THE ORIGINS OF CEREBRAL ASYMMETRY - A REVIEW OF EVIDENCE OF BEHAVIORAL AND BRAIN LATERALIZATION IN FISHES, REPTILES AND AMPHIBIANS

Early evidence for lateralization at a population and/or individual le vel in 'lower' vertebrates is reviewed. The lateralities include struc tural asymmetries in the epithalamus of several species of fish and am phibians, asymmetries in the location of both eyes on the same side of the head and of the dorsal/ventral crossing at optic chiasma in flatf ish, asymmetries in copulatory organs of several species of fishes, as ymmetries in lung size and direction of coiling in reptiles, and asymm etrical distribution of scarring in whitefish. More recent data on fun ctional lateralization at population level in lower vertebrates are al so reviewed. These include: lateral asymmetries in the direction of tu rning during escape behaviour and in eye use in poeciliid fish; latera lization of pectoral stridulation sounds in catfish; neural lateraliza tion for control of vocalization in the frogs; pawedness in toads; lat eralization of courtship behaviour in newts; and lateralization of agg ressive responses in lizards. Several cases of behavioural asymmetries at the individual level are also described, and possible relationship s between lateralization at the individual level and fluctuating asymm etries arising from reduced heterozygosity are discussed. It is argued that the overall evidence now available supports the hypothesis of an early origin of blain lateralization in vertebrates. (C) 1998 Elsevie r Science Ltd. All rights reserved.