Left frontal transcranial magnetic stimulation reduces contralesional extinction in patients with unilateral right brain damage

It has been demonstrated previously that transcranial magnetic stimulation (TMS) of the sensorimotor cortex can induce transient suppression of the pe rception of cutaneous near-threshold stimuli from fingers of the contralate ral hand in normal individuals, One explanation accounting for deficits in the exploration of contralateral space following a unilateral hemispheric l esion refers to a loss of the normal interhemispheric balance, with a resul tant hyperactivation of the unaffected hemisphere due to the release of rec iprocal inhibition by the affected one. In order to verify this hypothesis, we investigated the effects of a TMS-induced transient dysfunction of the normal hemisphere upon contralateral tactile extinctions in two groups: (i) 14 right brain-damaged patients and (ii) 14 left brain-damaged control pat ients. Single-pulse TMS was delivered to frontal and parietal scalp sites o f the unaffected hemisphere after an interval of 40 ms from an electrical u nimanual or bimanual digit stimulation. In right brain-damaged patients, le ft frontal TMS significantly reduced the rate of contralateral extinctions compared with controls, After left parietal TMS, the number of extinctions was comparable to the baseline. This pattern of increased sensitivity to cu taneous stimulation ipsilateral to TMS was not observed in left brain-damag ed control patients. In this group, right hemisphere TMS did not significan tly alter the recognition of bimanual stimuli delivered to the space contra lateral to the lesion, The suggestion is made that extinctions produced by right brain damage may be dependent on a breakdown in the balance of hemisp heric rivalry in directing spatial attention to contralateral hemispace, so that the unaffected hemisphere generates an unopposed orienting response t o the side of the lesion, The mechanisms whereby the left frontal TMS trans iently ameliorates these deficits may involve stimulus-induced removal of a left frontal-right parietal transcallosal inhibitory flow, although intera ctions at subcortical levels cannot be excluded.