Lateralization of brain function

The human brain is separated by a longitudinal fissure, separating the brain into two distinct cerebral hemispheres connected by the corpus callosum. The two sides of the brain are similar in appearance, and every structure in each hemisphere is generally mirrored on the other side. Despite these strong similarities, the functions of each cortical hemisphere are different.

Broad generalizations are often made in popular psychology about certain function (eg. logic, creativity) being lateralised, that is, located in the right or left side of the brain. These ideas need to be treated carefully because the popular lateralizations are often distributed across both sides.^[1] However, there is some division of mental processing. Probably most fundamental to brain lateralization is the fact that the lateral sulcus is generally longer in the left hemisphere than in the right hemisphere. Researchers have been investigating to what extent areas of the brain are specialized for certain functions. If a specific region of the brain is injured or destroyed, their functions can sometimes be recovered by neighboring brain regions - even opposite hemispheres. This depends more on the age and the damage occurred than anything else.

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 only" or "right-brained only" people.

Lateralization of brain functions is evident in the phenomena of right- or left-handedness, -earedness and -eyedness. But the handedness of a person is by no means a clear indication of location of brain function. While 95% of right handers have their language functions in the left hemisphere, only 18.8% of left-handers have their language function lateralized in the right hemisphere. Additionally, 19.8% of left-handers even have bilateral language functions.^[2]

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Which side?

Linear reasoning functions of language such as grammar and word production are often lateralized to the left hemisphere of the brain. Dyscalculia is a neurological syndrome associated with damage to the left temporal-pariet junction^[3]. This syndrome is associated with poor number manipulation, poor mental arithmetic, and an inability to understand or apply mathematical concepts.^[4]

In contrast, holistic reasoning functions of language such as intonation and emphasis are often lateralized to the right hemisphere of the brain. Functions such as the transduction of visual and musical stimuli such as spatial manipulation, facial perception, and artistic ability also seem to be lateralized to the right hemisphere.

Other integrative functions such as intuitive or heuristic arithmetic, binaural sound localization, emotions, etc. seem to be more bilaterally controlled.^[5]

^[6]

History

Speech and language

Speech consists of the mechanical process required for vocalizations, such as articulation and phonation. Language is the set of arbitrary 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 by French physician Paul Broca in 1861. Broca's research involved a patient nicknamed "Tan", who had a speech deficit (aphasia). One of the few words this patient could clearly articulate was "tan", 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—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. In clinical assessment of this condition, it is noted that the patient lacks clear articulation of the language being employed.

Wernicke

German physician Karl 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 some were 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.

Advance in imaging technique

These seminal works on hemispheric specialization were done on patients and/or postmortem brains, raising questions about the potential impact of pathology on the research findings. New methods permit the in vivo comparison of the hemispheres in healthy subjects. Particularly, magnetic resonance imaging (MRI) and positron emission tomography (PET) are important because of their high spatial resolution and ability to image subcortical brain structures.

Handedness and language

Broca's area and Wernicke’s area are linked by a white matter fiber tract called the arcuate fasciculus. This axonal tract allows the neurons in these two areas to work together to create vocal language. In more than 95% of right-handed males and more than 90% of right-handed females, language and speech are subserved by the left hemisphere of the brain. In left-handed people, the incidence of left-hemisphere language dominance is 73% ^[7] or 61%^[2], depending on the studies.

There are several ways 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 for such functions as language production and comprehension, verbal memory, and visual memory. More modern, less invasive and, in some cases, costlier techniques, such as functional magnetic resonance imaging and transcranial magnetic stimulation, can also be used to determine dominance, but their use is controversial and still considered experimental.

Movement and sensation

In the 1940s, Canadian neurosurgeon Wilder Penfield and his neurologist colleague Herbert Jasper developed a technique of brain mapping to help reduce side effects caused by surgery to treat epilepsy. They stimulated motor and somatosensory cortices of the brain with small electrical currents to activate discrete brain regions. They found that stimulation of one hemisphere's motor cortex could produce muscle contraction on the opposite side of the body. Furthermore, the functional map of the motor and sensory cortices is fairly consistent from person to person; Penfield and Jasper's famous pictures of the motor and sensory homunculi were the result.

Split-brain patients

Research by Michael Gazzaniga and Roger Wolcott Sperry 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 a large part of the corpus callosum. The corpus callosum connects the two hemispheres of the brain and allows them to communicate. When these connections are cut, the two halves of the brain have a reduced capacity to communicate with each other. This led to many interesting behavioral phenomena that allowed Gazzaniga and Sperry to study the contributions of each hemisphere to various cognitive and perceptual processes. One of their main findings was that the right hemisphere was capable of rudimentary language processing, but often has no lexical or grammatical abilities^[8].

Pseudoscientific exaggeration of the research

Hines (1987) states that the research on brain lateralization is valid as a research program, though commercial promoters have applied it to promote subjects and products far out of the implications of the research. For example, the implications of the research have no bearing on psychological interventions such as EMDR, brain training equipment, or management training. One explanation for being so prone to exaggeration and false application is that the left-right brain dichotomy is an easy-to-understand notion, yet is often grossly oversimplified and misused for promotion in the guise of science.^[9] The research on lateralization of brain functioning is ongoing, and its implications are always tightly delineated, whereas the pseudoscientific applications are exaggerated, and applied to an extremely wide range of situations.

See also

• Ambidexterity
• Cross-dominance
• Dual brain theory
• Eye dominance
• Handedness
• Laterality
• Left-handed
• Right-handed

References

• Goulven Josse, Nathalie Tzourio-Mazoyer (2003) Review: Hemispheric specialization for language. Brain Research Reviews 44 1–12.

• Hines, Terence (1987). Left Brain/Right Brain Mythology and Implications for Management and Training. The Academy of Management Review, 12:4, 600–606.

Further reading

• A.R. Luria, Higher cortical functions in man, Basic Books, 1966.
• Kandel E, Schwartz J, Jessel T. Principles of Neural Science. 4th ed. New York: McGraw-Hill; 2000. ISBN 0-8385-7701-6
• Gazzaniga, M.S., Ivry, R., & Mangun, G.R. Fundamentals of Cognitive Neuroscience. 2nd ed. W.W. Norton, 2002. ISBN 0-393-97777-3
• Betty Edwards, The New Drawing on the Right Side of the Brain, revised & expanded edition. New York: Tarcher; August 30, 1999. ISBN 0-87477-424-1