TRANSIENT AND SUSTAINED ACTIVITY IN A DISTRIBUTED NEURAL SYSTEM FOR HUMAN WORKING-MEMORY

Working memory involves the short-term maintenance of an active repres entation of information so that it is available for further processing . Visual working memory tasks, in which subjects retain the memory of a stimulus over brief delays, require both the perceptual encoding of the stimulus and the subsequent maintenance of its representation afte r the stimulus is removed from view. Such tasks activate multiple area s in visual and prefrontal cortices(1-9). To delineate the roles these areas play in perception and working memory maintenance, we used func tional magnetic resonance imaging (fMRI) to obtain dynamic measures of neural activity related to different components of a face working mem ory task-non-selective transient responses to visual stimuli, selectiv e transient responses to faces, and sustained responses over memory de lays. Three occipitotemporal areas in the ventral object vision pathwa y had mostly transient responses to stimuli, indicating their predomin ant role in perceptual processing, whereas three prefrontal areas demo nstrated sustained activity over memory delays, indicating their predo minant role in working memory. This distinction, however, was not abso lute. Additionally, the visual areas demonstrated different degrees of selectivity, and the prefrontal areas demonstrated different strength s of sustained activity, revealing a continuum of functional specializ ation, from occipital through multiple prefrontal areas, regarding eac h area's relative contribution to perceptual and mnemonic processing.