The evolution and genetics of cerebral asymmetry

Abstract

Handedness and cerebral asymmetry are commonly assumed to be uniquely human, and even defining characteristics of our species. This is increasingly refuted by the evidence of behavioural asymmetries in non-human species. Although complex manual skill and language are indeed unique to our species and are represented asymmetrically in the brain, some non-human asymmetries appear to be precursors, and others are shared between humans and non-humans. In all behavioural and cerebral asymmetries so far investigated, a minority of individuals reverse or negate the dominant asymmetry, suggesting that such asymmetries are best understood in the context of the overriding bilateral symmetry of the brain and body, and a trade-off between the relative advantages and disadvantages of symmetry and asymmetry. Genetic models of handedness, for example, typically postulate a gene with two alleles, one disposing towards right-handedness and the other imposing no directional influence. There is as yet no convincing evidence as to the location of this putative gene, suggesting that several genes may be involved, or that the gene may be monomorphic with variations due to environmental or epigenetic influences. Nevertheless, it is suggested that, in behavioural, neurological and evolutionary terms, it may be more profitable to examine the degree rather than the direction of asymmetry.

The Australian Imaging, Biomarkers and Lifestyle (AIBL) study of aging: methodology and baseline characteristics of 1112 individuals recruited for a longitudinal study of Alzheimer's disease

“…Results of the Wechsler Test of Adult Reading (WTAR) revealed estimated mean premorbid IQ scores of 101 for AD patients, 105 for MCI participants and 108 for HCs, with significant differences between each of the groups (HCs demonstrated significantly higher mean IQ than MCIs, with ADs scoring significantly lower than MCIs). The majority of participants were right handed (88% of HC, 86% of MCI, 87% of AD), in line with the world population proportion of right-handedness (Corballis, 2009). Table 2 presents the mean and standard deviation of normed and age-adjusted measures for the neuropsychological tasks, and the results of between-groups one-way analysis of variance (ANOVA) for each measure.…”

Embryonic exposure to predator odour modulates visual lateralization in cuttlefish

“…Given the advantages of being lateralized described both in vertebrates and invertebrates [7][8][9], the question arises as to why lateralization has not become fixed in the population. One possibility is that there may be compensatory advantages linked with the lack of asymmetry, such as more balanced awareness [41]. The effects of an environmental factor on the development offer a source of interindividual variation, potentially allowing selection for each individual of the more advantageous trade-off between being lateralized or not.…”

Neurophysiological origin of human brain asymmetry for speech and language

“…EEG/functional MRI | natural stimulation | resting state | oscillation A uditory asymmetry (1) and hand preference are traits humans share with other primate and nonprimate species (2)(3)(4)(5). Both have been proposed as the functional origin of human cerebral dominance in speech and language (6,7).…”