Properties of the stereoscopic (Cyclopean) motion aftereffect

Pergamon 0042-69W(93)EOOOl-N

Properties of the Stereoscopic Motion AfterefTect ROBERT PATTERSON,* CHRISTOPHER BOWD,* RAY PHINNEY,* WANDA J. BARTON-HOWARD,* MICHELLE ANGILLETTA*

Vision Res. Vol. 34, No. 9, pp. 1139-I 147, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-6989/94 $6.00 + 0.00

(Cyclopean) ROBERT POHNDORF,*

Received I July 1993; in revised form 30 September 1993

Across four experiments, this study investigated properties of the stereoscopic motion aftereffect (adaptation from moving retinal disparity information). The results showed that stereoscopic motion can induce an adaptation aftereffect across a wide range of conditions and observers, provided that the duration of adaptation is suflkiently long and a perceptually salient test pattern is viewed. Motion adaptation was found to transfer between the stereoscopic and luminance domains [replicating a previous report by Fox, Patterson and Lelunkuhle (1982) Inuestigathe Ophthulmo~ogy and Vimal Science (Suppl.), 22, 1441, suggesting that motion perception from stereoscopic (second-order) and luminance (first-order) attributes is mediated by a common neural substrate. Motion perception

Motion aftereffect Cyclopean Stereopsis

INTRODUCTION

After viewing for some time (e.g. several minutes) an object moving in a given direction, a subsequentlyviewed stationary object will appear to move in the opposite direction. This illusion of motion, the so-called “motion aftereffect”, has been studied for hundreds of years (e.g. Adams, 1834; Aristotle, cited in Wohlgemuth, 1911; Purkinje, 1825). In the contemporary literature, the motion aftereffect has been interpreted as reflecting an induced shift in the distribution of neural activity encoding object motion (e.g. Sutherland, 1961; Moulden, 1980; Wright & Johnston, 1985). This paper reports the results of a study investigating the stereoscopic motion aftereffect, motion adaptation from moving retinal disparity information. Stereoscopic motion is cyclopean, which refers to information existing at binocular-integration levels of vision. Julesz (1960, 1971) pioneered the investigation of cyclopean perception by developing computer-generated random-dot stereograms, dichoptic arrays of dots with embedded disparity which defines stereoscopic stimuli visible only to individuals with stereopsis. The concept of cyclopean vision is similar to the “purely binocular process”, a level of processing for which both eyes must be stimulated for activation of the process, equivalent to a logical “AND” operation (Wolfe, 1986). Changing the spatial position of the disparity information across time produces stereoscopic or cyclopean motion. Retinal disparity is one of several stimulus attributes or features whose displacement in space and time pro*Department of Psychology, Washington State University, Pullman, WA 99164-4820, U.S.A.

vides information for motion perception. Motion can be perceived from displacement of stimulus boundaries defined by differences in luminance, contrast, texture, disparity, and possibly color (Cavanagh & Mather, 1989; Chubb & Sperling, 1989; Nakayama, 1985; Patterson, Ricker, McGary & Rose, 1992; Turano & Pantle, 1989). One classification scheme of attributes, introduced by Julesz (1971) and elaborated by Cavanagh and Mather (1989), is based on geometrical probability. First-order attributes are defined by differences in first-order statistics, such as luminance differences, and second-order attributes are defined by differences in second-order statistics, such as texture or disparity (which are secondorder because they are defined by differences in the spatial arrangement of pattern elements, not by luminance differences which define individual elements; see Cavanagh & Mather, 1989). This study investigated whether adaptation to moving disparity information (i.e. stereoscopic motion) induces a motion aftereffect. The existence of stereoscopic motion aftereffects has been controversial. Studies by Steinbach and Anstis (1976, cited in Anstis, 1980), Papert (1964), and Zeevi and Geri (1985) have reported that stereoscopic motion induces little or no motion aftereffect. These results have been incorporated into a contemporary theory of motion perception by Anstis (1978, 1980), Braddick (1974, 1980), Nakayama (1985), and Petersik (1989, 1991). According to these authors, there exist two qualitatively different processes for motion perception. A lower-level sensory process (“short-range” process) computes motion from first-order luminance features across small spatial/temporal intervals, while a higher-level cognitive process (“long-range” process) mediates

