The perceived speed of drifting chromatic gratings is mechanism-dependent

The perceived speed of chromatic motion was investigated for gratings that stimulate each chromatic mechanism [L-M and S-(L+M)] in isolation and for gratings that stimulate both chromatic systems. The observers' task consisted of adjusting the speed of a drifting achromatic grating to match the perceived speed of an isoluminant chromatic grating, drifting at 8 deg/s (temporal frequency of 4 Hz). Every observer reported a substantial decrease in perceived speed for chromatic gratings modulated along the S-(L+M) (blue-yellow) cardinal axis compared to other directions in color space. One observer even reported motion standstill for gratings modulated along this axis. Further testing demonstrates that the perceived speed of an isoluminant chromatic grating depends solely on the extent to which it stimulates the L-M (red-green) mechanism. Thus, under the conditions that were tested, the S-(L+M) postreceptoral mechanism does not appear to contribute significantly to determining the perceived speed of chromatic motion.     

Perception of direction of motion reflects the early integration of first and second-order stimulus spatial properties

Second-order Type I and Type II plaids were constructed by combining two orientation-filtered random-dot gratings. Each component consisted of a dynamic filtered random-dot field, the contrast of which was modulated by a drifting sinusoidal grating. Orienting the two components suitably and interleaving at 120 Hz allowed us to produce a two-dimensional plaid pattern made from one-dimensional second-order components. The perceived direction of motion of both Type I and Type II plaids was measured as a function of the orientation content of the carrier, the contrast, and the duration of the stimulus. Type I plaids had a perceived direction close to the intersection of constraints/vector sum solution (which coincide for Type I patterns) for all conditions when the motion was visible. Type II plaids had a perceived direction that moved away from the vector sum and toward the intersection of constraints solution as the orientation bandwidth of the carrier increased. The data explain discrepancies in previous work using comparable stimuli and are consistent with recent evidence that the previously considered parallel pathways of form and motion have a strong influence upon one another from early stages of cortical visual processing.     

The oblique plaid effect

Plaids are ambiguous stimuli that can be perceived either as a coherent pattern moving rigidly or as two gratings sliding over each other. Here we report a new factor that affects the relative strength of coherency versus transparency: the global direction of motion of the plaid. Plaids moving in oblique directions are perceived as sliding more frequently than plaids moving in cardinal directions. We term this the oblique plaid effect. There is also a difference between the two cardinal directions: for most observers, plaids moving in horizontal directions cohere more than plaids moving in vertical directions. Two measures were used to quantify the relative strength of coherency vs. transparency: C/[C+T] and RTtransp. Those measures were derived from dynamics data obtained in long-duration trials (>1 min) where observers continually indicated their percept. The perception of plaids is bi-stable: over time it alternates between coherency and transparency, and the dynamics data reveal the relative strength of the two interpretations [Vision Research 43 (2003) 531]. C/[C+T] is the relative cumulative time spent perceiving coherency; RTtransp is the time between stimulus onset and the first report of transparency. The dynamics-based measures quantify the relative strength of coherency over a wider range of parameters than brief-presentation 2AFC methods, and exposed an oblique plaid effect in the entire range tested. There was no interaction between the effect of the global direction of motion and the effect of gratings' orientations. Thus, the oblique plaid effect is due to anisotropies inherent to motion mechanisms, not a bi-product of orientation anisotropies. The strong effect of a plaid's global direction on its tendency to cohere imposes new and important constraints on models of motion integration and transparency. Models that rely solely on relative differences in directions and/or orientations in the stimulus cannot predict our results. Instead, models should take into account anisotropies in the neuronal populations that represent the coherent percept (integrated motion) and those that represent the transparent percept (segmented motion). Furthermore, the oblique plaid effect could be used to test whether neuronal populations supposed to be involved in plaid perception display tuning biases in favor of cardinal directions.     

Exposure duration affects the perceived direction of cyclopean type II plaids

This study investigated the effect of exposure duration on the perceived direction of cyclopean Type I and Type II plaids moving in the X/Y plane. The cyclopean plaids were created from grating components defined by binocular disparity embedded in a dynamic random-dot stereogram. The results showed that the cyclopean Type I plaid appeared to move in the intersection-of-constraints (IOC) direction across the range of exposures tested. However, the cyclopean Type II plaids appeared to move in a direction different from the IOC with short exposures but near the IOC with long exposures. This perceived directional shift was also obtained with luminance-defined Type II plaids. A common pattern-motion mechanism that processes cyclopean and luminance motion signals appears responsible for the perceived directional shift of the Type II plaids.     

