Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Weak transcranial direct current stimulation (tDCS) can induce long lasting changes in cortical excitability. In the present study we asked whether tDCS applied to the left primary motor cortex (M1) also produces aftereffects distant from the site of the stimulating electrodes. We therefore tested corticospinal excitability in the left and the right M1 and transcallosal excitability between the two cortices using transcranial magnetic stimulation (TMS) before and after applying tDCS. ⋯ MEPs evoked from the right M1 were unchanged whilst the duration of transcallosal inhibition evoked from the right M1 was shortened after cathodal tDCS and prolonged after anodal tDCS. The duration of transcallosal inhibition returned to control values before the effect on the MEPs from the left M1 had recovered. These findings are compatible with the idea that tDCS-induced aftereffects in the cortical motor system are limited to the stimulated hemisphere, and that tDCS not only affects corticospinal circuits involved in producing MEPs but also inhibitory interneurons mediating transcallosal inhibition from the contralateral hemisphere.
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Blinks executed during eye movements affect kinetic eye movement parameters, e.g., peak velocity of saccades is decreased, their duration is increased, but their amplitude is not altered. This effect is mainly explained by the decreased activity of premotor neurons in the brainstem: omni-pause neurons (OPN) in the nucleus raphe interpositus. Previous studies examined the immediate effect of blinks directly on eye movements but not their effect when they are elicited several hundred milliseconds before the eye movements. ⋯ There was also no difference in blink amplitude and duration of pupil occlusion in the blink condition, neither in saccades nor in smooth pursuit. The latency reduction of smooth pursuit, but not of saccades, may neither be explained by the brief pupil occlusion nor by visual suppression, warning signals, or the startle response. Whether the effects are caused by the influence of blinks on OPNs or other premotor structures remains to be tested.