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Excerpt
Evidently from this recently published study, Rushworth et al. significantly enhance the potential of repetitive transcranial magnetic stimulation (rTMS) as a viable technique for distinguishing functionally-specific brain regions in humans. With the aid of an infra-red tracking device, it was possible to anatomically localise the TMS coil and co-register its position with high-resolution MRI structural images of the subject's head.
Using this technology, the paper examines whether orienting attention and motor attention, two processes intimately related functionally, are actually anatomically discrete. Previous studies have shown the ability to disengage covert attention was compromised in patients with parietal lesions in the area of the angular gyrus. However, evidence also suggests that covert orienting attention is dissociable from the subsequent preparation of an oculomotor response, and moreover that the mechanism for motor attention may indeed lie in a right parietal region more anterior to the angular gyrus.
During the systematic application of rTMS to transiently disrupt cortical functioning at sites in the left and right parietal cortices, reaction times (RTs) responses were recorded whilst subjects performed either an orienting attention task or a motor attention task. The RTs taken during rTMS trials were later compared to RT data collected during control experiments when no rTMS was involved. The results of the study reveal that not only are the modality-specific cortical processes involved in orienting attention and motor attention anatomically localised in the angular gyrus and supramarginal gyrus respectively, but also that the two attentional processes are indeed lateralised in humans; covert orienting being attributable to the region of the right angular gyrus and covert motor attention localised in the region of the left supramarginal gyrus.
In addition to presenting revealing evidence for the existence of a distinct anterior parietal mechanism for motor attention, Rushworth et al. have shown rTMS to be an important addition to the techniques available to cognitive neuroscientists, and one that can, when used imaginatively, consolidate and/or challenge the previous findings of lesion-deficit and animal models of cortical functioning.
