hemisphere is removed, and the patient is still able to speak, we can assume that the right hemisphere had at least some part in the language function before the surgery. The literature indicates that early removal of the left hemisphere for lesions incurred before age 5 does not generally result in aphasia. This result, like the data from unilateral lesions ((3) above) implies at least some right hemisphere participation in the language function in early years (Krashen, 1973a; Witelson, 1977).
The hemispherectomy data are clear only up to age 5 (Krashen, 1973a), as this surgical procedure is rarely used with older children.
5. EEG and AER. Research using EEG and AER techniques suggest that signs of hemispheric specialization are present even at birth. When infants as young as 2 weeks old are presented with verbal stimuli (e.g. the mother's voice), the auditory evoked response is greater over the left hemisphere; when musical stimuli are presented (e.g. a music box), the AER is greater over the right hemisphere (Molfese, 1976). EEG results are similar: we see left-right differences in infants as young as 5 months old (Gardiner and Walter, 1976).
6. Some researchers have looked for anatomical differences between the two hemispheres: slight differences do in fact exist in the adult brain (Geschwind and Levitsky, 1968) and it has been confirmed that similar left-right morphological differences exist in the infant and even in the pre-natal brain (Witelson and Pallie, 1973; Wada, Clarke, and Hamm, 1975), suggesting at least the potential of predisposition for hemispheric specialization.
What can we conclude about the development of cerebral dominance from this array of reports? We have seen some evidence for "lateralization by zero" (consistent with EEG, AER, and anatomical studies, as well as some dichotic listening studies and experiments using unimanual motor skills), some for "lateralization by 5" (clinical data on brain damage and hemispherectomy) and some for lateralization by puberty (dichotic studies using more difficult stimuli). A possible solution is to posit the following developmental course: