. 2. Cyclopean stimuli. Percentage of “group” responses for 6 interstimulus intervals and a frame duration of 184 msec. The data from 7 observers are shown in&+d~~y;

each datum is an average of either 10 or 20 trials. The observers: solid circle = PH; open circle = DN; solid square = GV; open square = WL; solid triangle = KH;

star = ET; x = RP.

2088 ROBERT PATTERSON et 01.

motion, whereas values shorter than 184 msec were too brief to permit a clear perception of motion with the cyclopean stimuli, regardless of practice. The decreased perceptual saliency of the briefly-presented cyclopean circles restricted the range of useful frame durations in our study. Nevertheless, the frame durations employed in the present investigation are comparable to those used in previous studies of bistable stroboscopic motion (e.g. 200 msec employed by Pantle & Picciano, 1976).*

Testing began by giving the observer the only set of instructions he/she was to receive during the study. The experimenter explained the perceptual criteria to be used for judging element and group movement (e.g. two stationary middle circles and end-to-end motion meant “element” movement). The observer’s task was to report on each trial whether the motion was “element” or “group”. The observer was told that either kind of motion could be ex- pected on any given trial, that there was no “correct” answer, and to simply report what was perceived.

On each trial, the two frames of the motion display were presented for four complete cycles only in order to prevent motion adaptation. The observer was instructed to fixate the center of the display screen, but no fixation point was provided in order to minimize position cues. The order of experimental conditions presented to each observer was haphazard with two excep- tions. First, all ISIS for a given frame duration were run before moving on to another frame duration. Second, testing always began with the luminance stimuli because we thought it was important to provide the observer with a clear understanding of what to judge before proceed-

*With the cyclopean stimuli, there is the question of what constitutes an ISI because these stimuli were presented on a raster-based display system for which it takes 16.7 msec for one frame to be painted. Because the typical definition of ISI is the interval between the end of one stimulus and the beginning of the next stimulus, we elected to define IS1 as the interval between the end of one frame and the beginning of the next frame (ignoring the few msec required for retrace). However, one could argue that IS1 should be defined as the interval between the end of one frame and the end of the next frame. If this definition is adopted, our functions for the cyclopean stimuli in Figs 2.4 and 5 shift to the right and one would conclude that a greater proportion of “element” responses occurred for the ISIS indicated on the abscissae. However, none of our conclusions of this study would be altered substantively by adopting this definition of ISI.

ing to the cyclopean stimuli. Ten trials were collected for each combination of frame duration and IS1 for the luminance stimuli, and either 10 or 20 trials were collected for each combination of frame duration and IS1 for the cyclopean stimuli.

During the first several trials involving the cyclopean stimuli, each observer stated that the stimuli were hard to see and that making the element-group discrimination was difficult. Some observers acted as if their decisions were random. (In an attempt to understand why cyclopean motion was difficult to perceive, we can speculate on the kind of manipulation that would impoverish, in similar fashion, the perception of luminance-domain motion. The decreased perceptual saliency of the cyclopean stimuli made them appear as if they were presented very briefly, thus the equivalent luminance-domain manipulation would be to present apparent-motion stimuli for very brief durations.) We therefore provided each observer with two practice sessions (about 240 trials) with the cyclopean stimuli, without feed- back, before formal data were collected. By the beginning of the third session, each observer’s responses had stabilized (he/she typically stated that the stimuli were now easier to perceive) and formal data collection continued. Such practice sessions were not necessary with the luminance stimuli.

During data collection with the cyclopean stimuli, each observer reported that the circles appeared as standing out in depth from background and moving either as element or as group (i.e. the shortest frame duration for cyclopean stimuli, 184 msec, was above threshold for perceiving depth of our stimuli). This indicated that the observer was basing his/her decisions on detecting disparity information, not decorrelation (with random-element stereograms, decorrelation is necessarily introduced to corresponding retinal areas when the subset of dots in one eye’s view is shifted to create disparity).

RESULTS

Figure 2 shows the percentage of “group” responses for each IS1 involving the cyclopean stimuli presented with the 184 msec frame duration, for the 7 observers individually. All observers perceived a small percentage of group movement, and therefore a high percentage of element movement, at the shorter ISIS (4 observers reported 100% element movement with

Cyclopean Temus display 2089

lNTERSTlMULUS INTERVAL (MS)

Fig. 3. Luminance stimuli. Percentage of “group” responses for 6 interstimulus intervals and a frame duration of 180 msec. The data from 7 observers are shown individually; each datum is an average of 10 trials. The observers: solid circle = PH; open circle = DN; solid square = GV; open square = WL; solid triangle = KH; star = ET; “ x ” = BP.

an IS1 of 0 msec). As IS1 increased, the percentage of group responses increased and element responses decreased. All observers perceived predominately group movement at the longer ISIS.

