Functions of the human brain can be studied non-invasively using various methods – electro-encephalography, functional MRI and non-invasive brain stimulations (NIBS). NIBS alter the activity of the brain by applying either mild electric current (transcranial electrical stimulations – tES) or magnetic field (transcranial magnetic stimulations – TMS) stimulations. Although NIBS techniques showed improvements of visual functions on neuro-developmental disorders such as amblyopia, a very sparse number of attempts have been made in the low vision populations such as those with macular degeneration, glaucoma and cortical blindness. To date, the effect of tES on the excitation of the visual cortex of individuals with low vision has not been studied extensively. Few attempts such as transpalpebral electrical stimulation on wet AMD and dry AMD in human participants, and repetitive TMS on retinal dystrophy in rat models showed some promising effects of NIBS on visual functions such as visual acuity and electroretinogram potentials, respectively.
In macular degeneration, most of the patients develop a motor adaptation strategy called Preferred Retinal Locus (PRL), using the retinal area that is near or outside the scotoma due to retinal degeneration/scar. The visual functions at the location of PRL are highly affected by visual crowding. Crowding is the inability of visual systems to recognize a target object if presented in clutter. This deficit is more pronounced in the peripheral vision than in the fovea. In an individual with normal vision or functional fovea, the crowding is less consequential as the eye movements like saccades would bring the image onto the fovea. However, in individuals who rely on their peripheral vision due to the less-functional fovea (e.g. macular degeneration), crowding is detrimental in tasks such as reading or recognition. There are many theories to explain the phenomenon of visual crowding (e.g. larger receptive field in the periphery, lack of attentional spatial resolution, increased fixational instability) (review by Levi, DM 2008). Recently, Maniglia et al. 2011 showed that crowding can be reduced by weakening the lateral inhibition through perceptual learning. Moreover, Reinhart et al. 2016 showed that after tDCS stimulation, Vernier acuity was improved by 15% in the peripheral vision. Thus, the results of these studies lead to a healthy speculation that after NIBS, other spatial functions such as crowding could also be altered at PRL location, which leads to enhancing the potential residual vision.
Most of the treatment options for ocular conditions (AMD, glaucoma, hemianopia) aim at stopping the progression of the disease. Therefore, the visual rehabilitation should be aimed at enhancing the residual vision. If we understand how the residual vision modulates the activity at the level of the visual cortex, then the visual rehabilitation can be provided more effectively. To study the concepts of tES not only helps in studying the activity of the brain in certain visual areas, but also helps in effectively strategizing the treatment plans in visual rehabilitation or vision therapy. Therefore, this proposed research provides insights to future treatment such as retinal prosthesis, and advanced visual rehabilitation.