Chromatic tuning of contour-shape mechanisms revealed through the shape-frequency and shape-amplitude after-effects

We investigated whether contour-shape processing mechanisms are selective for color direction using the shape-frequency and shape-amplitude after-effects, or SFAE and SAAE [Gheorghiu, E. & Kingdom, F. A. A. (2006). Luminance-contrast properties of contour-shape processing revealed through the shape-frequency after-effect. Vision Research, 46(21), 3603-3615. Gheorghiu, E. & Kingdom, F. A. A. (2007). The spatial feature underlying the shape-frequency and shape-amplitude after-effects. Vision Research, 47(6), 834-844]. All contours were defined along the 'red-green', 'blue-yellow' and 'luminance' axes of cardinal color space. Adapting and test contours were defined along the same or along opposite polarities within a cardinal axis, and along the same or along different cardinal axes. We found (i) little transfer of the after-effects across different within-axis polarities, for all cardinal axes and for both even-symmetric and odd-symmetric contours; (ii) little transfer between the red-green and blue-yellow cardinal axes; (iii) little transfer between the chromatic and luminance cardinal directions for the SAAE; (iv) large transfer between the chromatic and luminance cardinal directions for the SFAE. We conclude that contour-shape mechanisms are selective for within-cardinal axis polarity and for the chromatic axes within the isoluminant plane. However for certain types of contour-shape processing they are poorly selective along the chromatic versus luminance dimension. Overall our results suggest that contour-shape encoding mechanisms are selective for color direction and that color is important for contour-shape processing.     

The chromatic input to global motion perception

For over 30 years there has been a controversy over whether color-defined motion can be perceived by the human visual system. Some results suggest that there is no chromatic motion mechanism at all, whereas others do find evidence for a purely chromatic motion mechanism. Here we examine the chromatic input to global motion processing for a range of color directions in the photopic luminance range. We measure contrast thresholds for global motion identification and simple detection using sparse random-dot kinematograms. The results show a discrepancy between the two chromatic axes: whereas it is possible for observers to perform the global motion task for stimuli modulated along the red-green axis, we could not assess the contrast threshold required for stimuli modulated along the yellowish-violet axis. The contrast required for detection for both axes, however, are well below the contrasts required for global motion identification. We conclude that there is a significant red-green input to global motion processing providing further evidence for the involvement of the parvocellular pathway. The lack of S-cone input to global motion processing suggests that the koniocellular pathway mediates the detection but not the processing of complex motion for our parameter range.     

The chromatic selectivity of visual crowding

Purpose:  Precortical vision is mediated by three opponent mechanisms which linearly combine receptoral outputs to form a luminance channel (L + M) and two chromatic channels, red-green (L/M) and blue-yellow (S/L + M). These colour channels are known to undergo considerable cortical reorganisation in order to form multiple, 'higher-order' mechanisms tuned to a variety of axes in colour space. Here we ask the extent to which the basic colour-opponent mechanisms interact in the phenomenon of visual crowding, where nearby targets interfere with the spatial processing of a central test target.
Methods:  The task was to identify the orientation of a central Gabor patch whilst an annular plaid was positioned around the patch. The radius of the annulus was varied in order to produce different amounts of crowding. The chromatic content of the central patch and the annulus could be varied independently along the (L + M), (L/M) and (S/L + M) cardinal axes.
Results:  For all targets, when the target and crowding annulus were of the same chromaticity, the crowding effect increased with decreasing separation of the target and annulus. When the test and crowding stimuli had different chromatic properties, very little crowding was observed, even at the minimum separation of test target and crowding annulus.
Conclusions:  The crowding effect appears to be critically dependent on whether or not the target and crowding stimuli share a common chromatic axis. These results suggest a method by which performance in crowding-limited tasks might be enhanced through appropriate selection of chromatic stimulus content.     

Interactions between color and luminance in the perception of orientation

At the early stages of visual processing in humans and other primates, chromatic signals are carried to primary visual cortex (V1) via two chromatic channels and a third achromatic (luminance) channel. The sensitivities of the channels define the three cardinal axes of color space. A long-standing though controversial hypothesis is that the cortical pathways for color and form perception maintain this early segregation with the luminance channel dominating form perception and the chromatic channels driving color perception. Here we show that a simple interaction between orientation channels (the tilt illusion) is influenced by both chromatic and luminance mechanisms. We measured the effect of oriented surround gratings upon the perceived orientation of a test grating as a function of the axes of color space along which the gratings were modulated. We found that the effect of a surround stimulus on the perceived orientation of the test is largest when both are modulated along the same axis of color space, regardless of whether that is a cardinal axis. These results show that color and orientation are intimately coupled in visual processing. Further, they suggest that the cardinal chromatic axes have no special status at the level(s) of visual cortex at which the tilt illusion is mediated.     

