Visual Information Processing: The Many-Splendored Photoreceptor
One of the most striking aspects of the visual sense of animals is its ability to encompass an intensity range of a thousand million or more and yet maintain a very high sensitivity to small temporal or spatial variations about the mean intensity to which the animal is exposed at any time. Part of this task of adaptation to light and dark is performed by mechanical control of the light intensity incident on the retina, through pupil contraction and screening pigment migration. In higher animals, adaptation is aided by the specialization of photoreceptor cells to different parts of the intensity range: rods for weak light and cones for strong. A third component, at least in higher animals, is maintained by processing beyond the photoreceptor, mainly in the retina: absorption of photons in only 1% of the rods for example, can reduce the overall sensitivity of the system by a factor of three (Rushton, 1965).
KeywordsConditioning Stimulus Visual Information Processing Conditioning Intensity Light Intensity Incident Test Flash
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