CORTICOFUGAL FEEDBACK CAN REDUCE THE VISUAL LATENCY OF RESPONSES TO ANTAGONISTIC STIMULI

A biophysically realistical model of the primary visual pathway is des igned, including feedback connections from the visual cortex to the la teral geniculate nucleus (LGN) - the so-called corticofugal pathway. T he model comprises up to 10 000 retina and LGN cells divided into the ON and the OFF pathway according to their contrast response characteri stics. An additional 6000 cortical simple cells are modeled. Apart fro m the direct excitatory afferent pathway we include strong mutual inhi bition between the ON and the OFF subsystems. In addition, we propose a novel type of paradoxical corticofugal connection pattern which link s ON dominated cortical simple cells to OFF-center LGN cells and vice versa. In accordance with physiological findings these connections are weakly excitatory and do not interfere with the steady-state response s to constant illumination, because during the steady-state inhibition arising from the active pathway effectively silences the non-stimulat ed pathway. At the moment of a contrast reversal the effect of the par adoxical connection pattern comes into play and the depolarization of the previously silent channel is accelerated, leading to a latency red uction of up to 4 ms using moderate synaptic weights. With increased w eights reductions of more than 10 ms can be achieved. We introduce dif ferent synaptic characteristics for the feedback (AMPA, NMDA, AMPA + N MDA) and show that the strongest latency reduction is obtained for a c ombination of the membrane channels (i.e., AMPA + NMDA). The effect of the proposed paradoxical connection pattern is self-regulating; becau se the levels of inhibition and paradoxical excitation are always driv en by the same inputs (strong inhibition is counterbalanced by a stron ger paradoxical excitation and vice versa). In addition, the latency r eduction for a contrast inversion which ends at a small absolute contr ast level (small contrast step) is stronger than the reduction for an inversion with large final contrast (large contrast step). This leads to a more pronounced reduction in the reaction times for weak stimuli. Thus, reaction time differences for different contrast steps are smoo thed out.