Developmental dyslexia: Brain oscillations analysis

Abstract

Purpose: Developmental dyslexia is frequently characterized by co-morbid deficits in speech processing, phonological and morphological awareness. The Lexical Quality hypothesis suggests that the lack of tight integration of orthographic, phonological, and lexico-semantic information leads to poor reading and comprehension. Recent studies have suggested that theta frequency band is functionally related to early orthographic and phonological processing and retrieval of lexico-semantic information. But little is known about brain oscillation dynamics of these processes. Method: In this study we used event-related time-frequency analysis to investigate the awareness of morphological representations in children with dyslexia. Age-matched typically developing and dyslexic children were asked to verify whether a visually presented target word matched the preceding spoken word in a homophone verification task. The semantic congruency of heterographic homophone pairs was manipulated (e.g., [藍色嘅藍] - the blue of the color blue, followed by 藍 /laam4/ - blue or 籃 /laam4/ - basket). Results: We found that incongruent word pairs elicited greater theta power than congruent ones in the anterior sites. However, semantic congruency effects were only found in the control group. In addition, children with dyslexia elicited larger theta power than controls over the whole scalp, particularly in the anterior left region. Conclusions: In line with previous findings indicating the role of theta band in ambiguity resolution and early orthographic processing, we suggest that greater theta power reflects increased overall effort in lexical retrieval and semantic processing in dyslexia. Furthermore, the lack of congruency effects in dyslexic readers may suggest a weak integration among orthographic form, lexical semantic functional networks and working memory.