Light adaptation in motion direction judgments.

We examined the time course of light adaptation in the visual motion system. Subjects judged the direction of a two-frame apparent-motion display, with the two frames separated by a 50-ms interstimulus interval of the same mean luminance. The phase of the first frame was randomly determined on each trial. The grating presented in the second frame was phase shifted either leftward or rightward by pi/2 with respect to the grating in the first frame. At some variable point during the first frame, the mean luminance of the pattern increased or decreased by 1-3 log units. Mean luminance levels varied from scotopic or low mesopic to photopic levels. We found that the perceived direction of motion depended jointly on the luminance level of the first frame grating and the time at which the shift in average luminance occurs. When the average luminance increases from scotopic or mesopic to photopic levels at least 0.5 s before the offset of the first frame, motion in the 3pi/2 direction is perceived. When average luminance decreases to low mesopic or scotopic levels, motion in the pi/2 direction is perceived if the change occurs 1.0 s or more before first frame offset, depending on the size of the luminance step. Thus light adaptation in the visual motion system is essentially complete within 1 s. This suggests a rapid change in the shape (biphasic or monophasic) of the temporal impulse response functions that feed into a first-order motion mechanism.

Modulation of perceived contrast by a moving surround.

The apparent contrast of a center pattern depends on the contrast of its surround. To examine the suprathreshold perception of moving patterns, we measured the perceived contrast of a moving grating while the direction and speed of the surround patterns varied. Subjects matched the apparent contrast of a center patch embedded in surround patches to that of a patch with no surround pattern. Temporal frequency, Michelson contrast and movement direction of both center and surround patterns varied systematically. We found that: (1) contrast reduction is most prominent when the center and surround have the same velocity (velocity selectivity); (2) contrast enhancement occurs when the surround moves at a higher speed than the center, if the difference in temporal frequencies of center and surround exceeds 10-20, independent of the directional relationship between center and surround; (3) contrast reduction is stronger for higher surround contrasts with lower center contrasts; and (4) contrast enhancement is relatively unaffected by center and surround contrasts. We conclude that the contrast perception of moving patterns is influenced by directionally-selective mechanisms except at high temporal frequencies. Our results further suggest that there is not only the lateral inhibition often assumed to influence contrast gain control, but also an excitatory connection between motion encoding units.

Velocity discrimination in scotopic vision.

To characterize scotopic motion mechanisms, we examined how variation in average luminance affects the ability to discriminate velocity. Stimuli were drifting horizontal sine-wave gratings (0.25, 1.0 and 2.0 c/deg) viewed through a 2 mm artificial pupil and neutral density filters to produce mean adapting levels from 2.5 to -1.5 log photopic trolands. Drift temporal frequency varied from 0.5 to 36.0 Hz. Grating contrasts were either three or five times direction discrimination threshold contrasts at each adaptation level. Following 30 min adaptation, two drifting gratings were presented sequentially at the fovea. Subjects were asked to indicate which interval contained the faster moving stimulus. The Weber fraction for each base temporal frequency was determined using a staircase method. As previously reported, velocity discrimination performance was most acute at temporal frequencies of about 8.0 Hz and greater than 20.0 Hz (though there are individual differences), and fell off at both higher and lower temporal frequencies under photopic conditions. As adaptation level decreased, discrimination of high temporal frequencies in the central retina became increasingly worse, while discrimination of low temporal frequencies remained largely unaltered. The overall scotopic discrimination performance was best at about 3.0 Hz. These results can be explained by a motion mechanism comprising both low-pass and band-pass temporal filters whose peak and temporal cut-off shifts to lower temporal frequencies under scotopic conditions.

Contribution of S opponent cells to color appearance.

We measured the regions in isoluminant color space over which observers perceive red, yellow, green, and blue and examined the extent to which the colors vary in perceived amount within these regions. We compared color scaling of various isoluminant stimuli by using large spots, which activate all cone types, to that with tiny spots in the central foveola, where S cones, and thus S opponent (S(o)) cell activity, are largely or entirely absent. The addition of S(o) input to that from the L and M opponent cells changes the chromatic appearance of all colors, affecting each primary color in different chromatic regions in the directions and by the amount predicted by our color model. Shifts from white to the various chromatic stimuli we used produced sinusoidal variations in cone activation as a function of color angle for each cone type and in the responses of lateral geniculate cells. However, psychophysical color-scaling functions for 2 degrees spots were nonsinusoidal, being much more peaked. The color-scaling functions are well fit by sine waves raised to exponents between 1 and 3. The same is true for the color responses of a large subpopulation of striate cortex cells. The narrow color tuning, the discrepancies between the spectral loci of the peaks of the color-scaling curves and those of lateral geniculate cells, and the changes in color appearance produced by eliminating S(o) input provide evidence for a cortical processing stage at which the color axes are rotated by a combination of the outputs of S(o) cells with those of L and M opponent cells in the manner that we postulated earlier. There seems to be an expansive response nonlinearity at this stage.

Hue scaling of isoluminant and cone-specific lights.

Using a hue scaling technique, we have examined the appearance of colored spots produced by shifts from white to isoluminant stimuli along various color vectors in order to examine color appearance without the complications of the combined luminance and chromatic stimulation involved in most previous hue scaling studies, which have used flashes of monochromatic light. We also used spots lying along cone-isolating vectors in order to determine what hues would be reported with a change in activation of only single cone types or of only single geniculate opponent-cell types, an issue of direct relevance to any model of color vision. We find that: 1. Unique hues do not correspond either to the change in activation of single cone types or of single geniculate opponent-cell types. This is well known to be the case for yellow and blue, but we find it to be true for red and green as well. 2. These conclusions are not limited to the particular white (Illuminant C) used as an adapting background in most of the experiments. Shifts along the same cone-contrast vectors relative to different backgrounds lead to much the same hue percepts, independent of the starting white used. 3. The shifts of the perceptual colors from the geniculate axes are in the directions, and close to the absolute amounts, predicted by our [De Valois & De Valois (1993). Vision Research, 33, 1053-1065] multi-stage color model in which we postulate that the S-opponent cells are added to or subtracted from the M- and L-opponent cells to form the four perceptual color systems. 4. There are distinct asymmetries with respect to the extent to which various hues within each perceptual opponent system deviate from the geniculate opponent-cell axes. Blue is shifted more from the S-LM axis than is yellow; green is shifted more from the L-M axis than is red. There are also asymmetries in the angular extent of opponent color regions. Blue is seen over a larger range of color vectors than is yellow, and red over a slightly larger range than green. 5. Such asymmetries are not accounted for by any model that treats red-green and yellow-blue each as unitary, mirror-image opponent-color systems. Although red and green are perceptually opponent, the red and green perceptual systems do not appear to be constructed in a mirror-image fashion with respect to input from different cone types or from different geniculate opponent-cell types. The same is true for yellow and blue.

Motion-reversal reveals two motion mechanisms functioning in scotopic vision.

We studied scotopic motion mechanisms, using a two-frame sinusoidal grating separated by various ISIs equated for mean luminance level. Perceived direction of displacement varied with both ISI and luminance. As luminance decreased, apparent motion reversal disappeared. This is predicted by a first-order motion model if the underlying temporal impulse response function varies from biphasic under photopic conditions to monophasic under scotopic conditions. Performance at long (but not short) ISIs depends upon stimulus contrast, suggesting there is also a scotopic feature-tracking mechanism. With isoluminant and high spatial frequency gratings, where the temporal impulse response function is monophasic, no motion reversal was observed.

Figural aftereffects and spatial attention.

Distracting attention away from the location of an adaptation figure reduces the positional shift of a displaced test figure in the figural aftereffect (FAE). Participants performed an alignment task after adaptation involving various manipulations of spatial attention. In 1 condition, participants counted how often numbers occurred in an alphanumeric sequence presented during adaptation. (The sequence also appeared in a comparison condition, but no attention was required.) The FAE was reduced when the alphanumeric sequence attended to was in the center of the display while the adaptation figure was 3 degrees eccentric but not when the pattern was superimposed on the adaptation figure. Forced attention to 1 feature of the adaptation figure, its orientation, did not reduce the FAE (Experiment 3). To obtain a maximum FAE, the span of attention must cover the adaptation figure.

Motion contrast and motion integration.

