The functional roles of prefrontal cortex in episodic memory - I. Encoding

Functional neuroimaging studies of episodic memory consistently report an association between memory encoding operations and left prefrontal cortex (PFC) activation. Encoding-related activation has been described in dorsolateral, ventrolateral and anterior prefrontal regions. We tested the hypothesis that a specific component of this left PFC activation reflects organizational processes necessary for optimal memory encoding. Subjects underwent PET scans while learning auditorily presented word lists under dual task conditions. The degree to which they were required to organize word lists semantically was systematically varied across three experimental conditions. A task in which words were already organized produced the least degree of left PFC activity whereas a task requiring subjects to generate an organizational structure was associated with maximal activity in this region. This activation was localized to a region just above the inferior frontal sulcus. The functional specificity of this increased activity for organizational processes was tested using a concurrent distracting task known to disrupt these processes. Distraction resulted in a significant attenuation of this activation during the task emphasizing organizational processes but not other encoding tasks. In contrast, the distraction task resulted in reduced activity in a more ventral/anterior PFC region expressed equally for all memory tasks. The findings indicate that a key function of left dorsolateral PFC at encoding relates specifically to the use of executive processes necessary for the creation of an organizational structure. Activity in more ventral and anterior left PFC regions would appear to reflect a less specific component of episodic memory encoding.