Transcranial Direct Current Stimulation of the Healthy and Amblyopic Visual Cortex: Mechanisms and Action

Abstract

Amblyopia is a relatively common visual disorder characterised by an abnormal pattern of inhibition within the visual cortex. This impairment can affect quality of life and is associated with health risks. Current treatment approaches are based on patching or penalising the fellow fixing eye and are traditionally considered ineffective in adults, presumably due to insufficient visual cortex plasticity. The aim of this dissertation was to assess the effects of transcranial direct current stimulation (tDCS) on visual function within both the healthy and amblyopic visual cortex and evaluate its potential use in the treatment of adults with amblyopia. Four studies were conducted in total. Study 1 was conducted in healthy adults to investigate whether anodal tDCS is capable of decreasing the tone of inhibition in the visual cortex. The amount of intracortical inhibition, behaviourally manifested as surround suppression, was quantified with and without tDCS. Studies 2 and 3 aimed to assess the effects of a single dose of tDCS on the amblyopic visual cortex via behavioural (threshold contrast sensitivity) and functional (functional magnetic resonance imaging – fMRI) measures. The objective of study 4 was to translate these findings into a clinically applicable treatment approach combining tDCS and dichoptic perceptual training in adult patients with amblyopia. We found that anodal tDCS decreased the tone of intracortical inhibition within the healthy visual cortex manifested by a decreased amount of surround suppression. In a cohort of adult amblyopic patients, contrast sensitivity of the amblyopic eye improved and fMRI activation of the amblyopic visual cortex normalised after anodal tDCS. These data also indicated a reduction in tone of intracortical inhibition. The final experiment showed that a combination of anodal tDCS and dichoptic training can result in greater and/or more rapid improvements in vision in adults with amblyopia than dichoptic training alone. Our results indicate that anodal tDCS of the visual cortex has potential to be used as an adjunct therapy in the treatment of amblyopia in adults.