The Development of
New School for Social Research, New York
Massive evidence from studies of focal brain damage documents the many functional specializations in the human forebrain. Most if not all of these "modular" specializations are unilateral, that is, confined to one of the cerebral hemispheres. But little is known about the importance of the laterality involved. Is the asymmetry necessary for the modules to work effectively, or is it a trivial consequence of the fact that there are two cerebral hemispheres and that the modules do not need to be spread across both (even if they could be without detriment)?
Why might it be necessary for a module to be confined to one hemisphere? Perhaps the neuronal circuitry needs to be concentrated within a limited territory in order for its activity to be coordinated, or perhaps it is more feasible to insulate a focal facility from interfering informational neural cross-talk ( Kinsbourne & Hicks, 1978). Perhaps bilaterally represented facilities are apt to compete for control of output mechanisms, causing dysfluencies (i.e., impaired timing) in rapidly executed sequential skills ( Levy, 1969; Marler, 1970). If so, those control mechanisms that influence midline output (e.g., speech, song) should be the ones that are lateralized. Therefore, or for reasons that have not been articulated, it has been traditionally assumed that laterality, apparently so prominent in humans, in some way renders possible humans' superior behavioral control. Laterality might even itself be an emergent consequence of neural maturation. The older child is perhaps better able to process certain kinds of information precisely because the components of the relevant module have become concentrated within a hemisphere. But it is difficult to find out whether this is really so. Laterality cannot be manipulated within-