Activity in ventrolateral and mid-dorsolateral prefrontal cortex during nonspatial visual working memory processing: evidence from functional magnetic resonance imaging.

Whole-brain functional magnetic resonance imaging was used to study five healthy human subjects while they performed two nonspatial visual working memory tasks and one control task. In the first memory task, the subjects were required to view a sequence of three pattern stimuli, randomly selected from a familiar set of four stimuli, and then identify which one of three simultaneously presented stimuli was the one that had not been presented in the previous array. In the other task, the subjects were required to observe an identical sequence of three randomly selected pattern stimuli and then to respond by selecting those same stimuli in the order presented. In comparison to a baseline control task, increases in signal intensity were observed, bilaterally, in the mid-dorsolateral frontal cortex and in the right ventrolateral frontal cortex in both memory tasks. When the two tasks were compared directly, however, the first memory task, which had the higher monitoring requirement, yielded significantly greater signal intensity changes in area 9/46 of the right mid-dorsolateral frontal cortex. These results provide further evidence for the precise functional contribution made by the mid-dorsolateral frontal cortex in visual working memory tasks and concur closely with findings in nonhuman primates.

Functional organization of spatial and nonspatial working memory processing within the human lateral frontal cortex.

The present study used functional magnetic resonance imaging to demonstrate that performance of visual spatial and visual nonspatial working memory tasks involve the same regions of the lateral prefrontal cortex when all factors unrelated to the type of stimulus material are appropriately controlled. These results provide evidence that spatial and nonspatial working memory may not be mediated, respectively, by mid-dorsolateral and mid-ventrolateral regions of the frontal lobe, as widely assumed, and support the alternative notion that specific regions of the lateral prefrontal cortex make identical executive functional contributions to both spatial and nonspatial working memory.

Hyperacute stroke: evaluation with combined multisection diffusion-weighted and hemodynamically weighted echo-planar MR imaging.

PURPOSE: To evaluate acute stroke with conventional, multisection diffusion-weighted (DW), and hemodynamically weighted (HW) magnetic resonance (MR) imaging. MATERIALS AND METHODS: The three MR imaging techniques were performed in 11 patients within 10 hours of the onset of acute hemiparesis. The volume of DW and HW abnormalities were compared with infarct volumes depicted at initial and/or follow-up MR or computed tomography (CT). RESULTS: Findings at DW and HW imaging were abnormal in nine of the 11 patients, despite normal findings at initial CT and/or MR. In all nine patients, infarcts were depicted at follow-up CT or MR. The DW abnormality was generally smaller and the HW abnormality was generally larger than the infarct volume determined at subsequent imaging. In the two patients with normal findings at DW and HW imaging, symptoms resolved completely within 1-48 hours. CONCLUSION: Different aspects of hyperacute cerebral ischemia are depicted at DW and HW imaging before infarction is depicted at conventional MR or CT. These techniques may improve stroke diagnosis and may contribute to advances in treatment.

Visual motion aftereffect in human cortical area MT revealed by functional magnetic resonance imaging.

Functional magnetic resonance imaging (fMRI) was used to measure local haemodynamic changes (reflecting electrical activity) in human visual cortex during production of the visual motion aftereffect, also known as the waterfall illusion. As in previous studies, human cortical area MT (V5) responded much better to moving than to stationary visual stimuli. Here we demonstrate a clear increase in activity in MT when subjects viewed a stationary stimulus undergoing illusory motion, following adaptation to stimuli moving in a single local direction. Control stimuli moving in reversing, opposed directions produced neither a perceptual motion aftereffect nor elevated fMRI levels postadaptation. The time course of the motion aftereffect (measured in parallel psychophysical tests) was essentially identical to the time course of the fMRI motion aftereffect. Because the motion aftereffect is direction specific, this indicates that cells in human area MT are also direction specific. In five other retinotopically defined cortical areas, similar motion-specific aftereffects were smaller than those in MT or absent.