DISSOCIATING VERBAL AND SPATIAL WORKING-MEMORY USING PET

Three experiments used position emission tomography (PET) to study the neural basis of human working memory, These studies ask whether diffe rent neural circuits underly verbal and spatial memory. In Experiment 1, subjects had to retain for 3 sec. either the names of four letters (verbal memory) or the positions of three dots (spatial memory). The P ET results manifested a clear cut double dissociation, as the verbal t ask activated primarily left-hemisphere regions whereas the spatial ta sk activated only right-hemisphere regions, In Experiment 2, the ident ical sequence of letters was presented in all conditions, and what var ied was whether subjects had to remember the names of the letters (ver bal memory) or their positions in the display (spatial memory). In the verbal task, activation was concentrated more in the left than the ri ght hemisphere; in the spatial task, there was substantial activation in both hemispheres, though in key regions, there was more activation in the right than the left hemisphere. Experiment 3 studied only verba l memory, and showed that a continuous memory task activated the same regions as the discrete verbal task used in Experiment 1. Taken togeth er, these results indicate that verbal and spatial working memory are implemented by different neural structures.