Neural correlates of semantic and episodic memory retrieval

To investigate the functional neuroanatomy associated with retrieving seman tic and episodic memories, we measured changes in regional cerebral blood f low (rCBF) with positron emission tomography (PET) while subjects generated single word responses to achromatic line drawings of objects. During separ ate scans, subjects either named each object, retrieved a commonly associat ed color of each object (semantic condition), or recalled a previously stud ied uncommon color of each object (episodic condition). Subjects were also scanned while staring at Visual noise patterns to provide a low level perce ptual baseline. Relative to the low level baseline, all three conditions re vealed bilateral activations of posterior regions of the temporal lobes, ce rebellum, and left lateralized activations in frontal regions. Retrieving s emantic information, as compared to object naming, activated left inferior temporal, left superior parietal, and left frontal cortices. In addition, s mall regions of right frontal cortex were activated. Retrieving episodic in formation, as compared to object naming, activated bilateral medial parieta l cortex, bilateral retrosplenial cortex, right frontal cortex, thalamus, a nd cerebellum. Direct comparison of the semantic and episodic conditions re vealed bilateral activation in temporal and frontal lobes in the semantic t ask (left greater than right), and activation in medial parietal cortex, re trosplenial cortex, thalamus, and cerebellum (but not right frontal regions ) in the episodic task. These results support the assertion that distinct n eural structures mediate semantic and episodic memory retrieval. However, t hey also raise questions regarding the specific roles of left temporal and right frontal cortices during episodic memory retrieval, in particular. (C) 1998 Elsevier Science Ltd. All rights reserved.