Hippocampal and retrosplenial cortex contributions to learning, memory, and executive function in the non-human primate

The first chapter of this work addresses conflicting reports in the literature as to whether damage limited to the hippocampal formation (HF) in the monkey is sufficient to produce a deficit on the Delayed Nonmatching to Sample task (DNMS), a putative test of learning and memory. While some groups report no behavioral deficit relative to intact controls when subjects are trained to criterion prior to surgical ablation of the HF and retested following surgery, others report significant impairments relative to controls when subjects initially learn and perform these tasks postoperatively. Findings from the present investigation, consistent with those of prior studies, reveal that bilateral lesions of the hippocampal formation are sufficient to produce an impairment in learning when task rules are acquired postoperatively. The present findings also show that regardless of whether task rules were learned before or after surgery bilateral lesions of the HF produce a memory impairment when delays as brief as 120 seconds separate stimulus presentations. The second chapter is concerned with the unexpected but not unprecedented finding that monkeys with bilateral lesions of the HF are also impaired at performing a card sorting task, a conventional task used to assess executive system function, a cognitive domain thought to be mediated by the prefrontal cortex.;Taken together with previous work from our laboratory and the results from the HF lesion studies above, we hypothesize that the hippocampal formation may be functionally interactive with the prefrontal cortex. The third chapter of this work describes our investigation into this possibility; in a separate group of monkeys, we bilaterally ablated the retrosplenial cortex (RSp), an area known to have bilateral connections with the dorsolateral prefrontal cortex as well as the entorhinal cortex and parahippocampal gyms. In the final part of this study, we found that bilateral lesions of the RSp impairs performance of tasks of learning, memory, and executive function in the monkey. These findings provide further evidence that the hippocampus and prefrontal cortex, by virtue of their reciprocal connections with the RSp, may be functionally interdependent. The final chapter of this dissertation provides an integrated discussion of these findings.