Form And Color

The most difficult problem in color vision and vision as a whole is to understand how form perception occurs and how once an object's form is determined its color is appended. A reasonable explanation for form perception is Marr's idea of arrays of orientation-selective detectors coding for an object's configuration based on a retinotopic order. The initial detectors of contrast are retinogeniculate neurons with a center-surround receptive field organization based on energy contrast. The mathematical descriptions of such detectors are best represented by a difference of two-dimensional Gaussian-shaped fields. The central field is smaller and overlapped by a larger antagonistic field. The interaction of such detectors can lead to orientation-selective units that detect edges of contrast based on achromatic contrast. The neural machinery for doing this is highly developed in areas of visual cortex serving the fovea. Most of the neurons encountered in striate cortex of primates are sensitive to achromatic (energy) contrast and not selective for color. A smaller amount of neural machinery appears to be devoted to chromatic contrast and color vision in any one area of visual cortex, probably due to its lower spatial resolution.

It is reasonable to assume that chromatic detectors of contrast contribute to form perception in a similar way as achromatic detectors but as a parallel system. Here, wavelength rather than energy contrast determines the activation of the detectors established by cone opponent interactions as independent systems. One system detects blue-yellow chromatic contrasts and the other red-green contrasts. In general, the cues produced by energy contrasts are reinforced by chromatic contrast in detecting the same object. If there are ambiguities, the brain must make a decision favoring one or the other, perhaps favoring chromatic contrast because of its relative independence from shading. Other properties of an object, such as well-defined borders or texture, would be better determined by the achromatic system. In this scheme, three parallel systems for contrast detection are envisaged—one for achromatic (black-white-gray), one for blue-yellow, and one for red-green chromatic contrasts, each arranged with its own retinotopic order. Whether all three systems converge on a single neuron to determine the fused sense of color is unclear. For colors such as magenta, such interactions may occur. In addition, there seem to be neurons that respond to both chromatic and achromatic signals to perform neural functions but are not involved in the perception of color.

A clue to the cortical processing of vision comes from observations of multiple areas of prestriate cortex where the entire visual field is re-represented. These areas send and receive signals from each other. Working together they create a unified and presumably richer impression of the visual world. The actual role of each subarea is poorly understood.

Zeki proposed that one of these subareas, visual area 4, is unique for color processing. He argued that in this area cells correct for color constancy and respond to true color. At earlier stages cells may respond to wavelength contrast but not to color. It has been difficult to prove this hypothesis. Single neuron recordings from most visual areas reveal cells selective for chromatic contrast and color vision but usually in the minority. Visual area 4 has been reported to have a larger proportion ofcolor-selective cells but there is no universal agreement about this.

There is little doubt that the cortex works by dividing different aspects of visual function into anatomically distinct areas and that perception of a unified image depends on the multiple physiological linking of these separate subareas. A critical element in this reasoning is the question of retinotopic order, which seems to be the backbone of form perception.

The integration of form must be based on linking inputs labeled by retinal coordinates. Chromatic contrasts must be handled in a similar way as achromatic contrasts, each with its own retinal coordinates. Does the lower spatial resolving chromatic contrast system get funneled into a separate cortical area for color vision even though color vision depends as much on achromatic contrast as on chromatic contrast? It seems more likely that chromatic and achromatic processing occurs in all the areas to which the parvocellular system projects, and color discrimination as well as color constancy improve as increasingly more visual areas are involved in the processing.

Color discrimination continues to improve with the size of the object being judged. Surfaces subtending 20° and more of visual angle significantly improve color discrimination when compared with those subtending only a few degrees. Color discrimination can integrate over very large areas of visual space and consequently striate cortex. One of the characteristics of neurons in higher visual areas is the relatively large size of their receptive fields. They require inputs from large areas of the retina before deciding to respond. The larger size of visual stimuli activates more antagonistic interactions and therefore could increase the variety of experience. If one is judging a small object, he or she may not be using as much of his or her brain. If the object is enlarged, it begins to activate more cells in higher visual areas and its recognition is improved. This operation may depend on all subareas working in concert through feedback rather than a serial progression from lower to higher areas.

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