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The human brain is separated by a longitudinal fissure, separating the brain into two distinct cerebral hemispheres. The two sides of the brain are similar in appearance, and every structure in each hemisphere is mirrored on the other side, yet despite these gross similarities, the functions of each cortical hemisphere are 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.--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 on each side 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 are often lateralized to the left hemisphere of the brain. Dyscalculia is a neurological syndrome associated with damage to the left temporal-parietal junctionLevy LM, Reis IL, Grafman J. Metabolic abnormalities detected by 1H-MRS in dyscalculia and dysgraphia. Neurology. 1999 Aug 11;53(3):639-41. PMID 10449137. This syndrome is associated with poor number manipulation, poor mental arithmetic, and an inability to understand or apply mathematical concepts*.
In contrast, visual and music functions such as spatial manipulation, facial perception, and artistic ability 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.Dehaene S, Spelke E, Pinel P, Stanescu R, Tsivkin S. Sources of mathematical thinking: behavioral and brain-imaging evidence. Science. 1999 May 7;284(5416):970-4. PMID 10320379.
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 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.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 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 about 98% of right-handed males and 90-95% of right-handed females, language and speech are subserved by the left hemisphere of the brain. In left-handed people, language is subserved fairly equally by the left, right, or both hemispheres.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, and 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 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 act independently of one another. 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 abilitiesKandel E, Schwartz J, Jessel T. Principles of Neural Science. 4th ed. p1182. New York: McGraw-Hill; 2000. ISBN 0838577016.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.
Further reading
- AR Luria, - 1966 Higher cortical functions in man - Basic Books
- Kandel E, Schwartz J, Jessel T. Principles of Neural Science. 4th ed. New York: McGraw-Hill; 2000. ISBN 0838577016
- Gazzaniga, M.S., Ivry, R., & Mangun, G.R. Fundamentals of Cognitive Neuroscience. 2nd ed. W.W. Norton, 2002. ISBN 0393977773
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