Disparity Tuning of the Stereoscopic

Disparity Tuning of the Stereoscopic (Cyclopean) Motion Aftereffect ROBERT PATrERSON,*t CHRISTOPHER BOWD,* RAY PHINNEY,* ROBERT FOX,l: STEPHEN LEHMKUHLE§ Received 9 December 1994; in revised form 3 April 1995

Across five experiments this study investigated the disparity tuning of the stereoscopic motion aftereffect (adaptation from moving retinal disparity). Adapting and test stimuli were moving and stationary stereoscopic grating patterns, respectively, created from dynamic random-dot stereograms. Observers adapted to moving stereoscopic grating patterns presented with a given disparity and viewed stationary test patterns presented with the same or differing disparity to examine whether the motion aftereffect is disparity contingent. Across experiments aftereffect duration was greatest when adapting motion and test pattern both were presented with zero disparity and in the plane of fixation. Aftereffect declined as disparity of adapting motion and/or test pattern increased away from fixation, even under conditions in which depth position of adapt and test was equal. This argues against a relative depth separation explanation of the decline, and instead suggests that the amount of adaptable substrate decreases away from fixation.

Motion aftereffect Motionperception Stereopsis Random-dotstereograms Cyclopean

INTRODUCTION and Angilletta (1994) showed that significant stereo- The motion aftereffect (MAE) is the perception of scopic MAEs can be induced when adaptation duration is illusory motion of a stationary object following adapta- sufficiently long. Observers adapted to a moving tion to a moving object, with direction of illusory motion stereoscopic grating pattern and subsequently viewed a opposite to that of adapting motion. Studied for hundreds stationary test grating. Significant aftereffect durations of years (Aristotle, cited in Wohlgemuth, 1911; Purkinje, (e.g. 8 sec) were observed when adaptation duration was 1825), the MAE reflects properties of underlying 60 sec or greater, but not when it was 30 sec or less. mechanisms which mediate motion processing (Suther- Previous studies reporting significant aftereffects (Fox, land, 1961; Moulden, 1980; Wright & Johnston, 1985). Patterson & Lehmkuhle, 1982; Stork, Crowell & This study investigated MAEs induced from moving Levinson, 1985) typically used adaptation durations retinal disparity information, or stereoscopic MAEs. The longer than 30 sec while studies reporting weak after- locus of stereoscopic adaptation is binocular-integration, effects (Anstis, 1980; Papert, 1964; Zeevi & Geri, 1985) or cyclopean, levels of vision (Julesz, 1960; Julesz, 1971; typically used adaptation durations of 30 sec or less. Sekuler, 1975; Tyler, 1983). Stereoscopic motion is one The present study extended the Patterson et al. form of non-Fourier motion, which refers to motion from investigation by employing a long adaptation duration stimulus boundaries defined by differences in higher- to examine the stereoscopic, or Z-axis, properties of the order statistics or variables undetectable by motion MAE induced by frontoparallel stereoscopic motion. mechanisms sensing motion energy (Cavanagh & Specifically, we examined whether the MAE is disparity Mather, 1989; Chubb & Sperling, 1989). Other forms contingent by separating in disparity or depth the of non-Fourier motion include motion from contrast- adapting motion from the test pattern and measuring modulated patterns and relative motion cues. aftereffect duration. This question bears upon the idea Patterson, Bowd, Phinney, Pohndorf, Barton-Howard that the visual system contains cells which are selective for both direction of motion and retinal disparity (e.g. Maunsell & Van Essen, 1983). *Department of Psychology, Washington State University, Pullman, An effect of disparity or depth separation between WA 99164-4820, U.S.A. adapt and test would be interpreted as reflecting disparity tTo whom all correspondence should be addressed. tuning, the degree to which motion mechanisms suscep- :~Department of Psychology, Vanderbilt University, Nashville, tible to adaption are selective for disparity. Disparity TN 37240, U.S.A. §School of Optometry, University of Missouri, St Louis, MO 63121, tuning implies that common mechanisms are engaged U.S.A. only when adapt and test possess the same or similar 975 976 R. PA'I"rERSONet al. disparity whereas separate mechanisms are engaged METHODS when adapt and test possess very different disparities. If Observers disparity/depth separation decreases the aftereffect, we Fourteen observers (nine male and five female) served would conclude that it is possible to selectively adapt in one or more experiments. Of the 14, three males and different groups of cells. one female served in all experiments. All but two Disparity or depth-contingent MAEs have been studied observers were naive with regard to the purpose of this in the past. Anstis and Harris (1974) (see also Chase & study. The observers had normal or corrected-to-normal