Cretu, A. L. | Search bioRxiv

bioRxiv2020

Transcranial Random Noise Stimulation acutely lowers the response threshold of human motor circuits

AO_SCPLOWBSTRACTC_SCPLOWTranscranial random noise stimulation (tRNS) over cortical areas has been shown to acutely improve performance in sensory detection tasks. One explanation for this behavioural effect is stochastic resonance, a mechanism that explains how signal processing in non-linear systems can benefit from added noise. While acute noise benefits of electrical random noise stimulation have been demonstrated at the behavioural level as well as in in vitro preparations of neural tissue, it is currently largely unknown whether similar effects can be shown at the neural population level using neurophysiological readouts of human cortex. Here we hypothesized that acute tRNS will increase the responsiveness of primary motor cortex (M1) when probed with transcranial magnetic stimulation. Neural responsiveness was operationalized via the well-known concept of the resting motor threshold (RMT). We showed that tRNS acutely decreases RMT. This effect was small, but it was consistently replicated across four experiments including different cohorts (total N=81, 46 females, 35 males), two tRNS electrode montages, and different control conditions. Our experiments provide critical neurophysiological evidence that tRNS can acutely generate noise benefits by enhancing the neural population response of human M1. SO_SCPLOWIGNIFICANCEC_SCPLOWO_SCPCAP C_SCPCAPO_SCPLOWSTATEMENTC_SCPLOWA hallmark feature of stochastic resonance is that signal processing can benefit from added noise. This has mainly been demonstrated at the single-cell level in vitro where the neural response to weak input signals can be enhanced by simultaneously applying random noise. Our finding that tRNS acutely increases the excitability of corticomotor circuits extends the principle of noise benefits to the neural population level in human cortex. Our finding is in line with the notion that tRNS might affect cortical processing via the stochastic resonance phenomenon. It suggests that enhancing the response of cortical populations to an external stimulus might be one neurophysiological mechanism mediating performance improvements when tRNS is applied to sensory cortex during perception tasks.

neuroscience↗

bioRxiv2019

Integration of action value during action observation

Transcranial magnetic stimulation (TMS) studies demonstrated that observing the actions of other individuals leads to action-specific facilitation of primary motor cortex (M1) (i.e., "motor resonance"). Motor resonance is modulated by contextual information accompanying others actions, however, it is currently unknown whether action value influences behavioural and physiological outcomes during action observation in humans. Here we tested whether response times (RT) and muscle-specific changes of M1 excitability are modulated by the value an observer assigns to the action executed by another agent and whether this effect can be distinguished from attentional engagement. We show that observing highly-valued actions leads to a significant decrease in RT variability and a significant strengthening of action-specific neural representations in M1. This "sharpening" of behavioural and neural responses was observed over and beyond a control task requiring similar attentional engagement but did not include any rewards. Our finding that reward influences action specific representations in human M1 even if no motor response is required is new, suggesting that reward influences the transformation of action stimuli from the perceptual to the motor domain. We suggest that premotor areas are important for mediating the observed effect, most likely by optimizing grasp-specific PMv-M1 interactions which cause muscular facilitation patterns in M1 to be more distinct for rewarded actions.

neuroscience↗