Remembering the past: two 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: 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").


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
D000328:Adult; D001364:Awareness / Q000502:physiology; D001921:Brain / Q000000981:diagnostic imaging* / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D003243:Consciousness / Q000502:physiology; D004532:Ego; D005260:Female; D005608:Free Association; D005625:Frontal Lobe / Q000000981:diagnostic imaging / Q000502:physiology; D006801:Humans; D008297:Male; D008568:Memory / Q000502:physiology*; D011939:Mental Recall / Q000502:physiology; D010104:Oxygen Radioisotopes; D013850:Thinking / Q000502:physiology; D014055:Tomography, Emission-Computed; D014867:Water
D010104:Oxygen Radioisotopes; D014867:Water