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Daniel P. Spiegel, Robert F. Hess, Winston D. Byblow, Benjamin Thompson; Transcranial Direct Current Stimulation of the Visual Cortex in Adults with Amblyopia. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3894.
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It has been demonstrated that repetitive transcranial magnetic stimulation of the visual cortex can temporarily improve contrast sensitivity in adults with amblyopia, possibly by modulating the balance of neural excitation and inhibition. Transcranial direct current stimulation (tDCS) is an alternative technique for modulating neural excitation and inhibition. tDCS is advantageous as it is cheap, safe and portable making it convenient for clinical use. The aim of this study was to investigate whether tDCS of the visual cortex could improve contrast sensitive in adult amblyopia.
12 adults with amblyopia received 15 minutes of anodal or cathodal 2mA tDCS delivered over the occipital poles. Contrast detection thresholds for a sinusoidal grating stimulus with a patient-specific spatial frequency were measured directly before, during, after, and 30 minutes after stimulation. Participants were unaware of the type of stimulation used in each session.
Anodal stimulation tended to increase and cathodal stimulation tended to decrease contrast sensitivity in both eyes (F1, 11 = 13.9, p = 0.003). However, only the detrimental effect of cathodal stimulation on fellow eye contrast sensitivity was reliably different from baseline during (t11 = 2.669, p = 0.022) and immediately after (t11 = 3.147, p = 0.009) tDCS. A subgroup of 9 participants who responded to anodal tDCS were analysed separately. In this group there was a significant improvement in amblyopic eye contrast sensitivity immediately (t8 = 3.877, p = 0.005) and 30 minutes after (t8 = 3.315, p = 0.011) anodal tDCS. Across all participants, the amount of improvement in amblyopic eye contrast sensitivity after anodal tDCS was negatively correlated with the variance of the threshold measurements during (r = 0.683, p = 0.014), after (r = 0.611, p = 0.035) and 30 minutes after anodal tDCS (r = 0.804, p = 0.001).
tDCS did not reliably improve contrast sensitivity for the group of 12 observers with amblyopia as a whole, however improvements were found in a sub-group of 9 observers. In addition, improvements in contrast sensitivity were associated with a reduction in threshold variance. These data indicate that further investigation into the potential application of tDCS to amblyopia is warranted.
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