In order to investigate the development of lateralization of language functions, asymmetry of auditory perception was taken as an index of the lateralization in this report. It was measured by means of a dichotic listening test (DLT) consisting of 25 pairs of two-syllabic meaningful words and 10 pairs of two-syllabic meaningless words. Subjects were 120 normal children (aged 3 to 12) and 20 normal adults (aged 20 to 40).
The results were as follows:
1. Methodological problems.
a) It was found that DLT with meaningful words (DLTmf) was useful in detecting binaural differentiation, whereas DLT with meaningless words (DLTmI) was useful in detecting auditory asymmetry.
b) The results of lateralization in DLTmf coincided with those in DLTml at a rate of 91.5%. Therefore, it was suggested that both of DLTmf and DLTml should be undertaken to tell the lateraliaztion of the brain accurately. Furthermore, it seemed that the application of the tests was more appropriate for group studies than individual studies.
c) The laterality was detected sensitively (at 3 to 8 years of age) by DLTmf and (at 3 to 12 years of age) by DLTml.
The analysis of the laterality was found to be difficult in older children, mainly because of the ceiling effect.
2. Development of binaural differentiation (BD).
a) DLTmf showed that BD developed markedly in the period of 6 to 8 years.
b) Children of 3 to 5 years of age showed little both ear reaction, and even older children showed its development very slowly in DLTml. Besides, compound reaction was observed at a high rate in every age group in DLTml. Compound reaction was rarely found in DLTmf. Those facts suggested that binaural differentiation was related not only with the distinction of phonemes but also with the acquisition of semantics.
c) Both DLTmf and DLTmf showed no sex differences.
3. Auditory asymmetry.
a) DLTmf and DLTmf showed that right ear superiority (lateralization of the left hemisphere in language functions) had appeared by the age of 3. This was a piece of counterevidence against the equipotentiality hypothesis.
b) A laterality index in any age showed no significant increase.
c) The developmental rate of lateralization showed sex difference in neither DLTmf nor DLTml.
According to those results, methodological problems in DLT, correlation between development of binaural differentiation and that of cognitive functions, and auditory asymmetry were discussed with many literary references. The results did not support the equipotentiality hypothesis.
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