Journal of Vision (2007) 7(11):3, 1–16 http://journalofvision.org/7/11/3/ 1

Veridical perception of moving colors by trajectory integration of input signals

PRESTO, Japan Science & Technology Agency/ NTT Communication Science Laboratories, Junji Watanabe NTT Corporation, Kanagawa, Japan

NTT Communication Science Laboratories, Shin’ya Nishida NTT Corporation, Kanagawa, Japan

For rapid alternations of two colors (e.g., red and green), human observers see the mixed color (yellow). This chromatic flicker fusion has been considered to reflect neural integration of color signals presented successively at the same retinal location. If so, the retinal alternation rate should be a critical fusion parameter. However, here we show that temporal alternations of two colors on the retina are perceptually segregated more veridically when they are presented as moving patterns rather than as stationary alternations at the same rate. This finding is consistent with the hypothesis that the visual system integrates color signals along the motion trajectory, in addition to at the same retinal location, for reducing motion blur and seeing veridical colors of moving objects. This hypothesis is further supported by a covariation of perceived motion direction and perceived color in a multipath motion display. Keywords: motion, color, flicker, temporal frequency Citation: Watanabe, J., & Nishida, S. (2007). Veridical perception of moving colors by trajectory integration of input signals. Journal of Vision, 7(11):3, 1–16, http://journalofvision.org/7/11/3/, doi:10.1167/7.11.3.

magnitude of retinal fusion of drifting colors is not as Introduction strong as simply expected from flicker fusion. The motion-induced color segregation occurs regardless of When two colors (e.g., red and green) are presented with the color modulation axis. It cannot be ascribed to eye rapid alternations, human observers see the mixed color movements or to a change in total intensity. This finding (yellow). This is because the visual system integrates indicates the existence of a special neural mechanism in sensory signals over time, a property that is considered to human visual system that actively suppresses temporal improve the signal-to-noise ratio of neural color signals. blurs of moving colors. Previous studies (De Lange Dzn, 1958;Uchikawa&Ikeda, It is suggested that the visual system suppresses subjective 1986;Wisowaty,1981; Wyszecki & Stiles, 1982)have motion blur of luminance-defined patterns (Burr, 1980). A only considered temporal integration of color signals proposed mechanism for this motion deblurring is neural presented at the same retinal location. Longer temporal integration of visual signals along the motion trajectory integration time for color perception than for luminance (Burr, 1981; Burr & Ross, 1986; Burr & Ross, 2004). We perception is indicated, for instance, by relatively low recently found a phenomenon that suggests that the visual critical fusion frequency (at which rapidly alternating system also integrates color signals along the motion stimuli are perceptually fused into a stable pattern) for trajectory. Moving bars whose colors alternates between color perception (De Lange Dzn, 1958; Wisowaty, 1981). two colors are perceived as the mixed color to human A sluggish integration of color signals on the retinal observers although the two colors are not superimposed on coordinates should lead to severe image degradation when the retina (Nishida, Watanabe, Kuriki, & Tokimoto, 2007). the retinal color distribution changes rapidly as a conse- Theoretically, trajectory integration of color signals can quence of either object motion or the observer’s eye help us see veridical colors of moving objects, in which movements. This is comparable to the motion blur that colors are kept along motion trajectories, by reducing photographers often encounter when taking pictures with a motion blur while maintaining a high signal-to-noise ratio. long exposure time. Nevertheless, we do not normally We consider that the present finding of motion-induced perceive such color blurs. Why is this? segregation of colors, which are kept along the motion Here we show that alternations of different colors at the trajectory while alternated at the same retinal location, is same retinal location are more clearly segregated (i.e., less the evidence supporting this theoretical prediction. The mixed) in moving patterns than in stationary patterns contribution of motion to color perception is further flickering at the same alternation rate. This phenomenon, supported by an experiment reported in the second part of “motion-induced color segregation,” demonstrates that the this paperVusing a multipath motion stimulus, we doi: 10.1167/7.11.3 Received April 11, 2007; published August 16, 2007 ISSN 1534-7362 * ARVO

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observed a change in the perceived color in accordance green (x = 0.290, y = 0.606) phosphors. Two colors of a with a change in perceived motion direction in a way given red–green pair were aIred + bIgreen and bIred + aIgreen, predicted by the trajectory integration theory. where Ix is full contrast intensity of color x, a 9 b, and a + b = 1. The chromatic contrast, defined as a j b, was 100%, 75%, 50%, 25%, and 0% for M (mixture magnitude) = 1, 2, 3, 4, and 5, respectively. This gives rise to a pair of red and General methods green for M = 1 (perfect segregation) and a pair of identical yellows for M = 5 (perfect mixture). Under the maxlumi- nance condition, each phosphor was driven at maximum Observers 2 intensities of the monitor at which point M =1(Ired =28cd/m 2 and Igreen =84cd/m). Under the equiluminance condition, Observers in each experiment were two of the authors 2 and two to five volunteers. Most of the volunteers were Ired =28cd/m and Igreen was defined individually by means unaware of the purpose of the experiments. All the of flicker photometry (Wyszecki & Stiles, 1982), or by a observers had normal or corrected-to-normal vision. minimum motion method (Anstis & Cavanagh, 1983). The chromaticity of yellow varied depending on the luminance setting of Igreen. For S-axis modulation, a similar scale of Apparatus color mixture was made between (x = 0.409, y = 0.492) and (x = 0.233, y = 0.123), 30 cd/m2. Stimuli were displayed on a GDM-F520 CRT monitor (Sony), with a refresh rate of 160 Hz, driven by a VSG2/5 visual stimulus generator (Cambridge Research Systems Motion-induced color segregation Ltd.) installed in a Precision 350 workstation (Dell). The Phenomenon spatial resolution of the monitor was 800 Â 600 pixels, with each pixel subtending 1.5 min at the viewing distance of The basic phenomenon of motion-induced color segre- 113 cm. The observer viewed the monitor while sitting in gation is as follows. In a stimulus consisting of red and a dimly illuminated room with his or her head fixed on a green bars separated by constant intervals (Figure 1A), the chin rest. bars move at a constant speed without changing color (Figure 1B). Bright bars are presented on the dark background for producing strong luminance-based motion Color specification (Cavanagh & Mather, 1989). Given fixation of eyes, red and green stimulation alternates at each retinal location at A linear scale of color mixture was made by modulating a rate dependent on the interbar interval. Compare this the intensities of red (CIE, 1931; x =0.625,y = 0.341) and color-keeping motion stimulus with a flicker pattern, in which

