Mapping Brain Asymmetry

Arthur W. Toga and Paul M. Thompson

Laboratory of Neuro Imaging, Division of Brain Mapping,
UCLA School of Medicine, Los Angeles, CA

Review Article for Nature Reviews Neuroscience [to appear]
[Download Article, 413 KB, .pdf]

[Figures: Click on each Image for Larger Version]

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ABSTRACT

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Most biological systems demonstrate some degree of asymmetry (Geschwind and Galaburda, 1985). From humans to lower animals, normal variation and specialization produce asymmetries of function and structure. Even gross external features of the face and extremities evidence this asymmetry (Kimura, 1973). In humans and many other mammals, the two brain hemispheres differ in their anatomy and function. While cursory examination of the gross features of the human brain fails to expose profound left/right differences, careful examination of its structure reveals a variety of asymmetric features. This lateralized specialization is thought to originate from evolutionary, developmental, hereditary, experiential and pathological factors. For example, the evolutionary expansion of the left-hemisphere language cortices, in particular, may have led to marked volume asymmetries in Broca’s speech area, the planum temporale (an auditory processing structure in the posterior temporal lobe), and in other structures crucial for speech production, perception, and motor dominance. Asymmetries in the brain’s functional layout, cytoarchitecture, and neurochemistry have also been correlated with asymmetrical behavioral traits, such as handedness, auditory perception, motor preferences, and sensory acuity. Here, we review a variety of methods and their resulting observations about the structural and functional asymmetries in the brain. Brain mapping approaches, in particular, can detect and visualize patterns of asymmetries in whole populations, including subtle alterations in disease, with age, and during development. These and other tools show great promise for assessing factors that modulate cognitive specialization in the brain, including the ontogeny, phylogeny and genetic determinants of brain asymmetry.

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