Dissociating context and space within the hippocampus: Effects of complete, dorsal, and ventral excitotoxic hippocampal lesions on conditioned freezing and spatial learning

Rats with complete excitotoxic hippocampal lesions or selective damage to t he dorsal or ventral hippocampus were compared with controls on measures of contextually conditioned freezing in a signaled shock procedure and on a s patial water-maze task. Complete and ventral lesions produced equivalent, s ignificant anterograde deficits in conditioned freezing relative to both do rsal lesions and controls. Complete hippocampal lesions impaired water-maze performance; in contrast, ventral lesions improved performance relative to the dorsal group, which was itself unexpectedly unimpaired relative to con trols. Thus, the partial lesion effects seen in the 2 tasks never resembled each other. Anterograde impairments in contextual freezing and spatial lea rning do not share a common underlying neural basis; complete and ventral l esions may induce anterograde contextual freezing impairments by enhancing locomotor activity under conditions of mild stress.