Orientation Sensitive Properties of Visually Driven Neurons in Extrastriate Area 21a of the Cat's Cortex

Orientation sensitive properties of extrastriate area 21a neurons were investigated. Special attention was paid to the qualitative characteristics of neuron responses to the different orientations of visual stimulus motion across neurons classical receptive field (CRF). The results of experiments have shown that a group of neurons (31%) in area 21a with specialized responses to moving visual stimuli changed their direction selective (DS) characteristics depending on the orientation of the stimulus movement. Some neurons reveal an abrupt drop of the direction sensitivity index (DI) to certain orientation (58%), and some show significant increase of DI at one of the applied orientations (22%). Detailed investigations of the non-directional neurons have revealed qualitative differences in the response to different orientations of the stimulus. There is no correlation between the functional organization of neurons RF's and the qualitative modulations of neuron's response patterns. We suggest that the orientation discrimination is a complex process that includes quantitative as well as qualitative transformations of neuron activity.