Neurophysiological evidence of sensory and cognitive deficits in dyslexia
For those engaged in trying to understand the cause of dyslexia, these are interesting times. There is increasing evidence that dyslexia may result from a deficit in the brain's ability to process general visual and auditory information, which may subsequently contribute to observed language difficulties. While some suggest that this processing deficit is confined to lower perceptual levels, others propose that it extends to higher cognitive levels of attention and learning. So far there is surprisingly little evidence of research wherein both modalities, both processing levels and various stimulus features have been tested in the same set of dyslexics using electrophysiological measures. This was the purpose of this research. In four studies, event related potentials were recorded from dyslexic and control brains during the non-attentive and attentive discrimination of various visual and auditory stimuli. Average dyslexic-control ERP comparisons were made for sensory N 1 and MMN waves in the passive, and cognitive P2, N2 and P3 waves in the active response conditions. Dyslexics had attenuated MMNs during the pre-attentive discrimination of changes in peripheral visual field, auditory frequency and rapid auditory sequences but not auditory duration. Moreover, dyslexics had abnormal P2 or P3 waves during the attentive discrimination of all visual and auditory stimuli. Finally, the previously attenuated MMN to frequency discrimination was enhanced after attentive practice. The feature-specific MMN abnormalities suggest a highly selective, multi-modal, perceptual dysfunction in dyslexics, as predicted by the pan-sensory deficit theory. However, the ubiquitous task-related P2 and P3 abnormalities suggest that their deficits also extend to higher cognitive domains, as predicted by the automatization/cerebellar deficit theory. The subsequent MMN enhancement suggests practice-induced improvements in their perceptual acuity. These findings suggest that dyslexia is a multilevel syndrome: the same dyslexics have problems in both domains: visual and auditory, and at both processing levels: sensory and cognitive.