Maintenance and manipulation in spatial working memory: Dissociations in the prefrontal cortex

Two experiments were conducted to compare theories of the functional organization of spatial working memory within the human prefrontal cortex. In Experiment I, memory set size for locations was parametrically varied, allowing for the assessment of BOLD signal across maintenance requirements. In the second experiment, manipulation of spatial information held in working memory was contrasted with simple maintenance of that information. Both experiments evoked significant activity in a distributed spatial working memory network. Although dorsolateral prefrontal activation increased monotonically with memory set size, this region was differentially engaged in task conditions involving explicit manipulation of internal representations. Activation in the superior frontal sulcal region was associated with maintenance of spatial information, increasing with memory set size. In contrast, ventrolateral prefrontal activation was present only at the highest memory set size, possibly due to the differential use of organizational strategies with more complex stimuli. These results support claims that the dorsolateral prefrontal cortex is involved in the manipulation of internal representations and that the superior frontal sulcal region is involved in the maintenance of spatial information, but they suggest a complex role for the ventrolateral prefrontal region.