Imaging visual recognition memory network by PET in the baboon: perirhinal cortex heterogeneity and plasticity after perirhinal lesion.

We recently mapped the visual recognition memory network in the behaving baboon using a positron emission tomography (PET) activation paradigm with 18F-fluoro-deoxyglucose during a visual delayed matching-to-sample task. This study confirmed the key role of the perirhinal cortex and documented an unexpected left-sided advantage. Specific contribution of each subdivision of the perirhinal cortex has, however, never been investigated. Furthermore, although alteration to the perirhinal cortex has been implicated in several brain disorders, putative plasticity within the entire brain network after perirhinal damage remains largely unknown. To confirm our previous data and to investigate these latter issues, we used our PET activation paradigm on a second healthy baboon before and after 16 months after bilateral excitotoxic lesions of the perirhinal cortex. Activation common to our two healthy baboons occurred only in the left rostroventral perirhinal cortex (i.e., areas 36pm and rostral 36r) and insular cortex. Although histologic analysis disclosed that the perirhinal lesions achieved in the present baboon were essentially caudal to this preoperatively activated area, memory performance was severely impaired. Concomitant with this long-lasting cognitive deficit, changes in the neural network implicated in the task were observed, involving disappearance of the preoperative activations and appearance of a significant activation of the frontal and occipital cortices. However, different activation patterns were found in the first and last eight postoperative months. These findings highlight the functional heterogeneity of the perirhinal cortex and evidence progressive plasticity after perirhinal cortex damage.