Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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We investigated whether human attentional processes influence the activity of intracortical inhibitory and excitatory circuits--short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI), and the intracortical facilitation (ICF)--elicited by paired-pulse transcranial magnetic stimulation (TMS) in healthy subjects. In eight healthy subjects we tested SICI, LICI and ICF under different attention-demanding conditions: "relaxed", "target hand" and "non-target hand". To compare the effects of attentional levels on SICI, LICI and ICF with those produced on the MEPs elicited by repetitive TMS (rTMS), in the same subjects we also delivered supra-threshold 5-Hz rTMS under the same three experimental conditions. ⋯ Attentional levels had no influence on SICI, ICF and LICI activated by paired-pulse TMS, but increased the MEP facilitation elicited by 5-Hz rTMS. Varying the attention level left the findings from 1-Hz rTMS unchanged. The finding that attention leaves the activity of intracortical inhibitory and excitatory circuits elicited by paired-pulse TMS unchanged but influences the MEP facilitation elicited by 5-Hz rTMS suggests that attention operates only when the stimulation entrains neural circuits made up of a large number of cortical cells with plasticity properties.
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The aim of the current study was to examine the influence of visual feedback on compensatory variations in force within and between trials. In Experiment 1, the task was to maintain a constant force of 5 N for 15 s using both index fingers. In Experiment 2, the task was to produce discrete force pulses such that the peak value of the sum of the two finger forces was 5 N. ⋯ Similarly, between-trial correlations showed that the amount of error compensation decreased when feedback was removed, indicating that participants produced a smaller set of goal equivalent solutions. It appears that feedback has a dual role in coordination-removing the constraints on the degrees of freedom within trials, and facilitating the utilization of redundancy between trials. The distinction between these two classes of variation is central to understanding the redundancy problem in motor control.