When S-cones contribute to chromatic global motion processing

There is common consensus now that color-defined motion can be perceived by the human visual system. For global motion integration tasks based on isoluminant random dot kinematograms conflicting evidence exists, whether observers can (Ruppertsberg et al., 2003) or cannot (Bilodeau & Faubert, 1999) extract a common motion direction for stimuli modulated along the isoluminant red-green axis. Here we report conditions, in which S-cones contribute to chromatic global motion processing. When the display included extra-foveal regions, the individual elements were large ( approximately 0.3 degrees ) and the displacement was large ( approximately 1 degrees ), stimuli modulated along the yellowish-violet axis proved to be effective in a global motion task. The color contrast thresholds for detection for both color axes were well below the contrasts required for global motion integration, and therefore the discrimination-to-detection ratio was >1. We conclude that there is significant S-cone input to chromatic global motion processing and the extraction of global motion is not mediated by the same mechanism as simple detection. Whether the koniocellular or the magnocellular pathway is involved in transmitting S-cone signals is a topic of current debate (Chatterjee & Callaway, 2002).     

Evidence for a Feature Tracking Explanation of Why Type II Plaids Move in the Vector Sum Direction at Short Durations

When two moving sinusoidal gratings, with similar spatial frequency, contrast, phase, but different orientation are combined to form a plaid, their perceived direction of motion has been predicted by the intersection of constraints rule (IOC) (Adelson & Movshon, Nature, 300, 523–525, 1982). However, at short durations (60 msec) the direction of perceived motion has been predicted by the vector sum direction for “Type II” plaids (Yo & Wilson, Vision Research, 32, 1, 1992). Type II plaids are the set of plaids where the components are both located on one side of the resultant computed using the IOC rule. Yo and Wilson suggest that the vector sum direction is observed for Type II plaids at short durations because non-Fourier information is not available and direction is computed from Fourier information only. The first experiment in this study replicates the original Yo and Wilson result using similar stimuli but a simpler task; perceived direction was measured using a direction discrimination task instead of the method of adjustment used by Yo and Wilson. The second experiment provides evidence against generalizing the result to all Type II plaids. A systematic set of type II plaids that varied only in terms of the orientation of the second component provided an ideal set because their predicted motion direction followed very different patterns when predicted by the IOC and vector sum computations. The results obtained were predicted more accurately by the IOC than the vector sum. Experiment 3 provides further evidence that movement in the vector sum direction is not a general property of type II plaids. A small change to the velocity of one of the components of a plaid previously perceived in the vector sum direction had the effect of shifting the perceived motion in the IOC direction, despite increasing the difference between the IOC and VS predictions. This result is not consistent with Yo and Wilson's hypothesis that Type II plaids move in the vector sum direction because of a temporal delay between Fourier and non-Fourier information. Computational analysis of the stimuli used in both the current and original experiments revealed a possible explanation of the results in terms of a contribution from local feature tracking rather than a vector sum operation. Copyright © 1996 Elsevier Science Ltd.     

Analysis of the motion of 2-dimensional patterns: evidence for a second-order process.

The sum of two differently orientated moving sinusoidal gratings of similar spatial frequency, contrast, and velocity appears as a single coherent "plaid" pattern. The visual system is thought to analyse the motion of plaids in two stages, first analysing the motion of the (1-D) components, and then calculating a speed and direction which is consistent with those 1-D motions. We studied the apparent direction of motion of plaids made by adding two components that had the same spatial frequency and contrast, and were symmetrically oriented about the vertical axis. The gratings moved in jumps, and we studied the effect of varying the size of the jump, the angle between the component gratings, and the temporal interval between the jumps, on the perceived direction of motion. When the size of the jumps was increased to 3/8 of their spatial period, the perceived direction of motion of the plaid pattern reversed, although if one component were presented alone, its direction of movement did not reverse. Reversed motion of this type was consistently obtained if the angle between the components was greater than about 140 degrees, if the interval between jumps was at least 25 msec, and if the spatial frequency of the component gratings was less than about 4 c/deg. When the angle between the components was smaller, or the time between jumps was greater, most observers saw normal motion in the direction predicted by the two-stage hypothesis. When the spatial frequency was raised, observers saw no consistent motion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pre-exposure to contrast selectively compresses the achromatic half-axes of color space

The gamut of perceived colors can be represented in a space with bright-dark, red-green and blue-yellow axes. Pre-exposure to a field that changes periodically over time in luminance or along one of the color axes reduces vividness of colors along the entire axis [Webster and Mollon (1991) Nature, 349, 235-238]. But is it possible to reduce vividness or perceived contrast selectively for half-axes in color space? We assessed such selective compression of the bright-dark axis using a task where subjects matched tests in a pre-adapted region to ones in an un-adapted region. Tests were bright or dark pinstripes on a gray background, and pre-exposure was to multiple drifting pinstripes. Matches made after pre-exposure indicate a combination of symmetric and asymmetric compression, with more compression when adapting and test stimulus were similar in contrast polarity.     