Smith, 1981; Smith, 1976; Verstraten, Verlinde, Freder- visual acuity (tested with Ortho-Rater, Bausch and icksen & van de Grind, 1994) had observers alternately Lomb) and good binocular vision [tested with static adapt to clockwise rotary motion which appeared in random-dot stereograms (Julesz, 1971)]. The observers crossed depth and to counterclockwise motion which were tested to ensure that they could perceive stereo- appeared in uncrossed depth. They found that the scopic forms (e.g. grating patterns) in our dynamic direction of the aftereffect depended upon the depth of random-dot display before serving in the study. the stationary test stimulus. When the test appeared in crossed depth, the illusory motion appeared counter- Apparatus and stimuli clockwise, and when the test appeared in uncrossed Stereoscopic aftereffects were investigated by employ- depth, the illusory motion appeared clockwise. ing a dynamic random-dot stereogram generation system Using stereoscopic stimuli similar to those employed (Shetty, Brodersen & Fox, 1979; Fox & Patterson, 1981). in the present study, Lehmkuhle and Fox (1977) and Fox The observer viewed a 19-in. Sharp color monitor (model et al. (1982) had observers adapt to moving stereoscopic XM 1900; dimensions = 11.0 × 15.2 deg) from a distance of 1.5m (pixel size, 5.7minarc; stereogram grating patterns in one depth plane and view stationary display luminance, 46 cd/m2). The red and green guns stereoscopic test patterns in the same or different depth of the monitor were electronically controlled by a plane. Both studies found that depth separation between stereogram generator (hardwired device) to produce red adapt and test lessened the aftereffect duration. and green random-dot matrices (approx. 5000 dots each Across a series of five experiments we sought to learn matrix). Stereoscopic viewing was accomplished by more about depth separation effects on the stereoscopic placing red and green filters in front of the observer's MAE. Part of the motivation for this study was to attempt eyes. to explore two interpretations of the Lehmkuhle and Fox The stereogram generator produced random dots and and Fox et al. studies. These studies showed that the created disparity (background dots correlated between stereoscopic MAE is disparity or depth contingent, but eyes). All dots were replaced dynamically, with positions the exact nature of that contingency is unknown. For assigned randomly, at 60 Hz, which allowed the stimuli example, is the relative depth separation between to be moved without monocular cues (Julesz & Payne, adapting motion and test pattern the relevant variable, 1968). Two optical programmers (modified black and or is the disparity or depth of the stimuli from fixation white video cameras) scanned black and white square- (horopter) important? wave grating patterns, either stationary or moving Because previous studies have maintained adapt or test rightward, located on conveyor belts controlled by d.c. in the fixation plane while varying the depth of the other, motors. The voltage of the cameras (whose scan rate was synchronized with that of the monitor) was digitized and depth separation between adapt and test has involved used as code by the stereogram generator to specify displacing one stimulus away from fixation. Thus, these where disparity was inserted in the stereogram. The studies have confounded depth separation between two stereogram generator transformed the black and white stimuli with separating one of them from fixation. Across square-wave gratings into stationary or rightward moving the five experiments the disparity or depth relationships stereoscopic gratings as seen by the observers (half of the among adapting motion, test pattern, and fixation were bars of the stereoscopic grating had dots with crossed or manipulated to reveal their influence on the stereoscopic uncrossed disparity, while the remaining half had zero MAE. disparity, with a square-wave profile).*

General procedure *In a control experiment, Patterson et al. (1994) induced stereoscopic Testing began with several practice trials involving MAEs with bidirectional adapting motion, which should have stereoscopic gratings and luminance-defined gratings. minimized tracking eye movements. The adapting grating was presented as two separate panels of the display, one above and one Subsequent formal trials involved only stereoscopic below fixation. The direction of adapting motion was opposite in gratings. The observer was told that his/her task was to the two panels, rightward above fixation and leftward below report the duration, if any, of illusory motion on each fixation or vice versa. The test pattern was a stationarygrating also trial. The observer was told that the illusory motion may presented as two panels. The resulting MAE was bidirectional,with or may not occur, that there was no correct answer, and to illusory motion seen in opposite directions in the two panels. The duration of the bidirectional aftereffectsequalled that of unidirec- simply report what was perceived. The observer reported tional aftereffects,suggesting that eye movements do not play a the duration of aftereffect by activating and deactivating role in the induction of stereoscopic MAEs in our study. an electronic clock-counter. DISPARITY TUNING OF MAE 977

