Second Language Acquisition and Second ... - Stephen Krashen

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:
1. Most of us are born with a predisposition for left hemisphere
language, and there is thus some specialization right from the start,
enough to be detected by EEG and AER, and to influence the
development of unimanual motor skills.
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