Karen K. De Valois

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.

SPATIAL FREQUENCY ADAPTATION CAN ENHANCE CONTRAST SENSITIVITY

Abstract Adaptation to a high-contrast sinusoidal luminance grating produces a temporary, band limited loss in sensitivity centered around the adaptation frequency. The decrease appears to be both narrower and more symmetrical than earlier reports suggest. The effect falls to zero by f ± 1 octave and is not reliably present from f ± 1 octave to f ± 2 octaves. Enhancement of contrast sensitivity occurs for frequencies further removed, peaking at about f ± 2 built 34 to 3 octaves. This suggests mutual inhibitory interactions among spatial-frequency-selective units of varying filter characteristics. Longterm practice produces significantly higher contrast sensitivity functions and narrower bandwidths of the adaptational sensitivity loss.

Orientation and spatial frequency selectivity of adaptation to color and luminance gratings.

Prolonged viewing of sinusoidal luminance gratings produces elevated contrast detection thresholds for test gratings that are similar in spatial frequency and orientation to the adaptation stimulus. We have used this technique to investigate orientation and spatial frequency selectivity in the processing of color contrast information. Adaptation to isoluminant red-green gratings produces elevated color contrast thresholds that are selective for grating orientation and spatial frequency. Only small elevations in color contrast thresholds occur after adaptation to luminance gratings, and vice versa. Although the color adaptation effects appear slightly less selective than those for luminance, our results suggest similar spatial processing of color and luminance contrast patterns by early stages of the human visual system.

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.

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.

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.

Stimulus selectivity of figural aftereffects for faces

Viewing a distorted face induces large aftereffects in the appearance of an undistorted face. The authors examined the processes underlying this adaptation by comparing how selective the aftereffects are for different dimensions of the images including size, spatial frequency content, contrast, and color. Face aftereffects had weaker selectivity for changes in the size, contrast, or color of the images and stronger selectivity for changes in contrast polarity or spatial frequency. This pattern could arise if the adaptation is contingent on the perceived similarity of the stimuli as faces. Consistent with this, changing contrast polarity or spatial frequency had larger effects on the perceived identity of a face, and aftereffects were also selective for different individual faces. These results suggest that part of the sensitivity changes underlying the adaptation may arise at visual levels closely associated with the representation of faces.

Independence of black and white: phase-specific adaptation.

Abstract Following adaptation to a black/white rectangular-wave grating, the black and white bars of a square wave of the same period no longer appear equal. Black bars are shifted in apparent width away from the width of the black adaptation bars, and white bars appear to be shifted away from the width of the white adaptation bars. Tests with single adaptation and test bars (rather than gratings) confirm the contrast specificity of the effect—i.e. white test bars are reliably affected only by white adaptation bars. This suggests separate processing of black and white size-specific information, or phase specificity of spatial frequency channels.

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.

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.