RIGHT PREFRONTAL ACTIVATION DURING ENCODING, BUT NOT DURING RETRIEVAL, IN A NONVERBAL PAIRED-ASSOCIATES TASK

Brain imaging studies have shown that episodic encoding into long-term memory preferentially activates the left prefrontal cortex and retrie val activates the right prefrontal cortex. However, it is unclear to w hat degree verbal analysis contributes to the left prefrontal activati on during encoding. The present study was designed to avoid verbal ana lysis during encoding by using abstract pictures and computer-generate d sounds which were difficult to code verbally. Sounds and pictures we re grouped into six stimulus-stimulus pairs. When the sound from a pai r was presented, the subjects were instructed to recall and visualize the associated picture. After 2.0 s the associated picture and another picture appeared on the screen and the subjects were required to iden tify the associated picture. Feedback about the choice was then given. Regional cerebral blood flow (rCBF) was measured with [O-15]butanol a nd positron emission tomography (PET) in 10 subjects during initial tr aining on the paired-associates task (encoding scan) and after 35 min of training (retrieval scan). Performance during the encoding scan was 59% correct and during the retrieval scan 98% correct, with a mean re action time of 709 ms during retrieval. The rCBF was also measured dur ing a control condition without any instruction to encode or retrieve. Compared with retrieval, encoding showed significant activation of th e posterior part of the right middle frontal gyrus, the right inferior parietal cortex, the cingulate cortex, the left inferior parietal cor tex and the left inferior and middle temporal gyri. The rCBF increase during encoding was strongly correlated with the rate of encoding. Ret rieval was compared with both encoding and control. In none of these c omparisons was there any prefrontal activation. The lack of prefrontal activation during near-perfect performance of the retrieval task sugg ests that the prefrontal cortex is not necessarily active when retriev al is fast and accurate, or what might be called automatic. Encoding w as not associated with more activation of the left than the right pref rontal cortex. This result presents a limitation to the generality of left prefrontal activation during episodic encoding, which has been fo und in several previous brain imaging studies. Differences between stu dies in the relative activation of left and right prefrontal cortex du ring encoding and retrieval might he due to differences in paradigms, the type of stimulus used, and the demand for working memory and verba l analysis.