UNIT ANALYSIS OF VISUAL INPUT TO POSTERIOR LIMBIC CORTEX. I. PHOTIC STIMULATION

Abstract

This study" was undertaken to explore by means of microelectrode techniques the possibility of connections between the visual system and the limbic cortex located in the posterior cingulate gyrus, retrosplenial region, and posterior parts of the hippocampal gyrus and hippocampus. Experiments were performed on squirrel monkeys anesthetized with chloralose or sitting awake in a restraining chair. A brief intense flash of light evoked discharge from a large proportion of units in the retrosplenial region and in the posterior hippocampal gyrus and adjoining lingual cortex. Seventeen (26%) of 66 units in the latter location and 11 (22%) of 51 retrosplenial units responded with mean latencies, respectively, of 82 and 53 msec. The proportion of activated units for each area was statistically not significantly different from that of a sample population in the visual cortex. The units appeared to be modality-specific insofar as they were unaffected by somatic or auditory stimulation. Of 249 units tested during posterior cingulate exploration, only 5 (2%) were photically activated, and all were located near the cingulate sulcus. Two of these were also excited by somatic stimulation. Seventy-six hippocampal units were tested. Two located in the prosubiculum responded to a flash after a long latency; the others were unaffected. The findings establish the presence of visual connections with the limbic cortex located in the retrosplenial region and posterior hippocampal gyrus, which in turn are a source of afferents to the hippocampus. The range of latencies of the responding units is such as to allow both the possibility of direct relay from visual sub-cortical structures and indirect relay from the visual cortex. Experiments dealing with this question of hodology are reported in the subsequent paper.