Figure 1. Stimuli used to test motion-induced color segregation. (A) Spatial configuration and (B) space–time plot of the color-keeping motion stimulus. The color integrated over time at each retinal position (yellow) is shown at the bottom, and the colors integrated along the motion trajectory (green and red) are shown at the right. (C) Space–time plot of the control flicker stimulus, whose spatial configuration is the same as panel A.

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retinally local color changes in the motion pattern are viewed We called this phenomenon “motion-induced color through stationary slits (Figure 1C, see also Movie 1). The segregation” simply because alternations of different two stimuli should seem to have the same colors, if the colors at the same retinal location are phenomenologically apparent color mixture is solely determined by retinal more segregated in motion stimuli. We do not assume temporal integration because the colors alternate at the same such a complex process that different colors are once rate on the retina. However, under certain temporal integrated at the same retinal location by a peripheral conditions, whereas only a fluctuation between yellowish mechanism and then segregated again by a subsequent colors is seen in the flicker pattern, moving red and green motion-sensitive mechanism, which would be impossible bars are clearly visible in the motion pattern. in the present case (see Discussion). Our conjecture is that This phenomenon suggests that motion information integration of alternating colors along stationary trajectory makes the magnitude of color fusion less than expected may enhance color fusion for flicker stimuli. This is the from that of nonmoving flicker. Specifically, it can be same mechanism underlying color fusion along color- explained by a hypothesis that the color signals are alternating trajectories in “motion-induced color mixing” integrated along a motion trajectory, suggested by the color (Nishida et al., 2007). The two apparently opposite effects mixing phenomenon of the moving color-alternating bars (motion-induced color mixing and segregation) are com- (Nishida et al., 2007). According to this hypothesis, colors plementary functions of the same mechanism. should be veridically segregated for the motion case because the color does not change along the trajectory of moving bars (diagonal direction in the space–time plot), Rating whereas motion should be mixed for the flicker case because the color alternates along the trajectory of sta- We verified the observation of motion-induced segre- tionary bars (the vertical direction in the space–time plot). gation in a rating experiment.

Methods A (color-keeping) motion stimulus consisted of two horizontal arrays (20- in width) of vertical bright bars (each subtending 6 min in width, 1.0- in height) presented on a dark background 1.0- above and below fixation (Figures 1A and 1B). In each array, red and green bars (when M = 1 of the red–green condition) spatially alternated with a constant interval. The bars shifted every 6.25 ms by one bar width keeping color constant, in opposite directions in the upper and lower arrays. The color alternation rate at each retinal location increased from 6.7 to 40 Hz (for S-axis stimuli, from 4 to 26.7 Hz) as the interbar interval decreased from 66 to 6 min (from 114 to 12 min). Initial bar positions were randomized. The control flicker stimulus (Figure 1C) was made from the motion stimulus by removing bars to be presented in the interbar areas of a given frame. In each trial, a test stimulus was presented for 1000 ms, with 300-ms pre- and postmasks of stimulus onset/offset. The mask pattern was a uniform yellow presented in the area of the bar arrays. We asked observers to evaluate the magnitude of Movie 1. This is not a demonstration but an explanatory slow subjective color mixing by means of a 5-point rating scale movie of the stimuli for motion-induced color segregation. A (with referencing to physical samples of various chromatic moving pattern (Figure 1B) is shown above, and a flickering contrasts) for the motion and flicker stimuli while system- pattern (Figure 1C) is shown below. They have the same local atically changing the local alternation rate. In each trial, a color flicker (alternation) rate. The expected effect is stronger color stimulus presentation was followed by a presentation of a segregation in the moving pattern than in the flickering pattern. In static color sample. The sample consisted of five pairs of this pattern, the interbar interval was as three times wide as the colored bars showing five mixture magnitudes, and the bar width, which is the same as described in Figure 1. observer had to choose the sample number closest to the Demonstration of this effect requires fast frame rates (at least impression of perceived colors. Alternatively, the color 60 Hz, 160 Hz in our experiments). Demonstrations for Mac OS X sample was bicolor bar arrays presented with one of five and Windows developed using an Open-GL library are available mixture magnitudes, and the observer varied the mixture on a pubic database, Visiome Platform. magnitude to match sample colors to the perceived test

