Hemispheric specialization for speech perception in retarded children

Abstract

The relation between cerebral organization and cognitive and performance deficits in children may be examined using the dichotic-listening technique. Dichotic-listening ear advantages may reflect functional asymmetries favouring the hemisphere contralateral to the advantaged ear for the processing of the dichotically presented stimuli, although additionally they may be influenced by performance factors relating to the demands of the dichotic task. The research reported in the present thesis examined the dichotic-listening ear advantages of retarded children for verbal stimuli. Auditory-discrimination training using operant procedures prior to dichotic testing was designed to minimize the influence of performance factors on ear asymmetries and to allow the assessment of functional asymmetries of retarded children. Children were trained to make one response following the monaural presentation of any one of three digits and another response following any of three other digits (Experiment I), or to respond to pictures following monaurally presented words (Experiment II). Experiment I compared the ear asymmetries of a mixed group of retarded children and a group of normal children for three dichotic tests which followed training. In two tests, digits requiring competing responses during training were presented in single dichotic pairs, either randomly interspersed among monaural training trials or blocked in a single session. In a third test (word test) concrete nouns were presented in single dichotic pairs and children responded by pointing to pictures. The normal children showed a stronger right-ear advantage (REA) for the word test than for either digit test. The retarded children showed an advantage favouring the right ear which did not differ across tests, and which, for the word test, was significantly smaller than that of the normal children. However, absolute ear advantages (ear advantages independent of their direction) were of a similar order for normal and retarded children, and numbers of normal and retarded children failing to show an ear advantage did not differ. Experiment II compared the ear advantages of groups of developmentally retarded (undiagnosed retardation), Down's syndrome and normal children for repeated presentations of the Experiment I word test before and following training. The developmentally retarded children showed a mean REA of a similar order to that of the normal children, whereas the Down's syndrome children showed an advantage favouring the left ear. Absolute ear advantages were of a similar order for the three groups and were stronger for tests presented following training than for tests presented prior to training. Training reduced biases to respond to specific words independent of their ear of presentation. It was concluded that auditory-discrimination training allows the assessment of functional asymmetries of children with cognitive and performance deficits and reduces contamination of dichotic-listening ear asymmetries by stimulus bias. Furthermore, functional asymmetries were not smaller for retarded than for normal children: developmentally retarded children may be characterized by left-hemisphere specialization for speech perception, as are normal children, and Down's syndrome children are distinguished by their higher incidence of right-hemisphere specialization.