FUNCTIONAL-ANATOMIC STUDY OF EPISODIC RETRIEVAL II - SELECTIVE AVERAGING OF EVENT-RELATED FMRI TRIALS TO TEST THE RETRIEVAL SUCCESS HYPOTHESIS

In a companion paper (R. L. Buckner et al., 1998, NeuroImage 7, 151-16 2) we used fMRI to identify brain areas activated by episodic memory r etrieval. Prefrontal areas were shown to differentiate component proce sses related to retrieval success and retrieval effort in block-design ed paradigms. Importantly, a right anterior prefrontal area was most a ctive during task blocks involving greatest retrieval success, consist ent with an earlier PET study by M. D. Rugg ef al. (1996, Brain 119, 2 073-2083). However, manipulation of these variables within the context of blocked trials confounds differences related to varying levels of retrieval success with potential shifts in subjects' strategies due to changes in the probability of target events across blocks, To test mo re rigorously the hypothesis that certain areas are directly related t o retrieval success, we adopted recently developed procedures for even t-related fMRI. Fourteen subjects studied words under deep encoding an d were then tested in a mixed trial paradigm where old and new words w ere randomly presented. This recognition testing procedure activated s imilar areas to the blocked trial paradigm, with all areas showing sim ilar levels of activation across old and new items. Of critical import ance, significant activation was detected in right anterior prefrontal cortex for new items when subjects correctly indicated they were new (correct rejections). These findings go against the retrieval success hypothesis as formally proposed and provide an important constraint fo r interpretation of this region's role in episodic retrieval. Furtherm ore, anterior prefrontal activation was found to occur late, relative to other brain areas, suggesting that it may be involved in retrieval verification or monitoring processes or perhaps even in anticipation o f subsequent trial events (although an alternative possibility, that t he late onset is mediated by a late vascular response, cannot be ruled out). These findings and their relation to the results obtained in th e companion blocked-trial paradigm are discussed, a 1998 Academic Pres s.