Developmental versus individual differences in the ability of the hemispheres to operate independently.

We have claimed that the ability to use the two cerebral hemispheres independently improves during early adolescence: Adults benefited from division of conflicting tasks between the hemispheres (Liederman & Meehan, in press; Merola & Liederman, 1987), whereas 10-year-old children did not (Merola & Liederman, 1985). The current research was undertaken to rule out the possibility that differences in the degree of hemispheric independence between adults and children were due to: (a) differences in overall performance or (b) differences in cognitive ability. The performance of ten adults was reduced to that of the 10-year-olds (by inserting a visual mask between trials). Nonetheless, 90% of the adults showed hemispheric independence, whereas only 50% of the children did. In addition, when high scholastic aptitude children were compared to adults, the difference in hemispheric independence remained. A causal model indicated that age and scholastic aptitude have unique effects upon hemispheric independence. Thus, there are both developmental and individual differences in the degree of hemispheric independence.