- 1 Introduction
- 2 The History of Discoveries
- 3 Methods of Assessing Lateralization
- 4 Cerebral Dominance and Language Functions of the Left and Right Hemispheres
- 5 Anatomical Asymmetries
- 6 Proposed Correlations
- 7 Reorganization following brain injury
- 8 References
Hemispheric lateralization refers to the distinction between 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 is a function believed to be heavily lateralized: it is believed that certain aspects of language are localized in the left hemisphere, while others are found in the right, with the left hemisphere most often considered to be dominant. This was initially proposed by early lesion-deficit models and studies with split-brain patients, and has been shown in more recent years through tests like the Wada test and imaging studies. There have been studies which show that there are anatomic asymmetries located near and around the regions associated with language, and each hemisphere has shown to play its own but separate role in the production and comprehension of speech. The hemispheric lateralization of language functions has been suggested to be associated with both handedness, sex, bilingualism, sign-language, and a variance amongst cultures. It has also been proposed that a reorganization occurs following brain injury that involves a shifting of lateralized function, as long as the injury occurs early in life.
The History of Discoveries
Jean Baptiste Bouillaud and Simon Alexandre Ernest Aubertin
French physician Jean Baptiste Bouillaud (1796-1881) was one of the earliest proponents of hemispheric language lateralization. On February 21, 1825, Bouillaud presented a paper to the Royal Academy of Medicine in France which suggested that, because so many human tasks are performed using the right hand (such as writing), that the section of the brain which is in control of that hand might be the left hemisphere; this observation implies that language, at the core of writing, would be localized in the left hemisphere. It was already known at this time that motor function was primarily controlled by the hemisphere ipsilateral to the side of the body through lesion studies. Bouillaud also proposed that speech is localized in the frontal lobes, a theory that was carried on by Bouillaud’s son-in-law Simon Alexandre Ernest Aubertin (1825-1893), who went on to work with famed French neurologist Paul Broca in 1861. Together, Aubertin and Broca examined a patient with a left frontal lobe lesion who had lost nearly all ability to speak; this case and several others similar to it became the basis behind the earliest theories of language lateralization.
French neurologist Paul Broca (1824-1880) is often credited as being the first to expound upon this theory of language lateralization. In 1861, a 51 year old patient named Leborgne came to Broca; Leborgne was almost completely unable to speak and suffered from cellulitis of the right leg. Leborgne was able to comprehend language but was mostly unable to produce it. He responded to almost everything with the word “tan” and thus came to be known as Tan. Broca theorized that Tan must have a lesion of the left frontal lobe, and this theory was confirmed in autopsy when Tan died later that year (Bear et al., 2007). In 1863, Broca published a paper in which he described the eight cases of patients with damage to the left frontal lobe, all of whom had lost their ability to produce language, and included evidence of right frontal lesions having little effect on articulate speech (Bear et al., 2007). This led Broca to propose in 1864 that the expression of language is controlled by a specific hemisphere, most often the left (Bear et al., 2007). “On parle avec l’hemisphere gauche,” Broca concluded (Purves et al., 2008)- we speak with the left hemisphere.
German anatomist Carl Wernicke (1848-1904) is also known as an early supporter of the theory of language lateralization. In 1874, Wernicke found an area in the temporal lobe of the left hemisphere, thus distinct from that which Broca had described, which disrupted language capabilities (Bear et al., 2007). He then went on to provide the earliest map of left hemisphere language organization and processing.
Methods of Assessing Lateralization
A good deal of what we know regarding language lateralization comes from studying the loss of language abilities following brain injury (Bear et al., 2007). Aphasia, the partial or complete loss of language abilities occurring after brain damage, is the source of much of the information on this subject (Bear et al., 2007). As shown in the studies of Bouillaud, Aubertin, Broca and Wernicke described above, lesion studies combined with autopsy reports can tell us a good deal about localization of language, which ultimately has supplied information on lateralization. Lesion studies have shown that, not only is the left cerebral hemisphere most often dominant for language, but also that the right hemisphere generally is not, as lesions in the right hemisphere rarely disturb speech and language function (Bear et al., 2007).
The dangers of using lesion studies are, of course, that they may overemphasize the relevance of particular localized areas and their associated functions. The connection between brain regions and behaviours is not usually quite so simple, and is based on a larger network of connections. This is shown in the fact that the severity of an individual’s aphasia is often related to the amount of tissue damaged around the lesion itself (Bear et al., 2007). It is also known that there is a difference in the severity of the deficit depending on whether the area was removed surgically, or was caused by stroke. This is the case because strokes affect both the cortex and the subcortical structures due to the location of the middle cerebral artery, which supplies blood to the areas associated with language, as well as the basal ganglia, and is often the cause of stroke. As such, surgically produced lesions tend to have milder effects than those resulting from stroke (Bear et al., 2007).
