Lateralization of brain function
The lateralization of brain function refers to how some neural functions, or cognitive processes tend to be more dominant in one hemisphere than the other. The medial longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. Although the macrostructure of the two hemispheres appear to be almost identical, different composition of neuronal networks allow for specialized function that is different in each hemisphere.
The extent of any modularity, or specialization of brain function by area, remains under investigation.With two brain hemispheres, this allows for both specialization and redundancy of functions, which naturally allows some protection from damage. The hemispheres exhibit strong, but not complete, bilateral symmetry in both structure and function. With either side of the brain having a right and left hemispheric side of each cerebral lobe. However, despite the brain’s ability to compensate for some amount of damage through neural plasticity and bilateral symmetry some damage in specific areas of the brain can cause deficits in normal brain functioning, including certain language deficits. These include general aphasias, aphasias being a loss of language ability due to brain damage, and more specifically the aphasias associated with brain damage or lesions specific to Broca’s and Wernicke’s area.
Despite popular psychology accounts of right and left hemisphere having different functions of creativity or mathematical abilities the functions of the brain are more differentiated between the different cerebral lobes than right and left hemisphere. Short of having undergone a hemispherectomy (removal of a cerebral hemisphere), the ideas of a "left-brain only" or "right-brain only" person are unfounded in research.
Additionally, although some functions are lateralized, not every function has a specific area that executes the function in the brain. Individuals may also vary significantly as to how any specific function is implemented as well as the specific locations of functions. Also, many brain functions have implications of functioning in multiple areas and cerebral lobes of the brain. The areas of exploration of this causal or effectual difference of a particular brain function include 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.
Lateralization of brain function is demonstrated most conclusively in the left hemisphere dominant language areas of the brain, which includes Wernicke’s and Broca’s areas.
- 1 History of research on lateralization
- 2 Sex differences
- 3 Lateralized cognitive processes
- 4 Pathology
- 5 Misapplication of concept
- 6 Nonhuman brain lateralization
- 7 Advantages of brain lateralization
- 8 Additional images
- 9 See also
- 10 References
- 11 Further reading
- 12 External links
History of research on lateralization
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 expressive 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 expressive 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 receptive 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.
Movement and sensation
In the 1940s, 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. Eran Zaidel, however, also studied such patients and found some evidence for the right hemisphere having at least some syntactic ability.
As stated above, language is primarily localized in the left hemisphere. One of the experiments carried out by Gazzaniga involved a split-brain patient sitting in front of a computer screen while having words and images presented on either side of the screen and the visual stimuli would go to either the right or left eye, and thus the left or right brain, respectively. It was observed that if a patient was presented with an image to his left eye (right brain), he would report not seeing anything. However, if he was able to feel around for certain objects, he could accurately pick out the correct object, despite not having the ability to verbalize what he saw. This led to confirmation that the left brain is localized for language while the right brain does not have this capability, and when the corpus callosum is cut and the two hemispheres cannot communicate for the speech to be produced.
For example: patients with brain damage from surgery, stroke or infection sometimes develop a syndrome in which they feel sensations in a hand, but feel neither responsible for nor able to control its movements. In patients with a corpus callosotomy, this condition, alien hand syndrome, most often manifests as uncontrolled but purposeful movements of the nondominant hand.
Broad generalizations are often made in "pop" psychology about one side or the other having characteristic labels, such as "logical" for the left side or "creative" for the right. These labels are not supported by studies on lateralization, as lateralization does not add specialized usage from either hemisphere. Both hemispheres contribute to both kinds of processes, and experimental evidence provides little support for correlating the structural differences between the sides with such broadly defined functional differences.
Sex differences are apparent in almost every aspect of neural anatomy and physiological psychology. This is also true with regards to lateralization differences between men and women. It is generally accepted that male brains are typically much more lateralized than female brains, although this is challenged by a recent study.
Lateralized cognitive processes
For example, structurally, the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere, and functionally, Broca's area and Wernicke's area are located in the left cerebral hemisphere for about 95% of right-handers, but about 70% of left-handers.
