Functional anatomical studies of explicit and implicit memory retrieval tasks

Across three experiments, PET scans were obtained while subjects performed different word-stem completion and FIXATION control tasks designed to study the functional anatomy of memory retrieval. During each of three different word-stem completion scans, word-stem cues were visually presented in uppercase letters. The RECALL task required explicit retrieval of study words presented prior to the PET scan. The PRIMING task addressed the implicit effects of the prior study words without requiring intentional recall. The BASELINE task encouraged retrieval of information from a general knowledge store. Across experiments, the similarity between study words and word stems was manipulated by presenting prescan study words in either uppercase letters identical to the stems, in lowercase letters, or auditorily. The PRIMING task was not studied with auditory presentation. Many activations were consistent across experiments. The BASELINE task activated several regions in response to the reading and verbal-response demands of the task (visual, motor, and premotor cortices, cerebellum), as well as a left prefrontal region. The RECALL task additionally activated regions in anterior right prefrontal cortex. Bilateral occipitotemporal regions showed blood flow reductions during the PRIMING task as compared to the BASELINE task. Activation in the right hippocampal/parahippocampal region was observed only in one experiment, and no experiment showed activation in the left medial temporal lobe. These experiments suggest that areas of frontal cortex play a role in explicit recall and that an effect of priming may be to require less activation of perceptual regions for the processing of recently presented information.