Observers adapted to a moving stereoscopic grating was 22.8, 28.5 or 34.2 min arc. When adapting motion presented with a given disparity and subsequently tested was presented with the same crossed disparity from the for the aftereffect by viewing a stationary stereoscopic display screen as the fixation square, the motion had zero grating presented with the same or different disparity. On disparity relative to the observer's fixation (horopter).* each trial, the observer fixated a fixation square and When adapting motion was presented with greater adapted to stereoscopic motion (temporal frequency crossed disparity from the screen than the square, the 1.43 Hz, speed 5.13 deg/sec) for 2min. The fixation motion had crossed disparity relative to fixation. When square (1.0 dege) was a stereoscopic stimulus presented adapting motion was presented with lesser crossed in the middle of the display screen with a given crossed or disparity from the screen than the square, the motion uncrossed disparity relative to the screen. To prevent the had uncrossed disparity relative to fixation. grating pattern from occluding the fixation square or vice versa when the two stimuli had different magnitudes of disparity, the fixation square was seen through a small 12 window in the grating pattern (the window was stationary A FIXATION AND TEST DISPARITY EQUAL and slightly larger than the square). 11.4 rain test 10- 22.8 rain test Adapting motion was presented with the same . test disparity from the display screen as the fixation square (motion appearing in the fixation plane) or with greater or 8- lesser disparity than fixation (motion appearing in front of or behind fixation). Following adaptation, the observer viewed the stationary test pattern presented with the same 6- or different disparity from the display screen as the fixation square. Spatial frequency of adapting and test 4- gratings was 0.28 c/deg, orientation was vertical. Eight experimental trials were recorded under each 6" 2- • [] • condition by each observer with 4 min of rest taken LU between trials to allow the aftereffect to dissipate. About Z o 10-15 trials were performed each session. I-- i i i i i w. 11.4 17,1 22.8 28.5 34.2 ADAPTATION DISPARITY FROM DISPLAY SCREEN (MIN) r,, I..- o EXPERIMENT I u.I u- u. u.I 12 This experiment investigated the effect of depth n- LlU B I.- separation between adapting motion and test pattern on M. ,,¢ aftereffect duration. All stimuli were presented with 10 crossed disparity from the display and appeared in depth in front of it. Test disparity was equal to that of fixation while adapting disparity was varied across trials, either 8 greater than, equal to, or less than, that of fixation (and test). Three tuning curves were obtained, one for each of 6 three fixation/test disparities, a small disparity, an intermediate disparity, and a large disparity. (For the 4, small and large fixation/test disparities, only partial functions could be obtained because some observers have [] difficulty fusing disparities greater than about 40 rain- 2- arc.) The fixation square was presented with 11.4, 22.8, or 0 i i 0'. 0 i J 34.2 min arc of crossed disparity from the screen. For the 11,4 5.7 57 11.4 11.4 min arc fixation square, disparity of adapting motion UNCROSSED CROSSED ADAPTATION DISPARITY FROM FIXATION (MIN) from the screen was 11.4, 17.1, or 22.8 min arc. For the 22.8 min arc fixation square, disparity of adapting motion FIGURE 1. (A) Aftereffect duration as a function of disparity of was 11.4, 17.1, 22.8, 28.5, or 34.2 min arc. For the adapting motion for three test disparities (arrows) in crossed direction 34.2 min arc fixation square, disparity of adapting motion relative to display screen (all disparities appeared in front of the display). The test pattern was presented in the fixation plane. O, 11.4 min arc test disparity with 11.4, 17.1, and 22.8 min arc adapting disparities; IS], 22.8 min arc test disparity with 11.4, 17.1, 22.8, 28.5, *Strictly speaking, when adapting motion was presented with the same and 34.2 min arc adapting disparities; •, 34.2 rain arc test disparity disparity from the display screen as the fixation square, the moving with 22.8, 28.5, and 34.2 min arc adapting disparities. Each data point contours would not all lie in the horopter because the horopter is a is an average of eight observers. Error bars equal 1 SE. (B) Aftereffect concave curved line along the visual horizon and an inclined line in duration replotted as a function of disparity of adapting motion from the median plane, thus the moving contours would possess some the fixation plane and test pattern. O, 5, and • indicate test non-zero disparity in the quadrants of the display. disparities given in (A). Error bars equal 1 SE 978 R. PATI"ERSON et al.

