Cortical responses to local electrical stimulation of retina.

Abstract Extensive cortical excitation from punctiform photic stimuli has been difficult to interpret because of intraoptic diffusion of light. Direct electrical stimulation of the exposed retina was thus used in this attempt to determine the degree of spatial elaboration of stimuli in the retinocortical system. Multichannel CRO recording from cortex and optic tract show two types of response in anesthetized or “midpontine” animals. The “early” response is identical in form to potentials elicited by optic tract stimuli, but has a latency of 2 to 5.3 msec, depending upon distance of intraretinal conduction. Constancy of timing in the cortical spike sequence despite varying arrival latencies shows that this well-known series of spikes is not generated as a consequence of conduction velocity differences in optic nerve fibers. Early potentials can be highly restricted in cortical location and, save for their predominance in the lateral dorsal region of marginal gyrus, generally conform in distribution to Talbot and Marshall's scheme of retinocortical topographical relations. “Late” responses are complex, but most are similar to photically elicited responses in form and latency. Their threshold is usually lower than that for early responses. Their cortical distribution for a given retinal locus greatly exceeds that of the early response, sometimes including ipsilateral cortex even to stimulation of nasal retina.Since optic tract potentials continue for over 100 msec after the 1-msec retinal pulse, it is possible that some of the spatial elaboration of the late response occurs in the retina.