Recent studies with nonhuman primates have shown that lesions of the mid-do
rsolateral 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 an
d area 9), give rise to severe and long-lasting impairments on self-ordered
and externally ordered tasks designed to tax executive processing within w
orking memory, rather than short-term memory per se. Lesions limited to are
a 9 give rise to a mild impairment on these tasks. Thus, the mid-dorsolater
al prefrontal region has been shown to be critical for the monitoring of mu
ltiple events in working memory. The mid-dorsolateral prefrontal region rec
eives visuospatial input from the posterior dorsolateral region (areas 8 an
d 6) and from the cortex within the middle part (sulcal area 46) and the ca
udal part (area 8) of the sulcus principalis. Nonspatial visual input origi
nates from the ventrolateral prefrontal cortex. Thus, lesions focused on th
e middle to caudal part of the sulcus principalis would affect visuospatial
input, but would not affect the flow of nonspatial visual object informati
on that reaches the mid-dorsolateral prefrontal region from the ventrolater
al prefrontal cortex. Lesions of the sulcus principalis produce a spatially
selective impairment, whereas lesions of the mid-dorsolateral prefrontal r
egion produce a more general impairment of the monitoring and manipulation
of information in working memory. The results of recent functional neuroima
ging studies with human subjects are consistent with the above findings fro
m work with the monkey.