Functional segregation of memory encoding and retrieval in the prefrontal cortex

The activation of the prefrontal cortex is a consistent finding in neuroimaging studies of memory encoding and retrieval. Some of the studies have shown that the left prefrontal cortex is involved in the encoding of information, whereas the right prefrontal cortex is involved in the retrieval, which suggests distinct non-overlapping cortical processing of encoding and retrieval in the prefrontal cortex. Others support the contrasting view that similar or overlapping regions of the prefrontal lobe are involved in encoding and retrieval processes. To date, however, there has been no consensus yet. We addressed this issue in an event-related fMRI study by measuring the brain activity in 36 normal subjects. In our study, the experimental materials were designed as the graphic tasks. During the stage of encoding, subjects were required to try to read and comprehend the meaning of the graph, and then in the stage of retrieval, they were asked to make judgments in regard to the content of reading comprehension. The direct comparison between encoding and retrieval tasks showed that encoding significantly more activated in right medial prefrontal gyrus (BA8/9/10) than retrieval; in contrast, retrieval significantly more activated in left/right middle prefrontal gyrus (BA6/10/46). Our findings indicate that distinct regions in prefrontal cortical areas support the functional segregation of memory encoding and retrieval, and suggest that the medial prefrontal cortex is more involved in the encoding of information, whereas the lateral prefrontal cortex is more involved in the retrieval.