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.
"A human observer can detect periodic patterns modulated only in chromaticity34. The mechanisms which detect chromatic patterns also show a tuning to spatial frequency56789101112 and orientation5111213 like those that detect luminance patterns. However, while there are many studies on the properties of the early visual mechanism for extracting chromatic local features, how the visual system integrates these chromatic local features into a global pattern is less well understood. "
[Show abstract][Hide abstract] ABSTRACT: We investigated the role of color in the feature integration process for global form perception. For this, we used a 2AFC noise masking paradigm to measure the color selectivity of the symmetry detection mechanism. In each trial, a vertical symmetric target was randomly presented in one of the two intervals while a random dot control, in the other. The observers' task was to determine which interval contained the symmetric target. The image elements varied in chromaticity. The target density threshold was measured at various combinations of target and mask chromaticity. A noise mask with the same chromaticity as the target always produced the largest masking effect (threshold increment) on the detection on that target. The masking effect decreased as the difference in chromaticity between the target and mask increased. This suggests that the symmetry detection mechanisms are color selective and only extract local image features of a specific chromaticity.
"However, adaptation taking place elsewhere in the visual hierarchy could not be excluded. Bradley and colleagues (1988) used an adaptation paradigm to show that the presentation of color gratings has no effect on the sensitivity of luminance gratings, nor vice versa. To compound matters, robust masking of luminance by color has been reported, but not the other way around (De Valois and Switkes, 1983). "
[Show abstract][Hide abstract] ABSTRACT: Adaptation is widely used as a tool for studying selectivity to visual features. In these studies it is usually assumed that the loci of feature selective neural responses and adaptation coincide. We used an adaptation paradigm to investigate the relationship between response and adaptation selectivity in event-related potentials (ERPs). ERPs were evoked by the presentation of colored Glass patterns in a form discrimination task. Response selectivities to form and, to some extent, color of the patterns were reflected in the C1 and N1 ERP components. Adaptation selectivity to color was reflected in N1 and was followed by a late (300-500 ms after stimulus onset) effect of form adaptation. Thus for form, response and adaptation selectivity were manifested in non-overlapping intervals. These results indicate that adaptation and response selectivity can be associated with different processes. Therefore, inferring selectivity from an adaptation paradigm requires analysis of both adaptation and neural response data.
Frontiers in Human Neuroscience 04/2012; 6:89. DOI:10.3389/fnhum.2012.00089 · 3.63 Impact Factor
"Because simultaneous and opposite spatial frequency shifts (Favreau & Cavanagh, 1981) and tilt after effects (Flanagan, et al., 1990) were obtained in adaptation experiments with equiluminant-chromatic and achromatic gratings, it has been argued that a channel exists that codes spatial frequency and orientation solely based on chromatic information. Results from adaptation experiments measuring detection thresholds (Bradley, et al., 1988; Murasugi & Cavanagh, 1988) are consistent with this hypothesis, and results from visual search experiments (Cavanagh, et al., 1990) indicate that orientation and size are basic coding dimensions for equiluminous colors. These results point to the existence of a chrominance channel that analyzes shape information solely based on chromatic signals (Cavanagh, 1991). "
[Show abstract][Hide abstract] ABSTRACT: We report a novel class of V4 neuron in the macaque monkey that responds selectively to equiluminant colored form. These "equiluminance" cells stand apart because they violate the well established trend throughout the visual system that responses are minimal at low luminance contrast and grow and saturate as contrast increases. Equiluminance cells, which compose ∼22% of V4, exhibit the opposite behavior: responses are greatest near zero contrast and decrease as contrast increases. While equiluminance cells respond preferentially to equiluminant colored stimuli, strong hue tuning is not their distinguishing feature-some equiluminance cells do exhibit strong unimodal hue tuning, but many show little or no tuning for hue. We find that equiluminance cells are color and shape selective to a degree comparable with other classes of V4 cells with more conventional contrast response functions. Those more conventional cells respond equally well to achromatic luminance and equiluminant color stimuli, analogous to color luminance cells described in V1. The existence of equiluminance cells, which have not been reported in V1 or V2, suggests that chromatically defined boundaries and shapes are given special status in V4 and raises the possibility that form at equiluminance and form at higher contrasts are processed in separate channels in V4.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience 08/2011; 31(35):12398-412. DOI:10.1523/JNEUROSCI.1890-11.2011 · 6.34 Impact Factor
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