Modulating the excitability of the visual cortex using a stimulation priming paradigm.

BACKGROUND: Transcranial random noise stimulation (tRNS) can cause long term increase of corticospinal excitability when used to prime the motor cortex, before measuring the motor response in the hand muscles with TMS (Terney et al., 2008). In cognitive studies, tRNS has been used to improve visual attention and mathematical skills, an enhancement effect that might suggest sustained cortical plasticity changes (Cappelletti et al., 2013; Snowball et al., 2013). However, while the behavioral evidence of increased performance is becoming substantiated by empirical data, it still remains unclear whether tRNS over visual areas causes an increase in cortical excitability similar to what has been found in the motor cortex, and if that increase could be a potential physiological explanation for behavioral improvements found in visual tasks.

OBJECTIVE/HYPOTHESIS: In the present study, we aimed to investigate whether priming the visual cortex with tRNS leads to increased and sustained excitability as measured with visual phosphenes.

METHODS: We measured phosphene thresholds (PTs) using an objective staircase method to quantify the magnitude of cortical excitability changes. Single-pulse TMS was used to elicit phosphenes before, immediately after, and every 10 min up to one hour after the end of 20 min tRNS, anodal tDCS (a-tDCS) or sham.

RESULTS: Results showed that phosphene thresholds were significantly reduced up to 60 min post stimulation relative to baseline after tRNS, a behavioral marker of increased excitability of the visual cortex, while a-tDCS had no effect. This result is very similar in magnitude and duration to what has been found in the motor cortex.

CONCLUSIONS: Our findings demonstrate promising potential of tRNS as a tool to increase and sustain cortical excitability to promote improvement of cognitive functions.