COMPARISON OF SPATIAL FIRING CHARACTERISTICS OF UNITS IN DORSAL AND VENTRAL HIPPOCAMPUS OF THE RAT

The septal and temporal poles of the hippocampus differ markedly in th eir anatomical and neurochemical organization. Although it is well est ablished that the internal representation of space is a fundamental fu nction of hippocampal neurons, most of what is known about spatial cod ing in the hippocampus of freely moving animals has come from recordin gs from the dorsal one-third (largely for technical convenience). The present study therefore compared the spatial selectivity of CA 1 neuro ns in the dorsal and ventral hippocampi of rats during performance of a food reinforced, random search task in a square chamber containing s imple visual landmarks. Neural activity was recorded in the dorsal and ventral hippocampi of opposite hemispheres in the same rats, in many cases simultaneously. As in dorsal hippocampus, ventral CA 1 units cou ld be classified as ''complex spike'' (pyramidal) cells or ''theta'' i nterneurons. Both dorsal and ventral theta cells fired at relatively h igh rates and with low spatial selectivity in the apparatus. Of the po pulation of complex spike cells in the ventral hippocampus, a signific antly smaller number had ''place fields'' than in the dorsal hippocamp us, and the average spatial selectivity was of significantly lower res olution than that found among dorsal hippocampal complex spike cells. Thus, a septotemporal difference of spatial selectivity was found in t he CA 1 field of the rat hippocampus, complementing many other anatomi cal and neuropharmacological studies. A number of possible functional interpretations can be suggested from these results, including a compu tational advantage of representing space at different scales or a pree minence of essentially nonspatial information processing in the ventra l hippocampus.