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motion perception from first-order and second-order (e.g. disparity) features outside the range of the shortrange process. Because motion aftereffects are assumed to be mediated by motion sensors which are short-range and which require luminance contrast, this framework would predict that stereoscopic motion perception should not adapt. Studies by Lehmkuhle and Fox (1977) Fox, Patterson and Lehmkuhle (1982) and Stork, Crowell and Levinson (1985) have reported that stereoscopic motion induces strong aftereffects. These results have been largely ignored by the literature. Nonetheless, the theoretical view by Cavanagh and Mather (1989) is consistent with evidence for stereoscopic aftereffects. According to these authors, there exists a single process mediating motion perception from different stimulus attributes (for criticism of the two-process model, see Bischof & DiLollo, 1990; Cavanagh, 1991; Cavanagh & Mather, 1989). On this view, stereoscopic motion perception should adapt because it is computed by the same process as motion from first-order attributes which is known to adapt. One explanation of these contradictory results is that differences in stimulus parameters (e.g. speed, direction) among studies produced aftereffects of differing strength, although this hypothesis is difficult to assess because many of the methodological details were not published (three studies were published abstracts, one was a brief technical report, one was a paper presentation, and only one was a refereed journal article). One parameter to consider is the duration of adaptation. Of the three studies reporting weak or non-existent stereoscopic aftereffects, the two studies which cited the duration of adaptation employed durations of 30 s or less (Papert: 30 set; Zeevi & Geri: 25 set). Such durations may be too brief to produce reliable aftereffects from stereoscopic motion. Of the three studies reporting strong aftereffects, the two studies which reported the duration of adaptation used durations longer than 30 set (Lehmkuhle & Fox: 45 set; Fox et al.: 90 set). Such durations may be sufficient to produce reliable aftereffects from stereoscopic motion. It is also possible that individual differences may have produced aftereffects of differing strength across studies, with individuals in one study more susceptible to adaptation than individuals in another study. The present investigation examined the stereoscopic motion aftereffect under a wide range of conditions and observers to determine methods by which strong aftereffects may be induced. In addition, cross-domain adaptation was studied by examining whether the motion aftereffect transfers between stereoscopic and luminance domains (induction of aftereffect when stereoscopic and luminance stimuli are employed as adapting and test stimuli respectively, and vice versa). Crossdomain adaptation would suggest a common neural substrate underlying motion perception with stereoscopic and luminance stimuli. Fox et al. (1982) showed that cross-domain adaptation does occur. Therefore, this study is, in part, an extension of Fox et al.

et al.

There were four experiments. Experiments l-3 examined stimulus factors that affect the stereoscopic aftereffect. Experiment 1 investigated adaptation duration, Experiment 2 investigated temporal variation, and Experiment 3 investigated spatial frequency, disparity magnitude, and motion direction. Experiment 4 examined individual differences by investigating aftereffect strength in a large number of observers, Cross-domain adaptation between stereoscopic and luminance domains was also investigated in this experiment.

METHODS Observers Seventy-one observers (32 males and 39 females) served in one or more experiments. The observers were naive with regard to the purpose of this study, and they had normal or corrected-to-normal visual acuity (tested with Ortho-Rater, manufactured by Bausch and Lomb) and good binocular vision (tested with static random-dot stereograms in Julesz, 1971). In addition, all subjects were tested to ensure that they could perceive stereoscopic forms (e.g. grating patterns, squares) in our dynamic random-dot display before serving in the study. Apparatus

This study investigated stereoscopic aftereffects by employing a dynamic random-dot stereogram generation system whose prototype was described in detail by Shetty, Brodersen and Fox (1979) and Fox and Patterson (1981). The observer viewed a 19-in. Sharp color monitor (model XM 1900; dimensions = 11.Oby 15.2 arc deg) from a distance of 1.5 m (pixel size: 5.7 arcrnin; stereogram display luminance-i.e. average luminance of 50% density dots plus background: 46cd/m2). The red and green guns of the monitor were electronically controlled by a stereogram generator (hardwired device) to produce red and green random-dot matrices (approximately 5000 dots each matrix). Stereoscopic viewing was accomplished by placing red (Wratten No. 29) and green (Wratten No. 58) filters in front of the observer’s eyes. The stereogram generator generated the’ random dots and created the disparity, which produced a stereoscopic stimulus (background dots correlated between eyes). All dots were replaced dynamically, with positions assigned randomly, at 60 Hz, which allowed the stimuli to be moved without monocular cues (Jules2 & Payne, 1968). An optical programmer (modified black and white video camera) transformed two-dimensional achromatic stimuli it scanned (e.g. moving white and black bars) into a stereoscopic stimulus on the Sharp monitor. The voltage of the camera (whose scan rate was synchronized with that of the monitor) was digitzed and used as code to specify where disparity was inserted in the stereogram. The optical programmer scanned white and black square-wave grating patterns moving on a conveyor belt controlled by a d.c. motor. The temporal frequency of the grating (from which we derived speed) was measured with a special-purpose photoelectric device which

