Functional neuroimaging studies of long term memory in humans
This thesis explores contributions of prefrontal cortex (PFC) to memory using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). I begin by considering the cognitive neuroscience of memory processes and the impact that functional neuroimaging may have upon this. I then describe a series of PET and fMRI experiments concerned, primarily, with dissociating frontal contributions to encoding and retrieval processes. These initial studies show that left PFC activation predominates at encoding and right PFC activation at retrieval. Four further studies of left prefrontal activation at encoding are presented. Together, they show that left PFC is sensitive to tasks that require the organisation of encoded material according to its semantic attributes and that a more dorsal region of lateral PFC may specifically reflect the requirement to select from amongst semantic attributes in order to meet specific demands of the tasks. This region, and the behavioural performance associated with it, is shown to be sensitive to interference produced both by competing semantic attributes and by a simultaneously performed, distracting motor task. The two experiments on memory retrieval that are presented here provide evidence for distinctive roles of right dorsolateral and ventrolateral PFC during retrieval of verbal material. The ventrolateral region appears to reflect the changing specification of search parameters that occurs at the outset of a memory search and the dorsolateral PFC activation pattern is consistent with a role in monitoring and verification processes optimising the retrieval process. In conclusion, I review the broader literature on neuroimaging of memory-related frontal cortical function. While there are a number of inconsistencies, I suggest that the results presented here fit into an emerging pattern indicating the importance of PFC in memory encoding and retrieval and the distinctive roles of dorsolateral and ventrolateral regions within and between these memory stages.