Abstract
Size and duration of the neuroplastic effects of tDCS depend on stimulation parameters, including stimulation duration and intensity of current. The impact of stimulation parameters on physiological effects is partially non-linear. To improve the utility of this intervention, it is critical to gather information about the impact of stimulation duration and intensity on neuroplasticity, while expanding the parameter space to improve efficacy. Anodal tDCS of 1–3 mA current intensity was applied for 15–30 minutes to study motor cortex plasticity. Sixteen healthy right-handed non-smoking volunteers participated in 10 sessions (intensity-duration pairs) of stimulation in a randomized cross-over design. Transcranial magnetic stimulation (TMS)-induced motor-evoked potentials (MEP) were recorded as outcome measures of tDCS effects until next evening after tDCS. All active stimulation conditions enhanced motor cortex excitability within the first 2 hours after stimulation. We observed no significant differences between the three stimulation intensities and durations on cortical excitability. A trend for larger cortical excitability enhancements was however observed for higher current intensities (1 vs 3 mA). These results add information about intensified tDCS protocols and suggest that the impact of anodal tDCS on neuroplasticity is relatively robust with respect to gradual alterations of stimulation intensity, and duration.
Highlights
Size and duration of the neuroplastic effects of Transcranial direct current stimulation (tDCS) depend on stimulation parameters, including stimulation duration and intensity of current
Whereas the primary effects of tDCS are due to neuronal membrane polarization, after-effects are caused by a modification of synaptic strength related to long-term potentiation (LTP) and long-term depression (LTD). tDCS-induced plasticity depends on glutamatergic mechanisms
This study aims to expand and investigate systematically the parameter space of anodal tDCS with respect to current intensity and stimulation duration, to extend systematic information on optimally suited stimulation protocols for neuroplasticity induction based on the primary motor cortex model in healthy adults
Summary
Size and duration of the neuroplastic effects of tDCS depend on stimulation parameters, including stimulation duration and intensity of current. A trend for larger cortical excitability enhancements was observed for higher current intensities (1 vs 3 mA) These results add information about intensified tDCS protocols and suggest that the impact of anodal tDCS on neuroplasticity is relatively robust with respect to gradual alterations of stimulation intensity, and duration. Unlike other NIBS techniques, such as repetitive TMS, tDCS uses weak direct currents to induce gradual changes of the resting membrane potential of cortical neurons in a polarity dependent manner – anodal stimulation leads to a subthreshold depolarization whereas cathodal stimulation leads to hyperpolarization of neuronal compartments critical for the respective excitability, and neuronal activity alterations[11,12]. These recent studies stress the relevance of a systematic titration of tDCS parameters, to identify protocols that are resulting in stable and unidirectional effects
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