Dissociating aspects of verbal working memory within the human frontal lobe: Further evidence for a 'process-specific' model of lateral frontal organization

Abstract

There is now converging evidence that suggests that working memory processes within the dorsolateral and ventrolateral frontal cortices are organized 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. For example, recent positron emission tomography (PET) studies have demonstrated that either, or both, of these two lateral frontal areas can be activated in spatial working memory tasks, depending on the precise executive processes that are called upon by the task being performed. Moreover, in a recent study using functional magnetic resonance imaging, performances of visual spatial and visual nonspatial working memory tasks were shown to involve identical regions of the lateral prefrontal cortex when all the factors unrelated to the type of stimulus domain were appropriately controlled. These results concur fully with recent reviews of the imaging literature, which have demonstrated that spatial and nonspatial working memory studies, in general, have produced a widely distributed pattern of overlapping activation foci within these lateral frontal regions. In this study, the effects of varying the executive requirements of a simple verbal working memory task (forward vs. backward digit span) were explored in 8 subjects, using PET, in order to establish whether this model generalizes to the verbal domain. As was expected, during forward digit span, significant activation was observed within the midventrolateral frontal cortex, but not within the middorsolateral frontal cortex. In contrast, during backward digit span, significant activation was observed in both regions. The results provide further evidence that the middorsolateral and midventrolateral frontal cortical areas make distinct functional contributions to memory and that this corresponds, in psychological terms, to a fractionation of working memory processes.