Reversible inactivation of individual layers of the cat lateral geniculate and medial interlaminar nuclei was used to investigate the necessary and sufficient inputs for maintaining visually driven activity and receptive field properties in area 17. Neither orientation selectivity nor direction selectivity depends on any individual geniculate layer. We identified two groups of cortical layers on the basis of the pattern of thalamic inputs providing visual driving through the contralateral eye. One group, consisting of layers 4 and 6, has geniculate layer A as its only necessary and sufficient input. The other, consisting of supragranular layers, integrates at least two sufficient thalamic inputs, one of which is layer A. Several major receptive field properties are independently generated in these two groups of layers.