Asymmetric frontal activation during episodic memory: the effects of stimulus type on encoding and retrieval

Recent functional neuroimaging studies have suggested that the left prefron tal cortex is preferentially involved in the encoding of episodic memory wh ilst the right prefrontal cortex is preferentially involved in the retrieva l of episodic memory, irrespective of the type (e.g. modality) of informati on being remembered. In the present PET activation study, a 2 x 2 design wa s employed to investigate the relationship between encoding and retrieval o f verbal and non-verbal material in episodic memory. Accordingly, seven hea lthy volunteers were scanned whilst encoding and then recalling stimuli whi ch either emphasised visual or verbal processes. When encoding and retrieva l tasks were compared directly, significantly greater prefrontal activation was observed in the encoding conditions, regardless of modality, although these changes were bilaterally distributed. In contrast when the verbal and visual memory tasks were compared directly, the former was associated with rCBF changes that were predominantly located in the left lateral frontal c ortex whilst the latter was associated with rCBF changes that were predomin antly located in the right lateral frontal cortex. These results suggest th at encoding and retrieval may actually involve similar regions of the later al prefrontal cortex when all factors relating to the type of stimulus mate rial (i.e. modality), are appropriately controlled. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.