Abstract
Asymmetries in the functional and structural organization of the nervous system are widespread in the animal kingdom and especially characterize the human brain. Although there is little doubt that asymmetries arise through genetic and nongenetic factors, an overarching model to explain the development of functional lateralization patterns is still lacking. Current genetic psychology collects data on genes relevant to brain lateralizations, while animal research provides information on the cellular mechanisms mediating the effects of not only genetic but also environmental factors. This review combines data from human and animal research (especially on birds) and outlines a multi-level model for asymmetry formation. The relative impact of genetic and nongenetic factors varies between different developmental phases and neuronal structures. The basic lateralized organization of a brain is already established through genetically controlled embryonic events. During ongoing development, hemispheric specialization increases for specific functions and subsystems interact to shape the final functional organization of a brain. In particular, these developmental steps are influenced by environmental experiences, which regulate the fine-tuning of neural networks via processes that are referred to as ontogenetic plasticity. The plastic potential of the nervous system could be decisive for the evolutionary success of lateralized brains.
Highlights
Asymmetries in the functional and structural organization of the nervous system are widespread in the animal kingdom and especially characterize the human brain
Handedness is the most obvious asymmetry with about 90% of individual preferring to use their right hand for complex manual tasks like fine-tuned object manipulation or writing [32]
In several species, including humans, symmetry is broken by the rotation of motile cilia, which generate a directed flow that acts as a signal for the asymmetrical expression of a gene cascade, the Nodal signaling pathway. This pathway is remarkably conserved within bilaterian evolution [10,106,108]. This implies that asymmetry formation of body and brain starts with the action of cilia and, genes controlling generation and motility of cilia could play an early role in the development of neuronal asymmetries [38,110]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. “A number of embryonic events make up an integrated overture to the posthatching expression of lateralization” Lesley Rogers [1]
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