Visual Temporal Sampling and Oscillatory Entrainment in Developmental Dyslexia

Developmental dyslexia is the profound inability to learn to read, independent of intelligence or access to education. Affecting approximately 10% of the population, it is associated with poor educational outcomes, reduced mental health, and low psychosocial wellbeing. Despite the prevalence of the disorder, the underlying neurobiological causes of dyslexia remain unclear. As a result of this, therapeutic interventions that target the core neurological deficits of the disorder are non-existent. This thesis addresses both concerns. Firstly, it attempts to bridge a gap in the literature in our understanding of the visual deficits in dyslexia. Building on evidence of an auditory temporal sampling deficit in dyslexia, as well as evidence of a magnocellular processing deficit, this thesis presents a visual temporal sampling framework for dyslexia that exists specifically within the magnocellular network. It suggests that a top-down, bottom-up magnocellular circuit operates at a combined theta-gamma frequency in the temporal sampling of text and phase-locking and entrainment to the rhythm of incoming text stimuli. It provides preliminary evidence that normal reading is associated with eye movements that temporally sample text at a theta frequency, and that this temporal sampling is disrupted in poor readers with a high risk of dyslexia. It also provides evidence that adults with compensated dyslexia, that is people with dyslexia who have learned to read, improve dramatically in their sensitivity to gamma frequency stimuli. It suggests that compensated dyslexia may occur via perceptual learning due to repeated exposure to text stimuli over time. Building on this, the thesis then presents a series of experiments that aim to exploit the potential perceptual learning mechanism in compensated dyslexia through the use of sensory entrainment. Both visual and auditory entrainment are trialled. Theta auditory entrainment appeared to modulate visual spatial and temporal processing, and shift eye movements during reading in a manner that was predicted by reading ability. Cumulatively, the results point to theta frequency therapeutic sensory entrainment as a potential avenue for exploration in dyslexia.