Transcranial magnetic stimulation (TMS), a form of non-invasive brain stimulation, is known to activate inputs to the corticospinal neurons in motor cortex. It has long been known that these inputs are directionally selective, with anterior-posterior (AP) and posterior-anterior (PA) currents tending to activate separate sets of inputs.
The aim was to investigate whether a motor decision-making task integrating inputs from other cortical areas into M1 selectively employs the directionally-selective input pathways.
Participants performed a left/right choice reaction time task in response to a visual cue. Prior to the appearance of the cue, we presented a biased cue stimulus using random dot-motion paradigms. In order to test whether AP or PA motor cortex input pathways preferentially participated in the task, we selectively conditioned their excitability using a novel TMS device that could apply high frequency (theta burst) repetitive stimulation with either AP or PA pulses.
We measured the reaction times of left and right hands, and determined whether the effect of bias was differentially influenced after PA and AP stimulation. No effect of TMS current direction was demonstrated. However, stimulation was shown to strengthen the effect of the bias for the right hand only, indicating a strengthened bias effect specific to the stimulated hemisphere.
We propose that stimulation had an effect at the output level of left M1, increasing the background noise thereby causing threshold to be reached more quickly. This would account for the strengthened bias effect. Although this theta burst stimulation is conventionally seen as having inhibitory mechanisms, in this study we argue that it is capable of inducing a stochastic resonance effect, thereby demonstrating the potential for facilitatory effects.