TIME IS OF THE ESSENCE - A CONJECTURE THAT HEMISPHERIC-SPECIALIZATIONARISES FROM INTERHEMISPHERIC CONDUCTION DELAY

Tomasch (1954) and Aboitiz et al. (1992) found the majority of the fib ers of the human corpus callosum are under 1 mu m in diameter. Electro n microscopic studies of Swadlow et al. (1980) and the detailed study of LaMantia and Rakic (1990a) on macaques show the average size of the myelinated callosal axons also to be less than 1 mu m. In man, the av erage-sized myelinated fiber interconnecting the temporal lobes would have a one-way, interhemispheric delay of over 25 msec. Thus, finely d etailed, time-critical neuronal computations (i.e., tasks that strain the capacity of the callosum and hence could not be handled by just th e larger fibers) would be performed more quickly via shorter and faste r intrahemispheric circuits. While one transit across the commissural system might yield tolerable delays, multiple passes as in a system in volving ''settling'' would seem prohibitively slow. We suggest that th ese temporal limits will be avoided if the neural apparatus necessary to perform each high-resolution, time-critical task is gathered in one hemisphere. If the, presumably overlapping, neural assemblies needed to handle overlapping tasks are clustered together, this would lead to hemispheric specialization. The prediction follows that the large bra ins of mammals such as elephants and cetaceans will also manifest a hi gh degree of hemispheric specialization.