The electrophysiological correlates of episodic encoding and retrieval

As proposed by Tulving (1972), episodic memory involves encoding and retrieving events embedded within a temporal context that includes, but is not limited to, the time, place, and emotional state. One way to study episodic memory in the laboratory is to have subjects learn lists of words and then recall them in any order. Such free recall paradigms have been studied for over 100 years (Müller & Pilzecker, 1900), giving rise to a firm understanding of the behavioral mechanisms underlying episodic memory encoding and retrieval. A major goal of cognitive neuroscience is to map these behavioral dynamics onto the brain. Intracranial EEG (iEEG) recordings in patients undergoing monitoring for treatment of epilepsy provide a unique opportunity to bridge the gap between non-invasive measures of brain activity, such as fMRI and scalp EEG, and the multielectrode recordings typically attained in animals. I report the findings from oscillatory analysis of iEEG recordings from 52 epilepsy patients while they performed a free recall task. During encoding, increases in gamma (28--100 Hz) power in the hippocampus, left prefrontal cortex, and left temporal lobe distinguished words that were subsequently remembered from those that were not. The same pattern of gamma activity that predicted successful memory formation at encoding reemerged at retrieval to distinguish correct from incorrect responses. In addition to the gamma power increases, greater theta (4--8 Hz) power in the right temporal and prefrontal cortex prior to the response also differentiated correct from incorrect recalls. These results suggest that the item representation, associated with the study-list context, reemerges at retrieval to distinguish events that actually occurred from events that did not. In a parallel study, we recorded scalp EEG from 35 non-clinical subjects as they performed the identical free recall task as the epilepsy patients. Similar to the iEEG results, increases in gamma power during encoding predicted subsequent recall, but only for early serial positions. Consequently, these results reveal an oscillatory correlate of the primacy effect, the increased probability of recalling items from early serial positions, and indicate that items in early serial positions receive more focused encoding than later items, where attention is divided among numerous list items