The chromatic selectivity of visual crowding

Precortical vision is mediated by three opponent mechanisms that combine receptoral outputs to form a luminance channel (L + M) and two chromatic channels, red-green (L/M) and blue-yellow (S/L + M). Here we ask the extent to which these basic color opponent mechanisms interact in the phenomenon of crowding, where nearby targets interfere with the processing of a central test target. The task was to identify the orientation of a Gabor patch while an annular plaid surrounded the patch. The radius of the annulus was varied in order to produce different separations of the test and flanker. The chromatic content of the Gabor and the annulus could be varied independently along the (L + M), (L/M), and (S/L + M) cardinal axes. For all targets, when the target and flanker shared the same chromaticity, performance decreased with decreasing separation of the target and annulus, i.e., a typical crowding effect was seen. When the test and flanker isolated different chromatic mechanisms, very little crowding was observed, even at the minimum separation of test target and annulus. In addition to this, intermediate chromaticities were found to produce intermediate levels of crowding. Finally, crowding effects using "half-wave rectified" stimuli suggest a locus for crowding effects beyond the level of color opponent mechanisms.     

Ambiguity in the perception of moving stimuli is resolved in favour of the cardinal axes

The aim of this study was to determine whether there is a link between the statistical properties of natural scenes and our perception of moving surfaces. Accordingly, we devised an ambiguous moving stimulus that could be perceived as moving in one of three directions of motion. The stimulus was a circular patch containing three square-wave drifting gratings. One grating was always either horizontal or vertical; the other two had component directions of drift at 120 degrees to the first (and to each other), producing four possible stimulus geometries. These were presented in a pseudorandom sequence. In brief presentations, subjects always perceived two of the gratings to cohere and move as a pattern in one direction, and the third grating to move independently in the opposite direction (its component direction). Although there were three equally plausible axes (one cardinal and two oblique) along which the coherent and independent motions could occur, subjects routinely saw motion along one of the cardinal axes. Thus, the visual system preferentially combines the two oblique gratings to form a pattern that drifts in the opposite direction to the cardinal grating. It was only when the contrast of one of the oblique gratings was changed that an oblique axis of motion was perceived. This perceptual anisotropy can be related to naturally occurring bias in the visual environment, notably the predominance of horizontal and vertical contours in our visual world.     

Chromatic properties of the colour-shading effect

The 'colour-shading effect' describes the phenomenon whereby chromatic variations affect the magnitude of perceived shape-from-shading in luminance patterns. A previous study showed that in mixed colour-plus-luminance sine-wave plaids, impressions of depth in the luminance component were enhanced by non-aligned chromatic components, and suppressed by aligned chromatic components [Nature Neuroscience 6 (2003) 641-644]. Here we examine the chromatic determinants of these effects. Colour contrast was defined along the cardinal axes of colour space in order to isolate the L-M and S-(L+M) post-receptoral chromatic mechanisms. We found no difference in the potency of L-M-only and S-(L+M)-only gratings, either for enhancing or suppressing perceived depth. Moreover, the magnitude of depth-suppression was no different for any combination of depth-enhancing and depth-suppressing cardinal directions. Finally we tested whether the visual system carried the assumption that natural shading is tinged with blue, by measuring perceived depth in a colour-plus-luminance grating that was made to appear either bright-yellow/dark-blue or bright-blue/dark-yellow. However there was no difference in the magnitude of depth-suppression between conditions, suggesting that the visual system does not make any assumption about the colour of natural shading. Taken together, the results suggest that while the colour-shading effect is highly sensitive to colour contrast, it is agnostic with respect to colour direction.     

Cross-domain adaptation reveals that a common mechanism computes stereoscopic (cyclopean) and luminance plaid motion

Across three experiments, this study investigated the visual processing of moving stereoscopic plaid patterns (plaids created with cyclopean components defined by moving binocular disparity embedded in a dynamic random-dot stereogram). Results showed that adaptation to a moving stereoscopic plaid or its components affected the perceived coherence of a luminance test plaid, and vice versa. Cross-domain adaptation suggests that stereoscopic and luminance motion signals feed into a common pattern-motion mechanism, consistent with the idea that stereoscopic motion signals are computed early in the motion processing stream.     