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colors. We confirmed that the results obtained with this we also evaluated the effect of motion on color perception in “matching” procedure were not significantly different from terms of a more objective performance measure. Specifi- those obtained with the “rating” procedure. The observer cally, we measured the chromatic contrast detection thresh- could request replays of the stimulus sequence. old; that is, the contrast below which a color alternation was To check the reliability of the rating, we showed the perceptually indistinguishable from the mixed color. We stimulus with three magnitudes of color mixture, each expected that the stronger the magnitude of perceptual color corresponding to the reference sample colors for rating 1 segregation was, the lower would become the contrast (perfect segregation, equivalent to 100% chromatic con- required for correct judgment. trast), 3 (moderate mixture, 50%), and 5 (perfect mixture, 0%). Because these colors were shown in random order, the observers could not tell whether a given stimulus that Methods appeared yellow to them was indeed yellow or a mixture of We presented a color-keeping motion (or a control flicker red and green. stimulus) of a given chromatic contrast (target), together Between trials within a single session, the stimulus type, with a nontarget stimulus that was identical to the target the bar interval, the color mixture magnitude, and the except that the chromatic contrast was 0%. The background motion direction were randomly varied. was dark. The observers had to make a two-alternative forced choice judgment about which stimulus was the target. In a single trial, a target of a given chromatic contrast and a Results nontarget of 0% contrast were simultaneously presented, one As the local alternation rate was increased (by reducing above and the other below fixation. The observer had to keep the interbar interval), the perceived colors gradually fixating at the central point during stimulus presentation and changed from the veridical perception of the physical then indicated the target position by pressing one of two colors presented (indicated by an orange dotted line in buttons. The color condition was red–green equiluminance. each panel) to complete yellow, which ensures the Unless otherwise noted, other stimulus parameters were the accuracy and the consistency of the observers’ rating. same as those used for the rating experiments. Chromatic The increase of the mixture rating with increasing contrast was varied within a block, whereas stimulus type alternation rate for the flicker pattern can be accounted (color-keeping motion or flicker) and the alternation rate for by the known temporal summation properties of the (6.67 or 13.3 Hz) were varied between blocks. Alternatively, color system. The critical finding is that in comparison to the alternation rate was varied within a block whereas the the flicker pattern, the color mixture rating was lower chromatic contrast was kept at 50%. (thus color segregation was stronger) for the motion A preliminary test indicated that at very high temporal pattern for nearly all the alternation rates examined. frequencies, the drifting motion of color-keeping stimulus Similar results were obtained regardless of whether the was visible for high-contrast stimuli but not for 0% red and green phosphors were driven at the maximum contrast stimuli due to contribution of the chromatic intensities of the monitor (maxluminance, Figure 2A)or contrast (and residual luminance contrast) to motion the colors were subjectively equiluminant (Figure 2B), disambiguation. To avoid the difference in motion whereas the mixture rating was slightly higher in the latter strength as a discrimination cue, we added luminance case. Note that the background was dark in either case (yellow) noise to one of the color components. Specifi- (see the Control experiments section for the effect of cally, when the luminance noise was added to say target equiluminant background). Spatial scale is not a critical red (randomly chosen for each trial), target red = (a + Lt) factor. A strong motion-induced color segregation was Ired +(b + Lt)Igreen; target green = bIred + aIgreen; nontarget observed even when the bar width was increased from 6 to = 0.5Ired + 0.5Igreen; or (0.5 + Ln)Ired + (0.5 + Ln)Igreen, 18 min (Figure 2C). Color axis is neither a critical factor. where Lt and Ln are luminance noise amplitudes randomly Although the red–green color axis we used was close to and independently determined between 0.25 and 0.5. As a the L–M axis in the standard cone contrast color space result, the maximum chromatic contrast (a j b) was 50%. (Derrington, Krauskopf, & Lennie, 1984; MacLeod & With the addition of luminance noise of large and random Boynton, 1979), a similar effect was observed for the magnitudes, the observer saw motion for both the target S-axis stimuli (Figure 2D). and the nontarget, and the apparent motion strength was not correlated with stimulus chromatic contrasts.

Chromatic contrast detection Results Either at 6.67- or 13.3-Hz alternation, the threshold Although the rating method was good to directly evaluate chromatic contrast obtained under the motion condition motion-induced changes in subjective color appearance, it was only about one-third of that obtained under the control could introduce some ambiguity into quantitative evaluation flicker condition (Figure 3A), suggesting that the color- of the magnitude of the effect. To overcome this limitation, keeping motion facilitated color segregation in agreement

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Figure 2. Rating of color mixture as a function of the frequency of local color alternation. (A) Red–green maxluminance condition. (B) Red– green equiluminance condition. (C) Red–green equiluminance bar width Â3 (18 min). (D) S-cone modulation condition. Each data point shows the average for seven (A, B), five (C), or four (D) observers (six judgments for each). The results suggest that apparent color mixture is weaker for the motion stimulus than for the flicker stimulus.

with the rating results. We also varied the alternation interpretation that motion-induced color segregation is a frequency while remaining the chromatic contrast at 50% result of trajectory integration of color signals. (Figure 3B). The result showed that the threshold temporal frequency was considerably higher for the motion condition than for the flicker condition (24.2 vs. 13.8 Hz). Effects of eye movements

Firstly, one might suspect possible artifacts due to eye movements. If the observer’s eyes track the stimulus Control experiments movement, the conventional retinal color integration mechanism could account for our findings. However, our observers were asked to fixate the center of the display This section describes the results of control experiments and to simultaneously judge the colors of two arrays of that we carried out to address several concerns about our bars that moved in opposite directions. Subjectively, color

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for the lower one, based on visual memory. The positions of moving and flicker stimuli were exchanged between sessions. Because the pattern of eye movements should be always the same for the two stimuli, if colors are segregated in the motion stimulus because of any form of eye move- ments, colors should be similarly segregated in the simulta- neously presented flicker stimulus. However, the magnitude of motion-induced color segregation was as strong as that observed in the main experiment (Figure 4B).

