Reading words and pseudowords in dyslexia: ERP and behavioural tests in English-speaking adolescents.

The study reports neurophysiological and behavioural correlates of lexical decision processes in English speaking dyslexic and non-dyslexic readers. Nine dyslexic adolescents (ages 15.6-17.8) and 9 control (ages 15.4-19.3) adolescents were tested, and the event related potentials (ERPs) to words and pseudowords were recorded simultaneously with behavioural measures. As expected, both groups showed significantly lower accuracy and longer response times for the pseudowords. Furthermore, overall performance (in terms of lower accuracy and longer response times) was significantly worse for the dyslexic group. The ERP analysis indicated that the later positive peaks, P4 (around 400 ms from stimulus onset) and P5 (around 500 ms), were significantly delayed and attenuated for the dyslexic group. Analysis of the early ERP peaks recorded in the occipitotemporal region led to an interesting dissociation. The controls showed a left lateralised Condition effect, with the amplitude of P1 significantly smaller to words than pseudowords. By contrast, there was no such lexical effect for the dyslexic group, with equal P1 amplitudes for words and pseudowords, at the control level for pseudowords. The deviations in the early ERP components of dyslexics support the evidence of deficits in pre-lexical visual word form recognition within the first 110 ms of activation together with altered hemispheric asymmetry. In addition, the slowed and attenuated late ERP components and weaker behavioural performance of the dyslexic group highlight deficits in the later, cognitive, processing stages.