Research Project: Dyslexia and ERP
Left: The Geodesic Sensor Net, comprising 64 electrodes, is carefully placed on the subject's head
Centre: The subject performs a visual experiment while the 64 electrodes record his brain's EEG
Right: This EEG data is simultaneously displayed on a separate screen and saved for analysis
Electrophsiological Evidence of Sensory and Cognitive Deficits in Visual and Auditory Processing in Dyslexia
Aditi Shankardass
Background:
For those engaged in trying to understand the cause of dyslexia, these are interesting times. There is now increasing evidence that dyslexia may result from deficits in non-linguistic processing in the brain, which may then contribute to the observed linguistic difficulties. However, while some suggest that these deficits are confined to low-level auditory and visual processing, others propose that they are specific to higher-level cognitive processing. Moreover, while some report that the perceptual problem is confined to rapidly-presented and dynamic stimulus discrimination, others claim that it includes the discrimination of temporal and spectral features. 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 group of dyslexics, using neurophysiological measures. This was the central purpose of this research.
Method:
In four studies, event-related potentials (ERPs) were recorded from eight dyslexic and eight control brains during the preattentive and attentive discrimination of various visual and auditory stimulus features. Average dyslexic and control ERPs were compared for the perceptual N1 and MMN components in the preattentive, and the cognitive P2, N2 and P3 components in the attentive response conditions.
Results:
The dyslexics had abnormal MMNs during the automatic detection of changes in the peripheral visual field, auditory frequency and rapid auditory sequences, but not auditory duration. Moreover, they had abnormal P2 or P3 components during the conscious discrimination of all stimulus features. Finally, their MMNs during auditory frequency discrimination were no longer attenuated when reassessed in a second preattentive session following the attentive session.
Conclusion:
The feature-specific MMN abnormalities suggest low-level, perceptual deficits in dyslexics, as predicted by the multisensory/magnocellular deficit theory. However, the ubiquitous task-related P2 and P3 abnormalities suggest independent, high-level cognitive impairments, as predicted by the automatisation/cerebellar deficit theory. Lastly, the absence of abnormal MMNs after the attentive session suggests practice-induced improvements in their perceptual acuity. These findings suggest that dyslexia is a 'multilevel syndrome': the same group of dyslexics have improvable deficits at multisensory (visual and auditory) and cognitive processing levels.
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