PET STUDIES OF MEMORY - NOVEL AND PRACTICED FREE-RECALL OF COMPLEX NARRATIVES .1.

Positron Emission Tomography (PET) with the tracer (H2O)-O-15 was used to measure regional cerebral blood flow in 13 healthy volunteers duri ng two experimental memory tasks, one of which was well-practiced and the other of which was novel. The materials used for the memory tasks consisted of two complex narratives (Story A and Story B from the Wech sler Memory Scale). Natural language materials were chosen because the y similate experimentally the natural learning situation and permit st udy of the neural mechanisms by which recall memory becomes more fluid , automatic, or ''rote.'' One week before the PET study, subjects were trained to perfect recall of Story A, while they were exposed to Stor y B only 60 s prior to PET data acquisition. Despite the substantial d ifferences in level of familiarity (and in free recall performance), p atterns of activation were quite similar; activations presumed to refl ect recall in both tasks included frontal, inferior temporal, thalamic , anterior cingulate, and cerebellar regions, Many regions were smalle r during recall of the familiar story, however, presumably reflecting greater neural efficiency due to practice. In addition, the novel task activated an additional left frontal region that is presumed to refle ct more active encoding. The similarity and multiplicity of the activa tions in the two tasks suggest that the brain uses a multinodal genera l network for memory tasks such as free recall, while the differences suggest that some nodes in the network may be used for specific compon ents of memory such as encoding and retrieval. (C) 1995 Academic Press , Inc.