Perceptual Development: Color and Contrast

Spatial contrast is arguably the most fundamental variable in vision. It enables us, among other things, to discriminate figure from ground, to find the edges of objects so that those objects can be recognized by their shapes, to match regions in the two eyes to enable depth perception, and to segregate regions in the visual image that differ in texture. Without the ability to perceive contrast, these fundamental aspects of visual perception would be all but impossible. This entry focuses on the development of spatial contrast vision and color vision.

One can also make the argument that color plays an important role in many of the same aspects of visual perception. Regions in the visual image that have the same texture but different colors can be readily distinguished by a perceiver with normal color vision. Those regions probably differ in their material properties. Color also plays a role in object recognition and discrimination. Ripe bananas look very much like unripe bananas in their shapes, but they differ considerably in their colors. Even in the absence of luminance contrast, an organism with color vision can find edges in the visual image between regions that differ only in their hues.

Given the importance of color and contrast to vision in general, it is logical to ask about the development of color and spatial contrast mechanisms. Contrast here refers to spatial differences in luminance. In adults, sensitivity to contrast is measured [Page 739]by finding the smallest difference in luminance between adjacent light and dark bars that can be reliably detected. At their most sensitive, adults can detect a difference in the luminances of adjacent bars of as little as 0.1% (the inverse of this is taken as contrast sensitivity; in this case 1/.1% = 1000). The sizes of these bars can be varied to determine how contrast sensitivity varies with spatial scale. Spatial scale is typically quantified by the number of adjacent light and dark bars that fit within some distance, such as a degree of visual angle (approximately the distance covered by the width of your thumb viewed at arm's length). When quantified in this way, spatial scale is referred to as spatial frequency with units of cycles per degree (cpd). A pair of adjacent light/dark bars comprises one cycle, so spatial frequency refers to the number of such cycles in a degree of visual angle. High spatial frequencies refer to gratings with many of these light/dark cycles in a degree of visual angle. Adults with normal vision can see gratings that have as many as 60 of these light/dark cycles in a degree of visual angle (60 cycles per degree). When the spatial frequency exceeds 60 cpd, adults can no longer see such fine detail. This highest detectable spatial frequency is referred to as acuity.