Lateralization of speech perception and knowledge of number concepts in adolescents with Down syndrome

grantor: University of TorontoIndividuals with Down syndrome demonstrate significant language delays as compared to individuals with intellectual disabilities and children of similar mental age. The lateralization of speech perception and speech production in individuals with Down syndrome has been investigated to determine possible links between cerebral asymmetry and speech difficulties. The present study investigated the performance of individuals with Down syndrome, individuals with intellectual disabilities, and nondisabled children matched on mental age on dichotic listening tasks using digits and consonant vowel syllables as stimuli in both Free Report and Directed Attention conditions. Performance on the dichotic listening tasks was further examined in terms of the participants' knowledge or understanding of number concepts and the novelty of the task and stimulus conditions. Knowledge of number concepts was assessed using nine experimental tests developed by the author for this study. Ten adolescents with Down syndrome, ten non-Down syndrome adolescents with intellectual disabilities of unknown aetiologies, and nine nondisabled preschoolers participated in the present study. On the dichotic listening tasks, the three groups showed comparable patterns of responding to dichotically-presented pairs of stimuli. All groups showed a right hemisphere - left ear superiority for digits and a left hemisphere - right ear advantage for CVs. In terms of knowledge of number concepts, all three groups were equally skillful at counting. The adolescents with Down syndrome were significantly better than either of the other groups on numeral reading. All groups performed uniformly poorly on all tests of number word reading. The results of the correlation analyses of the relationships between the number concept tests and the lateralization indices provided tentative support for the hypotheses of a relationship between well developed knowledge of number concepts and left-hemisphere processing of digits and CVs. Goldberg and Costa's (1981) model of cerebral asymmetry accounts for the differential processing of the dichotic stimuli based on the difficulty of the dichotic task when digits are the stimuli. Furthermore, the findings are discussed in terms of Dehaene and Cohen's (1998) 'triple code' model for the representation of numbers. Task complexity, multiple dichotic trials, different dichotic listening procedures (i.e., stimulus detection), and more detailed analysis of cognitive and linguistic abilities are recommended as the focus of future research.Ph.D