Elsevier

Cortex

Volume 124, March 2020, Pages 274-285
Cortex

Special Issue “Brain and cognitive asymmetry”: Review
A function for the bicameral mind

https://doi.org/10.1016/j.cortex.2019.11.018Get rights and content

Abstract

Why do the left and right sides of the brain have different functions? Having a lateralized brain, in which each hemisphere processes sensory inputs differently and carries out different functions, is common in vertebrates, and it has now been reported for invertebrates too. Experiments with several animal species have shown that having a lateralized brain can enhance the capacity to perform two tasks at the same time. Thus, the different specializations of the left and right sides of the brain seem to increase brain efficiency. Other advantages may involve control of action that, in Bilateria, may be confounded by separate and independent sensory processing and motor outputs on the left and right sides. Also, the opportunity for increased perceptual training associated with preferential use of only one sensory or motoric organ may result in a time advantage for the dominant side. Although brain efficiency of individuals can be achieved without the need for alignment of lateralization in the population, lateral biases (such as preferences in the use of a laterally-placed eye) usually occur at the population level, with most individuals showing a similar direction of bias. Why is this the case? Not only humans, but also most non-human animals, show a similar pattern of population bias (i.e., directional asymmetry). For instance, in several vertebrate species (from fish to mammals) most individuals react faster when a predator approaches from their left side, although some individuals (a minority usually ranging from 10 to 35%) escape faster from predators arriving from their right side. Invoking individual efficiency (lateralization may increase fitness), evolutionary chance or simply genetic inheritance cannot explain this widespread pattern. Using mathematical theory of games, it has been argued that the population structure of lateralization (with either antisymmetry or directional asymmetry) may result from the type of interactions asymmetric organisms face with each other.

Introduction

In the last thirty years or so (actually, initial discoveries dated back to the early 1970s, see for historical accounts Bisazza et al., 1998, Rogers and Andrew, 2002, Vallortigara and Bisazza, 2002, Vallortigara et al., 2011; Vallortigara & Versace, 2017) comparative neuroscientists and behavioural biologists have come to realize that left/right asymmetries in brain and behaviour (so-called «lateralization»), long attributed only to the human species, are in fact widespread in the animal kingdom (Rogers, Vallortigara, & Andrew, 2013, p. 229). We know nowadays that asymmetries are apparent in all major taxonomic groups in vertebrates and in invertebrates as well. Fig. 1 schematizes the state of the art as to the phylogeny of brain and behavioural asymmetry.

In fact, if lateralization were a uniquely human characteristic, it would be extremely difficult for biologists to figure out (and experimentally prove) what is the advantage of having it. But luckily enough, we have now dozens of examples of asymmetries at different levels of organization in different animal models, and we can start reasoning and testing directly our hypotheses on what may be the benefits (and the costs) for bilaterian organisms of having a certain degree of asymmetry in their nervous systems.

Section snippets

Varieties of lateralization and their advantages

Possessing an asymmetrical brain may confer advantages. And this should have been occurring at least from 500 million years before the dawn of humans, in Cambrian times, when Anomalocaris, animals with limbs and thought to be closely related to ancestral arthropods, were preying on trilobites with a right-limbed bias (Babcock & Robinson, 1989; review in Ocklenburg & Güntürkün, 2018, p. 368).

It is apparent, however, that possessing a left-right asymmetry in overt behavior maybe a disadvantage.

Strength of lateralization: costs and benefits

Several mechanisms can be associated with advantages of individual-level lateralization. They could be specific, at least as to their original appearance during evolutionary times, to particular functions. An example is provided by lateralization of anti-predatory responses such as the C-start reaction of fish. Fish react to a threatening stimulus by bending their bodies into a C-shape with a unilateral contraction of their axial muscle, which then provides a sudden contralateral acceleration

Direction of lateralization: costs and benefits

We are thus facing an enigma. If possession of individual-level lateralization has benefits for cognitive functioning, whereas population-level lateralization does not add any further advantage, what could it be the biological function of population-level (i.e., directional) lateralization?

The different specializations of the left and right sides of the brain seems to increase brain efficiency. However, in the animal kingdom left/right biases in the use of an eye and, in some tasks, in the use

Conclusion

After noting the widespread presence of lateralization across vertebrate and invertebrate species, we have examined the evidence showing the advantages of having a lateralized brain. So far there is evidence that lateralization:

  • 1.

    Increases the processing capacity of the nervous system,

  • 2.

    Enhances discrimination performance, success in solving some problems and aids learning.

  • 3.

    Heightens ability to avoid predators,

  • 4.

    May improve motor control,

  • 5.

    Permits parallel and complementary processing of sensory inputs,

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