The effect of reversible cooling of cat's primary visual cortex on the responses of area 21a neurons.

1. Responses of sixty-four neurons in cortical area 21a were studied with areas 17 and 18 reversibly deactivated by cooling. From anatomical studies, most of area 21a input in the cat originates from these primary areas with no input from the A laminae of the lateral geniculate nucleus. 2. Both responses and spontaneous activity of all sixty-four area 21a neurons were markedly reduced when primary areas were cooled. In sixteen cells the responses were totally blocked. Temperatures of primary cortex required to produce total blockade varied between 25 and 4.5 degrees C. 3. The effect of cooling the primary visual cortex on the shape of orientation tuning curves was analysed in thirteen neurons from area 17 and in fifty-eight neurons from area 21a. In both areas, the width of the curve, when measured at its half-height, was preserved even when spike activity was reduced to below 10% of the original level. All neurons also retained their original directional preferences during cooling of the primary visual cortex. 4. The responsiveness of forty-seven neurons from area 21a was tested after rewarming of primary cortex. All but one neuron recovered their initial responsiveness within half an hour of restoring physiological temperature of primary visual cortex. 5. The results of the present study give an indication of the extent to which area 21a is sequentially related to the primary visual cortex in the processing of information.