The stationary test pattern was presented with the same relative to fixation (horopter) and test. Curves from Fig. crossed disparity from the display screen as the fixation I(A) for the 11.4, 22.8, and 34.2min arc test patterns square, 11.4, 22.8, or 34.2 min arc. The test pattern were superimposed onto one curve in Fig. I(B). Figure always had zero disparity relative to the observer's I(B) shows that aftereffect duration was greatest fixation. Five males and three females served. (7-9 sec) when adapting motion had zero disparity from fixation and test pattern. Aftereffect duration decreased Results symmetrically with increasing crossed or uncrossed For each condition and observer, aftereffect durations disparity of adapting motion from fixation, creating for the eight trials were averaged together to provide a disparity/depth differences between adapt and test. At the single estimate of aftereffect duration. Figure I(A) shows greatest disparity of adapting motion from fixation, aftereffect duration for differing disparities of adapting aftereffect duration was about 4see. Full disparity motion and fixation/test pattern from the display screen, bandwidth at half strength aftereffect would be 12 min - with disparity of fixation/test shown by arrows. The left arc or greater.* curve, with 0, shows three adapting disparities (11.4, Disparity/depth separation between adapting motion 17.1, and 22.8 min arc) each of which was paired with the and test pattern decreases duration of the MAE, for 11.4 min arc test pattern (arrow with 0). The middle stimuli presented in front of the display screen. curve, with [3, shows five adapting disparities (11.4, Note that variation in depth position of the stereoscopic 17.1, 22.8, 28.5, and 34.2 min arc) each of which was patterns induces changes in their apparent size owing to paired with the 22.8 min arc test pattern (arrow with [3). the operation of size constancy (i.e., closer stimuli appear The right curve, with Hi, shows three adapting disparities smaller while stimuli farther away appear larger). (22.8, 28.5, and 34.2 min arc) each of which was paired Variation in apparent size of the bars of the adapting with the 34.2minarc test pattern (arrow with II). grating due to their depth manipulation would produce a Aftereffect was longest when adapting motion had the mismatch in apparent spatial frequency which, in turn, same disparity as the test pattern, and aftereffect may have contributed to the decline in aftereffect decreased with differences in disparity/depth between duration with increasing disparity of adaptation. This adapt and test. issue of changes in apparent size is dispelled in Data shown in Fig. I(A) were statistically analyzed Experiment 3. using analysis of variance (ANOVA) for within subjects designs performed individually for each of the three EXPERIMENT 2 curves. For the curve on the left, depicted by O, the analysis revealed that the effect of disparity of adapting This experiment investigated depth separation and motion was reliable, F(2, 14) = 29.5, P < 0.01. A New- aftereffect duration using stimuli presented with un- man-Keuls post-hoe test showed that the 11.4 min arc crossed disparity from the display screen and appearing condition was reliably different from the 17.1 and in depth behind it. Methods were the same as those 22.8 min arc conditions (P < 0.05). employed in Experiment 1. Four males and four females For the middle curve, depicted by [3, the analysis served as subjects. revealed that the effect of disparity of adapting motion was reliable, F(4, 28) = 20.1, P < 0.01. Newman-Keuls Results testing showed that the 22.8 min arc condition was Figure 2(A) shows aftereffect duration for differing reliably different from the other four conditions, and disparities of adapt and fixation/test from the display that the 11.4 vs 17.1 min arc conditions, and the 28.5 vs screen, with disparity of fixation/test shown by arrows. 34.2minarc conditions, were significantly different The left curve, with O, shows three adapting disparities (t" < 0.05). (11.4, 17.1, and 22.8 min arc) each of which was paired For the curve on the right, depicted by I1, the analysis with the 11.4 min arc test pattern (arrow with Q). The revealed that the effect of disparity of adapting motion middle curve, with Fq, shows five adapting disparities was reliable, F(2, 14) = 31.0, P < 0.01. Newman-Keuls (11.4, 17.1, 22.8, 28.5, and 34.2 min arc) each of which testing showed that the 34.2rain arc condition was was paired with the 22.8 min arc test pattern (arrow with reliably different from the 28.5 and 22.8 min arc condi- [3). The right curve, with I, shows three adapting tions (P < 0.05). disparities (22.8, 28.5, and 34.2 min arc) each of which Figure I(B) shows data from Fig. I(A) recast as was paired with the 34.2 min arc test pattern (arrow with aftereffect duration for different disparities of adapt ID). Aftereffect was longest when adapting motion had the same disparity as the test pattern, and aftereffect decreased with differences in disparity/depth between *Our estimate of disparity bandwidth may be an underestimate because adapt and test. aftereffect duration may have been lower at more extreme disparity Data shown in Fig. 2(A) were statistically analyzed differences between adapt and test. Two observers (four trials each using ANOVA for within subjects designs performed condition each observer) reported aftereffect durations of only 1- individually for each of the three curves. For the curve on 2sec for disparity differences between adapt and test of 22.8 min arc. Lower aftereffect durations at the sides of the tuning the left, depicted by O, the analysis revealed that the functions shown in Fig. 1 and Fig. 2 would increase slightly the effect of disparity of adapting motion was reliable, estimate of bandwidth beyond a value of 12 min arc. F(2, 14) = 29.2, P < 0.01. Newman-Keuls testing showed DISPARITY TUNING OF MAE 979

