Development of Cerebral Lateralization in Children

Abstract

Lateral asymmetry prevails at levels of organization that range from subatomic particles to the human body and brain. At the subatomic level, asymmetry (‘‘chirality’’) seems to be the default option (Hegstrom & Kondepudi, 1990). In nature, bilateral symmetry appears in response to species-specific selection pressures, not because there is something ideal about symmetry, as has been held since classical times. That functional representation in the brain is bisymmetric was taken for granted, until Paul Broca finally overcame this strong bias as recalcitrant unilateral data from his aphasic hemisphere-damaged patients kept appearing. In 1862, he reluctantly acknowledged in public that although speech is located in the anterior cerebrum, as the phrenologists had claimed, it is represented in the front left, rather than equally on both sides. From then on, another equally unexamined assumption was substituted. It credited the left hemisphere with being the unique peak of the brain’s functional hierarchy. It seemed obvious that there had to be a leading hemisphere. The left hemisphere was considered major or dominant. The right, minor or subdominant, hemisphere was like the left, at the ‘‘animal’’ level, but unlike the left, it was devoid of those capabilities, like language, that elevate humans over animals. So while anatomical bisymmetry had to be abandoned, it took another century and much cultural change before Oliver Zangwill and his colleagues overthrew functional hierarchy in the late 1940s and early 1950s and established the current understanding that specialization is complementary between the hemispheres (Kinsbourne, 1982, 2000). Whether asymmetries at the lower levels of organization interact to generate asymmetries at the higher levels is unknown. However, the above sequence of historical events puts us on notice that, counterintuitively, both in evolution and in development, asymmetry may be the base state or default option, and bisymmetry the design characteristic that needs additional explanation. More generally, in view of the fiercely defended but baseless premises mentioned above, we might wonder, ‘‘Does the current Zeitgeist, in turn, blind us to still more promising ways of conceiving cerebral function?’’ (Kinsbourne, 2000).