Medial temporal and prefrontal contributions to working memory tasks with novel and familiar stimuli

Lesions of parahippocampal structures impair performance of delayed matching tasks in nonhuman primates, suggesting a role for these structures in the maintenance of items in working memory and short-term stimulus matching. However, most human functional imaging studies have not shown medial temporal activation during working memory tasks and have primarily focused on functional magnetic resonance imaging (fMRI) signal intensity changes in the prefrontal and posterior parietal cortex. The goal of this study was to test the hypothesis that the difference between the human and nonhuman primate data results from the use of highly familiar stimuli in human working memory studies and trial-unique stimuli in nonhuman primate studies. We used fMRI to examine prefrontal and temporal lobe activation during performance of a working memory (two-back) task, using blocks of novel and highly familiar complex pictures. Performance of the working memory task with novel complex pictures resulted in greater signal change within medial temporal lobe structures than performance of the task with familiar complex pictures. In contrast, the working memory task with highly familiar stimuli resulted in greater prefrontal activation. These results are consistent without hypothesis that the medial temporal lobe is recruited for the short-term maintenance of information that has no prior representation in the brain, whereas the prefrontal cortex is important for monitoring familiar stimuli that have a high degree of interference. A second set of tasks examined stimulus matching. Subjects performed a target-matching task, during which they identified a single target presented in blocks of novel or familiar stimuli. The results provide evidence of hippocampal and parahippocampal recruitment in the target-matching task with familiar stimuli. These results are consistent with prior animal studies and suggest that prefrontal regions may be important for the monitoring and matching of familiar stimuli which have a high potential for interference, whereas medial temporal regions may become proportionally more important for matching and maintenance of novel stimuli.