STEREQSCQPK

MUTlON

counted the number of white bars moving past a fixed p0iIIlt. the sweess of this study depends upon our ability to ensure that the motion a~er~~~t was truly cyclopean by ruling out the possibility that monocuiar cues contaminated our stimuli. A series of control trials was performed every session in which the observer wore either Rd or green $lters over both eyes during adaptation and tested for the after~~~t with the s~e~eo~o~ic test pattern (red and green filters over different eyes). On other control trials, the observer adapted to our dynamic display with the moving stereoscopic patterri set to zero disparity (red and green filters over different eyes) and again tested for the aftereffect with a non-zero disparity ste~~~pi~ test pattern. In all cases? observers never perceived any adapting pattern nor any afte~e~~t when viewing the test pattern, suggesting that monocular cues did not contaminate the stereoscopic aftereffect during formal data collection. (Other trials were also performed in which the observer wore either red OTgreeu filters over both eyes and attempted force-choice discrimination of the direction of motion of a stereoscopic pattern that moved eit.her rightward or leftward on each trial. In all cases, discrimination ~rfo~ance was at chance level.) In addition to motion aftereffects investigated with stereoscopic stimuli, we also examined after~ff~ts with l~i~an~-domain stimuli. This was necessary in Experiment 4, which investigated cross-damain adaptation between the stereoscopic and luminance domains. Two kinds of high-contrast luminance stimuli were used, which were well above d~tec~on threshold ~1~~~~ detectable). The first stimulus was a black and red squarewave grating pattern (luminance of black bars: 0.7 cd/m2; luminance of red bars: 1I .4 cd/m2; Michelson contrast: 88%). This pattern was defined by both luminance and chromatic borders. We tested subjects using this stimulus because it was easily generated and displayed OR the Sharp monitor and, therefore, easy to switch back and forth between the stereoscopic and luminance stimuli. This was important when testing cross-domain adaptation with a large number of naive observers. Dimensions (e.g. size, spacing) of the luminance stimuli were equal to the Dimensions of the stereoscopic stimuli in terms of angular subtense at the eye. To determine whether the magnitude of cross-domain effects obtained with the black and red pattern would

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WtlfR3~ fOr tracking e’e m~~~me~~~, wf ah? meas?xed M&Cm aftereffects in

four observers ~adaptation duration = t20 seq eight

trials per conditionper observer)when the adaptingpattern was PreSented as two separate panelsaf the display, one panel located above fixation and the other panel below fixation. The direction of adapting motion Was opposite in the two pa&s* either ~~t~a~ aboW fixatiorI arid leftwad below Sxation or vice versa, which shot&i minimize tracking. We found that tip. after&tit in&& with bidirectional motion was similar to that induced with nni-

directionalmotion (averagebidirectional effect = 5-4 set; average onidir~ti~~~~ effect = %sec), which suggests that tracking eye movements did not contaminate the unidirectional e&t in the fflain experiment.

AFTEREFFECT

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generaIize to achromatic patterns, we modifi~ the stereogram generator to generate and display a second kind of stimulus fol~o~ng formal data tx&XtiOn in ~x~~~rne~~ 4, This stimulus was a black and white square-wave sting pattern (luminan~ of black bars: 0.7 cd/m*; luminance of white ban: 52.5 cd/m2; contrast: 97%). This pattern was defined only by a luminance border. Next, cross-domain aftereRects were measured for four observers using either the stereoscopic pattern and btack and red pattern, or the stereoscopic pattern and black and white pattern. Adaptation duration was 120 sec. Eight trials were performed for each s~irn~lus pair under each cross-domain condition (i.e. stereoscopic adapt~lumi~ance test, or luminance adapt/stereoscopic test). Statistical analysis revealed no reliable difference in cross-domain aftere&cts between the st~eo~o~ic~black and white patterns (average duration = 12.5 set) and the stereoscopic/black and red patterns (average duration = 12.2 set}. These results suggest that the crossdomain results of Experiment 4 would likely generalize to other high contrast fuminance stimuli.