When a moving aperture contains a drifting grating, the perception of aperture movement is strongly affected by the grating movement. We have studied this interaction, using a moving circular patch of sinusoidal grating matched to the background in mean luminance. The circular window, or aperture, could be defined either by an abrupt transition from a full-contrast grating to the background (hard aperture) or by a two-dimensional Gaussian fall-off in contrast (soft aperture). The grating movement could be controlled independently of the aperture motion. Subjects judged the direction of the aperture movement (i.e. the movement of the patch as a whole). We find that an illusory motion of a stationary aperture can be induced depending on the direction of the grating drift. A hard aperture presented in the fovea appears to move in the direction opposite the grating movement, demonstrating simultaneous motion contrast. However, a soft aperture presented in the periphery appears to move in the same direction as the drifting grating, demonstrating motion integration (assimilation). These results are discussed in the context of interactions between short-range and long-range motion mechanisms and with respect to the significance of boundaries in determining the figure-ground relationship of motion signals.

A multi-stage color model.

The first stage of our model has three cone types, with L:M:S cones in ratios of 10:5:1. In the second stage, retinal connectivity leads to three pairs of cone-opponent, and one pair of cone-nonopponent systems. At a third (cortical) stage of color processing, the S-opponent cells are added to or subtracted from the L- and M-opponent units to split and rotate the one effective parvo geniculate response axis into separate RG and YB color axes, and separate luminance from color. We also discuss changes with eccentricity, and connectivity based on correlated neural activity.

Properties of the recombination of one-dimensional motion signals into a pattern motion signal.

We have examined the human ability to determine the direction of movement of a variety of plaid patterns. The plaids were composed of two orthogonal sine-wave gratings. When the plaid components are of unequal spatial frequency or sometimes of unequal contrast, observers judge the direction of movement incorrectly. In terms of the two-stage model of Adelson and Movshon (1982), these errors may result from either a misjudgment in the perceived speeds of each of the components or a failure in the combination of one-dimensional component movements into a coherent direction of motion of the two-dimensional plaid pattern, or both. A comparison of the perceived direction of motion of plaids with the relative perceived speeds of the plaid component gratings suggest that both failures occur, but in different circumstances. The relative perceived speed of the plaid components was measured with a spatial and a temporal forced-choice technique, the former leading to larger differences. Our results support the notion that the visual system decomposes a moving plaid into oriented components and subsequently recombines the component motions.

The role of color in the motion system.

We have examined the ability of observers to determine the direction of movement of a variety of colored plaid patterns. When the two plaid components are of unequal spatial frequency or of unequal luminance or chromatic contrast, observers judge the direction of movement incorrectly. These errors are correlated with a misjudgement of the speeds of the two components. Our results provide support for an initial decomposition into oriented components followed by a subsequent component-to-pattern recombination of moving equiluminant and colored plaids. At equal multiples of threshold contrast a moving luminance grating is about 8 times more powerful than a moving equiluminant grating in determining the apparent direction of motion of a plaid. When both are present, luminance and color do not interact linearly. Color and motion must be processed in parallel in at least partially separate pathways.

Higher-order factors influencing the perception of sliding and coherence of a plaid.

The effect of several new stimulus parameters on the perception of a moving plaid pattern (the sum of two sine-wave gratings) were tested. It was found that: (i) the degree of perceived sliding is strongly influenced by the aperture configuration through which the plaid is viewed; (ii) the chromaticity of the sinusoidal components affects coherence in that more sliding is observed when the plaid components differ in hue, and there is less sliding when they are of the same hue; (iii) equiluminant plaids made of components equal in color almost never show any sliding; and (iv) sliding increases with viewing time. The coherence-sliding percept must therefore be influenced by color, by global interactions, and by adaptation or learning effects, thus suggesting a higher-level influence. These results are most easily modelled by separating the decision to carry out recombination from the process of recombination.

Vernier acuity with stationary moving Gabors.

