Special Issue “Brain and cognitive asymmetry”: ReviewA function for the bicameral mind
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:
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Increases the processing capacity of the nervous system,
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Enhances discrimination performance, success in solving some problems and aids learning.
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Heightens ability to avoid predators,
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May improve motor control,
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Permits parallel and complementary processing of sensory inputs,
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2022, Applied Animal Behaviour ScienceCitation Excerpt :We noticed that the two yearlings (Pic-Nick and Lady) significantly preferred to use the left eye, and they were attracted by the younger visitors who elicited the penguins' interest by sliding their hands along the glass. A massive amount of research has verified that the typical scenario – regardless of the animals' clades – for species with monocular vision and almost complete decussation of the optical fibers consists of a preference to monitor the surroundings for predators with the left eye while looking elsewhere and doing other tasks with the right ocular field (reviewed by Vallortigara and Rogers, 2020). The functional lateralization of the visual system caused by the asymmetrical exposure to light during embryonic development (inducing an early differentiation with the left eye/right hemisphere prevailing over the right eye/left hemisphere for visual discrimination) has been observed in many species such as pigeons, chickens, zebrafish, and others (Andrew et al., 2009; Güntürkün and Manns, 2010; Chiandetti and Vallortigara, 2019; Güntürkün et al., 2020).
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