Perceived direction of plaid motion is not predicted by component speeds

It has been shown that the perceived direction of a plaid with components of unequal contrast is biased towards the direction of the higher-contrast component [Stone, L. S., Watson, A. B., & Mulligan, J. B. (1990). Effect of contrast on the perceived direction of a moving plaid. Vision Research 30, 1049-1067]. It was proposed that this effect is due to the influence of contrast on the perceived speed of the plaid components. This led to the conclusion that perceived plaid direction is computed by the intersection of constraints (IOC) of the perceived speed of the components rather than their physical speeds. We tested this proposal at a wider range of component speeds (2-16deg/s) than used previously, across which the effect of contrast on perceived speed is seen to reverse. We find that across this range, perceived plaid direction cannot be predicted either by a model which takes the IOC of physical or perceived component speed. Our results are consistent with an explanation of 2D motion perception proposed by [Bowns, L. (1996). Evidence for a feature tracking explanation of why Type II plaids move in the vector sum direction at short durations. Vision Research, 36, 3685-3694.] in which the motion of the zero-crossing edges of the features in the stimulus contribute to the perceived direction of motion.     

Moving two-dimensional patterns can capture the perceived directions of lower or higher spatial frequency gratings.

Coherent plaid motion is produced by superimposing two one-dimensional gratings of the same spatial frequency moving +/- 60 degrees from the intersection-of-constraints (IOC) resultant direction. These moving plaids were found to change the perceived direction of a third one-dimensional grating, either 6-fold lower or higher in spatial frequency, from traveling in one of the plaid's component direction to the IOC resultant direction. We describe this phenomenon as coherence capture. Coherence capture was found to be effective between plaids with 0.5, 1.0, and 1.5 c/deg components and gratings of 3.0, 6.0 and 9.0 c/deg respectively. It was also found to be effective between plaids with 3.0 c/deg components and gratings of 0.5 c/deg. However, coherence capture between higher spatial frequency plaids and lower spatial frequency gratings became less effective when the component spatial frequencies of the plaid increased.     

A model of plaid motion perception based on recursive Bayesian integration of the 1-D and 2-D motions of plaid features

We describe a theoretical and computational model of the perception of plaid pattern motion which fully accounts for the majority of cases in which misperception of the direction of motion of Type II plaids has been observed [Yo, C., & Wilson, H. (1992). Perceived direction of moving two-dimensional patterns depends on duration, contrast, and eccentricity. Vision Research 32, 135-147]. The model consists of two stages: in the first stage local motion detectors signal both the one-dimensional (1-D) and two-dimensional (2-D) motion of the high luminance features (blobs) in the plaid pattern; in the second stage these local motion signals are combined using a recursive Bayesian least squares estimation process. We demonstrate both theoretically and using simulations of the computational model that the estimated direction of the plaid motion for Type II plaids is initially dominated by the 1-D motion of the longer edges of the elongated blobs, which is in a direction close to the vector sum direction of the component gratings. The recursive estimation process which combines the local motion signals in the second stage of the model results in a dynamic shift in the estimated plaid direction towards the direction of the 2-D motion of the blobs, which corresponds to the veridical plaid direction.     

Comparison of color and luminance vision on a global shape discrimination task

We compared the performances of the blue-yellow, red-green and luminance systems on a shape discrimination task. Stimuli were radial frequency patterns (radially modulated fourth derivative of a Gaussian) with a peak spatial frequency of 0.75 cpd. Stimuli isolated the chromatic (red-green and blue-yellow) and achromatic post-receptoral mechanisms. We showed that in all cases performance, measured as a radial modulation threshold for discrimination between a circular and non-circular stimulus, improves with contrast. Performance was compared across radial frequencies with contrast matched in multiples of stimulus detection threshold. We find that blue-yellow color system performs the worse on this shape discrimination task, followed by the red-green, with the achromatic system performing best. The average difference is a factor of 2 between achromatic and blue-yellow performance, and a factor of 1.7 between red-green and achromatic. Despite these performance losses, chromatic shape discrimination can still reach hyperacuity performance levels. In a secondary experiment we contrast modulate the radial contour to eliminate either the "corners" or "sides" of an RF4 (square) pattern. We find that for the achromatic system, the sides are more important for the task than the corners. However, for the chromatic system, removal of sides or corners produces similar performance deficits. We conclude that color vision has a selective although relatively mild deficit for two-dimensional form perception.