Effects of spatiotemporal chromatic contrast energy

Stronger color segregation for the motion stimulus might be due to the increase in total stimulus energy relative to the flicker stimulus. We tested this possibility in a subsidiary experiment using stimuli that had the same total spatiotemporal energy as the motion stimulus. Observers rated the magnitude of color mixture for three types of flicker stimuli that had the same total spatiotem- poral energy as the motion stimulus. Stimulus color was equiluminance red–green, and the width of the total stimulus area was reduced to 6-. In data collection, all stimuli, including the original motion and flicker stimuli, Figure 3. Motion-induced color segregation indicated by differ- were randomly presented in the same session. ences in the threshold of discriminating stimuli with and without In the first condition (Figure 5A), the test was a chromatic contrast. The stimulus was a color-keeping motion continuous flicker stimulus. It was identical to the original (open blue square) or a control flicker stimulus (filled red circle). flicker stimulus except that each color flash was contin- Proportion correct of chromatic contrast detection as a function of uously presented until the onset of the next color. We the target chromatic contrast (A) or as a function of alternation found that the color mixture rating was just as high for the temporal frequency (B). Each data point shows the average for flicker stimulus. Note that the continuous flicker stimulus five observers (40 judgments for each). The smooth curves are has strong motion energy in the vertical direction (in the best-fit logistic functions, and the arrows indicate the 75% correct space–time plot), which is expected to strongly support points. That the proportion correct is slightly higher than the temporal integration of different color signals at the same chance level even at the highest temporal frequency in panel B retinal location. could be ascribed to occasional involuntary eye movements that In the second condition (Figure 5B), the test stimulus was we could not completely exclude. a counterphase red–green square wave grating, which was made from the original flicker stimulus by extending the appearance was similar for the two arrays and was stable width of each bar to fill the interbar gap. The mixture rating over the period of stimulus presentation. We further tested was higher than that for the motion stimulus, although only the possibility of eye movement artifacts in two additional slightly. The reason for the significant reduction in the experiments. mixture rating in comparison with the flicker stimulus In one experiment, we measured eye movements while could be that the counterphase grating is an ambiguous the observers performed the same tasks as in the rating motion stimulus, having substantial motion energy in experiment. Eye movements were monitored by an infra- diagonal directions that supports segregation of different red eye tracker (Iota Orbit 8) with the sampling rate of color signals, but little motion energy in the vertical 1 kHz. The results suggest that motion-induced color direction that supports color mixing (see next section for segregation does occur without tracking eye movements a related discussion about the continuous flicker condition (Figure 4A). The observers sometime made small invol- obtained with an equiluminant background). untary eye movements, but their occurrence did not In the third condition (Figure 5C), the test was a correlate with the magnitude of color mixing. uniform field flicker stimulus, consisting of flickering two In another experiment, we psychophysically evaluated the squares presented above and below the fixation. The contribution of eye movement. In each trial, motion and mixture rating was lower than that for the original bar flicker stimuli were simultaneously presented, one above flicker stimulus, but still significantly higher than that for and the other below fixation. The observer rated the the motion stimulus. It is well known for achromatic magnitude of color mixture first for the upper pattern, then flicker that the larger the test stimulus area, the higher the

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Figure 4. Results of experiments testing the effects of eye movements. (A) Separately for the three stimulus conditions, the horizontal position of the left eye measured in each trial is plotted as a function of time, with the color of the line indicating the rating of color mixing in the trial. The stimulus presentation started at time = 0. Diagonal lines indicate the trajectories of stimulus movements. The results of one observer, but similar results were obtained for the second observer. The data do not indicate such large eye movements as tracking the stimulus motion (eye movements parallel to the diagonal lines) nor indicate significant differences between the stimulus conditions. (B) Results of a psychophysical test for eye movement artifacts, in which motion and flicker stimuli were simultaneously presented. Each point shows the average for five observers (six judgments for each). Motion-induced color segregation remained to be observed.