12 FIXATION AND TEST DISPARITY EQUAL testing showed that the 34.2minarc condition was A reliably different from the 28.5 and 22.8 min arc condi- • 11.4 min test tions, and that the latter two conditions were different 10- 22.8 rain test -- 34.2 min lest from each other (P < 0.05). Figure 2(B) shows data from Fig. 2(A) recast as 8- aftereffect duration for different disparities of adapt from fixation (horopter) and test. Curves from Fig. 2(A) for 11.4, 22.8, and 34.2 min arc test patterns were superimposed onto one curve in Fig. 2(B). Figure 2(B) shows that aftereffect duration was greatest (about 6-7 sec) 4 ¸ when adapting motion had zero disparity from fixation and test pattern. Aftereffect duration decreased symme- • [] • ~" 2- trically with increasing crossed or uncrossed disparity of ill adapting motion from fixation, creating disparity/depth

Z differences between adapt and test. At the greatest o o I- disparity of adapting motion from fixation, aftereffect < I 1'.4 17.1' 22.8' 28.5' 34.2' rr ADAPTATION DISPARITY FROM DISPLAY SCREEN (MIN) duration was about 4 sec. a Disparity/depth separation between adapting motion k- O and test pattern decreases the aftereffect, for stimuli UJ ,"-,12 presented behind the display screen. w B I-- ~10 EXPERIMENT 3 Experiments 1 and 2 show that disparity/depth separation between adapting motion and test pattern lessens the aftereffect. Aftereffect duration is greatest when adapting motion and test pattern are presented with the same disparity, and aftereffect decreases with increasing disparity separation between adapt and test.

4 0 In the Lehmkuhle and Fox (1977) and Fox et al. (1982) studies, disparity of the test pattern was increased to increase depth separation between adapt and test, while in the present Experiments 1 and 2, disparity of adapting motion was increased to increase depth separation between adapt and test. In all cases aftereffect duration 0 11.4' 5.7' 0'.0 5 '.7 11.4' CROSSED UNCROSSED decreased with increasing depth separation, however, ADAPTATION DISPARITY FROM FIXATION (MIN) rather than a depth separation effect between stimuli, the FIGURE 2. (A) Aftereffect duration as a function of disparity of aftereffect may be governed by their disparity from adapting motion for three test disparities (arrows) in uncrossed fixation (horopter). direction relative to display screen (all disparities appeared behind the In this experiment we tested the depth separation display). The test pattern was presented in the fixation plane. O, [] and • indicate magnitude of test disparities given in Fig. I(A). Each data hypothesis by presenting adapt and test with the same point is an average of eight observers. Error bars equal 1 SE. non-zero disparity away from fixation (depth positions of (B) Aftereffect duration replotted as a function of disparity of adapting adapt and test were equal yet different from fixation). If motion from the fixation plane and test pattern. O, [] and • indicate the aftereffect is influenced by depth separation between test disparities given in Fig. I(A). Error bars equal 1 SE. stimuli, aftereffect duration across conditions should be significant and equal because adapt and test are not separated in depth. that the ll.4min arc condition was reliably different In order to compare results from this experiment to from the 17.1 and 22.8 min arc conditions, and that the those from Experiment 1 (shown in Fig. 1), we made latter two conditions were different from each other viewing conditions comparable by presenting the fixatiofi (P < 0.05). square with 22.8 min arc of crossed disparity in front of For the middle curve, depicted by [3, the analysis the display screen. The adapting motion as well as test revealed that the effect of disparity of adapting motion pattern were presented with either 11.4, 22.8, or was reliable, F(4, 28) = 8.2, P < 0.01. Newman-Keuls 34.2minarc crossed disparity from the screen and testing showed that the 22.8minarc condition was appeared in the same depth plane when presented. All reliably different from the other four conditions stimuli were presented with crossed disparity in front of (t" < 0.05). the display screen (i.e., display screen had 22.8 min arc of For the curve on the right, depicted by I, the analysis uncrossed disparity relative to fixation and uncrossed revealed that the effect of disparity of adapting motion disparity relative to all stimuli). Three males and one was reliable, F(2, 14)= 18.3, P < 0.01. Newman-Keuls female served as subjects. 980 R. PATrERSON et al.

