VERBAL WORKING-MEMORY LOAD AFFECTS REGIONAL BRAIN ACTIVATION AS MEASURED BY PET

We report an experiment that assesses the effect of variations in memo ry load on brain activations that mediate verbal working memory. The p aradigm that forms the basis of this experiment is the ''n-back'' task in which subjects must decide for each letter in a series whether it matches the one presented n items back in the series. This task is of interest because it recrdts processes involved in both the storage and manipulation of information in working memory. Variations in task dif ficulty were accomplished by varying the value of n. As n increased, s ubjects showed poorer behavioral performance as well as monotonically increasing magnitudes of brain activation in a large number of sites t hat together have been identified with verbal working-memory processes . By contrast, there was no reliable increase in activation in sites t hat are unrelated to working memory. These results validate the use of parametric manipulation of task variables in neuroimaging research, a nd they converge with the subtraction paradigm used most often in neur oimaging. In addition, the data support a model of working memory that includes both storage and executive processes that recruit a network of brain areas, all of which are involved in task performance.