Lateralization and the Critical Period

Abstract

The percent of errors method, described in Harshman and Krashen above, was applied to dichotic listening data for children between 5 and 10 years old. No significant change in degree of lateralization was found. This supports the hypothesis that the development of lateralization is complete by age five, rather than by puberty, as argued by Lenneberg. Data cited by Lenneberg, as well as additional cases, were examined. Analysis of cases of acquired aphasia in children resulting from localized lesions revealed that right-sided lesions do not cause more cases of speech disturbance after five than in adults, indicating that the involvement of the right hemisphere in the language function in children older than five is the same as in adults; this also supports the age five hypothesis. Not enough crucial data is available from hemispherectomies to decide between the two hypotheses. The completion of lateralization by five implies that lateralization reflects the maturation of some essential component of the language faculty, rather than the termination of “organizational plasticity,” as suggested by Lenneberg.