Evaluating hf-tRNS as a Tool for Enhancing Perceptual Learning: Evidence from a Multi-Session Orientation Discrimination Protocol

Visual perceptual performance can benefit from practice through experience-dependent plasticity in the adult brain, a phenomenon known as visual perceptual learning (VPL). High-frequency transcranial random noise stimulation (hf-tRNS) has been proposed as a tool to enhance visual perceptual learning, as this technique can modulate cortical excitability and facilitate plasticity-related processes, potentially through mechanisms such as stochastic resonance. This study aimed to investigate the effects of hf-tRNS on an orientation discrimination task featuring sequentially presented oriented stripes. Participants were randomly assigned to either active or sham stimulation and completed a multi-session protocol consisting of a baseline assessment session, eight training sessions with either active or sham stimulation, and a final post-training test. Sensitivity was estimated as d′ and analysed using linear mixed-effects models to account for inter-individual variability. The analyses showed a significant improvement in orientation sensitivity across training stages in the trained condition. Active hf-tRNS, compared with sham stimulation, was found to be associated with a modulation of orientation sensitivity during the training phase, as indicated by a significant interaction between group and angular difference in orientation. However, hf-tRNS did not reliably increase the rate of learning compared with sham stimulation. Additionally, the omnibus transfer analysis did not provide evidence that improvement differed reliably across conditions. Overall, these findings suggest that hf-tRNS may modulate training-phase orientation sensitivity as a function of angular difference magnitude, without accelerating the overall learning trajectory or enhancing transfer. Further optimization of stimulation protocols is needed before translation to clinical applications in VPL protocols.