SPATIAL-LEARNING WITH A MINISLAB IN THE DORSAL HIPPOCAMPUS

We have determined the volume and location of hippocampal tissue requi red for normal acquisition of a spatial memory task. Ibotenic acid was used to make bilateral symmetric lesions of 20-100% of hippocampal vo lume. Even a small transverse block (minislab) of the hippocampus (dow n to 26% of the total) could support spatial learning in a water maze, provided it was at the septal (dorsal) pole of the hippocampus. Lesio ns of the septal pole, leaving 60% of the hippocampi intact, caused a learning deficit, although normal electrophysiological responses, syna ptic plasticity, and preserved acetylcholinesterase staining argue for adequate function of the remaining tissue. Thus, with an otherwise no rmal brain, hippocampal-dependent spatial learning only requires a min islab of dorsal hippocampal tissue.