Evoked potentials in infants and kindergartners: Their contribution in detecting developmental dyslexia
Dyslexia, a specific reading disability, is one of the most common psychological problems which seriously impairs school achievement and interferes with the acquisition of knowledge of written sources. As the hereditary component of dyslexia is becoming more and more firmly established nowadays, there has been a growing interest in investigations of children at familial risk for dyslexia before the onset of reading instruction, given that infants and kindergartners born to families with a background of developmental dyslexia have an increased risk of becoming dyslexic. The aim of the present article is to review current electrophysiological studies of infants and kindergartners with and without familial risk for dyslexia in relation to reading acquisition.
Event-Related Potentials (ERPs) are relative small changes in the electroencephalogram in time locked to a given event or stimulus. ERP components can provide evidence for the existence, order and timing of the independent process of information processing. ERPs are shown to be effective indices of auditory processes involved in speech perception and thus, apparently also helpful in revealing the neuronal basis of language problems associated with difficulties in reading. The auditory processing of physical stimulus features can be measured by the mismatch negativity (MMN). MMN is a negative component of auditory ERP, which is elicited by any discriminable change (deviant stimulus) in some repetitive aspect (standard stimulus) in the ongoing auditory stimulation, reflecting the brain's automatic change detection process. Some studies have also measured the N100, a negative component elicited about 100msec, thought to represent the initial extraction of information from the sensory analysis of the stimulus.
Several ERP studies have found differences between infants and kindergartners at familial risk for dyslexia and controls in MMN and N100, during the processing of consonant-vowel syllables. The results of these studies demonstrate that infants born with a familiar risk for dyslexia process speech/auditory stimulus durations differently from controls. MMN and N100 have shown to be potential indicators, each revealing somewhat different aspects of subtle weaknesses of auditory processing which may reduce the accuracy of phonological representations thought to form a central core deficit in dyslexia. Common to these studies is the tendency of right hemisphere dominance in the ERP responses of the at-risk group and left hemisphere dominance in the typically developing group. Results of more recent studies also suggest that early deficits in children at-risk for dyslexia are not restricted to lower level auditory or phonological processing, but also involve higher-order language skills such as lexical and semantic processing. ERPs have also been found to relate to later language outcomes. Longitudinal studies which found out that electrophysiological measures obtained at birth successfully discriminate between infants who will display later, during their school years, different levels of reading skills, raise strong possibilities regarding the early identification of children with potential language and/or reading problems.
Overall, the ERP findings we discuss here are promising because they indicate that the ERP technique can reveal new and useful information about underlying brain function that is related to rapid perceptual processing in infants and kindergartners. This review also suggests that ERPs during infancy may -in the not too distant future- be a useful tool for early screening, which might be employed even at an individual level. Identification of early precursors would not only be crucial for the understanding of the nature of dyslexic disorders but also enable both an earlier than present onset of intervention, and a focus of intervention efforts on those children in the risk group that are in need of support. This opens up the possibility that reading and language problems could be tackled before becoming serious during the school years.
Key words: Evoked potentials, dyslexia, infants, kindergartners.