Visual cortex anodal tDCS does not alter reading performance for Chinese characters presented in left lateral peripheral vision

Purpose : When applied to primary visual cortex, anodal transcranial direct current stimulation (a-tDCS) has been reported to reduce crowding in peripheral vision indicating a potential therapeutic use for patients with central vision loss. To further assess the potential of a-tDCS as a vision rehabilitation tool, we tested the hypothesis that visual cortex a-tDCS, but not sham a-tDCS, would improve reading of Chinese characters presented to left lateral peripheral vision in normally sighted adults.

Methods : Sentences of Chinese characters were presented one character at a time 10o to the left of fixation at various speeds and print sizes. Participants (n = 10, 60 to 73 yrs. of age) read the sentences out loud and character recognition accuracy was recorded. Accuracy data were fitted with a continuous linear piecewise function that rises and then flattens to estimate all combinations of print sizes and speeds that elicit 55% accuracy. The point where both linear functions meet denotes the critical print size (CPS), defined as the smallest print size eliciting the participant’s fastest reading speed. Using a double-blind, within-subject design, participants were then tested with sentences of print size and reading speed corresponding to 0.2 logMAR below the CPS on 2 stimulation days on which 20 mins of active or sham a-tDCS was applied to the visual cortex. Each test consisted of 15 sentences, and one test was administered before, during, 5 mins post, and 30 mins post stimulation. Reading accuracies were compared across time-points.

Results : A within subjects ANOVA revealed no significant interaction between Stimulation Type (anodal vs. sham) and Time (pre, during, 5 min post and 30 min post stimulation) (F(3,27)=0.59, p=0.63).

Conclusions : A-tDCS did not improve reading of Chinese characters in left lateral peripheral vision in normally sighted individuals. The effect of a-tDCS on reading of Chinese characters or other writing systems presented to other regions of the visual field, including inferior peripheral vision, is yet to be determined.