The causal role of transcranial alternating current stimulation at alpha frequency in boosting visual perceptual learning

Abstract

Extensive training improves our ability to perceive visual contents around us, a phenomenon known as visual perceptual learning (VPL). Numerous studies have been conducted to understand the mechanisms of VPL, while the neural oscillatory mechanisms underpinning VPL has yet to be elucidated. To this end, we adopted transcranial alternating current stimulation (tACS), a neuromodulatory technique that can alter ongoing brain rhythms in a frequency-specific manner by applying external weak electric fields, to stimulate targeted cortical areas in human subjects while they performed an orientation discrimination learning task. Five groups of subjects undertook five daily training sessions to execute the task. Four groups received occipital tACS stimulation at 10 Hz (alpha band), 20 Hz (beta band), 40 Hz (gamma band), or sham 10 Hz (sham), and one group was stimulated at the sensorimotor regions by 10 Hz tACS. Compared with the sham stimulation, occipital tACS at 10 Hz, but not at 20 Hz or 40 Hz, increased both the learning rate and performance improvement. However, when 10 Hz tACS was delivered to the sensorimotor areas, the modulatory effects of tACS were absent, suggesting that tACS modulated the orientation discrimination learning in a frequency- and location-specific manner. Moreover, the tACS-induced enhancement lasted at least two months after the termination of training. Our findings provide strong evidence for the causal role of alpha oscillations in VPL and shed new light on the design of effective neuromodulation protocols that might facilitate rehabilitation for patients with neuro-ophthalmological disorders. Significance StatementPerformance of visual tasks can be enhanced substantially by training, which is known as visual perceptual learning (VPL). However, little is known about the neural oscillatory mechanisms underlying VPL. To probe the causal link between a given oscillatory frequency band and VPL, transcranial alternating current stimulation (tACS) was applied while subjects performed an orientation discrimination learning task. Our results revealed that tACS modulates VPL in a frequency- and location-specific manner. Specifically, only training coupled with 10 Hz tACS over the occipital cortex speeded up the learning process and amplified the performance gain. Our findings demonstrate the causal role of alpha oscillations in VPL, and provide insight into developing more effective and efficient remediation protocols for clinical applications, e.g., amblyopia.