Unilateral Application of Cathodal tDCS Reduces Transcallosal Inhibition and Improves Visual Acuity in Amblyopic Patients

Abstract

Objective: Amblyopia is a neurodevelopmental disorder characterized by visual acuity and contrast sensitivity loss, refractory to pharmacological and optical treatments in adulthood. In animals, the corpus callosum (CC) contributes to suppression of visual responses of the amblyopic eye. To investigate the role of interhemispheric pathways in amblyopic patients, we studied the response of the visual cortex to transcranial Direct Current Stimulation (tDCS) applied over the primary visual area (V1) contralateral to the “lazy eye.”Methods: Visual acuity (logMAR) was assessed before (T0), immediately after (T1) and 60’ following the application of cathodal tDCS (2.0 mA, 20’) in 12 amblyopic patients. At each time point, Visual Evoked Potentials (VEPs) triggered by grating stimuli of different contrasts (K90%, K20%) were recorded in both hemispheres and compared to those obtained in healthy volunteers.Results: Cathodal tDCS improved visual acuity respect to baseline (p < 0.0001), whereas sham polarization had no significant effect. At T1, tDCS induced an inhibitory effect on VEPs amplitudes at all contrasts in the targeted side and a facilitation of responses in the hemisphere ipsilateral to the amblyopic eye; compared with controls, the facilitation persisted at T2 for high contrasts (K90%; Holm–Sidak post hoc method, p < 0.001), while the stimulated hemisphere recovered more quickly from inhibition (Holm–Sidak post hoc method, p < 0.001).Conclusions: tDCS is a promising treatment for amblyopia in adults. The rapid recovery of excitability and the concurrent transcallosal disinhibition following perturbation of cortical activity may support a critical role of interhemispheric balance in the pathophysiology of amblyopia.

Multi-session visual cortex transcranial random noise stimulation in adults with amblyopia

“…Our observation that visual cortex tRNS improved amblyopic eye contrast sensitivity is consistent with a growing literature reporting improved contrast sensitivity, visual acuity, stereopsis, and an enhanced cortical response to amblyopic eye inputs following non-invasive visual cortex stimulation in adults with amblyopia [26,[37][38][39][40]66,67]. Two potential mechanisms have been proposed for tRNS effects: stochastic resonance and changes in the resting membrane potential [51,68].…”

Patching and Suppression in Amblyopia: One Mechanism or Two?

“…Later, by using daily continuous theta burst stimulation (cTBS) of the visual cortex, Clavagnier et al (2013) found that the effect of daily cTBS on contrast sensitivity could be accumulated and is long lasting. The findings were also reported by other studies (Ding et al, 2016;Bocci et al, 2018). This is suggested to be a result of altering the balance between excitation and inhibition of targeted brain areas by the non-invasive brain stimulation (Nitsche and Paulus, 2000;Campana et al, 2016).…”

Hemispheric Lateralization of Arithmetic Facts and Magnitude Processing for Two-Digit Numbers

“…In case arithmetic facts are indeed processed exclusively in the left hemisphere, unilateral input into the left visual hemifield and thus initial transmission to the right hemisphere should lead to a processing disadvantage, reflected by, for instance, longer response latencies and lower accuracy compared to unilateral presentation of the stimuli into the right visual hemifield. Evidence for interhemispheric processing and its modulation has been provided by several studies using tDCS on the non-dominant hemisphere for the task at hand in higher cognitive processing, such as arithmetic fact retrieval (e.g., Clemens et al, 2013; for anodal stimulation) and primary perceptual processing (e.g., Bocci et al, 2018; for vision acuity). Ratinckx et al (2006) demonstrated that the divided visual field paradigm showed differential effects for the case of the unit-decade compatibility effect in two-digit number magnitude comparison, which is supposed to be processed in the right hemisphere (Wood et al, 2006; for a review see Nuerk et al, 2011).…”