Brain responses to changes in tone and speech stimuli in infants with and without a risk for familial dyslexia

Abstract

Developmental dyslexia, a specific disorder of learning to read, is widely acknowledged as being often genetically transmitted, but as yet, little is known of the early precursors of the disorder. In the present study the brain electrical responses to changes in auditory oddball paradigms were studied both at birth and at the age of six months in a group of young infants from families with a genetic background of developmental dyslexia (at-risk group) and in a group of infants without such a background (control group). Both group differences and developmental features of event-related potentials (ERPs) were under focus. At birth, the ERPs to the deviating rare /ka/ stimuli with an interstimulus interval of 855 ms were different between the groups, but not with a shorter interval of 425 ms. At six months of age some differences between the groups were found in their responses to the frequent /kaa/ stimulus. At this age, more marked group differences were found in the ERPs to the duration change of a silence period, an acoustic cue for consonant lengths, embedded within a pseudoword (/ ata/ vs. / atta/). The results suggest, that the at-risk group differ from the control group both in the basic responsiveness to auditory/ speech stimuli and in the responses to changes in the temporal structure of speech stimuli, as well as in terms of how the stimulus presentation context affects the ERPs. The clearest developmental feature at birth was a waveform consisting of a long lasting positive deflection, which was generated by all deviant stimulus types, whether pure tones or speech stimuli differing in their temporal features. By the age of six months, this slow waveform pattern had been transformed into a wave complex with clearly distinguishable deflections, reflecting a more mature negative-positive-negative deflection structure. The responses to deviant stimuli were, typically still at this age, positively displaced in relation to those to the standard stimuli, suggesting maturational effects on early ERPs. Some evidence for an adult-like negatively displaced mismatch negativity (MMN) response was also found.