The Impact of Transcranial Direct Current Stimulation on Reading Performance in Individuals with Macular Degeneration: A Study of English and Chinese Writing Systems
Peripheral vision is often susceptible to crowding, a difficulty in distinguishing neighbouring objects or features, which can impair face and object recognition and reading in individuals with central vision loss. Anodal transcranial direct current stimulation (tDCS) of the primary visual cortex has been shown to reduce crowding in normal peripheral vision. A recent case series have also suggested that visual cortex tDCS reduces collinear inhibition, a phenomenon related to crowding, in patients with central vision loss due to macular degeneration.
To build on this previous research, a pre-registered, within-subjects, randomized, double-blind, placebo-controlled study was conducted to test the hypothesis that reading would be improved during, immediately after, and 30 minutes after visual cortex tDCS in adults with central vision loss due to macular degeneration. The study also aimed to assess whether the effect of tDCS would vary between two writing systems, the Roman alphabet and traditional Chinese characters.
Twenty-one individuals with macular degeneration participated in the study. Participants performed a rapid serial visual presentation (RSVP) verbal reading task, where each word or character in a sentence was presented sequentially. Reading accuracy was measured as the total proportion of words or characters read correctly. Participants completed both an active and a sham (placebo) tDCS session, with the order of stimulation type randomized. TDCS was administered using two electrodes and the anode was placed over the visual cortex and the cathode over a randomly selected cheek.
Results showed that there was a significant interaction between stimulation type and language, with active stimulation increasing reading accuracy for English sentences, but not for Chinese characters. Further analyses revealed no statistically significant effects for either language group, likely due to the limited sample size.
These results demonstrate that visual cortex tDCS influences reading performance in individuals with macular degeneration differently depending on the writing system. This information will be valuable in guiding the development of vision rehabilitation programs for macular degeneration that may utilize non-invasive brain stimulation. Further research is needed to explore the effect of tDCS on between-character crowding within strings of characters in Chinese characters.
In conclusion, the study provides evidence for the potential use of tDCS in enhancing reading performance in individuals with macular degeneration. However, more research is needed to fully understand the effects and potential applications of tDCS in this population.
Reference Paper
Silva, A.E. et al. (2022) “A differential effect of visual cortex tDCS on reading of English and Chinese in patients with Central Vision Loss,”
Brain Stimulation, 15(5), pp. 1215–1217.
Available at: https://doi.org/10.1016/j.brs.2022.08.016.