Split Brain Studies
Studies of patients who have had commissurotomies (split-brain patients) have provided significant information about language lateralization. Commissurotomy is a surgical procedure in which the hemispheres are disconnected by cutting the corpus callosum, the massive bundle of 200 million axons connecting the right and left hemisphere (Bear et al., 2007). Following this procedure, almost all communication between the hemispheres is lost, and each hemisphere then acts independently of the other. What is striking about split-brain patients with regards to the study of language lateralization is that a word may be presented to the right hemisphere of a patient whose left hemisphere is dominant, and when the patient is asked to name the word they will say that nothing is there, because although the right hemisphere “saw” the word, it is the left hemisphere which “speaks.” If that same word is presented to the left hemisphere, the patient is able to verbalize the response (Bear et al., 2007). As such, split-brain patients have presented substantial evidence that language function is generally lateralized in the left hemisphere.
The Wada test was created by Juhn Wada at the Montreal Neurological Institute in 1949, and was designed specifically to study lateralization. A fast-acting barbiturate such as sodium amytal is injected into the carotid artery on one side (although current procedures prefer to use a catheter which is inserted into the femoral artery), and is then transported to the cerebral hemisphere on the opposite side. It then serves to anaesthetize that side of the brain for approximately 10 minutes, after which it begins to wear off and the functions which were disrupted by the anaesthetic gradually return, often displaying aphasic errors (Bear et al., 2007; Wada and Rasmussen, 1960). During the time in which the patient is anaesthetized, they are assessed on their ability to use language. If the left hemisphere is anaesthetized and is the dominant hemisphere, the patient loses all ability to speak, whereas if the left hemisphere is anaesthetized but the right hemisphere is dominant, the patient will continue to speak throughout the procedure (Bear et al., 2007).
In a study published in 1977, Brenda Milner used the Wada test to demonstrate that 98% of left-handed people and 70% of right handed people have a dominant left hemisphere with regards to language and speech function. Her results also showed that 2% of right-handed people have a dominant right hemisphere, which is the same percentage of patients that display aphasia following a lesion to the right hemisphere (Branch et al., 1964).
This procedure is also used prior to brain surgery in order to determine the dominant hemisphere, so as to avoid removal of an area associated with speech and language.
Electrical stimulation, TMS and Imaging
Electrical stimulation was pioneered by Wilder Penfield and his colleagues at the Montreal Neurological Institute in the 1930’s, and helped to identify certain lateralized areas associated with speech and language. Electrical stimulation is the application of an electrical current directly to the cortical tissue of a patient who is conscious. Penfield found that stimulating the left frontal or temporal regions of the left hemisphere with an electrical current accelerated the production of speech (Penfield, 1961). He also found that stimulation can cause inhibition in complex functions like language, as applying a current to the areas associated with speech production in the left hemisphere while the patient is engaged in speech serves to disrupt this behaviour. This procedure is performed during surgery when the skull is removed, and as such it is not a commonly used method of assessment.
Transcranial Magnetic Stimulation (TMS) is a non-invasive procedure, often combined in studies with MRI, which has helped to map the regions associated with speech, showing lateralization to be dominant in the left hemisphere. TMS has also shown that, following brain injury, it is more likely that it is the tissue surrounding the lesion that acts in a compensatory way rather than the opposite hemisphere providing compensation (Kolb and Whishaw, 2009). The major drawback of TMS is, of course, the fact that the magnetic stimulation must pass through the scalp, skull, and meninges before stimulating the brain region of choice (Kolb and Whishaw, 2009).
Imaging studies have proven to be incredibly useful in determining lateralization of language abilities. Functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) have been able to show the complex circuitry associated with speech and language, and have also proven to be consistent with the findings from previous lesion studies, as well as Penfield’s electric stimulation (Bear et al., 2007). There has been some controversy regarding bilateral activation shown in fMRI studies, the reasons unknown, however it has been suggested that perhaps the right hemisphere is involved in aspects of speech that are not measured by such tests as the Wada procedure (Bear et al., 2007). A significant finding is that fMRI results during developmental years show activation during speech and the use of language mainly in the left hemisphere, providing further evidence in support of left hemisphere dominance (Bear et al., 2007).
Cerebral Dominance and Language Functions of the Left and Right Hemispheres
The left hemisphere is, in most people, dominant in terms of the capacity for speech. It is thought that areas involved in speech production and comprehension are both localized in the left cerebral hemisphere, around the Sylvian fissure. This may be the result of subtle anatomical asymmetries, discussed in more detail below.
The left hemisphere has been shown to be dominant in speech production and comprehension, represented physiologically in the areas known as Broca's and Wernicke's regions. Electrophysiological recording from George Ojemann and his colleagues at the University of Washington show that the perisylvian cortex of the left hemisphere is involved in language production (localized in Broca's region, at the left end of the left hemisphere's Sylvian fissure, in the inferior frontal gyrus) and comprehension (localized in Wernicke's region, in the right end of the left hemisphere's Sylvian fissure, in the temporal lobe) (Purves et al., 2008). Roger Sperry and his colleagues’ split-brain studies have shown that the left hemisphere is also responsible for lexical and syntactic language capabilities (grammatical rules, sentence structure), writing, and speech (Purves at al., 2008). Other aspects of language which are thought to be governed in most people by the left hemisphere include audition of language-related sounds, recognition of letters and words, phonetics and semantics.
