Lateralization of brain function

The human brain is separated by a longitudinal fissure, separating the brain into two distinct cerebral hemispheres. Both sides of the brain are similar in appearance, and every structure in each hemisphere is mirrored on both sides yet. Despite these gross similarities, the functions of each cortical hemisphere are somewhat different. Human neocortex--the outer, visible portions of the brain--is the source of all cognitive, sensory, linguistic, voluntary movement, and attentive processes. These higher-level functions--as opposed to reflexes, posture maintenance, etc.--that seem to preferentially exist in one half of the brain or the other. Deeper (subcortical) structures such as the basal ganglia and thalami seem to play the same role in each half of the brain, regardless of their hemisphere.

It is important to note that--while functions are indeed lateralized--these lateralizations are trends and do not apply to every person in every case. Short of having undergone a hemispherectomy (the removal of an entire cerebral hemisphere) there are no "left-brained" or "right-brained" people.

Which side?

Reasoning functions such as language and mathematics are often lateralized to the left hemisphere of the brain, whereas visual and music functions such as spatial manipulation, facial perception, and musical ability are lateralized to the right hemisphere.

History

Speech and language

Speech consists of the mechanical process required for vocalizations, such as articulation and phonation. Language is the set of symbols used for communication, often in the form of words strung together following syntactical rules.

Broca

One of the first indications of brain function laterality arose from research conducted by French physician Paul Pierre Broca in 1861. Broca's initial research involved a patient nicknamed "Tan". This patient had a speech deficit (aphasia) manifesting as a difficulty in speech production. One of the only words this patient could clearly articulate was "tan", thus leading to his nickname. Broca performed a post-mortem autopsy and determined that Tan had a lesion, caused by syphilis, in the left cerebral hemisphere. This brain area--located in the left frontal lobe--is known as Broca's area and is an important region for speech production. Deficits in speech production caused by damage to Broca’s area are known as Broca's aphasia.

Wernicke

German physician Carl Wernicke followed up on the work done by Broca by studying language deficits unlike those shown by Broca's aphasics. Wernicke noticed that not all deficits were in speech production, but rather linguistic. He found that damage to the left posterior, superior temporal gyrus resulted in deficits in language comprehension rather than speech production. This region is now referred to as Wernicke's area, and the associated syndrome is known as Wernicke's aphasia, for his discovery.

Handedness and language

Broca's area and Wernicke’s area are linked together by a white matter fiber tract called the arcuate fasciculus. This axonal tract allows the neurons between these two brain regions to work together to create vocal language. Further research indicates that, in approximately 98% of right-handed males and 90-95% of right-handed females, language and speech are subserved by the left hemisphere of the brain. Among left-handed people, language is subserved fairly equally by the left, right, or both hemispheres.

There are several methods of determining hemisphere dominance in a living human. The Wada test involves introducing an anesthetic into one hemisphere of the brain through one of the two carotid arteries. Once one hemisphere is anesthetized, a neuropsychological exam is performed to determine dominance. More modern and less invasive neuroimaging techniques such as functional magnetic resonance imaging can also easily determine dominance, but at a higher financial cost.

Movement and sensation

In the 1940s, Canadian neurosurgeon Wilder Penfield developed a technique of mapping out to help reduce side effects caused by surgery to treat epilepsy. To do this, he stimulated the motor and somatosensory cortices of the brain. He found that, by stimulating one hemisphere of the motor cortex he could elicit a muscle contraction on the opposite side of the body. Indeed, the entire right side of the body's musculature, as well as sensation of pain and touch, is located in the left hemisphere and vice versa.

Split brain patients

Research by Michael Gazzaniga and Roger Wolcott Sperry performed in the 1960s on split-brain patients led to an even greater understanding of functional laterality. Split-brain patients are patients who have undergone corpus callosotomy (usually as a treatment for severe epilepsy), a severing of the corpus callosum. The corpus callosum is a heavy white matter tract that connects the two hemispheres of the brain and allows them to communicate. By severing these connections, the two halves of the brain act independently of one another. This process lead to a wide array of interesting behavioral phenomena that allowed Gazzaniga and Sperry to research the contributions of each hemisphere in various cognitive and perceptual processes.