It is practically impossible that the two hemispheres will work in exactly the same manner; furthermore, in on togeny they might tend to deviate from each other in their processing strategies. With the increase in cortical areas in mammalian evolution, the two hemispheres perhaps tended to diverge in their processing sequences if not interacting properly. The corpus callosum may have compensated for intrinsic differences in neural processing between the hemispheres by monitoring parallel processing in the two sides (Zaidel et al., 1995) or through inhibitory interactions. Inhibitory connections may sometimes mask inherent asymmetries in the two hemispheres that are to the disadvantage of the animal (Denenberg, 1981). However, in some species, including hominids, lateralized functions appeared that were of high adaptive value, such as visuospatial processing in the right hemisphere and particularly linguistic processing in the left hemisphere. In this case, instead of compensating for the asymmetry generated in the two hemispheres, the corpus callosum worked as a pathway for transfer of relevant information to the specialized hemisphere. Inhibitory callosal interactions may have begun to block incompatible strategies in the two hemispheres. Other asymmetric functions remained processed in parallel in the left and the right sides, each hemisphere using a different strategy and being reciprocally monitored with the contralateral side at different computational stages through the corpus callosum (Zai-del et al., 1995).
The above scenario implies that although the corpus callosum has a role in the origin of lateralization, it is not the cause of it. Brain asymmetry might initially develop as a specialization of neural processing that is intrinsic to each hemisphere, in a manner analogous to the developmental model suggested above. A different, but not alternative, model (Ringo et al., 1994) suggests a direct role for the callosum in the evolutionary origin of lateralization. These authors proposed that in phylog-eny, a longer interhemispheric delay due to increased brain size might produce an emphasis on local processing within each hemisphere, resulting in hemispheric isolation, which in turn would facilitate hemispheric asymmetry and lateralization. Thus, according to Ringo and colleagues (1994), hemispheric lateralization for language might be partially a byproduct of having a larger brain. In this context, Jerison (1991) and later Schutz and Preissl (1996) reported no differences in average callosal fiber diameter between the mouse and the macaque, suggesting that conduction velocity remains constant despite an increased interhemispheric distance, thereby effectively increasing interhemispheric delay in larger brains. Preliminary findings by two of us (F. A. and R. O.) tend to support this conclusion, though also indicating that the largest callosal fibers tend to be thicker (and hence faster-conducting) in large-brained species compared to small-brained ones (Figure 2.6). This sug
the appearance of large-diameter fibers in the callosums of large-brained species. 15,400x E.M.
gests that a subpopulation of large, fast-conducting fibers tends to compensate for the increasing interhemi-spheric distance by increasing their axon diameter and conduction velocity.
Since the timing requirements are most stringent for sensorimotor areas, perhaps the population of large, fast-conducting fibers increases in size and numbers especially in callosal regions connecting these areas (Aboitiz et al., 1992a). Comparing the sizes of axons connecting visual areas 17/18 in the mouse and cat, Innocenti (1995a; see also Innocenti et al., 1995) reports an increase in fiber diameters in the cat. However, brain volume might not be the only factor, as behavioral specializations can also influence callosal fiber composition. Across species, no relationship was found between in-terhemispheric conduction delay in the 17/18 border and brain size when comparing studies performed by different authors (four species were compared: mouse, rabbit, cat, and monkey; see Innocenti, 1995a). Interestingly, the shortest interhemispheric delay (antidromic stimulation) is found in the cat (between 2 and 3 ms), and the monkey has a delay comparable to that of the mouse (7 to 8 ms) despite the approximately tenfold difference in interhemispheric distance. The rabbit has an unusually long interhemispheric delay (about 17 ms average), which perhaps relates to the poorly developed area of binocular vision and the strong emphasis on lateral vision in this species. The relatively short inter-hemispheric delays in cat and monkey may relate to the fact that both are frontally eyed species with a high degree of binocularity and depth perception, for which fast interhemispheric conduction may be especially useful. These results point to the possibility that besides brain size, callosal fiber composition may depend on behavioral and perceptual specializations such as the degree of stereopsis, again supporting the concept of an important role of the corpus callosum in midline fusion, especially in the visual system.
To what extent an increased interhemispheric distance plays a role in the evolutionary origin of brain lateralization needs to be determined by comparative studies analyzing interhemispheric transmission times in both sensory and higher-order areas and in species with different brain sizes and with different degrees of brain lateralization. In any case, Ringo and colleages (1994) leave the question open of why the left hemisphere specializes in linguistic-type task and the right in visuospa-tial tasks instead of vice versa. A right hemisphere superiority in visuospatial tasks has been observed in small mammals (Bradshaw and Rogers, 1993), indicating that at least there are other factors involved in the generation of brain asymmetry. Nevertheless, just as in the developmental model proposed above, in phylogeny a de creased callosal function (be it a consequence of less interhemispheric fibers or an increased transmission delay) may perhaps have served to enhance functional lateralization between the two hemispheres, particularly in the case of males (Witelson and Nowakowski, 1991; Aboitiz et al., 1992b, 1992c).
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