Dissociation of brain sites necessary for registration and storage of memory for a one-trial passive avoidance task in the chick.

Although memory formation occurs in a sequence of pharmacologically dissociable stages, many models assume that this sequence is located within a single neuronal ensemble. Three forebrain areas in the young chick-the intermediate medial hyperstriatum ventrale (IMHV), the lobus parolfactorius (LPO), and the paleostriatum augmentatum--have been identified as important structures in mediating learning and memory processes. It has been shown that pretraining lesions placed to the left IMHV produce amnesia for a 1-trial passive avoidance task. Posttraining lesions in the IMHV are not amnestic. The present study demonstrated that bilateral lesions to the LPO are amnestic but only if made subsequent to rather than before training. These results suggest that long-term storage of the memory is dependent on the LPO. Further experiments determined that in the absence of the LPO at training, the right IMHV can act a long-term memory storage site. In the absence of the right IMHV at training, other, as yet undetermined, areas can take up this role.