Prefrontal cortex and medial temporal lobe interactions in memory and executive function in the rhesus monkey

Previous studies have shown that the prefrontal cortex and hippocampal formation each contribute to different aspects of learning and memory function in the primate brain. But it is not known whether they interact in performing these functions. To address this, a disconnection paradigm was developed using the rhesus monkey as an experimental model. To test a possible interaction a unilateral lesion of the dorsolateral prefrontal cortex was made in the left hemisphere, a hippocampal lesion in the contralateral right hemisphere and the corpus callosum and anterior commissure were transected to effectively "disconnect" the remaining intact prefrontal cortex and hippocampus. To control for overall brain damage, another group of animals was prepared with unilateral hippocampal formation and unilateral dorsolateral prefrontal cortex lesions in the left hemisphere along with the commissural split. This matched the total damage in the experimental group (CFHS), but left the hippocampal formation and prefrontal cortex interconnected within the intact hemisphere (IFHS).; The Delayed Non-Match to Sample (DNMS) task and Delayed Recognition Span (DRST) task were administered to test rule learning and recognition memory while the Concurrent Object Discrimination Task (COD) was administered to assess discrimination learning. The CFHS animals were significantly impaired relative to animals in the IFHS and unoperated control (UC) groups on the acquisition (learning) and delay (memory) components of the DNMS task and on the spatial and object conditions of the DRST (short term memory capacity). In contrast, there were no significant differences between the groups on the COD task. In addition, animals were also tested on automated touch screen tests of Attention, Three-Choice Discrimination (TCD) and the Conceptual Set-Shifting Task (CSST). There were no significant differences among groups on these three tasks.; These results provide the first evidence that communication between prefrontal cortex and hippocampal formation is essential for efficient performance on rule learning and recognition memory, but is not essential to subserve associative learning or executive function.; The anatomical pathways that may underlie this prefrontal cortex and hippocampal formation interaction was also assessed using retrograde tract-tracing techniques.