Remembering the past: two facets of episodic memory explored with positron emission tomography

TLDR

The study used PET to compare focused episodic memory (history taking) and random episodic memory (free association) in psychiatric evaluation. Using 15O H₂O PET, 13 subjects performed focused episodic, random episodic, and semantic memory tasks, and the resulting cerebral blood flow maps were compared by subtracting conditions. Random episodic memory activated widespread association cortex, while focused episodic memory engaged medial inferior frontal, precuneus/retrosplenial cingulate, anterior cingulate, thalamus, and cerebellum; both recruited medial frontal and precuneus, whereas semantic memory uniquely activated Broca’s area and left frontal operculum, suggesting random episodic activity reflects retrieval and future planning and focused episodic activity supports personal identity and self‑awareness.

Abstract

This study used positron emission tomography to examine two kinds of personal memory that are used in psychiatric evaluation: focused episodic memory (recall of past experience, employed in "taking a history") and random episodic memory (uncensored thinking about experience, examined during analytic therapy using free association). For comparison, a third memory task was used to tap impersonal memory that represents general information about the world ("semantic memory").Thirteen subjects were studied using the [15O]H2O method to obtain quantitative measurements of cerebral blood flow. The three conditions were subtracted and their relative relationships examined.The random episodic condition produced activations in widely distributed association cortex (right and left frontal, parietal, angular/supramarginal, and posterior inferior temporal regions). Focused episodic memory engaged a network that included the medial inferior frontal regions, precuneus/retrosplenial cingulate, anterior cingulate, thalamus, and cerebellum. The use of medial frontal regions and the precuneus/retrosplenial cingulate was common to both focused and random episodic memory. The major difference between semantic and episodic memory was activation of Broca's area and the left frontal operculum by semantic memory.These results indicate that free-ranging mental activity (random episodic memory) produces large activations in association cortex and may reflect both active retrieval of past experiences and planning of future experiences. Focused episodic memory shares some components of this circuit (inferior frontal and precuneus), which 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.