Interactions of the processing of letters and speech sounds as reflected by event-related brain potentials
Year: 2014
Type: dissertation
Abstract: The processing of audiovisual information is ubiquitous in our daily life. As such, understanding the cortical correlates of audiovisual processing and its interactions offers a promise of practical interventions in many real-life settings. Reading, as one example, relies on the formation of artificial audiovisual associations and requires adaptions from brain mechanisms in order to process and integrate these connections effortlessly. In dyslexia, reading problems are associated with a failure in forming those associations, and neural changes and improvements of reading skills in children with dyslexia were reported after interventions ameliorated those processes. The present thesis investigates the neural networks associated with speech sound processing and discrimination when accompanied by printed text. In all studies, a high-density EEG system was utilized, enabling the examination of spatio-temporal dynamics of audiovisual processing in adult fluent readers and in readers with dyslexia. In fluent adult readers, change-related responses to consonant and pitch changes were greater when presented with printed text than with scrambled images, suggesting that letters modulate speech sound discrimination at an early cortical processing stage. This integration was sensitive to precise temporal alignment between the sounds and printed text, as it broke down when a time delay between the sounds and print was introduced. In contrast to fluent readers, adult readers with dyslexia showed a general attenuated discrimination of speech sounds when presented with print. Their neural responses for speech sounds presented with print did not differ from those presented with scrambled images. Our results, therefore, suggest that audiovisual processing is generally impaired in dyslexia, and support the notion that letter representations are poorer in readers with dyslexia than fluent readers. In addition, audiovisual processing was delayed in readers with dyslexia, suggesting a deficit in concurrent processing of multiple sensory cues. The studies of this thesis also show that attention to one of the modalities is needed for the audiovisual integration to occur, and, moreover, that audiovisual attention boosts the integration. Furthermore, our results reveal that, in addition to attention, the phonological content of the task modulates letter-speech sound processing. The studies presented in the present thesis confirmed, with a more controlled methodology, that letters modulate speech sound discrimination at an early neural level. The present results illuminate the way these processes are impaired in dyslexia, and, further, that audiovisual attention is most beneficial for such an integration to occur. To conclude, the studies at hand have shed novel light on the basic and aberrant mechanisms of letter-speech sound processing, and can be used, for instance, in training programs to promote accurate mapping of letters and speech sounds, and, consequently, reading skills in individuals with dyslexia.
Source: Työväentutkimus Vuosikirja
Author: Maria Mittag
Language: English
Subfield: Developmental and Educational Psychology
Field: Psychology
Domain: Social Sciences
Open Access status: green
OpenAlex ID: W1499929670