The role of the lateral frontal cortex in mnemonic processing: the contribution of functional neuroimaging

Evidence is now converging which suggests that, at the area level, working-memory processes within the dorsolateral and ventrolateral frontal cortices are organised according to the type of processing required, rather than according to the nature (i.e. domain) of the information being processed, as has been widely assumed. In a recent study using functional magnetic-resonance imaging (fMRI), performance of visual spatial and visual non-spatial working-memory tasks was shown to involve identical regions of the lateral prefrontal cortex when all factors unrelated to the type of stimulus material were appropriately controlled. These results concur fully with recent reviews of the imaging literature, which demonstrate that spatial, visuospatial and verbal working-memory studies have produced distributed patterns of overlapping activation foci within these lateral frontal regions. Moreover, two recent positron-emission tomography studies have demonstrated that either, or both, the ventrolateral and dorsolateral frontal regions can be activated in spatial working-memory tasks, depending on the precise executive processes that are called upon by the task being performed. Similarly, when the executive requirements of a simple verbal working-memory task (e.g. forwards versus backwards digit span) are manipulated, differential activation within these two frontal cortical areas is observed. The results provide further evidence that the mid-dorsolateral and mid-ventrolateral frontal cortical areas make distinct "executive" contributions to memory and correspond with a fractionation of working-memory processes in psychological terms.