critical flicker fusion frequency (Harvey, 1970). A similar the original dark background (Figure 6C) using the same effect was reported also for chromatic fusion (Vul & procedure and observers. MacLeod, 2006). Some effect of spatial scale on critical A rating experiment was conducted with equiluminant fusion frequency may result from motion-based signal yellow and dark backgrounds in separate sessions. integration along stationary trajectory because a slim Motion, flicker, and continuous flicker stimuli made of trajectory will be advantageous for signal integration. equiluminant red–green bars at three color mixture levels Taken together, the results of the subsidiary experiment were randomly presented from trial to trial. To reduce a indicate that it is not the reduction of total stimulus energy display artifact visible at vertical chromatic borders, we but integration along the apparent motion trajectory that rotated the orientation of the stimulus by 90-. makes the flicker stimulus appear more color-mixed than Even with the equiluminant background, the color the motion stimulus in motion-induced color segregation. mixture rating was significantly higher for the flicker stimuli than for motion stimuli. With regard to these two stimuli, overall ratings were higher for the equiluminant Effects of background luminance background than for the dark background, which could be ascribed to the background mixing effect. Flicker bars We presented bright color stimuli on a dark background were hard to detect even at lower temporal frequencies to optimally drive motion mechanisms that prefer achro- due to insufficient chromatic energy. On the other hand, matic signals, as well as to minimize the effect of moving color bars were still clearly visible for a wide spatiotemporal integration with color signals between the range of temporal frequency. This might at least partially bars and the background (background mixing effect). To result from color signal integration along motion trajec- relate our findings with other color studies, however, one tory, where motion information was carried by chromatic might want to know what happens when the background is signals, residual achromatic signals, or both. replaced by a more conventional equiluminant yellow However, only from the results obtained with the (Figure 6A). We therefore measured the magnitude of equiluminant background, we cannot conclude that stron- color mixture for motion, flicker, and continuous flicker ger color segregation for motion stimuli is caused by the stimuli with an equiluminant background (Figure 6B) and integration of color signals along the motion trajectory.

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Figure 5. Results of an experiment testing the effect of spatiotemporal chromatic contrast energy. Apparent color mixture was measured with test stimuli that have the same total spatiotemporal energy as the motion stimulus. (A) The test was a continuous flicker stimulus. (B) The test was a counterphase red–green square wave grating. (C) The test was a uniform field flicker. The results indicate that it is neither the increase of total stimulus energy nor the increase in apparent stimulus size that makes the motion stimulus appear less color-mixed than the flicker stimulus. Each point shows the average for five observers (six judgments for each).

This is because apparent color segregation was even stimuli), a color transition reverses the sign of chromatic stronger for nonmoving continuous flicker stimuli whose contrast, and possibly the sign of residual luminance total chromatic energy was identical to the motion stimuli. contrast. Therefore, motion signals supporting the stationary This is in contrast to the results obtained with the dark trajectories must be very weak, if they exit at all. As a result, background, where color mixing for the continuous flicker our theory predicts that red and green signals are not strongly stimuli was as strong as that for the flicker stimuli. integrated with the equiluminant background. Weak color mixing (stronger color segregation) observed In summary, using equiluminant background, a standard with the continuous flicker stimuli presented on the procedure to isolate chromatic mechanism, makes the equiluminant background can be ascribed to an increase in effect of motion on color perception less visible. local chromatic contrast energy and a decrease in back- ground mixing effect in comparison with the motion stimuli. Yet the notion of trajectory integration of color signals is Effects of chromatic aberration useful to account for the difference between the two background conditions. With the dark background, steady In our previous study on motion-induced color mixing luminance increments clearly define stationary bar trajecto- (Nishida et al., 2007), we used an aperture viewing ries. Along them, the visual system can integrate red and condition to exclude contributions of optical blur arising green signals. This color mixing is strong enough to form chromatic aberration (Flitcroft, 1989). Using a similar overcome the effect of an increase in local chromatic energy. viewing condition, we show here that chromatic aberration With the equiluminant background, on the other hand, the had little effect on motion-induced color segregation. steady luminance increments of bars are almost excluded. In We had observers monocularly view the stimuli through addition, along the stationary color-alternating trajectory a small artificial pupil (2 mm in diameter), and we (but not along the color-keeping trajectory of the motion confined stimulus presentation in the central 3- Â 3- area.

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Figure 6. Results of an experiment testing the effects of equiluminant background. (A) Space–time plot of three types of stimuli (motion, flicker, and continuous flicker). (B) The rating results obtained with the equiluminant background. (C) The control results obtained with the dark background.

We found that this procedure did not affect the magnitude flicker to be equivalent with other motion trajectories. To of motion-induced color segregation both in rating experi- clear these concerns, we made a more critical test of the ment (Figures 7A and 7B) and in chromatic contrast notion of trajectory color integration. We used a multipath detection (Figure 7C), which indicates that chromatic motion display in which color was alternated at every aberration plays no significant role in motion-induced jump along one path but kept the same along another path. color segregation. We asked the observers to rate the perceived motion direction and color mixture while we manipulated the perceived motion direction by changing the jump size of the color-keeping path and luminance condition. The Covariation of motion and color question was whether color perception changed in perception correlation with the direction change. Our hypothesis predicts that colors should be preserved when motion perception prefers the color-keeping path whereas mixed Our interpretation of motion-induced color segregation when motion perception prefers the color-alternating path. is that although retinal color alternation is the same between the motion and the flicker stimuli, perceived motion trajectory is different, and thus the way of color Methods integration is different. However, one might doubt the existence of causal relationship between the change in The methods were identical to those used in the color perception and the change in motion perception. In experiments described in previous sections except for the addition, one might be reluctant to consider stationary following.

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Figure 7. Results of experiments testing the effects of optical blur arising from chromatic aberration. For reduction of this factor, observers monocularly viewed the stimulus (presented only in the central 3- Â 3- area) through a small artificial pupil (2 mm in diameter) placed in front of their dominant eyes. (A) The results of a rating experiment for RG maxluminance stimuli. (B) The results of a rating experiment for S-cone modulation stimuli. (C) The chromatic contrast detection performance obtained with the aperture viewing and RG equiluminance stimuli.

