Is Cerebral Lateralization a Graded or a Discrete Characteristic?

Abstract

The human body is replete with paired organs, both externally (eyes and ears, for example), and internally (the kidneys). The anatomical similarity between the members of such pairs leads us to expect that they will have similar or even identical functions. This expectation is born out in fact, although some mechanisms of depth perception and sound localization demand that both eyes and both ears are respectively operative. At the level of gross anatomy, the human brain likewise shows every appearance of being a double organ, and it is thus hardly suprising that, until the time of Broca (1865), the two hemispheres were usually regarded as functional duplicates of each other. Broca’s discovery that a left unilateral lesion could severely impair speech production (and the many later reports of cognitive deficit subsequent upon either left or right unilateral damage) dealt the duplicate model a blow from which it has never recovered. It was initially replaced, however, by an equally simple dichotomous model. The notion of complementary specialization was often taken to imply that, for many higher functions, one hemisphere and one hemisphere alone possessed the relevant underlying computational capacities, the other hemisphere being totally inert within that domain of processing.