Humbert Suarez, Christof Koch, Rodney Douglas
Almost all models of orientation and direction selectivity in visual cortex are based on feedforward connection schemes, where genicu(cid:173) late input provides all excitation to both pyramidal and inhibitory neurons. The latter neurons then suppress the response of the for(cid:173) mer for non-optimal stimuli. However, anatomical studies show that up to 90 % of the excitatory synaptic input onto any corti(cid:173) cal cell is provided by other cortical cells. The massive excitatory feedback nature of cortical circuits is embedded in the canonical microcircuit of Douglas &. Martin (1991). We here investigate ana(cid:173) lytically and through biologically realistic simulations the function(cid:173) ing of a detailed model of this circuitry, operating in a hysteretic mode. In the model, weak geniculate input is dramatically ampli(cid:173) fied by intracortical excitation, while inhibition has a dual role: (i) to prevent the early geniculate-induced excitation in the null di(cid:173) rection and (ii) to restrain excitation and ensure that the neurons fire only when the stimulus is in their receptive-field. Among the
Humbert Suarez, Christo! Koch, Rodney Douglas
insights gained are the possibility that hysteresis underlies visual cortical function, paralleling proposals for short-term memory, and strong limitations on linearity tests that use gratings. Properties of visual cortical neurons are compared in detail to this model and to a classical model of direction selectivity that does not include excitatory corti co-cortical connections. The model explain a num(cid:173) ber of puzzling features of direction-selective simple cells, includ(cid:173) ing the small somatic input conductance changes that have been measured experimentally during stimulation in the null direction. The model also allows us to understand why the velocity-response curve of area 17 neurons is different from that of their LG N affer(cid:173) ents, and the origin of expansive and compressive nonlinearities in the contrast-response curve of striate cortical neurons.