Expansion of Visual Receptive Fields in the Extrastriate Visual Cortex: Dependence on the Trajectory of a Moving Stimulus

Regularities of spatial expansion of receptive fields (RFs) of visually sensitive neurons upon application of moving visual images were investigated in the cat extrastriate visual cortex (area 21a). The RF size and their spatial infrastructure were first defined by presentation of stationary flashing stimuli, and horizontal and vertical axes (HA and VA, respectively) of classical RFs were determined. Then the lengths of the above axes were carefully measured by spatial scanning of the RFs with moving visual stimuli. It was found that dynamic expansion of the RF sizes was, as a rule, linked to the trajectory of a moving stimulus across the RF. Stimulus motion along the RF HA resulted in significant extension of this axis but not of the VA, while the its motion along the RF VA usually caused extension of only this axis, while another axis underwent negligible changes. These results demonstrate that spatial expansion of the RFs correlates mainly with the trajectory of stimulus motion across the RF. Such an effect probably results from excitation of the neuron under study by the influences coming from adjacent cortical neural networks. Thus, neural circuits localized outside the RF play a decisive role in modulation of the qualitative and quantitative characteristics of classical RFs, hence ensuring precise central processing of incoming visual information.