Abstract
Purpose :
Anodal transcranial direct current stimulation (a-tDCS) of the primary visual cortex can alter visual perception in individuals with normal vision and in those with amblyopia. The mechanisms that underpin this effect are unknown. When applied to the primary motor cortex, a-tDCS reduces the concentration of the neurotransmitter GABA within the stimulated region. We investigated whether this effect occurs when a-tDCS is delivered to the primary visual cortex.
Methods :
Thirteen adults with normal vision participated in a within-subjects study design consisting of two MRI sessions, one involving active a-tDCS and the other a placebo (sham) a-tDCS to the visual cortex. Each session began with the measurement of binocular rivalry dynamics (oblique gratings, 6.1° size, 0.5 cpd, 60-second viewing time). GABA and glutamate concentrations (magnetic resonance spectroscopy; MRS) and resting state functional connectivity (functional MRI) were then measured before (baseline) and immediately after application of a-tDCS. For MRS, a 2.5 cm3 voxel-of-interest (VOI) was centered on the calcarine sulcus and positioned so that it covered only brain tissue. For functional MRI, brain volumes were acquired using whole-brain echo-planar imaging with a T2-weighted sequence.
Results :
Active a-tDCS stimulation had no effect on visual cortex GABA concentration nor on resting state connectivity. However, there was a trend towards increased glutamate concentration following active stimulation (p=0.045). In addition, higher visual cortex glutamate concentrations at baseline were associated with longer piecemeal percepts during binocular rivalry (r=0.561, p=0.046).
Conclusions :
Unlike motor cortex, visual cortex a-tDCS does not appear to be mediated by a reduction in GABA concentration. Unexpectedly, temporal dynamics of binocular rivalry are associated with higher visual cortex glutamate concentration.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.