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Michael A Nitsche

Publications and source records attributed to Michael A Nitsche.

41 records · Page 3Linked to original sources
PubMed2002

Pulse configuration-dependent effects of repetitive transcranial magnetic stimulation on visual perception.

Transcranial magnetic stimulation (TMS) is a noninvasive technique for direct stimulation of the neocortex. In the last two decades it is successfully applied in the study of motor and sensory physiology. TMS uses the indirect induction of electrical fields in the brain generated by intense changes of magnetic fields applied to the scalp. It encompasses two widely used waveform configurations: mono-phasic magnetic pulses induce a single current in the brain while biphasic pulses induce at least two currents of inverse direction. As has been shown for the motor cortex, efficacy of repetitive transcranial magnetic stimulation (rTMS) may depend on pulse configuration. In order to clarify this question with regard to visual perception, static contrast sensitivities (sCS) were evaluated before, during, immediately after and 10 minutes after monophasic and biphasic low frequency (1 Hz) rTMS applied to the occipital cortex of 15 healthy subjects. The intensity of stimulation was the phosphene threshold of each individual subject. Using 4 c/d spatial frequency, significant sCS loss was found during and immediately after 10 min of monophasic stimulation, while biphasic stimulation resulted in no significant effect. Ten minutes after the end of stimulation, the sCS values were at baseline level again. However, reversed current flow direction resulted in an increased efficacy of biphasic and decreased efficacy of monophasic stimulation. Our results are in agreement with previous findings showing that primary visual functions, such as contrast detection, can be transiently altered by low frequency transcranial magnetic stimulation. However the effect of modulation significantly depends on the current waveform and direction.

Adult↗
PubMed2002

Pharmacological approach to the mechanisms of transcranial DC-stimulation-induced after-effects of human motor cortex excitability.

Weak transcranial direct current stimulation (tDCS) induces persisting excitability changes in the human motor cortex. These plastic excitability changes are selectively controlled by the polarity, duration and current strength of stimulation. To reveal the underlying mechanisms of direct current (DC)-induced neuroplasticity, we combined tDCS of the motor cortex with the application of Na(+)-channel-blocking carbamazepine (CBZ) and the N-methyl-D-aspartate (NMDA)-receptor antagonist dextromethorphan (DMO). Monitored by transcranial magnetic stimulation (TMS), motor cortical excitability changes of up to 40% were achieved in the drug-free condition. Increase of cortical excitability could be selected by anodal stimulation, and decrease by cathodal stimulation. Both types of excitability change lasted several minutes after cessation of current stimulation. DMO suppressed the post-stimulation effects of both anodal and cathodal DC stimulation, strongly suggesting the involvement of NMDA receptors in both types of DC-induced neuroplasticity. In contrast, CBZ selectively eliminated anodal effects. Since CBZ stabilizes the membrane potential voltage-dependently, the results reveal that after-effects of anodal tDCS require a depolarization of membrane potentials. Similar to the induction of established types of short- or long-term neuroplasticity, a combination of glutamatergic and membrane mechanisms is necessary to induce the after-effects of tDCS. On the basis of these results, we suggest that polarity-driven alterations of resting membrane potentials represent the crucial mechanisms of the DC-induced after-effects, leading to both an alteration of spontaneous discharge rates and to a change in NMDA-receptor activation.

Adult↗