Using tDCS and tACS to understand the role of dorsal processing and theta signals in word recognition and natural reading

Abstract

The dorsal visual pathway has long been associated with reading and dyslexia, however the mechanism by which the dorsal pathway is involved in reading is unclear. The most robust theory is that the dorsal pathway is responsible for visuo-spatial encoding of letters within words, and words within text. Complementing this is a recent theory suggesting specifically that the process of reading generates a theta signal via the dorsal pathway which is intrinsic to normal reading development. We ran 2 studies; a tDCS study to explore the role of the dorsal pathway in spatial encoding within words, and a tACS study to determine whether an entrained theta signal enhances the mechanics of reading. Study 1 used anodal, cathodal and sham tDCS with anodal stimulation over the right posterior parietal cortex. Participants engaged in a single-word reading task where the letters within words were displaced vertically in order to enhance visuo-spatial processing, and eye movements were minimised. Responses under anodal stimulation were significantly slower than sham in session 1. However this had reversed by session 3 where responses were faster, and spatially altered text was affected relatively more than normally presented words. Here then, stimulation disproportionally affected words with altered spatial properties, supporting the proposal that the dorsal visual pathway has functional responsibility over the spatial encoding of letters in reading. In Study 2, participants read blocks of text while tACS selectively entrained a 5Hz theta signal over the frontal eye fields. Compared with the sham condition, the 5Hz signal enhanced reading speed, and decreased the number of fixations and dwell time on words. The results from these studies are consistent with the proposal that the visual system, via the dorsal pathway, samples text at a theta rhythm in a way that facilitates spatial encoding when reading.