Stimulus value of SOA. Each observer made consistent direction The spatiotemporal structure of the multipath stimulus is ratings for a given path–length ratio across trials. illustrated in Figure 8A. The spatial configuration was similar to the stimuli in Figure 1A. The stimulus consisted Procedure of two arrays of bright bars, each subtending 9 min in In a trial, two arrays of a multipath stimulus moving in width, 1.0- in height, presented on the dark background. opposite directions were presented for 1000 ms. The The bar arrays were presented 2.0- above and below observer first rated the perceived motion direction using a fixation. In each array, red and green bars spatially 5-point scale (j2 = the upper array moved unambigu- alternated (Figure 8A). The center-to-center separation ously leftward; j1 = dominantly leftward; 0 = totally between adjacent bars was 9 min  (path–length ratio + 1). ambiguous; +1 = dominantly rightward; +2 = unambigu- The bars shifted by one bar width in opposite directions in ously rightward) and then made a 5-point scale rating the upper and lower arrays. As a result, the displacement size about the perceived color mixture. In each trial (of the of color-alternating path was always 9 min, whereas that of absolute rating condition), a stimulus presentation was color-keeping path was 9 min  (path–length ratio). Each followed by a presentation of a static color sample. The stimulus frame was presented for 6.25 ms with a blank sample consisted of five pairs of colored bars showing five interstimulus interval. The stimulus-onset asynchrony mixture magnitudes, and the observer had to choose the (SOA) was 52 ms. sample number closest to the impression of perceived colors. A preliminary test indicated that with the shortest SOA, The observer could request replays of the stimulus sequence. the observer often saw unexpected motions corresponding to The path–length ratio was randomly varied across trials. low-speed paths in which bars of a given frame jumped to Four conditions were carried out in separate sessions: two nearby bars presented two or more frames ahead (Burt & conditions of luminance setting (maxluminance or equilumi- Sperling, 1981). On the other hand, longer SOAs made the nance) combined with two conditions of color rating perceived motion path perceptually multistable. This is an (absolute or relative). For the absolute rating, we presented interesting situation in which the covariation of motion and the color sample, as described above. For the relative rating, color perceptions could be tested without changing stim- without showing the sample, we asked the observer to fully ulus parameters (Shim & Cavanagh, 2004), but longer use the five scales for the stimuli presented in that session and SOAs also made the magnitude of color mixing too small normalized the obtained responses (excluding those of the to reliably measure. We therefore used an intermediate first block) into the range between 0 and 1 for each condition

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Figure 8. Covariation of motion and color perceptions. (A) Space–time plots of multipath displays in which integration of color signals along a rightward color-alternating path results in color mixing, whereas integration along a leftward color-keeping path results in color segregation. When the path–length ratio of the color-keeping path was 1 (left), the color-keeping path predominates in motion perception. When the path– length ratio was 4 (right), the color-alternating path predominates. (B and C) The results of double rating experiment. The ratings of motion direction (first row) and color mixture (second row) are plotted as functions of the path–length ratio. The scale of subjective color mixture was the same as in the other experiment (absolute scale, B) or relative within each luminance condition (C). Each data point shows the average for four observers (six judgments for each). In the third row, the two ratings are plotted against each other. The observed covariation of motion and color perceptions is consistent with the notion of integration of color signals along motion trajectories.

and each observer. The magnitude of the physical color that when the path–length ratio of the color-keeping path to mixture was 1, 3, or 5 for the absolute rating condition, the color-alternating path was 1, bicolor stimuli appeared to whereas the magnitude of the physical color mixture was move dominantly in the direction of the color-keeping path only 1 for the relative rating condition. because the motion system uses luminance and color as correspondence cues (Cavanagh & Anstis, 1991; Nishida & Results Takeuchi, 1990). With an increase in the path–length ratio, the perceived motion gradually changed toward the The direction judgments (Figure 8B, top row) obtained direction of the color-alternating path because the motion under the maxluminance condition (open squares) indicate system prefers shorter motion paths (Burt & Sperling,

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1981; Ullman, 1979). In accordance with this change in perceived direction, the color rating, which indicated no Discussion mixture for the path–length ratio of 1, gradually increased with the path–length ratio (middle row). The covariation Temporal limit of color perception of the motion and color ratings (bottom row) is consistent with the expected change in the way color signals are The motion-induced color segregation demonstrates that integrated (see also Movies 2A and 2B). color alternations at the same rate are perceived more In comparison with the maxluminance condition, the veridically in moving patterns than in nonmoving flicker- motion direction rating obtained under the equiluminance ing patterns. If temporal color mixing is exclusively condition (filled circles in Figure 8B) shifted as a whole in determined by peripheral motion-insensitive mechanism, favor of the color-alternating path due to minimization of it would be impossible to see color segregation in motion the luminance cue. In accordance with this change, the stimuli when the two colors alternate at the frequency color mixture rating also increased. As a result, the beyond the temporal limit indicated by flicker stimuli that patterns of covariation for the two luminance-conditions have the same pattern of local color alternation. The color were very similar. signals must be segregated in early stages at least up to the Because the color alternation rate along the color- temporal limit indicated by the motion stimuli. The alternating path (10 Hz) was not fast enough to induce difference in color mixing between motion and flicker perfect color mixing, the change in the mixture rating stimuli should be ascribed to a subsequent process, where was relatively small when we used the absolute scale we propose that color signals are integrated along the that assigned one for no mixture and five for perfect motion trajectory. mixture. We therefore replicated a part of the experiment Physiological findings (Gur & Snodderly, 1997; Lee, (physical mixture = 1) by asking the observers to assign Martin, & Valberg, 1989), as well as a psychophysical the full rating scale to the range of the color mixtures finding (Vul & MacLeod, 2006), indicate that color- seen in the session (relative rating). The results show the selective neural mechanisms in early visual processing same pattern (Figure 8C). The small discrepancy between can carry chromatic temporal modulations well above the the luminance conditions in the bottom panel can be perceptual fusion limit. Considering the efficiency of ascribed to the mismatch of the smallest magnitude of neural coding, one would naturally wonder why the brain color mixture. has to encode invisible high-frequency signals if the