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I I I I I 11.4' 5'.7 0,0' 5 i 7 11.4' 11.4 5.7 0.0 5.7 1 1.4 UNCROSSED CROSSED UNCROSSED CROSSED ADAPTATION/TEST DISPARITY FROM FIXATION (MIN) TEST DISPARrP/FROM FIXATION (MIN) FIGURE 3. Aftereffectduration as a function of disparity of adapt/test FIGURE 4. Aftereffectduration as a function of disparity of test from from fixation. Fixation was 22.8 rain arc crossed disparity from the fixation. The adapting motion was presented in the fixation plane, display (all disparitiesappeared in front of the display). Each data point 22.8 min arc crossed from the display (all disparities appeared in front is an average of four observers. Error bars equal 1 SE. of the display). Each data point is an average of four observers. Error bars equal 1 SE.

Results governed by the disparity of adaptation, and possibly that of test, from fixation (horopter). Figure 3 shows aftereffect duration when adapt and test were presented with differing disparity from fixation. Aftereffect was greatest (9 sec) when adapt and test were EXPERIMENT 4 in the fixation plane. Aftereffect decreased to about 5 sec Although Experiments 1 and 2 showed that disparity of when adapt and test had crossed or uncrossed disparity adaptation from fixation influences the aftereffect, part of from fixation. the decline in aftereffect duration shown in Experiment 3 The data shown in Fig. 3 were statistically analyzed may have been due to variation of disparity of the test using ANOVA for within subjects designs. The analysis pattern from fixation. To test this idea, we presented the revealed that effect of disparity was reliable, F(2, 4) test pattern with differing amounts of disparity from =12.3, P < 0.025. Newman-Keuls testing showed that the fixation while the disparity of adapting motion was equal 11.4 and 34.2 min arc conditions were reliably less than to fixation. These conditions replicate the basic design of the 22.8 min arc condition (P < 0.05). Lehmkuhle and Fox (1977) and Fox et al. (1982), who Increases in disparity of adapt and test from fixation kept adapting motion in the fixation plane and varied decreases aftereffect duration, even though depth posi- disparity of the test. tions of the stimuli were equal. This result argues against The fixation square and adapting motion were changes in apparent size as an explanation for Experi- presented with 22.8 min arc of crossed disparity in front ments 1 and 2. In the present experiment there was no of the display screen. The test pattern was presented with difference in depth position between adapt and test within either 11.4, 22.8, or 34.2 min arc crossed disparity from a given trial, thus no mismatch was created in their the screen. Three males and one female served as apparent size or spatial frequency, owing to size subjects. constancy. Results An analysis of variance was computed on the data from Figure 4 shows aftereffect duration for differing Experiment 3 (shown in Fig. 3) combined with the data disparities of test pattern from fixation. Aftereffect was from the 22.8 min arc test conditions from Experiment 1 greatest (8 sec) when test had zero disparity from [shown in Fig. I(B); the 5.7 min arc conditions were fixation. Aftereffect decreased to about 6 sec when test dropped from the analysis to make the conditions had crossed or uncrossed disparity from fixation. comparable across experiments]. No reliable differences The data shown in Fig. 4 were statistically analyzed were found between the results of the two experiments, using ANOVA for within subjects designs. The analysis F(1, 30) = 0.07, P > 0.05, indicating that relative depth revealed that the effect of disparity of test pattern was between adapt and test is not important for the decline in reliable, F(2, 6) = 10.29, P < 0.025. Newman--Keuls test- aftereffect duration. ing showed that the 11.4 and 34.2min arc conditions The results of this experiment argue against a depth were reliably different from the 22.8 min arc condition separation explanation of aftereffect decline in Experi- (P < 0.05). ments 1 and 2, and support the idea that the aftereffect is Increasing disparity of the test pattern away from DISPARITY TUNING OF MAE 981

