Abstract
Transcranial random noise stimulation (tRNS) is a recent neuro-modulation technique whose effects at both behavioural and neural level are still debated. Here we employed the well-known phenomenon of motion after-effect (MAE) in order to investigate the effects of high- vs. low-frequency tRNS on motion adaptation and recovery. Participants were asked to estimate the MAE duration following prolonged adaptation (20 s) to a complex moving pattern, while being stimulated with either sham or tRNS across different blocks. Different groups were administered with either high- or low-frequency tRNS. Stimulation sites were either bilateral human MT complex (hMT+) or frontal areas. The results showed that, whereas no effects on MAE duration were induced by stimulating frontal areas, when applied to the bilateral hMT+, high-frequency tRNS caused a significant decrease in MAE duration whereas low-frequency tRNS caused a significant corresponding increase in MAE duration. These findings indicate that high- and low-frequency tRNS have opposed effects on the adaptation-dependent unbalance between neurons tuned to opposite motion directions, and thus on neuronal excitability.
Highlights
Cause of this perceptual illusion[17,18,19,20]
We investigated the effects of lf- and hf-transcranial random noise stimulation (tRNS) using a standard motion adaptation paradigm
The results of the main Experiment showed a double dissociation between the effect of hf- and lf-tRNS over bilateral hMT+: whereby high-frequency tRNS (hf-tRNS) decreased MAE duration, and lf-tRNS over the same areas increased the perceived MAE duration by a similar amount
Summary
Cause of this perceptual illusion[17,18,19,20]. Adaptation is considered a form of gain control that increases the efficiency of stimulus encoding, and it takes place at multiple levels of visual motion processing[17,21,22]. Rather than producing an imbalance between opponent pairs of motion detectors, suppression of motion detectors during adaptation resulted in a shift of the population response ( involving detectors tuned to the orthogonal directions) of motion sensitive neurons in hMT+. The processing of complex motion is mainly mediated by the area hMT+38,39, and the MAE produced by adaptation to a complex moving pattern is likely to be subserved by the same regions[23]. In order to understand the neural effects of lf- and hf-tRNS on visual areas, we employed the MAE resulting from adaptation to a complex moving pattern, and measured the MAE duration while participants were administered either Sham stimulation, lf-tRNS or hf-tRNS. In the present study we assess whether lf- and hf-tRNS produce the same or different effects on MAE duration, and whether the effect consists of either a decrease (as found with both tDCS and TMS) or an increase in MAE duration
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