Visual cortex cTBS increases mixed percept duration while a-tDCS has no effect on binocular rivalry

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.

Fig 1

a-tDCS (A) and cTBS (B) experiment protocols. Binocular rivalry dynamics were recorded for 6 minutes at baseline (pre), during, 5 minutes post and 30 minutes post a-tDCS. A-tDCS electrodes were placed on the head following the baseline measure. Similarly, for cTBS, binocular rivalry dynamics were recorded pre, 5 minutes post and 30 minutes post stimulation. Motor thresholding was completed on the first visit following the baseline measure.

Fig 2

Average time spent in mixed percept (A), ocular dominance indices (B), and alternation rates (C) for 15 participants pre, during, 5 minutes and 30 minutes post a-tDCS. Error bars = SEM. No statistically significant effects were observed.

Fig 3

Average time spent in mixed percept (A), ocular dominance indices (B), and alternation rates (C) for 15 participants pre-stimulation, 5 minutes and 30 minutes post cTBS. Error bars = SEM. * p<0.05.