To provide the observer with a clear understanding of what to judge when reporting a stereoscopic aftereffect, testing always began with two practice trials involving the luminance stimuli. The observer viewed the display by fixating a small black fixation dot in the center of the screen during every trial; the fixatian dot and monitor frame served to guide the subject”s fixation.* The observer adapted to lumin~ce motion for 120 set (30 set for some ~~~ditiu~s in Ex~~rn~t 4), then viewed a stationary luminance test pattern and reported what was perceived. Virtually all observers reported seeing illusory motion of the test pattern in a dir~tion opposite that of the adapting motion (three observers reported no aftereffect during practice or formal data collection with the N-see adaptation duration). ~ollo~n~ the practide trials, the observer was told that his/her task was to report the duration, if any, of illusory motion on each trial using the same criterion as adopted in the practice trials. The observer was told that the illusory motion may or may not occur on each trial, that there was no correct answer, and to simply report what was perceived. The observer reported the duration of the aftereffect by activating and deactivating an electronic clock-counter. Except where otherwise noted, the observer adapted to a moving stereoscopic square-wave grating pattern on each triaf. Following ~dap~~o~, the observer viewed a stationary ste~o~opic grating pattern (test pattern) of the same spatial frequency and disparity value. hour trials were recorded under each condition by each observer in Experiments 1 and 2; six trials were recorded per condition per observer in ~x~r~en~ 3 and 4, Four minutes of rest were taken between trials to allow the aftereffect to dissipate (in ~~~rni~~ry work, we found that four minutes of rest is more than enough time to dlOW aftereffects to dissipate completeiy when adap tntion time is 3 min or less). About 10-15 trials wexy3 performed each session.

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not studied; all disparities were crossed (i.e. hatf of the bars of the stereoscopic grating had dots with crossed disparity, while the remaining half had zero disparity, with a square-wave profile),*

EXPERIMENT

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FIGURE 1. Aftereffect duration for five values of adaptation duration. Each datum is an average of nine observers, and each error bar equals 1 SE. Spatial frequency of adapting and test pattern was 0.28 c/deg, and disparity was 11.4arcmin. Temporal frequency of the adapting pattern was 2.2 Hz (speed was 7.9 de&see), and direction of motion was rightward.

Before formal data colkction, we performed preliminary studies of spatial and temporal factors afkcting stereoscopic motion processing. Stereoscopic processing involves poor spatiotemporal resolution, which limits the range of usable stimulus parameters. For example, we found that the range of usable stereoscopic spatial frequencies was about 0.10-l ,Oc/de@, generally consistent with Schumer and Ganz (1979) and Tyler (1974). The range of stereoscopic temporal frequencies was about OS-8-O Hz, consistent with Patterson et al. (1992). These values indicate that the range of usable speeds was from OS to about 8Odegjsec Xn our study, we strived to employ stimulus parameters that made our stimuli easy to perceive. Motion perception from stereoscopic stimuli defined by uncrossed disparity was poor and therefore

*We have observed in a number of contexts (e.g. apparent motion in a Ternus display; Patterson, Hart & Nowak, 1991) that the perceived quaiity of motion from uncrossed disparity is always paar, which may be related to occIusion. Background elements in a random-dot stereogram appear as an occluding surface behind which uncrossed stimuli are seen. Typically, the boundaries of uncrossed stimufi appear as if they belong to the background and not to the stimuli (i.e. boundaries appear “extrinsic?). Motion processing may be degraded because stimulus boundaries are perceptually unde~ned. Such occlusion cues would not arise when stimuli are defined by crossed disparity. t&cause motion direction was constant across trials, observer expectations or carry-over effects may haoe i&enced the aftereffect, although we believe this to be unlikely. To control for this possibility, we measured motion aftereffects in four observers (adaptation duration = 120 set; eight trials per condition per observer) when rightward motion was intermixed with other directions of motion (up, down, Ieftward) in the same block of trials, which should eliminate expectations/carry-over effects. We found that the aftere&zct induced under these conditions was simifar to that induced when dire&an of motion was exclusiveiy rightward (average e&c& = 5.3 vs 5.6 set, respectively), which suggests that expectations or carry-over effects did not affect results of the main experiment.

