Abstract
According to current theory, orientation selectivity in cortical simple cells is critically dependent on intracortical synaptic inhibition. In particular, it is thought that IPSPs evoked by stimuli of the nonpreferred orientation are required to prevent a neuron from responding to a broadly tuned excitatory input at any but the preferred orientation. Yet EPSPs recorded in simple cells are in themselves highly orientation-selective. How is this possible, when excitation arises primarily from relay cells of the lateral geniculate nucleus (LGN), which are largely insensitive to orientation? In this paper, the properties of EPSPs are compared with the predictions of a model of geniculate excitation of simple cells. The model, which is derived from the suggestions of Hubel and Wiesel (1962), relies on the now familiar arrangement of the receptive fields of presynaptic geniculate cells into lines parallel to the axis of orientation of the postsynaptic cell. Several properties of the EPSPs observed in simple cells, and the orientation tuning of simple cells observed in extracellular experiments, can be accounted for without resorting to intracortical inhibition.