Theta-burst stimulation: remote physiological and local behavioral after-effects.

Theta-burst stimulation (TBS), a novel repetitive transcranial magnetic stimulation (TMS) protocol, is capable of suppressing the amplitude of contralateral motor-evoked potentials (MEP) for several minutes after the end of a conditioning train over the motor cortex. It remains unknown whether TBS leads to effects on motor cortical excitability when applied to contralateral brain sites distant but connected to motor cortex and whether TBS triggers measurable changes in force control. Subjects received bursts (50 Hz) of three subthreshold magnetic stimuli repeated at 5 Hz for 20 s (TBS-300) or 40 s (TBS-600) over the hand area of the left motor cortex (M1(LEFT)). With TBS-300, conditioning of right motor cortex (M1(RIGHT)), right dorsal premotor cortex (PMd(RIGHT)), and a mid-occipital (MO) region also were tested. Corticospinal excitability was probed by evoking MEPs in abductor pollicis brevis (APB) muscle by single suprathreshold stimuli over M1(LEFT) or M1(RIGHT) before and after TBS. Force level control was assessed in an isometric right thumb abduction task. With TBS-600, the time course of physiological and behavioral changes was monitored. TBS over either of the motor cortices reduced the amplitude of MEP in the contralateral APB and increased it in the ipsilateral APB. In contrast, conditioning TBS over PMd(RIGHT) or MO did not modify MEP size. Post-TBS right thumb force level control was impaired, with contralateral M1(LEFT) stimulation only, for a duration of at least 5 min. TBS may induce remote physiological effects and reveals local functional properties of the underlying brain region.