12 combinations of disparities of adapt and test relative to fixation. Aftereffect was greatest (9 sec) when adapt and 10 test had zero disparity from fixation. Aftereffect 6" I,LI decreased symmetrically as disparity of adaptation became increasingly crossed while disparity of test z 8 O became increasingly uncrossed, and vice versa. At the I-- t,v. greatest disparity separation between adapt and test, aftereffect duration was about 4 sec. I.-- Data shown in Fig. 5 were statistically analyzed using tO I,LI ANOVA for within-subjects designs. Analysis revealed I,LI that the effect of disparity was reliable, F(4, 12) = 27.3, tll llI- P<0.001. Newman-Keuls testing showed that all conditions were reliably different from one another (t" < 0.05). Stereoscopic MAEs occur when adapt and test are 0' ADAPT: 11.4 UX 5.7 UX 0 5.7 X 11.4 X presented with opposite signs of disparity. TEST: 11.4X 5.7 X 5.7 UX 11.4UX ADAPTATION/TEST DISPARITY FROM FIXATION (MIN) GENERAL DISCUSSION FIGURE 5. Aftereffect duration as a function of crossed disparity of adapt and uncrossed disparity of test, or vice versa, from fixation. The results of all experiments show that aftereffect Fixation was 22.8 min arc crossed from the display (all disparities duration is greatest when both adapting motion and test appeared in front of the display). X, crossed disparity; UX, uncrossed pattern are presented with zero disparity and appear in the disparity. Each data point is an average of four observers. Error bars equal 1 SE. plane of fixation. The aftereffect declines as disparity of adapt and/or test increases away from the horopter. It is likely that we were able to achieve significant stereo- fixation decreases the aftereffect. These results replicate scopic MAEs because our display flickered. Although the Lehmkuhle and Fox (1977) and Fox et al. (1982). stereoscopic test pattern did not flicker, the small luminance dots making up the stereogram display were dynamic. Studies (Hiris & Blake, 1992; von Grunau, EXPERIMENT 5 1986) have shown that dynamic test patterns may be In this experiment, we placed adapting motion in important for MAEs, especially with non-Fourier stimuli crossed disparity while the test pattern was placed in (Nishida & Sato, 1993; Turano, 1991). uncrossed disparity (relative to fixation), or vice versa; The decline in aftereffect with increasing disparity depth positions of adapt and test straddled the horopter. occurs even though the apparent depth positions of adapt We did so because a theory of stereopsis by Richards and test are equal in Experiment 3. This argues against a (1970, 1971; see also Mustillo, 1985) posits that detection depth separation explanation of the decline because it of crossed and uncrossed disparity involves separate occurs without depth separation. The effects of depth classes of detector and that perceived depth depends upon separation on aftereffect duration reported by Lehmkuhle their metameric combination (similar to color perception and Fox (1977) and Fox et al. (1982) (possibly also arising from metamerism of responses of three cone Verstraten et al., 1994) likely were produced by changes types). According to this theory, aftereffects between in disparity of the test pattern from fixation. In the study crossed and uncrossed disparity would be expected in the by Anstis and Harris (1974), a disparity-contingent MAE disparity domain. The present study investigated whether was clearly demonstrated (for luminance-domain crossed and uncrossed mechanisms interact to produce motion). These authors did not include a condition in aftereffects in the motion domain by determining whether which adapt and test were presented in the fixation plane, MAEs could be induced when adapt and test were thus the effect of varying disparity of their stimuli from presented with opposite signs of disparity. fixation could not be considered. Stimuli were presented with crossed disparity relative This decline in aftereffect when adapt and test have to the display screen. The fixation square was presented equal disparity and depth (Experiment 3) is inconsistent with a disparity of 22.8 min arc from the screen. Adapting with a ratio model of aftereffect magnitude (e.g. motion was presented with disparities of 11.4, 17.1, 22.8, Moulden, 1980). According to this model, aftereffect 28.5, or 34.2 min arc from the screen. Corresponding magnitude is governed by the ratio of units adapted and disparities of the test pattern from the screen were 34.2, tested to those tested; if the ratio is large, aftereffect 28.5, 22.8, 17.1, or 11.4 min arc. The sign of the disparity