AN INTRACELLULAR STUDY OF THE CONTRAST-DEPENDENCE OF NEURONAL-ACTIVITY IN CAT VISUAL-CORTEX

Extracellular recordings indicate that mechanisms that control contras t gain of neuronal discharge are found in the retina, thalamus and cor tex. In addition, the cortex is able to adapt its contrast response fu nction to match the average local contrast. Here we examine the neuron al mechanism of contrast adaptation by direct intracellular recordings in vivo. Both simple (n = 3) and complex cells (n = 4) show contrast adaptation during intracellular recording. For simple cells, that the amplitude of fluctuations in membrane potential induced by a drifting grating stimulus follows a contrast response relation similar to later al geniculate relay cells, and does not reflect the high gain and adap tive properties seen in the action potential discharge of the neurons. We found no evidence of significant shunting inhibition that could ex plain these results. In complex cells there was no change in the mean membrane potential for different contrast stimuli or different states of adaptation, despite marked changes in discharge rate. We use a simp lified electronic model to discuss the central features of our results and to explain the disparity between the contrast response functions of the membrane potential and action potential discharge in simple cel ls.