Patients affected by right parietal lobe lesion can be severely impaired in sustained attention tasks, particularly in the left visual field. For example, patients with right parietal stroke are commonly limited in their ability to attentionally track multiple moving objects in their left visual field when competing stimuli are simultaneously presented in the right, ipsilesional visual field. This is a hallmark of visual extinction, a failure to respond to contralesional visual stimuli, when competing stimuli are presented in the good hemifield. It has been hypothesized that post-stroke hyperactivity of the undamaged left hemisphere leads to excessive cross-hemispheric inhibition of the damaged right hemisphere, thus exacerbating the attentional deficits. However, there has been no direct physiological demonstration of this hypothesis, as most of the studies are conducted using unilateral tasks, a condition not sufficient to drive inter-hemispheric competition. The inter-hemispheric inhibition hypothesis also raises the possibility that if hyperactivity of the healthy hemisphere were reduced, this could relieve inter-hemispheric inhibition, disinhibiting the damaged hemisphere and potentially restoring some function. To test this hypothesis, and to examine whether we could relieve deficits in sustained attention in right parietal patients, we used repetitive transcranial magnetic stimulation (rTMS) to reduce the activity of the left, healthy hemisphere. Six patients suffering from visual extinction underwent two counterbalanced sessions: low frequency rTMS over the left parietal lobe and sham control stimulation. The patients' performance in an attentional tracking task significantly improved in the contralesional visual field immediately after rTMS, but not after sham. Performance remained unaltered in the ipsilesional field. We hypothesize that rTMS temporarily releases the damaged right hemisphere from excessive cross-hemispheric inhibition by the hyperactive healthy hemisphere, leading to some cognitive recovery after cortical lesion.
Keywords: Interhemispheric balance; Right parietal lesion; Sustained attention; TMS; Visual extinction.
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