Brain activity associated with episodic memory : similarities and differences between encoding and retrieval

Sammanfattning: Understanding the mnemonic functions of the brain has been extensively facilitated by the development of functional neuroimaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). The present thesis aims at investigating the neural mechanisms underlying memory for personally experienced events (episodic memory), using PET. In paper I, similarities between encoding and retrieval of enacted (motor) information were explored. We observed increased retrieval activation in right premotor areas in the brain when sentences encoded by motor enactment and sentences encoded by maintenance rehearsal were contrasted. In paper II, overlap between encoding and retrieval was explicitly tested for three types of event information: spatial, item, and temporal. Using conjunction analyses, we found that encoding and retrieval of spatial information was associated with increased brain activity in bilateral inferior parietal regions. Encoding and retrieval of item information were related to increased activation in right inferior temporal cortex, and encoding and retrieval of temporal information were associated with increased activation in left inferior temporal and left inferior frontal cortex. In paper III, brain activity associated with retrieval success was examined. Conditions included three levels of retrieval success (high, medium, and low level), for two types of information (pictures and sentences). The results showed a pattern of activation that distinguished between brain regions involved in processing of sentences vs. processing of pictures. A second pattern that distinguished between brain regions involved in encoding vs. retrieval processes, irrespectively of material (sentences and pictures) and retrieval success, was also found. The manipulation of retrieval success was associated with systematic changes in the correlation between material specific regions and other areas of the brain. In study IV, changes in activation related to successful retrieval of pictures were investigated. More specifically, we expected to find decreases in infero-temporal (IT) regions of the brain that were associated with successful recognition memory. As expected, we found a region in left IT cortex that showed decreased activation related to memory for event information. This decrease in activation could be dissociated from responses related to novelty detection, and perceptual priming. The results from study I and II are discussed in relation to findings and theories regarding similarities between encoding and retrieval processes, and reactivation of modality-specific brain areas important for memory storage. The results from studies III and IV are discussed in relation to differences between encoding and retrieval processes, e.g. asymmetric frontal activation and sub-processes of episodic memory, such as retrieval mode, retrieval success, and novelty detection. Taken together, the studies show that different episodic memory processes are correlated with distinct brain areas, hence supporting the view that remembering is based on multiple component processes.