We examined the ability of observers to determine the vertical alignment of three Gabor patches (cosine gratings tapered in X and Y by Gaussians) when the grating within the middle patch was moving right or left. The comparison patches were flickered in counterphase, as was the test patch in a control condition. In all conditions, the Gabor patch itself (the envelope) was stationary. Vernier acuity (i.e. sensitivity) was almost as good with the moving as with the flickering Gabors, but there was a very pronounced positional bias in the case of the patterns in which the internal gratings were moving. The (stationary) patches appeared to be displaced in the direction of the grating movement. Thus if the grating were drifting rightwards, the observer would see the patches as being aligned only when the test patch position in fact was shifted far over to the left. This movement-related bias increased rapidly with retinal eccentricity, reaching 15 min at 8 deg eccentricity. The bias was greatest at 4-8 Hz temporal frequency, and at low spatial frequencies. Whether the patterns were on the horizontal or the vertical meridian was largely irrelevant, but larger biases were found with patterns moving towards or away from the fovea than with those moving in a tangential direction.

Orientation and spatial-frequency discrimination for luminance and chromatic gratings.

We have examined the accuracy of orientation and spatial-frequency discrimination for sine-wave gratings that vary in either luminance or color. The equiluminant chromatic gratings were modulated along either a tritanopic confusion axis (so that they were detectable on the basis of activity in only the short-wavelength-sensitive cones) or an axis of constant short-wavelength-sensitive cone excitation (so that they could be detected on the basis of opposing activity in only the long- and medium-wavelength-sensitive cones). Grating contrasts ranged from the detection threshold to the highest levels that we could produce; the contrasts of the luminance and color patterns were equated for equal multiples of their respective detection thresholds. Discrimination thresholds for all patterns showed a similar dependence on stimulus contrast, rising sharply at low contrasts and becoming nearly asymptotic at moderate contrasts. However, even at threshold contrasts, observers could still reliably discriminate sufficiently large differences in the orientation or spatial frequency of all patterns, and they could also reliably identify the type of variation (luminance or which color) defining the grafting. For most conditions the discrimination thresholds did not differ from the two types of color grafting and reached values as low as 1 deg (orientation) or 4% (spatial frequency). Thus observers were able to make accurate spatial judgments on the basis of either type of chromatic information. However, these thresholds were slightly but consistently higher than the thresholds for comparable luminance graftings. This difference in the color and luminance discrimination thresholds may reflect somewhat coarser orientation and spatial-frequency selectivity in the mechanisms encoding the chromatic patterns.

Discrimination of relative spatial position.

Subjects can accurately discriminate small changes in the relative position of features within a pattern. Simple patterns (intersected line segments) can undergo magnification and a variety of transformations without significantly affecting the discrimination thresholds. Features to be localized and compared need not be similar. We suggest that such relative position discriminations could support complex object identification.

Contrast dependence and mechanisms of masking interactions among chromatic and luminance gratings.

The contrast dependence of simultaneous masking has been measured using isochromatic yellow-black luminance sinusoids and isoluminant red-green chrominance gratings. Masking functions for all four combinations of chromatic and luminance masks and tests are reported. In the two same-on-same conditions (luminance mask/luminance test and chromatic mask/chromatic test) these functions (increment threshold contrast versus mask contrast) have the typical dipper shape and are almost identical when test and mask contrasts are normalized to the unmasked contrast thresholds. The contrast dependence of the luminance mask/color test and color mask/luminance test functions are quite different. The luminance mask/color test shows facilitation over a broad range of both subthreshold and suprathreshold contrasts of the luminance mask. In the color mask/luminance test condition facilitation is never observed, but at suprathreshold contrasts a 2-cycle/degree (c/deg) chromatic grating masks a 2-c/deg luminance grating as strongly as does a luminance mask. The luminance mask/chromatic test results are invariant over the 0.25-2-c/deg spatial-frequency range, whereas the robust masking of luminance by color at 2 c/deg diminishes at lower spatial frequencies. The spatial-frequency selectivity of the luminance-facilitates-color interaction is much broader than facilitatory interactions in either the color-color or luminance-luminance conditions. Possible mechanisms of color-luminance interactions are considered. The lack of facilitation in the color mask/luminance test condition precludes a simple pedestal interpretation of this masking interaction. The data are, however, consistent with models that invoke inhibitory or more elaborate excitatory masking interactions.

Temporal properties of brightness and color induction.

