The neural response to transcranial magnetic stimulation of the human motor cortex. II. Thalamocortical contributions

Beta oscillations (15-30 Hz) constitute an important electrophysiological signal recorded in the resting state over the human precentral gyrus. The brain circuitry involved in generating the beta oscillations is not well understood but appears to involve both cortical and subcortical structures. We have shown that single pulses of transcranial magnetic stimulation (TMS) applied over the primary motor cortex consistently elicit a brief beta oscillation. Reducing the local cortical excitability using low-frequency repetitive TMS does not change the amplitude of the induced beta oscillation (Van Der Werf and Paus in Exp Brain Res DOI 10.1007/s00221-006-0551-2). Here, we investigated the possible involvement of the thalamus in the cortically expressed beta response to single-pulse TMS. We included eight patients with Parkinson's disease who had undergone unilateral surgical lesioning of the ventrolateral nucleus of the thalamus. We administered 50 single pulses of TMS, at an intensity of 120% of resting motor threshold, over the left and right primary motor cortex and, at the same time, recorded the electroencephalogram (EEG) using a 60-electrode cap. We were able to perform analyses on seven EEG data sets and found that stimulation of the unoperated hemisphere (with thalamus) resulted in higher amplitudes of the single-trial induced beta oscillations than in the operated hemisphere (with thalamotomy). The beta oscillation obtained in response to pulses applied over the unoperated hemisphere was also higher than that obtained in healthy controls. We suggest that (1) the beta oscillatory response to pulses of TMS applied over the primary motor cortex is higher in Parkinson's disease patients, (2) thalamotomy serves to reduce the abnormally high TMS-induced beta oscillations, and (3) the motor thalamus facilitates the cortically generated oscillation, through cortico-subcortico-cortical feedback loops.