1

Our first experiment investigated one sEim~lus parameter we considered important for stereoscopic motion adaptation, namely, duration of adaptation. We examined stereoscopic aftereffects with a number of adaptation durations, the results of which would be used to set adaptation duration in the subsequent experiments. The adapting and test stimulus was a vertically-oriented grating pattern of spatial frequency 0.28 c/deg, During adaptation, the pattern moved ri~twards at a temporal frequency of 2.2 Hz which corresponded to a speed of 7.9 deg/sec.t Duration of adaptation was either 15, 30, 60, 90, or 120 sec. Disparity was 11.4 arcmin. Nine observers served,

Figure 1 shows the relation between aftereffect duration and adaptatjon dura~on. Stereoscopic aftereffects are brief (e.g. several seconds) with 1.5 and 30 set of adaptation, and aftereffects are quite Iong (e.g. almost IO see) with the 60, 90 and 120 set of adaptation. The large standard errors show that there were large individual differences, with aftereffect durations ranging from about 2 to 20 set or greater in all but the 15 set adaptation condition (in that condition, aftereffects ranged from less than 1 see to less than 7 set). These data were analyzed with a one-way analysis of variance (ANOVA) for thin-subj~ts designs. The analysis revealed a reliable effect of adaptation duration &‘(4, 32) = 8.45, P K 0.011. A Ne~an-Keys multiple comparison test showed that the 1.5 and 30 set conditions are reliably different from the 60,90, and 120 set conditions, and that the 30 and 60 set conditions are reliably different from the 90 and 120 set conditions (P < 0.05)” The results show that reliable stereoscopic motion aftereffects exist when adaptation duration is sapiently long to induce them. When investigating other properties of stereoscopic aftereffects in subsequent experiments, adaptation duration was set to 120 set except where otherwise noted.

This experiment investigated the effects of temporal variation of the adapting pattern on aftereffect duration. To manipulate temporal variation, we varied the speed of drift of the stereoscopic pattern which produced changes in its temporal frequency. The adapting and test stimulus was a vertically-o~e~ted grating pattern of spatial frequency 0.28 cjdeg. During adaptation, the pattern moved ~ghtwards at a temporal frequency of either 0.55, l.t, 2.2, 4.4, or 8.8 Hz which corresponded

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STEREOSCOPIC MOTION AFTEREFFECT 12

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low temporal frequency/speed, with increase in frequency/speed inducing greater aftereffect durations. When investigating other properties of stereoscopic aftereffects in the next two ex~~ments, temporai frequency was set to 2.2 Hz (speed of 7.9 deg/sec). EXPERIMENT 3 now to other properties of the stereoscopic aftereffect, this experiment investigated the effects of spatial frequency, disparity magnitude, and direction of adapting motion on aftereffect duration. Spatial frequency of the adapting and test grating pattern was either 0.14, 0.28, or 0.56 c/deg. During adaptation, the pattern moved at a temporal frequency of 2.2 Hz; speed was either 15.7,7.9, or 3.9 deg/sec. Motion direction was either left, right, up, or down (orientation of adapting and test gratings was always perpendicular to direction of adapting motion). Disparity was either 11.4 or 22.8 arcmin. Four observers served. Turning

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FIGURE 2. Aftereffect duration for five values of temporal frequency of the adapting pattern (corresponding speed was 2.0, 3.9, 7.9, 15.7, and 3 1.4 deg/sec, respectively). Each datum is an average of seven observers, and each error bar equals 1 SE. Spatial frequency of adapting and test pattern was O.ZSc/deg, and disparity was 11.4 arcmin. Direction of motion of the adapting pattern was rightward, and adaptation duration was 120 sec.

to a speed of 2.0, 3.9, 7.9, 15.7, or 31.4deg/sec, respectively. Temporal frequencies greater than 8.8 Hz could not be used because observers would experience temporal summation of disparity information. Disparity was 11.4 arcmin. Seven observers served. [For one observer, temporal summation of disparity information (perceptual smearing of stimulus features and loss of motion sensation) occurred at a temporal frequency of about 8.0 Hz, while for the other six observers summation occurred at slightly higher frequencies (above 8.8 Hz). For the observer experiencing summation at 8.0 Hz, we lowered temporal frequency of the adapting pattern slightly to a point where he could just perceive spatial structure and motion, and he adapted to the lesser frequency. Thus, average temporal frequency in the 8.8 Hz condition was actually slightly less than 8.8 Hz.)