Figure 3 shows the percentage of “group” responses for each IS1 involving the luminance stimuli presented with the 180 msec frame duration, for the same 7 observers. The relation between IS1 and percentage group responses is similar to that found for the cyclopean stimuli.

Figure 4 shows the data from Figs 1 and 2 averaged and plotted on the same graph. The two functions are similar; the only difference is that the function for the luminance stimuli is

2 2 100 CYCLOPEAN - 0

n. 1 v 10

I I I I

40 60 80 100

INTERSTIMULUS INTERVAL (MS)

Fig. 4. Both cyclopean and luminance stimuli. Percentage of “group” responses for 6 interstimulus intervals and a frame duration of 184 msec (cyclopean) or 180 msec (luminance). The data points are averages of the 7 observers taken from

Figs 2 and 3. The SE bar depicts the median SE.

M v) loo- CYCLOPEAN-o

9. _ LUMINANCE-.

bj :I$/, , ( , ,I 20 40 60 80 100

INTERSTIMULUS INTERVAL (MS)

Fig. 5. Both cyclopean and luminance stimuli. Percentage of “group” responses for 6 interstimulus intervals and a frame duration of 284 msec (cyclopean) or 280 msec (luminance). The data points are averages of the 7 observers. The SE bar

depicts the median SE.

steeper than that for the cyclopean stimuli; a greater percentage of element responses occurred at the shorter ISIS, and a greater percentage of group responses occurred at the longer ISIS, with the luminance stimuli. The 50% transition point is at an IS1 of about 40-45 msec for the luminance and cyclopean stimuli, re- spectively.

Figure 5 reveals the data (average values) obtained with the cyclopean and luminance stimuli presented with the 284 msec (cyclopean) or 280 msec (luminance) frame duration. Both functions have shifted toward the left slightly, relative to the curves depicted in Fig. 4. This shift indicates that a greater percentage of group responses, and a lesser percentage of element responses, were reported with the longer frame durations than with the shorter frame durations.

These data were analyzed with a three- way (frame duration x IS1 x type-of-stimulus) analysis of variance for within-subjects designs. The analysis revealed reliable effects of IS1 [F(5,30) = 70.46, P < O.OOOl] and the inter- action of IS1 with type-of-stimulus [F(5,30) = 7.80, P < O.OOl]. The main effect of frame duration approached significance [F( 1,6) = 4.8 1, P = 0.07) Neither the main effect of type-of- stimulus, nor the interactions of frame duration with type-of-stimulus, frame duration with ISI, or frame duration with type-of-stimulus with ISI, were reliable (P > 0.10).

We made two additional observations involving cyclopean motion. First, the perception of element movement was perturbed when the two frames of the motion display were presented with different values of disparity. For example,

2090 ROBERT PATTHWN et al

when the frames were presented with an IS1 of 0 msec and the same disparity value (e.g. 22.8 min arc), the perception of element movement predominated. When the frames were presented with the 0 msec ISI but different values of disparity, the percentage of element movement decreased (group movement increased) with in- creasing differences in disparity. The percentage of element movement was 0 (group movement 100%) when the disparity difference was 11.4 min arc or greater. Second, the overall quality of cyclopean motion (either element or group) was much better in the crossed disparity direction relative to the uncrossed direction, supporting the idea that the two directions are mediated by separate mechanisms (Mustillo, 1985).

DISCUSSION

The principal result of this study is that the perception of element vs group movement in a Ternus display occurs for cyclopean stimuli in a manner similar to that for luminance stimuli. The predominance of element movement at short ISIS gives way to a predominance of group movement at longer ISIS. Also, the percentage of group movement is greater with a longer frame duration. The main difference between the two stimulus domains is that the slope of the relation between IS1 and percentage of group responses is less for the cyclopean case. Although differences in stimulus parameters between the two stimulus domains may contribute to the difference in slope, we believe that the lesser slope for the cyclopean case is produced by the difficulty in perceiving the briefly- presented cyclopean circles and in making the element-group discrimination. Recall that the observers stated that the cyclopean stimuli were difficult to perceive, and that frame durations briefer than 184 msec could not be employed, problems not occurring with the luminance stimuli. We conclude that the difference between cyclopean and luminance motion perception observed in this study is due to a difference in perceptual saliency.