Movie 2. Movie demonstration of covariation of perceived motion direction and perceived color (Figure 8). These two movies have different path–length ratios. (Left panel) Path–length ratio is 1. Expected dominant motion path is the color-keeping path, and expected color perception is color segregation. (Right panel) Path–length ratio is 10. Expected dominant motion path is color-alternating path, and expected color perception is color mixing.

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subsequent processing simply filters them out. This physically superimposed stimuli (flash-lag effect). In inconsistency may be explained, at least partially, by our either case, the visual system decomposes a physically hypothesis that the temporal limit of chromatic perception mixed color into two component colors. Undoing color is determined by a higher level process that flexibly mixing, which sounds logically impossible, is accom- integrates color signals along the trajectory indicated by plished by the aid of spatiotemporal contexts that indicate motion signals. the component colors. On the other hand, in our case, if Our findings suggest that the standard method of using red and green signals from our color-keeping motion chromatic flicker fusion may underestimate the upper stimulus were completely mixed in earlier processes, the temporal resolution of color perception. In particular, subsequent process would only see a simple achromatic when small flickering elements are presented on the dark pattern with no contextual cues, thus being unable to background (e.g., Wisowaty, 1981), color integration recover original colors. The color-segregation mechanism along a stationary trajectory will enhance color fusion. indicated by our phenomenon, which requires segregation On the other hand, when we presented continuous flicker of color signals in early processes, is different from those stimuli on equiluminant background, the temporal color indicated by previous findings (Nijhawan, 1997; Wallach, segregation limit is as high as those for motion stimuli 1935). (Figure 6). We suspect this is because motion energy along stationary trajectory is very week. Wisowaty (1981) suggested that the increase in chromatic fusion frequency Neural correlates by addition of a bright (including equiluminant) surround should be ascribed to a luminance artifact. However, we Given that visual motion detectors do not exist before did not measure flicker detection thresholds, but we used a the visual cortex in humans, our findings suggest a rating of color mixture, which is similar to the perception significant contribution from cortical processing to color of chromatic alternation (PCA) criterion that Wisowaty mixing and segregation. As we suggested before (Nishida used to avoid the luminance artifact. et al., 2007), a possible mechanism underlying motion- Vul and MacLeod (2006) found induction of a weak but based color processing is local spatiotemporal integration significant amount of orientation-contingent color after- of color information by neurons sensitive to both motion effect (McCollough effect) even when adaptation colors direction and color (Burr & Ross, 1986; Burr & Ross, alternated at rates (25 and 50 Hz) higher than the 2004; Gegenfurtner, Kiper, & Fenstemaker, 1996; chromatic fusion limit. Although the present findings Tamura, Sato, Katsuyama, Hata, & Tsumoto, 1996). indicate that chromatic fusion may underestimate the Alternatively, the visual system may implement motion- temporal limit of conscious color perception, whether the based chromatic integration as a more global interaction best temporal resolution of conscious color perception can between motion and color processing subsystems. Specif- be as high as the limit of the adaptation effect remains an ically, feedback signals from motion analysis in the open question. Although our data (Figure 3) showed that luminance-sensitive pathway might modulate the pattern chromatic contrast detection for 40 Hz motion stimulus of spatiotemporal integration of neurons in the pathway was significantly above chance, we cannot exclude representing color, by way of dynamically changing contribution of involuntary eye movements to this effect. connection weights implemented by shunting inhibition (Blomfield, 1974), neural synchrony (Engel, Fries, & Singer, 2001), or other gating mechanisms. In relation to Related studies this hypothesis, it is intriguing that neurons in monkey V4, an area often associated with color processing, are In comparison with a large body of literature on the effects narrowly tuned to motion speed (Cheng, Hasegawa, of color on motion perception (e.g., Cavanagh & Anstis, Saleem, & Tanaka, 1994). Although not many monkey 1991; Croner & Albright, 1997; Dobkins & Albright, V4 neurons show direction-selective response (Ferrera, 2004; Kool & De Valois, 1992;Krauskopf&Farell,1990; Rudolph, & Maunsell, 1994), a motion adaptation effect is Lu, Lesmes, & Sperling, 1999; Takeuchi, De Valois, & direction selective even for non-direction-selective V4 Hardy, 2003), only a small number of studies have made on neurons (Tolias, Keliris, Smirnakis, & Logothetis, 2005; the effect of motion on color perception (Chen & Cicerone, Tolias, Smirnakis, Augath, Trinath, & Logothetis, 2001). 2002; Cicerone, Hoffman, Gowdy, & Kim, 1995;Hepler, Human imaging studies suggest that motion adaptation 1968; Moller & Hurlbert, 1997; Monnier & Shevell, 2004; effect in V4 is direction selective as in the case of monkey Nijhawan, 1997; Wallach, 1935). Among these studies, as (Huk, Ress, & Heeger, 2001; Nishida, Sasaki, Murakami, far as we notice, there were two reports of motion-induced Watanabe, & Tootell, 2003). These findings may imply color segregation. Wallach (1935) found perceptual color that V4 neurons receive balanced inputs from motion- decomposition occurring when motion transparency is sensitive mechanisms tuned to different directions in MT/ seen for orthogonal gratings of different colorings. V5 (Tolias et al., 2005), and that these connections are Nijhawan (1997) found decomposition of yellow into red responsible for spatiotemporal integration of chromatic and green when motion induces an apparent shift between signals in the stimulus motion direction.

