MODELING DIRECTION SELECTIVITY OF SIMPLE CELLS IN STRIATE VISUAL-CORTEX WITHIN THE FRAMEWORK OF THE CANONICAL MICROCIRCUIT

Nearly all models of direction selectivity (DS) in visual cortex are b ased on feedforward connection schemes, where geniculate input provide s all excitatory synaptic input to both pyramidal and inhibitory neuro ns. Feedforward inhibition then suppresses feedforward excitation for nonoptimal stimuli. Anatomically, however, the majority of asymmetric, excitatory, synaptic contacts onto cortical cells is provided by othe r cortical neurons, as embodied in the Canonical Microcircuit of Dougl as and Martin (1991). In this view, weak geniculate input is strongly amplified in the preferred direction by the action of intracortical ex citatory connections, while in the null direction inhibition reduces g eniculate-induced excitation. We investigate analytically and through biologically realistic computer simulations the functioning of a corti cal network based on massive excitatory, cortico-cortical feedback. Th e behavior of this network is compared to physiological data as well a s to the behavior of a purely feedforward model of DS based on nonlagg ed input. Our model explains a number of puzzling features of directio n selective simple cells, including the small somatic input conductanc e changes that have been measured experimentally during stimulation in the null direction, and the persistence of DS while fully blocking in hibition in a single cell. Although the operation at the heart of our network is amplification, the network passes the linearity test of (Ja gadeesh et al., 1993). We make specific predictions concerning the eff ect of selective blockade of cortical inhibition on the velocity-respo nse curve.