The important result is the similarity between the two stimulus domains. For both domains, the perception of element vs group movement is controlled in the same way by IS1 and frame duration, factors known to affect the perception of bistable motion (e.g. Pantle & Picciano, 1976; Petersik & Pantle, 1979; Breitmayer & Ritter, 1986; Petersik, 1989). This suggests that the same

mechanism mediates bistable motion perception in both domains. It is interesting to note that a study by Ritter and Breitmeyer (1989) showed that under certain conditions both element and group movement can be perceived with luminance stimuli when the frames of the Ternus display are presented dichoptically. These authors concluded that the process responsible for the perception of element vs group movement occurs subsequent to the site of binocular integration, which is consistent with the results of the present investigation. But what do these results mean for the interpretation of bistable motion that assumes element movement is a signature for the lower-level, SR process while group movement is a signature for the higher- level, LR process? Are there other, plausible interpretations of the Ternus phenomenon?

One alternative interpretation is that the perception of both element and group motion are derived from the LR system. A study by Petersik, Hicks and Pantle (1978) is relevant to this idea. They investigated the perception of apparent motion from successively presented subjective figures. Each figure was created from the temporal alternation of a pair of random- dot patterns of the kind examined by Braddick (e.g. 1974). Both members of the pair of patterns contained a subset of dots that were correlated, and a background set of dots that were uncorrelated, across the two patterns (or vice versa: subset of dots uncorrelated, background dots correlated). Upon alternation of the patterns, a subjective figure was perceived as a static form seen against a dynamic background (or a dynamic form seen against a static background). Petersik et al. successively presented sets ol subjective figures in a Ternus-like arrangement. and had subjects report on their perception of apparent movement. The authors found that both element movement (called “type A” motion in their study) and group movement (“type B” motion) were perceived, depending upon stimulus conditions. According to Petersik er al., perception of each subjective figure was produced by a lower-order motion process which entails a spatiotemporal cross-correlation of the intensity distributions in the patterns comprising the subjective figure, whereas perception of the apparent motion of those figures was produced by a higher-order process Assuming that this analysis is correct, both element and group percepts occur with higher- order motion perception, which is consistent with the results of the present study.

Cyclopean Temus display 209 1

In fact, we betieve that the perception of both element and group motion may be a feature af any legitimate motion-processing system, not just a LR system. On this idea, element and group percepts may be indicators of a process that can take on different perceptual states. This process may be responsible for the establishment of perceptual correspondence between successive images of a moving object, a process which should occur prior to the computation of motion direction and speed. For example, element movement is perceived when correspondence is established between the two inner circles of the Ternus display. This pattern of correspondence is established at short ISIS, possibly due to the existence of visual persistence which preserves phenomenal identity, as postu- lated by Breitmeyer and Ritter (1986) for luminance stimuli. Breitmeyer and Ritter showed that the range of ISIS across which the transition from element to group movement occurs de- pends upon frame duration and circle size. As frame duration and size decrease, element movement is perceived at longer ISIS. Because pattern persistence in many cases is inversely related to duration and size, such persistence may contribute to the perception of element movement. Group movement is perceived at longer ISIs, possibly because the duration of persistence does not extend across long temporal intervals.

This explanation may be applicable ta cyclopean motion as well because there is evidence for the existence of cyclopean persistence (Engel, 1970). That the percentage of element movement decreases with disparity differences between frames of the motion display supports this conjecture if it is assumed that cyclopean persistence occurs within separate disparity channels; switching channels could disrupt the persistence responsible for element motion, But regardless of the validity of this persistence hypothesis, the present results cast suspicion on the inte~retation of bistable motion based on low-level (short-range) vs high-level (long-range) processing. This is because both element and group percepts are engendered at binocular-integration levels of vision using stereoscopic stimuli, and in the traditional SR!LR framework retinal disparity provides input only to the LR process (Anstis, 1980; Braddick, 1980); thus the perception of element motion, like group motion, is a LR phenomenon.

A~ow~edg~~s~u~o~ for this invest&ion was Pro- G&d by NSFBPSCoR granr 290565. We thank Dr J. Timothy Pete&k and two reviewers for helpfut comments on an earlier version of this manuscript. The data reported in this paper were collected at Montana State University, Bozeman, Mont.

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The cyclopean ternus display and the perception of element versus group movement

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