abstract The nature of the information traversing the fore-brain commissures is poorly understood but certainly shares in the complexity of cortical computations, as emphasized by ipsicortical collaterals of many callosal axons. In larger brains, or in time-critical functions, the slow velocity of most callosal axons makes intracortical processing more efficient than bilateral calculation, thus promoting functional specialization within a single hemisphere. The consistency with which the right or left hemisphere becomes superior to, or dominant over, the other in humans and certain animals for a given function remains puzzling, and an example is given for a macaque in which well-established dominance spontaneously reversed. While it is possible to produce unilateral engrams using electrical excitation of cortex as the input signal, the more common mode for vision is that limitation of input to a single hemisphere nonetheless produces a bilateral memory trace. When the forebrain commissures are absent (the split brain), significant subcortical modulation of the hemispheres remains and seems to unite into a single trace input that is received simultaneously by the two isolated hemispheres. Given the functional specialization of the hemispheres, some cost in time and efficiency can be expected when processing must be switched from one hemisphere to the other.
As multicellular organisms discovered the advantages of moving through space, presumably at first along a substrate, they inevitably encountered the problem of attractions and threats that lay to either side of the direction of motion. Turning thus became desirable, and laterality came into being, with receptors to detect what lay to right or left and muscular contractility and inhibition to provide appropriate steering. As a consequence, duplicate neuronal systems arose on each side of the body. With the singular exception of radial symmetry, a secondary overlay in echinoderms, this pattern persists from flatworms to humans.
Despite recent compelling evidence, it is not commonly appreciated how profound this duplication of robert w. doty Department of Neurobiology and Anatomy, University of Rochester School of Medicine and Dentistry, Rochester, New York.
neuronal organization remains, even in human beings. There are two dramatic demonstrations that either cerebral hemisphere is unequivocally human: the now extensive literature on hemispherectomy, showing that individuals with a single surviving hemisphere can function in society, sometimes at quite high levels (e.g., Griffith and Davidson, 1966; Gott, 1973; Austin, Hay-ward, and Rouhe, 1974; Smith, 1974; Schepelmann et al., 1976; Patterson, Vargha-Khadem, and Polkey, 1989; Ogden, 1996), and the thoroughly studied cases in whom the forebrain commissures have been transected, resulting in two undeniably human entities within the single individual (e.g., Sperry, Zaidel, and Zaidel, 1979; Sperry, 1984). From the latter it must be concluded that the forebrain commissures somehow achieve the mental unity enjoyed by most normal, bihemispheric human beings.
There has in the past been some argument as to whether, for intact individuals, the left, "speaking" (linguistically talented) hemisphere is actually the only one that is truly conscious in the human sense. The evidence just alluded to clearly denies this, as does a huge body of data on localized lesions in either hemisphere, now extensively supported by imaging of differential blood flow in the active, thinking brain. Perhaps the latest evidence here is the most convincing, showing that contemplation of events unique to one's personal history yields distinct activation of areas in the right (nonlinguistic) hemisphere (Fink et al., 1996).
Three primary questions arise from these facts:
1. What is the nature of the exchange between the hemispheres provided by the corpus callosum that accounts for unity of conscious experience in the face of the potentially independent mental life of each hemisphere alone?
2. Why does this unification nevertheless leave each hemisphere with distinctly different propensities and skills?
3. By what process is the "decision" achieved as to which hemisphere will lead at any given moment; that is, "who is in charge?"
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