With a matching procedure, we studied the temporal properties of direct brightness (or lightness) and chromatic changes (produced by modulation of the region being matched) and induced brightness and chromatic changes (produced by modulation of the surround of the region being matched). The amount of direct brightness and color change was found to vary only slightly with temporal frequency over the 0.5-8 Hz range studied, whereas induced changes were found to occur only at low temporal frequencies, below about 2.5 Hz. With high temporal-frequency modulation of the surround, the induced patterns appeared to flicker but not to change in brightness or color. Despite the fact that chrominance and luminance temporal contrast sensitivity functions are very different, the temporal induction curves for color and brightness were very similar. However, brightness induction was found to increase approximately linearly with increasing surround modulation up to very high levels, whereas the amount of color induction was much less dependent on the modulation depth of the surround.

Spatial-frequency-specific inhibition in cat striate cortex cells.

Responses to single and multiple spatial frequency gratings were recorded from eighty-eight cat striate cortex cells. A cell's response to a grating of its optimum spatial frequency (f) was examined both alone and in the presence of gratings of 1/4, 1/3, 1/2, 2, 3 and 4f, respectively. Some 97% (thirty-seven of thirty-eight) of all simple cells showed significant inhibition of f by one or more of the other frequencies. This inhibition was usually fairly narrowly tuned, with only one or two spatial frequencies producing significant inhibition. Thirty-four simple cells were maximally inhibited by a higher frequency, three by a lower spatial frequency. By far the most common interaction was a considerable inhibition of f by 2f and/or 3f. Of the thirty-seven simple cells showing inhibition to a complex grating, seventeen responded in a manner dependent on the relative phases of the two components. Some showed only inhibition of f; in others, the response to f was either increased or decreased, depending on the relative phase of the two frequencies. The other twenty simple cells showed phase-independent inhibition: the inhibition was of approximately equal amplitude regardless of the relative phase angle of the two grating components. Such phase-independent inhibition cannot be accounted for by linear summation within classical cortical receptive fields. Only eighteen of forty-eight (38%) of the complex cells showed significant inhibition of f by one or more other spatial frequencies. Fourteen of these (29%) were maximally inhibited by a higher spatial frequency, four (8%) by a lower spatial frequency. Inhibitory interactions in complex cells were never dependent on the relative phase of the two component gratings. Six simple cells (16%) and fourteen complex cells (29%) showed significant facilitation of the response to f by one or more (most often lower) spatial frequencies. This enhanced response was greater than the sum of the responses to each component alone, was usually broadly tuned for spatial frequency, and did not depend on the relative phase of the two components. It thus differs from the increased response sometimes seen in a phase-dependent interaction. Some of the observed spatial-frequency-specific interactions are incompatible with either a strictly hierarchical model of cortical architecture or a simple linear filter model of visual cortical processing. The asymmetry of inhibition suggests that it subserves some function other than (or in addition to) the narrowing of spatial frequency tuning functions.

Simultaneous masking interactions between chromatic and luminance gratings.

Simultaneous masking using test and mask gratings composed of isochromatic luminance variations and isoluminant chromatic variations was studied. Masking of chromatic gratings by chromatic gratings shows less spatial-frequency specificity than does masking of luminance gratings by luminance gratings. Luminance gratings mask chromatic gratings of identical space-average luminance and chromaticity little and only when the spatial frequencies of the test and mask gratings are similar. Chromatic gratings, however, profoundly mask luminance gratings with a degree of spatial-frequency specificity akin to that of luminance-luminance masking. The insensitivity of the luminance-color masking results to the relative phase of the chromatic and luminance gratings indicates that the observed asymmetry is not due to local interactions.

Spatial frequency specific interaction of dot patterns and gratings.

Adaptation to patterns of paired random dots produces loss of contrast sensitivity to sinusoidal luminance gratings oriented perpendicularly to the dot-pair direction. This adaptation loss is spatial frequency- and orientation-specific and varies with dot-pair separation in a manner predictable from the Fourier spectra of the stimuli and observed characteristics of the visual system. These results support the idea that the visual system acts as a periodicity analyzer with known restrictions and cannot be accounted for by a feature-detector model. When the bars of the test gratings are aligned in the dot-pair direction, there is no adaptational loss at any frequency despite the fact that the adaptation pattern contains significant spectral power at all frequencies in this orientation. This lack of adaptation may be due to inhibitory interactions among channels or to nonlinear effects within local receptive fields.