Online research in philosophy

Vallortigara & Rogers's (V&R's) proposal that directional asymmetries evolved under social pressures raises questions about the ontogenetic mechanisms subserving the alignment of asymmetries in a population. Neuro-ontogenetic principles suggest that epigenetic factors are decisively involved in the determination of individual lateralization and that genetic factors align their direction. Clearly, directional asymmetry has an epigenetic trait.

Corballis presents a plausible evolutionary mechanism to explain the tight linkage between cerebral lateralization for language and for handedness in humans. This argument may be bolstered by invoking Stokoe's notion of semantic phonology to explain the role of Broca's area in grammatical functions.

Primate research suggests that affiliation is a highly complex construct. Studies of primate affiliation demonstrate the need to distinguish between various affiliative behaviors, consider relationships as emergent properties of these behaviors, define affiliation in the context of general environmental responsiveness, and address developmental changes in affiliation across the lifespan.

Recent computer simulations of evolving neural networks have shown that population-level behavioral asymmetries can arise without social interactions. Although these models are quite limited at present, they support the hypothesis that social pressures can be sufficient but are not necessary for population lateralization to occur, and they provide a framework for further theoretical investigation of this issue.

I do not disagree with the argument that human-population right-handedness may in some way be a consequence of the population-level left-lateralization of language. But I suggest that the human functional lateralization is not dependent on the structural left-right brain asymmetries to which Corballis refers.

Similar to directional asymmetries in animals, language lateralization in humans follows a bimodal distribution. A majority of individuals are lateralized to the left and a minority of individuals are lateralized to the right side of the brain. However, a biological advantage for either lateralization is lacking. The scenario outlined by Vallortigara & Rogers (V&R) suggests that language lateralization in humans is not specific to language or human speciation but simply follows an evolutionarily conserved organizational principle of the brain.

Can we understand brain lateralization in humans by analysis in terms of an evolutionarily stable strategy? The attempt to demonstrate a link between lateralization in humans and that in, for example, fish appears to hinge critically on whether the isomorphism is viewed as a matter of homology or homoplasy. Consideration of human handedness presents a number of challenges to the proposed framework.

Direction of the embyro's head rotation is determined by asymmetrical expression of several genes (such as shh, Nodal, lefty, and FGF8) in Hensen's node. This genetically determined head-turning bias provides a base for light-aligned population lateralization in chicks, in which the direction of the lateralization is determined by genetic factors and the degree of the lateralization is determined by environmental factors.

Recent evidence in natural and semi-natural settings has revealed a variety of left-right perceptual asymmetries among vertebrates. These include preferential use of the left or right visual hemifield during activities such as searching for food, agonistic responses, or escape from predators in animals as different as fish, amphibians, reptiles, birds, and mammals. There are obvious disadvantages in showing such directional asymmetries because relevant stimuli may be located to the animal's left or right at random; there is no a priori association between the meaning of a stimulus (e.g., its being a predator or a food item) and its being located to the animal's left or right. Moreover, other organisms (e.g., 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 counteracting 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 population. 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. Key Words: asymmetry; brain evolution; brain lateralization; development; hemispheric specialization; laterality; lateralization of behavior; social behavior; theory of games.

The present response elaborates and defends the main theses advanced in the target article: namely, that in order to provide an evolutionary account of brain lateralization, we should consider advantages and disadvantages associated both with the individual possession of an asymmetrical brain and with the alignment of the direction of lateralization at the population level. We explain why we believe that the hypothesis that directional lateralization evolved as an evolutionarily stable strategy may provide a better account than alternative hypotheses. We also further our discussion of the influence of stimulation and experience in early life on lateralization, and thereby show that our hypothesis is not deterministic. We also consider some novel data and ideas in support of our main thesis.