Brain asymmetry

• Neuroanatomical differences between the left and right sides of the brain
• Lateralized functional differences: lateralization of brain function

Neuroanatomical differences themselves exist on different scales, from neuronal densities, to the size of regions such as the planum temporale, to—at the largest scale—the torsion or "wind" in the human brain, reflected shape of the skull, which reflects a backward (posterior) protrusion of the left occipital bone and a forward (anterior) protrusion of the right frontal bone. In addition to gross size differences, both neurochemical and structural differences have been found between the hemispheres. Asymmetries appear in the spacing of cortical columns, as well as dendritic structure and complexity. Larger cell sizes are also found in layer III of Broca's area.

The human brain has an overall leftward posterior and rightward anterior asymmetry (or brain torque). There are particularly large asymmetries in the frontal, temporal and occipital lobes, which increase in asymmetry in the antero-posterior direction beginning at the central region. Leftward asymmetry can be seen in the Heschl gyrus, parietal operculum, Silvian fissure, left cingulate gyrus, temporo-parietal region and planum temporale. Rightward asymmetry can be seen in the right central sulcus (potentially suggesting increased connectivity between motor and somatosensory cortices in the left side of the brain), lateral ventricle, entorhinal cortex, amygdala and temporo-parieto-occipital area. Sex-dependent brain asymmetries are also common. For example, human male brains are more asymmetrically lateralized than those of females. However, gene expression studies done by Hawrylycz and colleagues and Pletikos and colleagues, were not able to detect asymmetry between the hemispheres on the population level.

People with autism have much more symmetrical brains than people without it.

History

In the mid-19th century scientists first began to make discoveries regarding lateralization of the brain, or differences in anatomy and corresponding function between the brain's two hemispheres. Franz Gall, a German anatomist, was the first to describe what is now known as the Doctrine of Cerebral Localization. Gall believed that, rather than the brain operating as a single, whole entity, different mental functions could be attributed to different parts of the brain. He was also the first to suggest language processing happened in the frontal lobes. However, Gall's theories were controversial among many scientists at the time. Others were convinced by experiments such as those conducted by Marie-Jean-Pierre Flourens, in which he demonstrated lesions to bird brains caused irreparable damage to vital functions. Flourens's methods, however, were not precise; the crude methodology employed in his experiments actually caused damage to several areas of the tiny brains of the avian models.

In 1861, surgeon Paul Broca provided evidence that supported Gall's theories. Broca discovered that two of his patients who had had speech loss had similar lesions in the same area of the left frontal lobe. While this was compelling evidence for localization of function, the connection to "sidedness" was not made immediately. As Broca continued to study similar patients, he made the connection that all of the cases involved damage to the left hemisphere, and in 1864 noted the significance of these findings—that this must be a specialized region. He also—incorrectly—proposed theories about the relationship of speech areas to "handedness".

Accordingly, some of the most famous early studies on brain asymmetry involved speech processing. Asymmetry in the Sylvian fissure (also known as the lateral sulcus), which separates the frontal and parietal lobes from the temporal lobe, was one of the first incongruencies to be discovered. Its anatomical variances are related to the size and location of two areas of the human brain that are important for language processing, Broca's area and Wernicke's area, both in the left hemisphere.

Around the same time that Broca and Wernicke made their discoveries, neurologist Hughlings Jackson suggested the idea of a "leading hemisphere"—or, one side of the brain that played a more significant role in overall function—which would eventually pave the way for understanding hemispheric "dominance" for various processes. Several years later, in the mid-20th century, critical understanding of hemispheric lateralization for visuospatial, attention and perception, auditory, linguistic and emotional processing came from patients who underwent split-brain procedures to treat disorders such as epilepsy. In split-brain patients, the corpus callosum is cut, severing the main structure for communication between the two hemispheres. The first modern split-brain patient was a war veteran known as Patient W.J., whose case contributed to further understanding of asymmetry.

This work led to Roger W. Sperry receiving the 1981 Nobel Prize in Physiology or Medicine, but also generated exaggerated or poorly supported speculation about the complementary functions of the cerebral hemispheres.

Brain asymmetry is not unique to humans. In addition to studies on human patients with various diseases of the brain, much of what is understood today about asymmetries and lateralization of function has been learned through both invertebrate and vertebrate animal models, including zebrafish, pigeons, rats, and many others. For example, more recent studies revealing sexual dimorphism in brain asymmetries in the cerebral cortex and hypothalamus of rats show that sex differences emerging from hormonal signaling can be an important influence on brain structure and function. Work with zebrafish has been especially informative because this species provides the best model for directly linking asymmetric gene expression with asymmetric morphology, and for behavioral analyses.