The role of the mid-dorsolateral prefrontal cortex in working memory.
Recent studies with nonhuman primates have shown that lesions of the mid-dorsolateral prefrontal cortex, which extends from the lip of the dorsal bank of the sulcus principalis to the midline (i.e., dorsal area 46 and 9/46 and area 9), give rise to severe and long-lasting impairments on self-ordered and externally ordered tasks designed to tax executive processing within working memory, rather than short-term memory per se. Lesions limited to area 9 give rise to a mild impairment on these tasks. Thus, the mid-dorsolateral prefrontal region has been shown to be critical for the monitoring of multiple events in working memory. The mid-dorsolateral prefrontal region receives visuospatial input from the posterior dorsolateral region (areas 8 and 6) and from the cortex within the middle part (sulcal area 46) and the caudal part (area 8) of the sulcus principalis. Nonspatial visual input originates from the ventrolateral prefrontal cortex. Thus, lesions focused on the middle to caudal part of the sulcus principalis would affect visuospatial input, but would not affect the flow of nonspatial visual object information that reaches the mid-dorsolateral prefrontal region from the ventrolateral prefrontal cortex. Lesions of the sulcus principalis produce a spatially selective impairment, whereas lesions of the mid-dorsolateral prefrontal region produce a more general impairment of the monitoring and manipulation of information in working memory. The results of recent functional neuroimaging studies with human subjects are consistent with the above findings from work with the monkey.