Time and its consequences

As was just noted, the conduction velocity of callosal axons is exceedingly slow. This arises, of course, from the fact that these axons are very small, probably as a price for there being so many of them! Aboitiz and colleagues (1992) confirm the earlier estimate of Tomasch that there are roughly 200,000,000 callosal axons in humans, and there are 40 times as many having an axon greater than 0.6 mm as with an axon greater than 1.5 mm in diameter. Interestingly, the human callosal axons are, on average, smaller than those of macaques (LaMantia and Rakic, 1990). Thus, it would seem, the trade-off between number of connections and rapidity of communication as brains increase in size is being decided in favor of number. As Ringo and colleagues (1994) noted, a trade-off is inevitable because of the compounding effect that increasing axon size will in turn increase conduction distance, and so on. In this regard, cetaceans seem simply to have given up, for while the brain of the dolphin is about 75% of that of the human, the size of its callosum is less than 20% of that of the human (Nieto, Nieto, and Pacheco, 1976). This paucity of hemispheric interconnection may underlie the remarkable ability of these creatures to sleep with one hemisphere at a time (e.g., Mukhametov, 1987).

In any event, it is clear that a one-way message across the human callosum will, on average, given the distances involved and the size of the fibers, require on the order of 25 ms (Aboitiz et al., 1992; Ringo et al., 1994); and, of course, with longer interhemispheric paths and the large number of smaller fibers, such communication in many instances may take 100 ms or more! Ringo and colleagues (1994) proposed that such long delays for a one-way passage essentially preclude repeated back-and-forth exchange between the hemispheres in neuronal computation. Thus, in the interest of time, each hemisphere must achieve most of its multistage synaptic arguments locally, and as a consequence, each hemisphere will develop its own modus operandi. While this hypothesis offers an explanation for hemispheric specialization in larger brains or where temporal demands are critical, it says nothing as to why the right and left hemispheres each have the same individual propensities for the great majority of human beings, for example, linguistic talent in the left hemisphere and visual perceptual skills in the right.

It must also be recognized that there are some large, fast fibers in the human callosum, some 160,000 >4.6 mm according to Aboitiz and colleagues (1992). Typical of these is a group probably connecting the cortices of the vertical meridian (Shoumura, Ando, and Kato, 1975). This small contingent of fast fibers is undoubtedly responsible for the brevity seen for interhemispherically evoked potentials or for reaction times to simple stimuli. Lest it be thought that a ''mere'' 80,000 fibers provides but a feeble and primitive input from one hemisphere to the other, it need but be recalled that this is roughly the population of fibers from the eye of the cat projecting to the forebrain (Illing and Wassle, 1981), a system that is undeniably capable of intricate signaling.

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