Achromatic Versus Chromatic Contrast

Achromatic contrast detects spatial differences, which depend on the distribution of light energy in the retinal image. For daylight vision, it is mediated by the two longer wavelength-sensitive cones. It is well developed in the fovea, mediating our highest spatial resolution, and represented in relatively large areas of visual cortex. The foveal area of striate cortex is about 36 times larger than that of striate cortex serving peripheral vision. In the fovea, the responses of neighboring cones are compared for achromatic contrast (Fig. 6). Extrafoveally, spatial resolution is reduced because ganglion cells collect synergistic signals from more than one cone. Achromatic contrasts establish local lightness and darkness, which input orientation-selective neurons in striate cortex and undoubtedly contribute to form perception. Lightness or darkness is determined entirely by simultaneous contrast. Absolute values of light energy are discarded by antagonistic interactions between neurons representing different areas of visual space. An object is light or dark depending entirely on its background.

Chromatic (wavelength) contrast is established by eliminating the effects of energy contrast. It compares the responses of one set of cones to those of another in the same area of space (Fig. 6) and then compares this with comparisons obtained in neighboring areas of space (Fig. 7). Chromatic contrast depends on the difference between cone responses in a unit area of chromatic visual space and not on the absolute responses of the cones. Chromatic contrast does not resolve the detail offered by a single cone because a cone cannot provide an unambiguous clue to wavelength contrast as it can to energy contrast.

A cone's response depends on the energy absorbed, regardless of wavelength. Wavelength determines the probability with which a quantum is absorbed. A response to any wavelength can be reproduced by any other wavelength by varying energy. A mosaic of at least two different types of cones must be sampled to distinguish wavelength contrast. Wavelength contrast needs a double comparison. First, the responses of two different sets of cones in one area of visual space must be compared, and then this comparison must be compared with other areas of visual space to establish spatial contrast based on gradients of wavelength. If one compared only two neighboring cones, a gradient of energy contrast would create an ambiguous signal. For this reason, a unit area of chromatic space is larger than that of achromatic space. The lower spatial resolution of chromatic contrast has been demonstrated psychophysically. The spatial resolution of chromatic contrast for all three cone mechanisms appears to be identical. Figure 8 shows the difference between the spatial resolution of chromatic and achromatic

UNIT AREA OF UNIT AREAS OF

CHROMATIC SPACE ACHROMATIC SPACE

Figure 6 This figure illustrates that the unit areas of chromatic and achromatic space differ because of the nature of the neural comparison. For chromatic contrast, the responses of a group of L cones must be compared with the responses of a neighboring group of M cones in the same area of visual space. For achromatic contrast a single cone can be compared with a neighboring L or M cone.

UNIT AREA OF UNIT AREAS OF

CHROMATIC SPACE ACHROMATIC SPACE

Figure 6 This figure illustrates that the unit areas of chromatic and achromatic space differ because of the nature of the neural comparison. For chromatic contrast, the responses of a group of L cones must be compared with the responses of a neighboring group of M cones in the same area of visual space. For achromatic contrast a single cone can be compared with a neighboring L or M cone.

AREA A AREA B

AREA A AREA B

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Understanding And Treating Autism

Understanding And Treating Autism

Whenever a doctor informs the parents that their child is suffering with Autism, the first & foremost question that is thrown over him is - How did it happen? How did my child get this disease? Well, there is no definite answer to what are the exact causes of Autism.

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