Effects Of Dorsal Hippocampal Lesions Research Articles

Medial Septal Lesions Mimic Effects of Both Selective Dorsal and Ventral Hippocampal Lesions.

Electrolytic medial septal (MS) lesions, which depleted acetylcholinesterase staining in both dorsal and ventral hippocampus, produced a constellation of behaviors, combining aspects of both selective dorsal and ventral hippocampal lesion effects. MS lesions impaired spatial working memory on the T maze, thus resembling the effects of dorsal hippocampal lesions. In addition, MS lesions reduced anxiety during successive alleys (a modified form of the elevated plus-maze), social interaction, and hyponeophagia tests. MS lesions also reduced postshock freezing. These effects more closely resemble those of ventral hippocampal lesions. Therefore, the effects of electrolytic MS lesions derive from the resulting combined deafferentation of dorsal and ventral hippocampal regions, suggesting that previously reported effects of cytotoxic dorsal hippocampal lesions are unlikely to be due to a demyelination of fibers of passage coursing through the septal pole.

Retention and acquisition of classical trace conditioned responses by rabbits with hippocampal lesions.

The effects of dorsal hippocampal lesions on retention of classical trace conditioned responses were examined using the rabbit nictitating membrane preparation. Animals were trained to criteria and then lesioned either in the cortex or in the hippocampus and the cortex. Hippocampal damage had no effect on the retention of responses but produced significantly longer onset latencies. A control group of hippocampal animals acquired conditioned responses (CRs) at least as quickly as the prelesion subjects, and they also exhibited longer response onset latency. A second experiment evaluated the performance of hippocampal lesioned animals in classical trace conditioning with either a low-intensity periorbital shock or corneal air puff as the unconditioned stimulus (UCS). Hippocampal animals successfully acquired CRs under both conditions but exhibited an alteration of response onset which was dependent on the form of the UCS. Hippocampal animals displayed shorter response onset in the air-puff condition and longer response onset in the shock condition. Cortical animals timed responses consistently regardless of the UCS. These findings strongly suggest that the hippocampus modulates temporal characteristics of learned behavior.

The effects of dorsal hippocampal lesions on reinforcement shifts

This experiment attempted to clarify the effects of dorsal hippocampal lesions on a lick-rate response to reinforcement shifts. Rats received preshift solutions of either 4% or 32% sucrose and were then shifted to the opposite concentration. The results indicated (1) that Ss shifted to the increased concentration quickly adjusted to the control levels, and (2) a negative contrast effect (NCE) for both hippocampal and sham downshift groups. These results are in contrast to earlier failures to find a NCE in hippocampal Ss (Franchina & Brown, 1971; Murphy & Brown, 1970). It was proposed that the immediate feedback inherent in the licking response resulted in the standard contrast effect not found when longer ITIs are introduced.