Language functions such as grammar, vocabulary and literal meaning are typically lateralized to the left hemisphere, especially in right handed individuals. While language production is left-lateralized in up to 90% of right-handed subjects, it is more bilateral, or even right lateralized in approximately 50% of left-handers. In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain.
The processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally, but may show a right hemisphere superiority. Numerical estimation, comparison and online calculation depend on bilateral parietal regions while exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing. 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.
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". 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." The delusional misidentification syndromes, reduplicative paramnesia and Capgras delusion are also often the result of right hemisphere lesions. There is evidence 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.
Lateralization of Language Processes
Hemispheric lateralization refers to the distinction of functions of the right and left hemispheres of the brain. If one hemisphere is more heavily involved in a specific function, it is often referred to as being dominant (Bear et al., 2007). Language and speech understanding and function is commonly accepted by linguists and neuroscientists to be a heavily lateralized function. Many specific aspects of language are found to be localized in the left hemisphere, while less so in the right hemisphere as the left hemisphere is most often dominant. This was proposed first through early work in patients with aphasia and language deficits found to have specific areas with lesions and damage. These lesion deficit-models and work with split-brain patients as well as work in recent years through new imaging options (fMRI, MRI, PET scans) and studies employing the Wada test have also corroborated these anatomic asymmetries and differences in function in these regions that are associated with language. With each hemisphere playing a specific but separate role in the understanding, production and use of language and speech.
This is especially evident when looking at patients that have unilateral hemisphere damage, in either the right or left hemisphere and their language deficits can be studied. For example; when the left hemisphere has been damaged or lesioned, the right hemisphere is being used to take over some functions via brain plasticity, and this damage of the one hemisphere and compensation by the opposite hemisphere creates language understanding and production changes and deficits that can be studied to examine and determine the basis and interaction of brain areas in language processes.
The production of language and language comprehension require the coordination of different subprocesses in time. Though there is debate on how these subprocesses work together and how thinking and comprehending can change, the anatomical basis and role of a loop involving Wernicke’s and Broca’s area is usually agreed upon.
Neuroscientists generally agree that around the lateral sulcus (or Sylvian Fissure) in the left hemisphere of the brain, there is a neural loop involved both in understanding and producing spoken language. At the front end or beginning of this loop lies Broca's area, which is usually associated with the production of language, or language outputs. At the other end, or specifically in the superior posterior temporal lobe, lies Wernicke's area, which is associated with the processing of words that we hear being spoken, or language inputs. Broca's area and Wernicke's area are connected by a large bundle of nerve fibres called the arcuate fasciculus.
Handedness and language
Broca's area and Wernicke's area are linked by a white matter fiber tract, the arcuate fasciculus.[dubious ] 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, the left hemisphere is dominant in certain aspects of language and speech processing. In left-handed people, the incidence of left-hemisphere language dominance has been reported as 73% and 61%, suggesting left handed people tend to be less lateralized than right-handed people. In general, however, neuroimaging methods such as functional magnetic resonance imaging and magnetoencephalography show involvement of both hemispheres in many aspects of language processing, and the "dominance" of one hemisphere just refers to more brain activation relative to the other hemisphere (or better performance by that hemisphere on psycholinguistic tasks such as dichotic listening); it is not the case that language is "localized" in any one hemisphere laterally.
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, 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of the left-handed have bilateral language functions. Even within various language functions (e.g., semantics, syntax, prosody), degree (and even hemisphere) of dominance may differ.
Methods of study
There are ways of determining whether particular cognitive functions tend to be lateralized to one cerebral hemisphere. 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 whether cognitive functions such as language production, language comprehension, verbal memory, or visual memory are retained. Another common way to study neural deficits is to identify the deficits a person exhibits in relation to lesions in different areas of the brain.
Less invasive techniques, such as functional magnetic resonance imaging and transcranial magnetic stimulation may also be used to investigate the role of a particular cerebral hemisphere in a particular task, although these methods are costly. The divided visual field paradigm is another technique that has contributed to the study of hemispheric specialization.CAT scans, PET scans and EEG are also used to study the brain. CAT scans use tomography to create a 3D image of the brain, which provides insights about neural anatomy, but it is unable to show the brain functioning in real time. PET scans image areas of high metabolic activity and neural activity by scanning for an active substance that has been tagged with positron emitting isotopes, that has been ingested by the patient. Finally, EEGs collect data from the electric fields that are produced by the brain.
