Spatial contrast adaptation characteristics of neurones recorded in the cat's visual cortex.

Abstract

Spatial contrast adaptation, produced by prolonged exposure to high contrast grating patterns, has become an important psychophysical method for isolating spatial and orientation selective channels in the human visual system. This adaptation may be fundamentally dependent upon the activity of neurons in the striate cortex. To test the validity of this hypothesis, the general adaptation characteristics of 144 striate neurons were measured using a stimulus protocol comparable to the typical psychophysical methods. In general, during prolonged high contrast stimulation, the responses of most cells exponentially decayed from a transient peak response to a sustained plateau response; following adaptation, the responses to lower contrasts were depressed relative to the unadapted state but then gradually recovered from the transient depression to a sustained plateau. Such adaptation was a property common to both simple and complex cells (the distributions of the quantitative indices of adaptation were overlapping); there were, however, small but reliable differences. The neurophysiological contrast adaptation was compared with 2 psychophysical estimates of human contrast adaptation (threshold contrast elevation and apparent contrast reduction) and found that the time courses and the magnitudes were quite similar. The effect of contrast adaptation on the spatial frequency tuning was assessed by measuring the contrast response function at several different test spatial frequencies before and after adaptation at the optimum center frequency. The effect of adaptation decreased as the difference between the test and adaptation frequency increased. Grating contrast adaptation has been alternatively described as constructive gain control and as deleterious fatigue. The effect of contrast adaptation on the contrast response function was tested. Adaptation shifts the curves vertically downward parallel to the response axis (thus reflecting a decrease in the maximum rate of firing and a deleterious compression of the response range) and horizontally to the right parallel to the contrast axis (reflecting a true sensitivity shift of the remaining response range for constructive maintenance of high differential sensitivity around the prevailing background level). To specifically test whether contrast adaptation could have advantageous consequences in terms of the maintenance of high differential contrast sensitivity, we measured the responses during prolonged sequential contrast alternation and found that adaptation did enhance the differential response modulation at higher contrasts.