Abstract
Neuromodulation of the primary visual cortex using anodal transcranial direct current stimulation (a-tDCS) can alter visual perception and enhance neuroplasticity. However, the mechanisms that underpin these effects are currently unknown. When applied to the motor cortex, a-tDCS reduces the concentration of the inhibitory neurotransmitter gamma aminobutyric acid (GABA), an effect that has been linked to increased neuroplasticity. The aim of this study was to assess whether a-tDCS also reduces GABA-mediated inhibition when applied to the human visual cortex. Changes in visual cortex inhibition were measured using the mixed percept duration in binocular rivalry. Binocular rivalry mixed percept duration has recently been advocated as a direct and sensitive measure of visual cortex inhibition whereby GABA agonists decrease mixed percept durations and agonists of the excitatory neurotransmitter acetylcholine (ACH) increase them. Our hypothesis was that visual cortex a-tDCS would increase mixed percept duration by reducing GABA-mediated inhibition and increasing cortical excitation. In addition, we measured the effect of continuous theta-burst transcranial magnetic stimulation (cTBS) of the visual cortex on binocular rivalry dynamics. When applied to the motor or visual cortex, cTBS increases GABA concentration and we therefore hypothesized that visual cortex cTBS would decrease the mixed percept duration. Binocular rivalry dynamics were recorded before and after active and sham a-tDCS (N = 15) or cTBS (N = 15). Contrary to our hypotheses, a-tDCS had no effect, whereas cTBS increased mixed percepts during rivalry. These results suggest that the neurochemical mechanisms of a-tDCS may differ between the motor and visual cortices.
Highlights
Anodal transcranial direct current stimulation (a-tDCS) is a non-invasive electrical brain stimulation technique that can modulate neural excitability and promote neuroplasticity
We tested the hypothesis that visual cortex a-tDCS, which has been observed to reduce regional gamma aminobutyric acid (GABA) concentration [13,17], acts to reduce GABA-mediated inhibition within visual cortex as evidenced by increased binocular rivalry mixed percept duration [27]
Mixed percept duration during binocular rivalry has been causally linked to visual cortex GABA concentration through pharmacological antagonism of GABAa and GABAb receptors [27]
Summary
Anodal transcranial direct current stimulation (a-tDCS) is a non-invasive electrical brain stimulation technique that can modulate neural excitability and promote neuroplasticity. When applied to the visual cortex, a-tDCS can increase contrast sensitivity [1,2,3,4], improve visual acuity [5,6], and enhance perceptual learning [7,8] in patients with amblyopia, a neurodevelopmental disorder that affects binocular vision, as well as in controls. Visual cortex cTBS increases mixed percept duration
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