Figure 2 shows duration of aftereff~t for five values of temporal frequency/speed. Because Pantle (1974) has shown that the strength of the luminance motion aftereffect is governed by temporal frequency, not speed, we represent temporal variation in Fig. 2 as temporal frequency. The duration of the aftereffect increases with increases in temporal frequency/s~d, from about 2 see at 0.55 Hz (2.0deg/sec) to about 8 see at 8.8 Hz (31.4 deg/sec). Again, the large standard> errors show that there were large individual differences. The data were analyzed with a one-way ANOVA for within-subjects designs. The analysis revealed a reliable effect of temporal frequency/speed [F(4, 24) = 7.14, P < O.OOl].A Newman-Keuls multiple comparison test showed that the 0.55 Hz condition is reliably different from the 1.1, 2.2, 4.4, and 8.8 Hz conditions. The results show that the aftereffect is very short at a

Results Aftereffect duration was affected by motion direction but unaffected by spatial frequency or disparity (see results below), therefore we plot in Fig. 3 the duration of aftereffect for the four directions of motion, collapsing across spatial frequency and disparity. Aftereffect duration was about 12 set for adapting motion in the left and right directions, and it was 20 set or greater for adapting motion in the up and down directions. As in Experiments 1 and 2, there were large individual differences. These data were analyzed with a three-way ANOVA for within-subjects factorial designs. The analysis revealed a reliable effect of motion direction [F(3, 9) = 8.94, P < 0.011. A Newman-Keuls multiple comparison test showed that the down condition was reliably different from the left/right conditions. There were no other reliable main effects or interactions (P > 0.05).

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Right

UP

Motion direction FIGURE 3. Aftereffect duration of four directions of adapting motion. Each bar of the histogram is an average of four observers, and each error bar equals 1 SE. Adaptation duration was 120 sec. See text for details of other stimulus parameters.

The results show that the direction of adapting motion iafluences after&ect duration, with longer aftereffects indue& by rno~~~ &#ng the TpertiCa% axis rek&tive to the ~~~~~~ta~ axis. {This ver~~~b~~~on~~ a~~§~tr~~~ a$rpears u~re~at~ to eye m~v~rn~nts because it exists when adapting motion is bidirect~o~~~,) The next experiment examined individual differences in stereoscopic motion adaptation. ~~~~~~~

ii

This experimtsflt investigated SndividuaE d~~~~~~~~~in susceptibility to stereoscopic adaptation by measuring aftere%z&s in a large number of obsemers for one o,f two gyrations of ~~~~~a~~on~30 or I2G seG*The 3Qset i&ration was used because previous studies ~~~~~~g weak aftere~~ts implored adaptation durations close to 30 set; the 120 set adaptation duration was selected because it produces a strong aftereffect. We were also ~~t~e~~d in ~~~~s~~t~~~ ~ross-d~ma~~ ~~t~~~ a&ptation between s~er~o~~~~ and ~~~a~~ donE&s_ The ~da~~n~ and test st~rn~~us was a vertkaftyoriented grating pattern of’ spatial. frequency 0.28 c/deg, During adaptation, the pattern moved rightwards at a temporal frequency of 2.2 Hz which corresponded to a speed of 7.9 degjke. Fom ~~~~~~ns qere e~~~o~e~ f i ) ~~~~~~~ ~a~~~~~ ~~~~~~~~~~~~ kst ~~~~~~ (2) ster~~o~~c ~d~~~~~~ ~attern~ste~o~o~~~ test pattern, (3) stereoscopic adapting patte~~luminan~e test pattern, (4) luminance adapting pattern/stereoscopic test pattern, For the stereosoopio stimuli, disparity was 113%awtin*

