The effect of transcranial direct current stimulation on contrast sensitivity and visual evoked potential amplitude in adults with amblyopia

Abstract

Amblyopia is a neurodevelopmental disorder of vision that occurs when the visual cortex receives decorrelated inputs from the two eyes during an early critical period of development. Amblyopic eyes are subject to suppression from the fellow eye, generate weaker visual evoked potentials (VEPs) than fellow eyes and have multiple visual deficits including impairments in visual acuity and contrast sensitivity. Primate models and human psychophysics indicate that stronger suppression is associated with greater deficits in amblyopic eye contrast sensitivity and visual acuity. We tested whether transcranial direct current stimulation (tDCS) of the visual cortex would modulate VEP amplitude and contrast sensitivity in adults with amblyopia. tDCS can transiently alter cortical excitability and may influence suppressive neural interactions. Twenty-one patients with amblyopia and twenty-seven controls completed separate sessions of anodal (a-), cathodal (c-) and sham (s-) visual cortex tDCS. A-tDCS transiently and significantly increased VEP amplitudes for amblyopic, fellow and control eyes and contrast sensitivity for amblyopic and control eyes. C-tDCS decreased VEP amplitude and contrast sensitivity and s-tDCS had no effect. These results suggest that tDCS can modulate visual cortex responses to information from adult amblyopic eyes and provide a foundation for future clinical studies of tDCS in adults with amblyopia.

Figure 1. A-tDCS increased the amplitude of the pattern reversal VEP.

The change in VEP amplitude from baseline after 20 minutes of anodal, cathodal or sham tDCS is shown for amblyopic (A; n = 21), non-amblyopic (B; n = 21) and control (C; n = 12) eyes. Positive values indicate an improvement. Measurements were made directly after (Post) and 30 minutes after (Post30) simulation. For controls, reducing the simulation duration to 10 minutes did not change the pattern of tDCS effects (D; n = 15). * = significant change from baseline, “ = significant difference from sham (paired t-test, p < 0.05).

Figure 2. Example VEP Reponses for an amblyopic eye (Patient 19), a fellow eye (Patient 19) and a control eye before (baseline), after and 30 minutes after a-tDCS.

The right column identifies the size of the check in the VEP stimulus (either 60′ or 15′) and the contrast (either 50% or 100%). Each waveform is the average of 64 repetitions.

Figure 3. A-tDCS enhanced contrast sensitivity.

The change in log contrast sensitivity relative to baseline after 20 minutes of anodal, cathodal or sham tDCS is shown for amblyopic (A), non-amblyopic (B) and control (C) eyes. Positive values indicate an improvement. Measurements were made during (Dur), directly after (Post) and 30 minutes after (Post30) simulation. For controls, reducing the stimulation duration to 10 minutes did not change the pattern of results (D). * = Significant change from baseline, “ = significant difference from sham (paired t-test, p < 0.05).

Figure 4

Relationships between amblyopia severity and tDCS induced changes in VEP amplitude (A) and contrast sensitivity (B).

Figure 5

Correlations between the effects of anodal tDCS on contrast sensitivity and VEP amplitude for amblyopic eyes (Panel A), fellow fixing eyes (Panel B), control eyes 10 min tDCS (Panel C) and control eyes 20 minutes tDCS (Panel D). The change in contrast sensitivity and VEP amplitude were collapsed across time points.