On asymmetries exhibiting a near-equiprobable distribution of directions

The origins of cerebral asymmetry: a review of evidence of behavioural and brain lateralization in fishes, reptiles and amphibians

Early evidence for lateralization at a population and/or individual level in 'lower' vertebrates is reviewed. The lateralities include structural asymmetries in the epithalamus of several species of fish and amphibians, asymmetries in the location of both eyes on the same side of the head and of the dorsal/ventral crossing at optic-chiasma in flatfish, asymmetries in copulatory organs of several species of fishes, asymmetries in lung size and direction of coiling in reptiles, and asymmetrical distribution of scarring in whitefish. More recent data on functional lateralization at population level in lower vertebrates are also reviewed. These include: lateral asymmetries in the direction of turning during escape behaviour and in eye use in poeciliid fish; lateralization of pectoral stridulation sounds in catfish; neural lateralization for control of vocalization in the frogs; pawedness in toads; lateralization of courtship behaviour in newts; and lateralization of aggressive responses in lizards. Several cases of behavioural asymmetries at the individual level are also described, and possible relationships between lateralization at the individual level and fluctuating asymmetries arising from reduced heterozygosity are discussed. It is argued that the overall evidence now available supports the hypothesis of an early origin of brain lateralization in vertebrates.

Lateral asymmetries during escape behavior in a species of teleost fish (Jenynsia lineata)

Lateral asymmetries in the direction of turning during escape behavior in a species of teleost fish, Jenynsia lineata, are reported. When faced with the visual image of a simulated predator, approximately half of the individuals exhibited a significant bias to turn rightwards or leftwards, and the asymmetry tended to be retained when the same fish were retested 1 month later. Some morphological characters (pectoral fin rays, scales in natural row, supraorbital, preopercular, and postotic pores) were measured to check whether the degree of behavioral asymmetry was correlated with morphological fluctuating asymmetries associated with environmental stress or reduced heterozygosis. The results showed that it was not. The implications of these results for the interpretation of behavioral lateralization at the individual and population level are discussed.

Cerebral lateralization as a source of interindividual differences in behavior

Cerebral laterality can no longer be considered an exclusively human trait, as over the last 15 years there has been an emergence of data to suggest that animal brains are also lateralized. Morphologic, chemical and behavioral indices of brain asymmetry in the rodent have been reported, and it is suggested that variations in the magnitude and direction of these indices are determined by a complex interaction of genetic, hormonal and experiential factors. Interindividual differences in cerebral laterality have been shown to covary with, or predict, individual differences in spatial behavior and stress reactivity, as well as susceptibility to stress pathology and drug sensitivity. Such findings suggest that it is possible to study individual differences in lateralized brain function through the use of animal models.

Light experience during development affects asymmetry of forebrain function in chickens

The chicken brain has asymmetry of forebrain function of attack and copulation behaviors. This study reports that this asymmetry, revealed by intracranial injections of glutamate, is affected by light exposure. Eggs were incubated in 3 forced-draught incubators matched for noise, humidity and temperature control, so that light and laboratory noise were the only systematic variables. All eggs were incubated in darkness for the first 17 days of incubation after which they were allocated to one of 3 incubating conditions, L+S+, L-S+, L+S-, where + or - indicates the presence or absence of light (L) or laboratory noise (S). Light exposure was necessary to establish lateralization of attack and copulation, sound was not since only the L+S+ and L+S- groups were lateralized. The lateralization of auditory habituation was not affected by these incubating conditions. Further experiments established that on day 19 of incubation exposure for 4 h to a light of constant intensity produced lateralization. 1 h exposure to a light of constant intensity or to a flickering light (1.7 Hz) failed to produce lateralization. Examination of the data for individuals suggests that light exposure synchronizes the direction of lateralization rather than generates its presence. Environmental and genetic factors interact to produce the lateralization of attack and copulation.