PubMed · 11273628
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D D Hoffman. 2001. The data problem for color objectivism.. https://doi.org/10.1006/ccog.2000.0476
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The aim of this work was to study the relationship between cortical activity and visual perception. To do so, we developed a psychophysical technique that is able to dissociate the visual percept from the visual stimulus and thus distinguish brain activity reflecting the perceptual state from that reflecting other stages of stimulus processing. We used dichoptic color fusion to make identical monocular stimuli of opposite color contrast "disappear" at the binocular level and thus become "invisible" as far as conscious visual perception is concerned. By imaging brain activity in subjects during a discrimination task between face and house stimuli presented in this way, we found that house-specific and face-specific brain areas are always activated in a stimulus-specific way regardless of whether the stimuli are perceived. Absolute levels of cortical activation, however, were lower with invisible stimulation compared with visible stimulation. We conclude that there is no terminal "perceptual" area in the visual brain, but that the brain regions involved in processing a visual stimulus are also involved in its perception, the difference between the two being dictated by a higher level of activity in the specific brain region when the stimulus is perceived.
When a stimulus oscillates in both colour and direction of motion, changes in colour must lag behind those in direction if they are to be seen as concurrent. It has been argued that this lag is the consequence of asynchronous visual processing, with colour being processed more rapidly than motion. This proposal is contentious: it has been criticized on the basis that the time-course of cortical activity may not correlate directly with that of perceptual experience. Here, we demonstrate that the extent of the apparent asynchrony can vary according to the prevailing stimulus conditions. The apparent asynchrony is greatest if the stimulus is composed of opponent directions of motion and is reduced if the angular difference between the directions is reduced. This pattern of results suggests that asynchronous neural activity arises, in part, as a consequence of differential levels of inhibition within relatively independent cortical structures.
The human visual system shows a relatively greater response to low spatial frequencies of chromatic spatial modulation than to luminance spatial modulation. However, previous work has shown that this differential sensitivity to low spatial frequencies is not reflected in any differential luminance and chromatic content of general natural scenes. This is contrary to the prevailing assumption that the spatial properties of human vision ought to reflect the structure of natural scenes. Now, colorimetric measures of scenes suggest that red-green (and perhaps blue-yellow) color discrimination in primates is particularly suited to the encoding of specific scenes: reddish or yellowish objects on a background of leaves. We therefore ask whether the spatial, as well as chromatic, properties of such scenes are matched to the different spatial-encoding properties of color and luminance modulation in human vision. We show that the spatiochromatic properties of a wide class of scenes, which contain reddish objects (e.g., fruit) on a background of leaves, correspond well to the properties of the red-green (but not blue-yellow) systems in human vision, at viewing distances commensurate with typical grasping distance. This implies that the red-green system is particularly suited to encoding both the spatial and the chromatic structure of such scenes.