sixty-t&r=

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St&e ~~~p~~~~~ ~~~~~~~~ md ~~~~~~-~~~in the- 1xbx adaptation ~~~ditio~~. Results The resu&s showed &at tke were zarge ~~~~~~d~~ ~~~~ren~s in ~~~r~~~t d~~~o~ even IEKkr fixed stimufus ~o~~~o~s. With 31)see of ~a~~~~o~~ ~~tere~~~ duration ridged from zero to 15 set: or ~r~~t~ across conditions, The condition involving stereoscopic adapting motion and stereoscopic test pattern is particularly interestiag beoause It is ~~~~~arto ~~d~~o~s of previous st?zdies r~or~~g no ~e~s~o~~~ a~te~e~~t. Of the 28 observers in this condition, five reported ?ero af$ereEoct, 17 reported aftereffect dnratio~s greater than zero but less than 3 set, and only 6 observers reported aftereffect durations greater than three seconds. W&b ~2~~~ of a~~t~~o~~ a~~e~~~ d~~~~~~~ ranged from zePo or &se to zero to over 25 See-;fcTosS conditions. Of the 35 ~nd~vid~a~s in the st~~os~opj~ adapt/stereoscopic test condition, five reported zero aftereffect, 15 reported aftereffect durations greater than i see but kss &an jOsee, 13 reported ~f~~~e~e~t durations greater than $0 see IX&4ess &a?%B se%&arrd SVo observers reported aft~re~~t durations greater t&n 20 sec. Figures 4 and 5 show average aftereffect duration under the four adaptation conditions for the two adaptation d~~~t~o~s~ XI and 12Osec, respectively. Average

STEREOSCOPIC MOTION AFTEREFFECT

condition (called lum/lum in Figs 4 and 5) induces the greatest aftereffect, followed closely by the stereoscopic adapt/luminance test condition (called cyc/lum in the figures); the stereoscopic adapt/ste~o~opic test condition and the luminance adapt/stereoscopic test condition (called cyc/cyc and lym/cyc, respectively, in the figures) induce the weakest aftereffect. Interestingly, there is a 400% increase in aftereffect duration induced by stereoscopic motion with 120 set of adaptation and a luminance test pattern (cyc/lum of Fig. 5) relative to 30 set of adaptation and a stereoscopic test pattern (cyc(cyc of Fig. 4).* These data were analyzed with a two-way ANOVA for mixed designs. The analysis revealed a reliable effect of adaptation duration [F(l, 61) = 11.62, P < O,Ol] and of adaptation condition [F(3, 183) = 11.92, P < O.OOl],but no significant interaction between the two factors. We next performed three post-hoc comparisons employing the Scheffe test. (1) Aftereffect duration in the intradomain conditions (lum/lum and cyc/cyc) was averaged together and compared to average aftereffect duration in the interdomain conditions (cyc/lum and lum/ cyc). Scheffe’s test showed there were no reliable differences between intradomain and interdomain aftereffects. (2) Aftereffect duration using luminan~ adapting motion (lum/lum and lum/cyc) was averaged together and compared to average aftereffect duration using stereoscopic adapting motion (cycicyc and cyc/lum). There were no reliable differences between stereoscopic and luminance motion. (3) Aftereffect duration using the luminance test pattern (lum/l~ and cyc/Ium) was averaged together and compared to average aftereffect duration using the stereoscopic test pattern (cyc/cyc and lum/cyc). The stereoscopic test pattern produces significantly shorter aftereffects than the luminance test pattern, S = 3.97, P < 0.05 (S,,, = 1.55).

GENERAL DiSCUSSlON This study confirms that stereoscopic motion induces an adaptation aftereffect. This suggests that the process mediating stereoscopic motion perception possesses sensory characteristics because adaptation is a signature of sensory processing. This result rejects a key aspect of the short-range/long-range theory (e.g. Anstis, 1978, 1980; Braddick, 1980), namely, its assertion that stereoscopic motion perception is mediated by a process which does not adapt. *To control for the possibility that the luminance dots of the stereoscopic test pattern may elicit an aftereffect independent of disparity (i.e. luminance aftereffect), we measured the motion aftereffect in three observers (adaptation duration = 120 set; four trials per observer) with a luminance adapting pattern and a stereoscopic test pattern set to zero disparity (i.e. test pattern was the dots of the display). Ail observers reported an average aftereffect of zero or CIOSCto zero duration (i.e. less than 0.2 see), which suggests that the stereoscopic bars themselves elicit the aftereffect (the very small aftereffect was likely produced by a brief transient that occurred when the display was switched from the adapting pattern to the test pattern).

