Figure 2.3. Diagram indicating the task of midline fusion for callosal cells in the primary visual cortex. In each hemisphere the visual cortex has a representation of the contralateral visual hemifield that is connected to the contralateral hemisphere through callosal fibers, thus establishing a continuity between the two hemirepresentations.

sensory or motor hemirepresentations in both hemispheres, while in auditory areas they may represent an additional stage of fast bilateral interaction in the auditory pathway that serves to localize sounds in space (Aboitiz et al., 1992a). Early stages of sensory processing probably require fast interhemispheric interaction, especially in regions representing the visual midline, where depth perception is achieved. In higher-order cortical areas, where intrinsic processing may take long enough to render the interhemispheric delay less critical, conduction velocity may not be such a stringent requirement. Note that fibers connecting prefrontal areas, which participate in long-term organization of behavior, are the thinnest and bear the highest proportion of un-myelinated fibers of the whole corpus callosum.

The interhemispheric distance between sensory areas is around 100—130 mm in the human. Interhemispheric transmission delay can be estimated to be of about 19— 25 ms for the most abundant fibers between 0.4 and 1 mm in observed diameter. This fits the reported in-terhemispheric transmission times reported by evoked potentials (see Aboitiz et al., 1992a). Behavioral experiments in humans indicate that the shortest interhemi-spheric delays for simple motor reaction time tasks take around 3 ms, a time that fits the population of large-diameter fibers (larger than 3 mm) that tend to connect sensorimotor areas, while delays for more complex tasks take on average around 45 ms, fitting the population of thinnest myelinated and unmyelinated fibers that correspond to association or higher-order areas. However, delays of around 45 ms may be an overestimate, since, especially for complex tasks, it is sometimes difficult to distinguish interhemispheric transmission times from inherent hemispheric differences in computational speed (Aboitiz et al., 1992a). In this context, it is of interest to mention that Grafstein (1963) found that unmyelinated callosal fibers produce an evoked potential of longer latency and of opposite sign than myelinated fibers. Keeping in mind the difficulties of interpreting these results, one role of unmyelinated fibers may perhaps be thought of as an aftereffect of the excitation or inhibition (via excitation of inhibitory interneurons) produced by faster, myelinated fibers, thereby modulating the temporal dimension of the interhemispheric stimulus.

We (Aboitiz et al., 1996) have recently observed an increase in the numbers of relatively large (larger than 1 mm) and very large-diameter (larger than 3 mm) cal-losal fibers with age (at least until 68 years old). At least some interhemispheric functions may increase transmission velocity with age, perhaps related to the automatization of certain neural strategies after continued use. It is known that the course of myelination depends on the functional state of the nerve fibers in the optic nerve (Fernandez et al., 1993). Therefore, it is possible that, in general, large-diameter callosal fibers correspond to relatively automatic neural circuits that become established early in ontogeny, such as those involved in midline fusion in primary sensory areas. Some other automatic circuits may become established later and even during adult life, thus increasing the proportions of large-diameter fibers with age.

After separating by sex and considering the different segments of the corpus callosum, it was found that in females fibers larger than 1 mm in diameter increased in numbers in the anterior and posterior thirds only. In the midbody (but not in the splenium), fibers larger than 3 mm in diameter also increased their numbers with age. In males, however, the relationship between very large-diameter fibers and age disappeared after the callosum was divided into distinct segments. This indicates that in females there is an age-related increase of relatively fast interhemispheric transfer (fibers larger than 1 mm) in regions connecting higher-order areas of the frontal and temporoparietal lobes (corresponding to fibers larger than 1 mm of the anterior and posterior thirds), while the largest fibers involved in very fast transfer increase in the callosal midbody that connects motor, somatosensory, and auditory areas. However, there is no significant age dependency in the very large visual fibers of the posterior splenium that represent visual areas. On the other hand, although in males gigantic fibers (larger than 3 mm)

increased with age in the whole callosum, we could not detect any callosal region in which this relationship was concentrated. Perhaps in males the age relationship with these very large fibers is dispersed along the corpus callosum, not relating to a particular, localized kind of function. The possibility that females and males differ in the fiber types that increase with age is reminiscent of other findings in the rat (Juraska and Kopcic, 1988) in which different fiber types respond to different specific environmental conditions in the two sexes (see below).

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