magnitude should be large. The results of Experiment 3 of the test relative to fixation was opposite to that of argue against this model because both adapt and test adaptation. Three males and one female served as should engage the same disparity detecting cells, thus the subjects. ratio of cells adapted and tested to those tested should be unity. This, in turn, should lead to a large aftereffect with Results no decline, a prediction that is disconfirmed. Figure 5 shows aftereffect duration for differing An alternative interpretation of our results is that the 982 R. PAqTERSON et al. absolute amount of adaptable substrate for stereoscopic Proceedings of the National Academy of Science U.S.A., 86, motion may decrease away from the horopter, such that 2985-2989. less substrate is adapted by large disparities; more Cormack, L. K., Stevenson, S. B. & Schor, C. M. (1993). Disparity- tuned channels of the human visual system. Visual Neuroscience, 10, substrate is adapted and engaged by stereoscopic 585-596. stimulation close to the horopter. This idea may be Fox, R. & Patterson, R. (1981). Depth separation and lateral related to studies by Beverley and Regan (Beverley & interference. Perception & Psychophysics, 30, 513-520. Regan, 1973; Regan & Beverley, 1973; Regan & Fox, R., Patterson, R. & Lehmkuhle, S. W. (1982). Effect of depth Beverley, 1980) who showed that the system responsible position on the motion aftereffect. Investigative Ophthalmology & for computing motion-in-depth (motion towards or away Visual Science (Suppl.), 22, 144. von Grunau, M. W. (1986). A motion aftereffect for long-range from the observer) can be dissociated from the system stroboscopic apparent motion. Perception & Psychophysics, 40, 31- that computes sideways motion. Our stereoscopic motion 38. stimuli may have adapted binocular processes that Hiris, E. & Blake, R. (1992). Another perspective on the visual motion compute lateral movements of disparity-defined bound- after effect. Proceedings of the National Academy of Science U.S.A., aries in order to disambiguate targets from background 89, 9025-9028. and reinforce motion processing from other cues Julesz, B. (1960). Binocular depth perception of computer-generated patterns. Bell System Technical Journal, 30, 1125-1162. (Cavanagh & Mather, 1989), but the bulk of such Julesz, B. (1971). Foundations ofcyclopean perception. Chicago, IL.: processing occurs for stimuli only near the plane of University of Chicago Press. fixation. Julesz, B. & Payne, R. A. (1968). Differences between monocular and Experiment 5 shows that stereoscopic MAEs do occur binocular stroboscopic movement perception. Vision Research, 8, when disparity of adapt and test are of opposite signs and 433-444. Lehmkuhle, S. & Fox, R. (1977). Stereoscopic motion aftereffects. depth positions straddle the horopter. This shows that Paper presented to Midwestern Psychological Association, Chicago, crossed and uncrossed mechanisms interact to produce IL. aftereffects in the motion domain, an extension of Maunsell, J. H. R. & Van Essen, D. C. (1983). Functional properties of Richards (1970, 1971) idea of metamerism between neurons in middle temporal area of the Macaque monkey. II. classes of disparity detectors which would predict Binocular interactions and sensitivity to binocular disparity. Journal aftereffects in the disparity domain [but note that of Neurophysiology, 49, 1148. Moulden, B. (1980). After effects and the integration of patterns of Richards' theory has been rejected for other reasons neural activity within a channel. Philosophical Transactions of the (see Cormack, Stevenson & Schor, 1993; Patterson & Royal Society of London B, 290, 39-55. Fox, 1984; Patterson, Cayko, Short, Flanagan, Moe, Mustillo, P. (1985). Binocular mechanisms mediating crossed and Taylor & Day, 1995)]. uncrossed stereopsis. Psychological Bulletin, 97, 187. Overall, the MAE was reduced in Experiment 2 Nishida, S. & Sato, T. (1993). Two kinds of motion aftereffect reveal (stimuli behind the display screen) relative to Experiment different types of motion processing. Investigative Ophthalmology & Visual Science (Suppl.), 34, 1363. 1 (stimuli in front of the screen). Reduced aftereffect may Papert, S. (1964). Stereoscopic synthesis as a technique for localizing be related to degraded stereoscopic motion perception visual mechanisms. M.L T. 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