PET ACTIVATION OF THE MEDIAL TEMPORAL-LOBE IN LEARNING

Regional cerebral blood flow was examined during multiple-trial learni ng in healthy volunteers. On the basis that incremental learning from trial to trial is severely impaired in neuropsychological studies of p atients with medial temporal lesions, we predicted that medial tempora l activation might be particularly associated with incremental gains i n learning, On the other hand, we predicted that frontal activations w ould not show any increase during incremental learning, and might even diminish. PET recordings were undertaken while subjects were presente d visually with a 15-word list in one of three conditions: a list in w hich a single word was repeated 15 times (S), a list of novel words (N ), and a list which was repeated from before (R), We demonstrated that statistically significant incremental learning did occur when word li sts were repeated in (R) trials. The subtraction of novel minus repeat ed conditions (N - R) was associated with left medial temporal as well as left prefrontal activations, whereas the opposite (R - N) subtract ion gave rise to right prefrontal and precuneal activations. In partic ular, incremental learning during the repeated trials (R) identified a left medial temporal activation, as predicted, but the left frontal a ctivation was no longer evident. We suggest that the left medial tempo ral region is not only activated by novel, to-be-learned stimuli, but it also contributes to incremental teaming as part of a network involv ed in 'binding' or 'consolidating' new memories. The right frontal and precuneal regions, which participate in the repeated retrieval and re hearsal of already learned memories, are also involved in this network . The left frontal region is implicated in the more 'effortful' or ela borative aspects of memory.