Recent evidence in natural and semi-natural settings has revealed a variety of left-right perceptual asymmetries among ver- tebrates These include preferential use of the left or right visual hemifield during activities such as searching for food, agonistic re- sponses, or escape from predators in animals as different as fish, amphibians, reptiles, birds, and mammals There are obvious disad- vantages in showing such directional asymmetries because relevant stimuli may be located to the animal's left or right at rando m; there is no a priori association between the meaning of a stimulus (eg, its being a predator or a food item) and its being located to the ani- mal's left or right Moreover, other organisms (eg, predators) could exploit the predictability of behavior that arises from population- level lateral biases It might be argued that lateralization of function enhances cognitive capacity and efficiency of the brain, thus coun- teracting the ecological disadvantages of lateral biases in behavior However, such an increase in brain efficiency could be obtained by each individual being lateralized without any need to align the direction of the asymmetry in the majority of the individuals of the pop- ulation Here we argue that the alignment of the direction of behavioral asymmetries at the population level arises as an "evolutionarily stable strategy" under "social" pressures occurring when individually asymmetrical organisms must coordinate their behavior with the behavior of other asymmetrical organisms of the same or different species

/pdf/survival-with-an-asymmetrical-brain-advantages-and-4nfldlpjsp.pdf

Survival with an asymmetrical brain: Advantages and disadvantages of cerebral lateralization

Women and men exhibit different cortical activation patterns during mental rotation tasks.

Schizophrenia as the price that homo sapiens pays for language: a resolution of the central paradox in the origin of the species.

In recent years, it has become apparent that behavioural and brain lateralization at the population level is the rule rather than the exception among vertebrates. The study of these phenomena has so far been the province of neurology and neuropsychology. Here, we show how such research can be integrated with evolutionary biology to understand lateralization more fully. In particular, we address the fact that, within a species, left- and right-type individuals often occur in proportions different from one-half (e.g. hand use in humans). The traditional explanations offered for lateralization of brain function (that it may avoid unnecessary duplication of neural circuitry and reduce interference between functions) cannot account for this fact, because increased individual efficiency is unrelated to the alignment of lateralization at the population level. A further puzzle is that such an alignment may even be disadvantageous, as it makes individual behaviour more predictable to other organisms. Here, we show that alignment of the direction of behavioural asymmetries in a population can arise as an evolutionarily stable strategy when individual asymmetrical organisms must coordinate their behaviour with that of other asymmetrical organisms. Brain and behavioural lateralization, as we know it in humans and other vertebrates, may have evolved under basically 'social' selection pressures.

/pdf/the-evolution-of-brain-lateralization-a-game-theoretical-493nurkucz.pdf

The evolution of brain lateralization: a game-theoretical analysis of population structure

Relative hand skill predicts academic ability: global deficits at the point of hemispheric indecision

The right shift (RS) theory of handedness (Annett, 1972, 1985) suggested that the high prevalence of human right-handedness and left hemisphere specialization for speech is due to a single factor which induces the typical pattern of cerebral specialization and incidentally displaces a chance distribution of manual asymmetry toward dextrality. The factor could be a single gene (rs+). An estimate of genotype frequencies based on the percentage of right hemisphere speakers suggested a genetic balanced polymorphism with heterozygote advantage. Studies of actual hand skill suggest that strong dextrality depends on a process which induces left hand weakness, probably associated with impairment of right hemisphere function. Non gene-carriers (rs-genotypes) avoid right hemisphere handicap but are at risk for the development of speech and speech dependent language processes. People with specific deficits of phonological processing have reduced bias to dextrality, as expected if they lack the rs+ gene. The presence of the rs+ gene is associated with poorer spatial ability and this is true for left-handers as well as right-handers. The higher proportion of nonright-handers among mathematicians and other talented groups may be due to the absence of rs++ genotypes, rather than to an intrinsic advantage for left-handers. Evidence for heterozygote advantage has been found in studies of educational achievement, especially in reading. Poor readers are more prevalent at both extremes of the laterality distribution. A critical test of the balanced polymorphism hypothesis asked whether a double dissociation exists between people with specific deficits of phonological versus visual memory for words, and reduced versus strong bias to dextrality respectively. Evidence for this double dissociation was found in school and university samples. Applying the model to dyslexia, children with specific reading difficulties classified for presence versus absence of poor phonology were found to differ for handedness as expected if the former are less dextral and the latter more dextral than controls. These findings are not due to pathology, but they are associated with differences in the handedness of relatives, as expected by the RS model. Criticisms of the balanced polymorphism model are shown to be ill-founded. Questions of interpretation of the theory are considered and needs for further research are indicated.

The right shift theory of a genetic balanced polymorphism for cerebral dominance and cognitive processing. Commentaries. Author's reply

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The right shift theory of a genetic balanced polymorphism for cerebral dominance and cognitive processing. Commentaries. Author's reply