Brain asymmetry has been observed in humans and other animals in terms of structure, function and behaviour. This lateralization is thought to reflect evolutionary, developmental, hereditary, experiential and pathological factors. Language and handedness are well-known behaviours that provide clues to the structural and functional lateralization of the human brain. Language production and some aspects of syntactic processing are localized primarily to areas of the anterior left hemisphere, including Broca's area, whereas language comprehension is confined primarily to the left posterior temporal–parietal region, including Wernicke's area. Hand preference correlates strongly with structural and functional asymmetries in language-processing structures, such as the planum temporale. Among the most prominent observations of anatomical brain asymmetry are the right frontal and left occipital petalia-impressions on the inner surface of the skull that reflect protrusions of the right frontal pole and left occipital pole beyond their counterparts in the opposite hemisphere. A twisting effect is also seen, known as Yakovlevian anticlockwise torque, in which structures surrounding the right Sylvian fissure are 'torqued forward' relative to those on the left. The asymmetrical trajectory of the Sylvian fissure was one of the first anatomical asymmetries to be described. The height of the end-point of the Sylvian fissure is negatively correlated with the volume of the planum temporale, an extension of Wernicke's posterior receptive language area. In humans, the left planum temporale is up to ten times larger than the right. Broca's speech area is also larger in volume than its homologue in the right hemisphere. Heschl's gyrus, which corresponds to the primary auditory cortex, is larger on the left side. By contrast, the central sulcus, which houses the primary motor cortex, is reported to be deeper and larger in the right hemisphere. Advances in brain-mapping methods have enabled us to detect and visualize patterns of asymmetry in whole populations. These approaches have led to a more detailed description of the anatomical organization of the brain, allowing us to identify subtle variations in asymmetry that occur during development,with age and in disease. Among the diseases that have been associated with aberrant brain asymmetries are Alzheimer's disease, in which left-hemisphere regions are affected earlier and more severely, and developmental dyslexia, in which reduced or reversed asymmetry of the planum temporale has been reported. Male–female differences in brain asymmetry have also been detected,with some evidence to suggest that the male brain is more lateralized than that of the female. The degree to which functional asymmetries parallel those observed anatomically has been investigated using a variety of methods, including positron emission tomography and functional magnetic resonance imaging. These studies have provided further insights into brain asymmetry, describing features of left-hemisphere language localization and right-hemisphere dominance for certain visuospatial tasks. Studies of the cellular and molecular mechanisms that underpin the formation of brain asymmetries are in their infancy. Future investigations will be led by a detailed knowledge of how the brain deviates from symmetry both in healthy individuals and in disease. Brain-mapping approaches show great promise for assessing factors that modulate the lateralization of the brain, including the ontogeny, phylogeny and genetic determinants of brain asymmetry.
