(e.g. the pair "2" (right ear) and "9" (left ear) would be presented, followed one half-second later by "6" (right ear) and "4" (left ear), followed by "1" (right ear) and "3" (left ear). The subject is asked to recall as many digits as possible out of the six presented.) In most other studies, one single pair of syllables (e.g. "ba" and "ga") or words were used, a lower short-term memory load. This raises the possibility that there might be two different developmental courses for two different kinds of language processing, one complete lateralized to the left hemisphere by puberty and the other much earlier in life.
2. Motor skills. Early completion of the development of cerebral dominance is also indicated by studies examining the development of unimanual motor skills. Caplan and Kinsbourne (1976), in a paper appropriately titled "Baby drops the rattle", provide an interesting example. The experimenters gave rattles to twenty-one infants (average age 21 months, 21 days) and found that the babies tended to hold the rattles longer when they were placed in their right hands (average duration of grasp = 62 seconds for the right hand and 41 seconds for the left hand), suggesting early lateral specialization of the central nervous system. Witelson (1977) has reviewed many studies of this sort using children age 7 and younger and concludes that "right-hand or right-sided superiority was observed in the large majority of the 34 studies" (p. 236). This supports the hypothesis that there is an early propensity for unimanual performance in motor tasks, which is consistent with the hypothesis that cerebral dominance is established early in life.
3. Brain damage. Studies examining the effects of unilateral brain damage on language in children also imply that lateralization is complete earlier than puberty. In later childhood (age 5 and older), just as in adults, aphasia is primarily the result of left hemisphere lesions. It appears to be the case, however, that before age 5 aphasia may result from right hemisphere lesions somewhat more frequently (about 30 per cent of the time, as compared with 3 per cent in adults and older children). Still, even for very young children, aphasia is associated more often with left lesions than with right lesions (for reviews, see Witelson, 1977; Krashen, 1973a; Hecean, 1976).
4. Hemispherectomy. The removal of an entire hemisphere, hemispherectomy, is perhaps the ultimate test of laterality. If the left