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The integration of color signals along the motion We thank I. Kuriki, M. Terao, I. Murakami, I. trajectory is a useful mechanism for seeing colors in Motoyoshi, T. Tokimoto, A. Johnston, D. MacLeod, and dynamic scenes. Given that most objects do not change S. Tachi for their help, comments, and support. Demon- color as they move, this mechanism can improve the strations of the phenomena reported in this paper are signal-to-noise ratio by excluding rapid fluctuations of available on a pubic database, Visiome Platform. color signals over the motion trajectory, which poten- tially reflect spatial inhomogeneity in the sensitivity of Commercial relationships: none. photosensors or subsequent neural mechanisms or reflect Corresponding author: Junji Watanabe. spatial variations in illumination irrelevant to object Email: [email protected]. color per se. Motion-based integration has also been Address: 3-1 Morinosato Wakamiya, Atsugi, Kanagawa suggested for luminance-defined spatial pattern signals 243-0198, Japan. (Burr, 1981; Burr & Ross, 1986; Burr & Ross, 2004)to account for motion deblurring (Burr, 1980) and for the perception of a shape moving behind multiple slits (spatiotemporal pattern interpolation; Burr, 1979; Nishida, References 2004). There has been a debate as to whether motion Anderson, C. H., Van Essen, D. C., & Gallant, J. L. deblurring is a result of blur removal by some active (1990). Blur into focus. Nature, 343, 419–420. process involving motion-based signal integration or just [PubMed] a change in subjective appearance because the visual system is unable to perform the discrimination necessary Anstis, S. M., & Cavanagh, P. (1983). A minimum motion to decide the sharpness of the moving object (e.g., technique for judging equiluminance. In J. Mollon & Anderson, Van-Essen, & Gallant, 1990; Burr & Morgan, R. T. Sharpe (Eds.), Colour vision: Physiology and 1997; Ramachandran, Rao, & Vidyasagar, 1974). Although psychophysics (pp. 155–166). New York: Academic. the latter hypothesis remains as a general explanation of Blomfield, S. (1974). Arithmetical operations performed perceptual sharpening, changes in objective perceptual by nerve cells. Brain Research, 69, 115–124. performance in color perception (chromatic contrast [PubMed] detection experiment of our study), as well as those in Burr, D. C. (1979). Acuity for apparent vernier offset. pattern perception (Nishida, 2004), indicates the operation Vision Research, 19, 835–837. [PubMed] of active motion deblurring processes at least under some specific circumstances. Burr, D. C. (1980). Motion smear. Nature, 284, 164–165. We conjecture that motion may generally play a role in [PubMed] guiding how patterns and colors are spatiotemporally Burr, D. C. (1981). Temporal summation of moving bound to object representations. Our findings have at least images by the human visual system. Proceedings of functional if not mechanistic relevance to such phenom- the Royal Society of London Series B: Biological ena as object-specific feature integration mediated by Sciences, 211, 321–339. [PubMed] apparent motion (Kahneman, Treisman, & Gibbs, 1992; Burr, D. C., & Morgan, M. J. (1997). Motion deblurring in Og˘men, Otto, & Herzog, 2006; Otto, Og˘men, & Herzog, human vision. Proceedings of the Royal Society B: 2006; Shimozaki, Eckstein, & Thomas, 1999), object Biological Sciences, 264, 431–436. [PubMed] updating in apparent motion (Moore & Enns, 2004), [Article] temporal misbinding of features in visual masking (Enns, 2002), and integration of sensory information through a Burr, D. C., & Ross, J. (1986). Visual processing of moving window of attention (Cavanagh & Holcombe, motion. Trends in Neuroscience, 9, 304–307. 2005). Our hypothesis is also conceptually related to the Burr, D. C., & Ross, J. (2004). Vision: The world through consideration of the visual persistence as active integra- picket fences. Current Biology, 14, R381–R382. tion, not passive continuation, of visual signals (Dixon & [Article] Di Lollo, 1994). Burt, P., & Sperling, G. (1981). Time, distance, and A functional drawback of our hypothetical mechanism feature trade-offs in visual apparent motion. Psycho- is the difficulty of localizing moving features at the correct logical Review, 88, 171–195. [PubMed] space–time position. We speculate that the mislocaliza- tions of moving stimuli and features in the direction of Cai, R. H., & Schlag, J. (2001). Asynchronous feature motion (Cai & Schlag, 2001; De Valois & De Valois, binding and the flash-lag illusion. Investigative 1991; Nijhawan, 1994) may be explained as epipheno- Ophthalmology & Visual Science, 42, 3830. mena of the integration of sensory signals along motion Cavanagh, P., & Anstis, S. (1991). The contribution of trajectories. color to motion in normal and color-deficient

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