A longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The sides resemble each other and each hemisphere's structure is generally mirrored by the other side. Yet despite the strong anatomical similarities, the functions of each cortical hemisphere are managed differently. For example, the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere.
Broad generalizations are often made in popular psychology about one side or the other having characteristic labels such as "logical" or "creative". These labels need to be treated carefully; although a lateral dominance is measurable, these characteristics are in fact existent in both sides,[1] and experimental evidence provides little support for correlating the structural differences between the sides with functional differences.[2]
The extent of any modularity, or specialization of brain function by area, remains under investigation. If a specific region of the brain or even an entire hemisphere[3] is either injured or destroyed, its functions can sometimes be assumed by a neighboring region, even in the opposite hemisphere, depending upon the area damaged and the patient's age. When injury interferes with pathways from one area to another, alternative (indirect) connections may come to exist to communicate information with detached areas, despite the inefficiencies.
While functions are lateralized, these are only a tendency. The trend across many individuals may also vary significantly as to how any specific function is implemented. The areas of exploration of this causal or effectual difference of a particular brain function includes its gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variance of a particular brain function, between the two hemispheres of one brain or between the same hemisphere of two different brains, is still being studied. Short of having undergone a hemispherectomy (removal of a cerebral hemisphere), no one is a "left-brain only" or "right-brain only" person.[4]
Brain function lateralization 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 dominance for language, only 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of the left-handed have bilateral language functions.[5] Even within various language functions (e.g., semantics, syntax, prosody), degree (and even hemisphere) of dominance may differ.[6]
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Linear reasoning[7] and language functions such as grammar and vocabulary[8] often are lateralized to the left hemisphere of the brain. Dyscalculia is a neurological syndrome associated with damage to the left temporo-parietal junction.[9] This syndrome is associated with poor numeric manipulation, poor mental arithmetic skill, and the inability to either understand or apply mathematical concepts.[10]
In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.[11][12] The processing of visual and audiological stimuli, spatial manipulation, facial perception, and artistic ability seem to be functions of the right hemisphere. Depression is linked with a hyperactive right hemisphere, with evidence of selective involvement in "processing negative emotions, pessimistic thoughts and unconstructive thinking styles", as well as vigilance, arousal and self-reflection, and a relatively hypoactive left hemisphere, "specifically involved in processing pleasurable experiences" and "relatively more involved in decision-making processes".[13] Additionally, "left hemisphere lesions result in an omissive response bias or error pattern whereas right hemisphere lesions result in a commissive response bias or error pattern." [14] The delusional misidentification syndromes reduplicative paramnesia and Capgras delusion are also often the result of right hemisphere lesions.[15][16] There is evidence[17] that the right hemisphere is more involved in processing novel situations, while the left hemisphere is most involved when routine or well rehearsed processing is called for.
Other integrative functions, including arithmetic,[18][19] binaural sound localization, and emotions (lateralization of emotion), seem more bilaterally controlled.
| Left hemisphere functions | Right hemisphere functions |
| numerical computation (exact calculation, numerical comparison, estimation) left hemisphere only: direct fact retrieval[18][19] |
numerical computation (approximate calculation, numerical comparison, estimation)[18][19] |
| language: grammar/vocabulary, literal[20] | language: intonation/accentuation, prosody, pragmatic, contextual[20] |
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's aphasia. 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.
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.
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% of right-handed men, and more than 90% 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 has been reported as 73%[21] and 61%.[5]
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.
In the 1940s, American born, Montreal based 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 produces 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.
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.[22] Eran Zaidel, however, also studied such patients and found some evidence for the right hemisphere having at least some syntactic ability.
For example: Patients with brain damage from surgery, stroke or infection sometimes develop a syndrome in which they can feel sensations in their hand, but they don't feel responsible for nor able to control its movements. In patients with a corpus callosotomy, alien hand syndrome most often manifests as uncontrolled but purposeful movements of the nondominant hand.
Terence Hines 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 outside the implications of the research.[23] For example, the implications of the research have no bearing on psychological interventions such as EMDR and neurolinguistic programming,[24] brain training equipment, or management training.[25]
Specialization of the two hemispheres is general in vertebrates including fish, frogs, reptiles, birds and mammals with the left hemisphere being specialized to categorize information and control everyday, routine behavior, with the right hemisphere responsible for responses to novel events and behavior in emergencies including the expression of intense emotions. An example of a routine left hemisphere behavior is feeding behavior whereas as a right hemisphere is escape from predators and attacks from conspecifics.[26]
| Side | Left | Both | Right |
|---|---|---|---|
| General | Ambidexterity | ||
| In cognitive abilities | Geschwind–Galaburda hypothesis | ||
| In brain | Brain asymmetry · Dual brain theory · Bicameralism | ||
| In eyes | Ocular dominance | ||
| In hands | Left-handedness | Cross-dominance | Right-handedness |
| Handedness in boxing | Southpaw stance | Orthodox stance | |
| Handedness in people | Musicians · US presidents | ||
| Handedness related to | Sex · Maths | ||
| Handedness measurement | Edinburgh Handedness Inventory | ||
| Handedness genetics | LRRTM1 | ||
| In heart | Levocardia | Dextrocardia | |
| In major viscera | Situs solitus | Situs ambiguus | Situs inversus |
| In feet | Footedness | ||