Brain asymmetry

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.[5] 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.[6] 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.

Paul Broca was among the first to offer compelling evidence for localization of function when he identified an area of the brain related to speech.

In 1861 surgeon Paul Broca provided evidence that supported Gall's theories. Broca discovered that two of his patients who had suffered from speech loss had similar lesions in the same area of the left frontal lobe.[5] 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.[7]

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.,[8] whose case contributed to further understanding of asymmetry.

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.[9] 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.[10]

Lateralization of function and asymmetry in the human brain continues to propel a popular branch of neuroscientific and psychological inquiry. Technological advancements for brain mapping have enabled researchers to see more parts of the brain more clearly, which has illuminated previously undetected lateralization differences that occur during different life stages.[7] As more information emerges, researchers are finding insights into how and why early human brains may have evolved the way that they did to adapt to social, environmental and pathological changes. This information provides clues regarding plasticity, or how different parts of the brain can sometimes be recruited for different functions.[24]

Continued study of brain asymmetry also contributes to the understanding and treatment of complex diseases. Neuroimaging in patients with Alzheimer's disease, for example, shows significant deterioration in the left hemisphere, along with a rightward hemispheric dominance—which could relate to recruitment of resources to that side of the brain in the face of damage to the left.[25] These hemispheric changes have been connected to performance on memory tasks.[26]

As has been the case in the past, studies on language processing and the implications of left- and right- handedness also dominate current research on brain asymmetry.

• Split-brain
• Laterality
• Lateralization of brain function
• Alien hand syndrome
• Michael Gazzaniga
• Roger Wolcott Sperry