Effects of Feedback Projection From Cortical Area PMLS on Response Properties of Striate Neurons*

Abstract

In the nervous system,numerous feedforward and feedback circuits are formed by descending projections together with ascending inputs.It has been shown that the transmission and processing of neural signals are modulated by these extremely complicated pathways,but little is known about the functional roles of feedback projections in the cortex.Area posteromedial lateral suprasylvian(PMLS) in the extrastriate cortex of the cat was inactivated locally and reversibly by microinjection of inhibitory neurotransmitter GABA,and extracellular recording was performed to investigate the changes in visual response properties of neurons in the striate cortex.The PMLS inactivation resulted in an overall reduction in the responses of striate neurons to moving stimuli,accompanied by little change in the relative stability of responsiveness and a decline in the ratio of maximal firing rate to spontaneous activity level.In the meanwhile,the direction selectivity index decreased,but the change of orientation tuning strength was insignificant,the preferred direction kept unvaried except for a few bi-directional cells.Further analysis revealed that the neuronal responses were reduced in all the stimulus directions,but most largely in the preferred direction,which might serve as the key factor to weaken the direction selectivity.These findings indicate that the feedback projection from PMLS can enhance the direction selectivity of striate neurons but exert little effect on the orientation tuning.It appears that the feedback modulation is characterized by the important role of PMLS in visual motion processing in the cerebral cortex.