Spatial learning impairment parallels the magnitude of dorsal hippocampal lesions, but is hardly present following ventral lesions

TLDR

The hippocampus plays an essential role in spatial learning. The study compared the effects of variously sized and localized hippocampal aspiration lesions on spatial learning in a Morris water maze to determine whether the whole structure is equally important. Lesion volumes were measured for each aspiration lesion in the hippocampus. Dorsal hippocampal lesions caused greater spatial learning deficits than ventral lesions, with impairments correlating with lesion volume and a threshold of about 20 % of total hippocampal volume required to affect escape latencies; ventral lesions only impaired learning when nearly the entire ventral half was removed, and cortical damage alone had no effect.

Abstract

The hippocampus plays an essential role in spatial learning. To investigate whether the whole structure is equally important, we compared the effects of variously sized and localized hippocampal aspiration lesions on spatial learning in a Morris water maze. The volume of all hippocampal lesions was determined. Dorsal hippocampal lesions consistently impaired spatial learning more than equally large ventral lesions. The dorsal lesions had to be larger than 20% of the total hippocampal volume to prolong final escape latencies. The acquisition rate and precision on a probe test without platform were sensitive to even smaller dorsal lesions. The degree of impairment correlated with the lesion volume. In contrast, the lesions of the ventral half of the hippocampus spared both the rate and the precision of learning unless nearly all of the ventral half was removed. There was no significant effect of the location (dorsal or ventral) of damage to the overlying neocortex only. In conclusion, the dorsal half of the hippocampus appears more important for spatial learning than the ventral half. The spatial learning ability seems related to the amount of damaged dorsal hippocampal tissue, with a threshold at about 20% of the total hippocampal volume, under which normal learning can occur.