Popular psychology tends to make broad and sometimes pseudoscientific generalizations about certain functions (e.g. logic, creativity) being lateral, that is, located in either the right or the left side of the brain. Researchers often criticize popular psychology for this, because the popular lateralizations often are distributed across both hemispheres, although mental processing is divided between them.
Fundamental to brain process lateralization is the fact that the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere. The extent of specialized brain function by area remains under investigation. If a specific region of the brain is either injured or destroyed, its functions can sometimes be recovered by a neighboring region, even in the opposite hemisphere, depending upon the area damaged and the patient's age.
While functions are lateralized, the lateralizations are functional trends, and are not applicable in every case. Short of having undergone a hemispherectomy (removal of a cerebral hemisphere), no one is a "left-brain only" or "right-brain only" person.
Brain function laterization is evident in the phenomena of right- or left-handedness and of right or left ear preference, but a person's preferred hand is not a clear indication of the location of brain function. Although 95% of right-handed people have left-hemisphere language function, only 18.8% of left-handed people have right-hemisphere language function. Additionally, 19.8% of the left-handed have bilateral language functions.
Linear reasoning and language functions such as grammar and vocabulary often are lateralized to the left hemisphere of the brain . Dyscalculia is a neurological syndrome associated with damage to the left temporo-parietal junction. This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to either understand or apply mathematical concepts.
In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain. Functions such as the processing of visual and musical stimuli, spatial manipulation, facial perception, and artistic ability seem to be functions of the right hemisphere.
Other integrative functions, including arithmetic, binaural sound localization, and emotions, seem more bilaterally controlled.
|Left hemisphere functions||Right hemisphere functions|
| mathematics (exact calculation, numerical comparison, estimation) |
left hemisphere only: direct fact retrieval
|mathematics (approximate calculation, numerical comparison, estimation)|
|present and past||present and future|
|language: grammar/vocabulary, literal||language: intonation/accentuation, prosody, pragmatic, contextual|
Speech consists of the mechanical processes required for vocalization, such as articulation and phonation. Language is the set of arbitrary symbols used for communication, often in the form of words strung together per syntactical rules.
One of the first indications of brain function lateralization resulted from the research of French physician Pierre Paul Broca, in 1861. His research involved the male patient nicknamed "Tan", who suffered a speech deficit (aphasia); "tan" was one of the few words he could articulate, hence his nickname. In Tan's autopsy, Broca determined he had a syphilitic lesion in the left cerebral hemisphere. This left frontal lobe brain area (Broca's Area) is an important speech production region. The motor aspects of speech production deficits caused by damage to Broca’s Area are known as Broca's aphasia. In clinical assessment of this aphasia, it is noted that the patient cannot clearly articulate the language being employed.
German physician Karl Wernicke continued in the vein of Broca's research by studying language deficits unlike Broca aphasias. Wernicke noted that not every deficit was in speech production; some were linguistic. He found that damage to the left posterior, superior temporal gyrus (Wernicke's area) caused language comprehension deficits rather than speech production deficits, a syndrome known as Wernicke's aphasia.
Broca's Area and Wernicke’s Area are linked by a white matter fiber tract, the arcuate fasciculus. This axonal tract allows the neurons in the two areas to work together in creating vocal language. In more than 95 per cent of right-handed men, and more than 90 per cent of right-handed women, language and speech are subserved by the brain's left hemisphere. In left-handed people, the incidence of left-hemisphere language dominance is 73 per cent or 61%, depending on the studies.
There are ways of determining hemispheric dominance in a person. The Wada Test introduces an anesthetic to one hemisphere of the brain via one of the two carotid arteries. Once the hemisphere is anesthetized, a neuropsychological examination is effected to determine dominance for language production, language comprehension, verbal memory, and visual memory functions. Less invasive (sometimes costlier) techniques, such as functional magnetic resonance imaging and Transcranial magnetic stimulation, also are used to determine hemispheric dominance; usage remains controversial for being experimental.
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 and neurolinguistic programming (Drenth 2003:53), brain training equipment, or management training. One explanation for why research on lateralization is so prone to exaggeration and false application is that the left-right brain dichotomy is an easy-to-understand notion, which can be oversimplified and misused for promotion in the guise of science. 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.