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Experiment l-3 revealed several factors of adaptation that influence aftereffect duration: adaptation duration, temporal frequency/speed, and direction of motion. Aftereffects are greatest when adaptation duration is long, temporal fr~uency~s~ is high, and motion direction is up or down. All experiments, especially Experiment 4, showed that even under fixed stimulus conditions, inter-observer differences are quite large, with differences among observers of 10 set being common. In Experiment 4, motion adaptation transferred between the stereoscopic and luminance domains, which replicates Fox et af. (1982). Moreover, there were no reliable differences between intradomain and interdomain conditions and no reliable differences between stereoscopic versus luminance adapting motion. Cross-domain adaptation indicates that motion processing from stereoscopic (second-order) and luminance (first-order) attributes is mediated by a common neural substrate. This idea is consistent with results of Cavanagh, Arguin and von Grunau (1989), who showed that apparent motion can be perceived when first-order and second-order att~butes are intermixed across frames of a motion sequence. Also, in a neurophysiologic study, Albright (1992) showed that neurons located in middle temporal area of primate cortex demonstrate similar directional tuning for both luminance and texture stimuli (because MT contains cells activated by both disparity and motion, it may provide the substrate for stereoscopic motion perception; see Maunsell & Van Essen, 1983). Cross-domain results of this and other studies support strongly the theory by Cavanagh and Mather (1989; Cavanagh, 1991) that motion perception from different stimulus attributes is computed by the same mechanism. Experiment 4 revealed an important factor of testing that affects aftereffect duration: the kind of test pattern employed. The stereoscopic test pattern produces a significantly shorter aftereffect than the luminance test pattern. This suggests that stereoscopic patterns are perceptually weak for supporting a motion signal. Indeed, Patterson et af. (1991), in a study investigating stereoscopic bistable motion perception (Ternus display), reported that briefly-presented stereoscopic stimuli undergoing apparent motion were perceived weakly relative to luminance stimuli. Our results provide insight into the controversy regarding the existence of stereoscopic motion adaptation. Factors such as duration of adaptation, temporal frequency/speed, and motion direction affect aftereffect duration, and previous studies have employed different values of these factors which may have induced aftereffects of differing strength, although recall that this idea is difficult to assess because many details of these studies were not publish~. However, two studies reporting little stereoscopic aftereffect (Papert, 1964; Zeevi & Geri, 1985) employed adaptation durations of 30 see or less, durations found in Experiments 1 and 4 to be too brief to induce signific~t aftereff~ts in many observers (i.e. a “floor effect”). Two studies reporting strong

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ROBERT PATTERSON et al.

aftereffects (Lehmkuhle & Fox, 1977; Fox et al., ‘1982) used adaptation durations longer than 30 set, and a floor effect would be less likely to occur under these conditions. Large differences among observers across all four experiments of this study suggest that individual differences may have led to aftereffects of differing strengths across previous studies. Note, however, that the d~ation of adaptation is critical in producing a significant aftereffect on& when a stereoscopic test pattern is used. In some way the stereoscopic pattern is weak for supporting a motion signal, leading to a brief aftereffect. With stereoscopic adapting motion and a high-contrast luminance test pattern, the aftereffect is robust. Note that our stereoscopic test patterns may elicit weak aftereffects only when they are static. They might produce motion aftereffects equivalent to those elicited by luminance test patterns if the disparity bars were dynamic (e.g. counterphased flickered) (Hiris & Blake, 1992; Turano, 1991; von Grunau, 1986). Dynamic test patterns may be especially important for aftereffects with second-order stimuli (Nishida & Sato, 1993). Our results are consistent with two recent studies. Raymond (1993) showed that thresholds for detecting the direction of motion in partially-coherent random-dot kinematograms are elevated following motion adaptation, and that such adaptation transfers interocularly. Carney and Shadlen (1993) reported that motion aftereffects can be induced by cyclopean motion created from dichoptic flickering displays. Both studies suggest that a binocular substrate exists for motion adaptation. While Carny and Shadlen’s motion was cyclopean, it would be classified as first-order in the sense that the stimuli were defined by luminance contrast; our cyclopean stimuli were second-order because they were defined by disparity contrast.

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Acknowledgements-The authors Randy Blake, Mike Donnelly, and on the manuscript. This research awarded to R. P. by Washington

would like to thank Tim Petersik, two reviewers for helpful comments was supported by grant No. 30023 State University.