The main conclusion of this study is that dyslexic subjects are less sensitive than control subjects at detecting both visual coherent motion in RDK stimuli and low rates of auditory FM (2 Hz and 40 Hz), whereas their thresholds for 240 Hz auditory FM are not significantly different. Furthermore, for both dyslexic and normal readers, low frequency FM sensitivity and visual-motion sensitivity correlated highly. Finally, both our auditory and visual measures of temporal perception predicted significant proportions of the variance in phonological skills of both dyslexic and control subjects, as assessed by nonword reading.
Timing of stimuli in the visual system is dependent upon a system of large neurons comprising the magnocellular retinocortical stream, one of the two main parallel visual pathways [25] and [27]. High temporal sensitivity is a characteristic property of the receptive fields of these magnocells [25] and [27]. They have large cell bodies, thick myelinated axons and, consequently, high conduction velocities that provide an efficient substrate for encoding information about stimulus timing and change. Lesions to cortical and sub-cortical visual areas which receive predominant magnocellular input therefore cause selective deficits in the detection of time-varying stimuli, such as motion and flicker, without imparing non-temporal visual functions, such as colour and form discrimination [26], [34] and [37]. There is now considerable evidence that dyslexic individuals have impairment of the visual magnocellular system (see [22] for review), and our demonstration here that dyslexic individuals have lower sensitivity to visual motion provides further support for this idea.