REMEMBERING THE PAST - 2 FACETS OF EPISODIC MEMORY EXPLORED WITH POSITRON EMISSION TOMOGRAPHY

Objective: This study used positron emission tomography to examine two kinds of personal memory that are used in psychiatric evaluation: foc used episodic memory (recall of past experience, employed in ''taking a history'') and random episodic memory (uncensored thinking about exp erience, examined during analytic therapy using free association). For comparison, a third memory task was used to tap impersonal memory tha t represents general information about the world (''semantic memory'') . Method: Thirteen subjects were studied using the [O-15]H2O method to obtain quantitative measurements of cerebral blood flow. The three co nditions were subtracted and their relative relationships examined. Re sults: The random episodic condition produced activations in widely di stributed association cortex (right and left frontal, parietal, angula r/supramarginal, and posterior inferior temporal regions). Focused epi sodic memory engaged a network that included the medial inferior front al region, precuneus/retrosplenial cingulate, anterior cingulate, thal amus, and cerebellum. The use of medial frontal regions and the precun eus/retrosplenial cingulate was common to both focused and random epis odic memory. The major difference between semantic and episodic memory was activation of Broca's area and the left frontal operculum by sema ntic memory. Conclusions: These results indicate that free-ranging men tal activity (random episodic memory) produces large activations in as sociation cortex and may reflect both active retrieval of past experie nces and planning of future experiences. Focused episodic memory share s some components of this circuit (inferior frontal and precuneus), wh ich may reflect the time-linked components of both aspects of episodic memory, and which permit human beings to experience personal identity , consciousness, and self-awareness.