Evidence for quantitative domain dominance for verbal and spatial working memory in frontal and parietal cortex

Neuroimaging studies in humans have shown that different working memory (WM) tasks recruit a common bilateral fronto-parietal cortical network. Animal studies as well as neuroimaging studies in humans have suggested that this network, in particular the prefrontal cortex, is preferentially recruited when material from different domains (e.g. spatial information or verbal/object information) has to be memorized. Early imaging studies have suggested qualitative dissociations in the prefrontal cortex for spatial and object/verbal WM, either in a left-right or a ventral-dorsal dimension. However, results from different studies are inconsistent. Moreover, recent fMRI studies have failed to find evidence for domain dependent dissociations of WM-related activity in prefrontal cortex. Here we present evidence from two independent fMRI studies using physically identical stimuli in a verbal and spatial WM task showing that domain dominance for WM does indeed exist, although only in the form of quantitative differences in activation and not in the form of a dissociation with different prefrontal regions showing mutually exclusive activation in different domains. Our results support a mixed dimension model of domain dominance for WM within the prefrontal cortex, with left ventral prefrontal cortex (PFC) supporting preferentially verbal WM and right dorsal PFC supporting preferentially spatial WM. The concept of domain dominance is discussed in the light of recent theories of prefrontal cortex function.