Lexical decision making in adults with dyslexia: An event-related potential study

Abstract

Performance on a lexical decision task was investigated in 12 English speaking adults with dyslexia. Two age-matched comparison groups of unimpaired readers were included:14 monolingual adults and 15 late proficient bilinguals. The aim of the study was to determine the timing of neural events with event-related potentials (ERPs) during lexical decisionmaking between individuals with dyslexia and unimpaired readers (both unilingual and bilingual). ERPs were calculated for posterior sites in the left and right hemispheres and the P1 and N170 components were compared between groups. Eventrelated EEG coherence (measuring the synchrony of neural events during lexical tasks both between and within cerebral hemispheres) was also calculated for seven electrode pairs (three pairs at symmetrical locations between hemispheres, and two pairs within each hemisphere). We chose to recruit two comparison groups of unimpaired readers to better clarify the findings resulting from the right hemisphere (EEG) coherence analysis. That is, both late-proficient bilinguals and adults with dyslexia are thought to rely on right hemisphere resources during reading. We hypothesized that those with dyslexia would show less within-hemisphere coherence and more betweenhemisphere coherence than bilingual individuals. Dyslexics had both lower amplitude and longer latency N170 activation than unimpaired readers, suggesting asynchronous neural activity. Dyslexics showed greater synchrony between hemispheres in gamma range frequencies whereas the bilingual group showed greater synchrony in the theta frequency band (both within and between hemispheres). This study demonstrates that individuals with developmental dyslexia have reduced amplitudes in the N170 and higher synchrony between hemispheres during a reading task. The differences may be due to an asynchrony of neuronal activity at the point where word form features are distinguished.