Pre- and post-training lesions of the intermediate medial hyperstriatum ventrale and passive avoidance learning in the chick.

Three distinct nuclei of the chick forebrain--the intermediate medial hyperstriatum ventrale (IMHV), lobus parolfactorius (LPO), and paleostriatum augmentatum (PA)--show metabolic, morphological, and neurophysiological changes following training on a passive avoidance task, suggesting that these and other areas of the chick forebrain participate in memory formation for this task. Considerable evidence exists for lateralization of memory processes in the chick. Several experiments examined the effects of lesions in the IMHV on the ability of chicks to learn and retain the avoidance task. Pre-training bilateral lesions in the IMHV produced an impairment in avoidance responding tested three hours after training. Pre-training unilateral lesions in the left but not the right IMHV resulted in a similar impairment. However, bilateral IMHV ablations, given either 1 or 6 h post-training, did not impair retention. IMHV lesions did not impair retention of a simple escape learning task. These results are consistent with other studies that have examined the effects of bilateral IMHV lesions on acquisition of passive avoidance and extend these findings by demonstrating lateralization of acquisition involving the left IMHV. The results also suggest that, as early as one hour post-training, the IMHV is not necessary to retain the memory and indicate that other forebrain structures, possibly the LPO or PA, may maintain the memory trace following training. Hypotheses to account for these results and indications of future research are discussed.