Damage to either the right or left hemisphere, and its resulting deficits provide insight into the function of the damaged area. Right hemisphere damage has many effects on language production and perception. Damage or lesions to the right hemisphere can result in a lack of emotional prosody or intonation when speaking. Right hemisphere damage also has monumental effects on understanding discourse. People with damage to the right hemisphere have a reduced ability to generate inferences, comprehend and produce main concepts and a reduced ability to manage alternative meanings. Furthermore, when engaging in discourse people with right hemisphere damage, their discourse is often abrupt and perfunctory or verbose and excessive. They can also have pragmatic deficits in situations of turn taking, topic maintenance and shared knowledge.
Lateral brain damage can also have effects on spatial frequency. People with left hemisphere damage are only able to see low frequency, or big picture, parts of an image. Right hemisphere damage causes damage to low spatial frequency, so people with right hemisphere damage can only see the details of an image, or the high frequency parts of an image.
If a specific region of the brain, or even an entire hemisphere, is injured or destroyed, its functions can sometimes be assumed by a neighboring region in the same hemisphere or the corresponding region in the other 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 develop to communicate information with detached areas, despite the inefficiencies.
Broca’s aphasia is a specific type of expressive aphasia and is so named due to the aphasia that results from damage or lesions to the Broca’s area of the brain, that exists specifically in the left inferior frontal hemisphere. Thus, the aphasia that develops from the lack of functioning of the Broca’s area is an expressives and non-fluent aphasia. It is called ‘non-fluent’ due the issues that arise because Broca’s area is critical for language pronunciation and production. The area controls some motor aspects of speech production and articulation of thoughts to words and as such lesions to the area result in the specific non-fluent aphasia.
Wernicke’s aphasia is the result of damage to the area of the brain that is in the left hemisphere above the sylvian fissure but the exact location is still not defined. Damage to this area causes many deficits in language production and cognition. Although the speech produced by a person with Wernicke’s aphasia sounds like regular speech, it is riddled with mistakes. They include mild impairments in word selection, grammar, and segmental phonology. Wernicke's aphasia is characterized by phonemic paraphasias, neologism or jargon. Comprehension of spoken language is also mildly impaired in people with Wernicke's aphasia. Another characteristic of a person with Wernicke’s aphasia is that they are unconcerned by the mistakes that they are making.
Misapplication of concept
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. For example, the implications of the research have no bearing on psychological interventions such as EMDR and neurolinguistic programming, brain training equipment, or management training.
Nonhuman brain lateralization
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.
Advantages of brain lateralization
The widespread lateralization of many vertebrate animals indicates an evolutionary advantage associated with the specialization of each hemisphere. In one experiment, baby chicks were lateralized before hatching by exposing their eggs to light. These chicks were set to a task of picking out food from a bed of pebbles. Neither the lateralized, nor the non-lateralized chicks had a problem with this task, but the lateralized chicks only used the eye on the side of which they were lateralized to pick up the pebbles. When presented with a second task of watching for a cutout of a predatory hawk, the discrepancy between lateralized and non-lateralized chicks became evident. Lateralized chicks could pick food out of the pebbles with one eye and one half of the brain while using the other eye and other half of their brain to monitor the skies for predators. Not only could non-lateralized chicks not complete the two tasks simultaneously, but their performance of the single task deteriorated. This suggests that the evolutionary advantage of lateralization comes from the capacity to perform separate parallel tasks in each hemisphere of the brain.
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|In cognitive abilities||Geschwind–Galaburda hypothesis|
|In eyes||Ocular dominance|
|Handedness in boxing||Southpaw stance||Orthodox stance|
|Handedness in people|
|Handedness related to|
|Handedness measurement||Edinburgh Handedness Inventory|
|In major viscera||Situs solitus||Situs ambiguus||Situs inversus|