Lateralisaation

Lateralisaation, or brain lateralization, refers to the tendency for certain cognitive functions and neural processes to be more dominant in one cerebral hemisphere than the other. While many functions recruit both hemispheres, others show a consistent bias toward the left or right side across individuals or populations.

Language is the most well-documented example, typically localized to the left hemisphere in the majority of right-handed people, with regions such as Broca's area and Wernicke's area involved in speech production and comprehension. Motor control is contralateral, so movements on one side of the body are predominantly controlled by the opposite hemisphere. Visuospatial processing, attention, and certain aspects of emotion tend to involve the right hemisphere more strongly, though there is considerable overlap and variation among individuals.

Anatomical and functional evidence for lateralization comes from structural asymmetries (for example, the planum temporale often being larger on the left in humans), lesion studies, and neuroimaging techniques such as fMRI and PET, as well as electrophysiological methods like EEG and non-invasive brain stimulation such as transcranial magnetic stimulation that perturb or measure hemispheric activity.

Developmentally, lateralization emerges during prenatal development and early childhood, influenced by genetic factors and environmental experience. The prevalence of right-handedness is high in most populations (roughly 85–90%), with left-handedness showing more variability. Atypical or reduced lateralization has been studied in conditions such as dyslexia, autism spectrum disorder, and schizophrenia, though findings are not uniform.

Why lateralization evolved remains a topic of research. Proposed benefits include increased processing efficiency, parallel specialization, and reduced inter-hemispheric interference. Lateralization patterns vary among individuals and across species, reflecting a combination of genetic, developmental, and ecological factors.