Asymmetry in the epithalamus of vertebrates

Lateralization of the brain appeared early in evolution and many of its features appear to have been retained, possibly even in humans. We now have a considerable amount of information on the different forms of lateralization in a number of species, and the commonalities of these are discussed, but there has been relatively little investigation of the advantages of being lateralized. This article reports new findings on the differences between lateralized and nonlateralized chicks. The lateralized chicks were exposed to light for 24 h on day 19 of incubation, a treatment known to lead to lateralization of a number of visually guided responses, and the nonlateralized chicks were incubated in the dark. When they were feeding, the lateralized chicks were found to detect a stimulus resembling a raptor with shorter latency than nonlateralized chicks. This difference was not a nonspecific effect caused by the light-exposed chicks being more distressed by the stimulus. Instead, it appears to be a genuine advantage conferred by having a lateralized brain. It is suggested that having a lateralized brain allows dual attention to the tasks of feeding (right eye and left hemisphere) and vigilance for predators (left eye and right hemisphere). Nonlateralized chicks appear to perform these dual tasks less efficiently than lateralized ones. Reference is made to other species in discussing these results. Copyright 2000 Academic Press.

The afferent connections of the habenular complex in the rat were examined by injecting horseradish peroxidase (HRP) into discrete portions of the habenular nuclei by microelectrophoresis. 1. HRP deposits confined to the lateral half of the lateral habenular nucleus labeled a multitude of cells in the entopeduncular nucleus. Numerous labeled cells also appeared in such cases in the lateral hypothalamus, indicating that the lateral habenular nucleus is a major convergence point of projections from these otherwise apparently quite separate cell regions. Moderate-to-small numbers of labeled cells were also found in the nuclei of the diagonal band, substantia innominata, lateral preoptic area and more caudally, in the ventral tegmental area, the region of the mesencephalic raphe, and the central gray substance. 2. HRP injected into the medial part of the lateral habenular nucleus labeled cells in the same regions, but more in the diagonal band and fewer in the entopeduncular nucleus than were labeled by more lateral injections. The contrast suggests that the projections from the basal forebrain and entopeduncular nucleus to the lateral habenular nucleus are somewhat topographically organized. 3. Injections of the medial habenular nucleus labeled an abundance of cells in the posterior parts of the supracommissural septum, but also a small number of cells in the diagonal band and mesencephalic raphe. 4. HRP injected into the stria medullaris labeled cells in all of the afore-mentioned areas and, in addition, cells in several olfactory structures, confirming that HRP may be taken up by fibers of passage and label their cells of origin, and suggesting that olfactory structures contribute fibers to the stria medullaris that do not terminate in the habenula.