The right hemisphere, though generally not dominant in terms of linguistic ability, has its role in the use of language. Although the right hemisphere has no “speech,” it is still able to understand language through the auditory system, the evidence of which can be found in split-brain studies, and also has shows a small amount of reading ability and word recognition. It is suggested that pitch perception is dependent on areas in the right hemisphere (Zatorre et al., 2002), which plays an important role in the understanding of prosody and emotion in language. Lesion studies of patients who have right hemisphere lesions show a reduction in verbal fluency and deficits in the understanding and use of prosody, indicating tha such functions are likely lateralized in the right hemisphere. Patients who have had their right hemisphere surgically removed (hemispherectomy) show no aphasia, but do show less obvious deficiencies in areas such as verbal selection and understanding of metaphor. It has thus been concluded that the right hemisphere is most often responsible for the prosodic and emotional elements of speech and language (Purves et al., 2008).
It has been suggested by Zatorre et al. (2002) that auditory information is processed in stages, with both cerebral hemispheres contributing to different aspects. Their study (2002)proposes that phonetic decisions are made in Broca's area in the left hemisphere, following which phonetic analysis and comprehension of phonetic segments is made in the left posterior temporal region. Phonetic processing is suggested to occur in the left supramarginal gyrus. According to their study, the right hemisphere controls the judgment of pitch.
The structural differences between the right and left hemisphere may play a role in the lateralization of language. In the nineteenth century, anatomists observed that the left hemisphere’s sylvian fissure is longer and less steep than that of the right (Bear et al., 2007). In 1980, Graham Ratcliffe and his colleagues used evidence of this asymmetry of the sylvian fissure, shown in carotid angiogram, with results of Wada testing and found that those with speech regions located in the left hemisphere had a mean difference of 27 degrees in the angle of the blood vessels leaving the posterior end of the sylvian fissure, while those with language located in the right hemisphere had a mean angle of zero degrees (Kolb and Whishaw, 2009).
In the 1960s, Norman Geschwind and his colleagues at Harvard Medical School found that the planum temporal, the superior portion of the temporal lobe, is larger in the left hemisphere in almost two thirds of humans, an observation with was later confirmed with MRI (Bear et al., 2007; Purves et al., 2008). This asymmetry exists even in the brain of the human fetus (Bear et al., 2007). The correlation of this asymmetry with the left hemisphere’s language dominance is refuted by many due to the fact that 67% of people show this structural asymmetry, while 97% show left hemispheric dominance.
The correlation between handedness and hemispheric lateralization is described in the results of the Wada test, described above. The majority of the population is right handed (approximately 90%), and the Wada test results propose that 93% of people’s left hemisphere is dominant for language (Bear et al., 2007). But correlation does not necessarily imply causation, and it is believed that there is no direct relationship between handedness and language, as the majority of left-handers also have their language lateralized in the left hemisphere (Purves et al., 2008). It is, however, a physical example of functional asymmetry, and it is certainly possible that a more substantial connection between handedness and language will be found.
The tendency for women to score higher than men on language-related tasks is perhaps the result of the fact that women also tend to have a larger corpus callosum than men, indicating more neural connections between the right and left hemispheres (Jay, 2003). fMRI studies show that women have more bilateral activation than men when performing rhyming tasks, and PET studies show that women have more bilateral activation than men during reading tasks (Jay, 2003). Perhaps the bilateral activation implies the use of what are thought to be right hemisphere language abilities, such as prosody and intonation. Research has also shown that women have a greater ability to recover from left hemisphere brain damage; the evidence provided by the imaging studies in combination with the results of recovery following injury have led to the controversial suggestion that language is more unilateral in men than in women (Jay, 2003).
Sign Language and Bilingualism
Sign language has shown to be lateralized in the left hemisphere of the brain, in the left frontal and temporal lobes. This is known through the use of lesion studies in which the patient had left hemisphere lesion in the areas associated with language which impaired their ability to sign, while right hemisphere lesions in the same areas show no linguistic deficit (Purves et al., 2008). Lesions in the right hemisphere of signers did, however, show a limited use of spatial information encoded iconically (which is when the sign is similar looking to its referent) (Jay, 2003). This is in keeping with the belief that visuo-spatial ability is a right hemisphere function and suggests that the role of the right hemisphere in sign language is in the non-linguistic features of sign language (Jay, 2003).
Bilingualism is thought to be an overlapping of populations of neurons corresponding to each language, all of which are located in the frontal and temporal regions of the left hemisphere associated with speech comprehension and speech production (Jay, 2003).
Culture and Language Lateralization
When thinking of language there is a tendency to focus on that language in which you think, however it has been proposed that lateralization of language functions can vary from culture to culture. Asian languages show more bilateral activation during speech than European languages, likely because Asian languages employ a far greater use of right hemisphere abilities, for example prosody, and the use of spatial processing for the more “pictorial” Chinese characters; Native American languages also show a good deal of bilateral activity (Kolb and Whishaw, 2009).
Reorganization following brain injury
Studies have been done following brain injury to determine the level of recovery of language and speech ability, and whether or not recovery is based on lateralized function. Bryan Woods and Hans-Leukas Teuber looked at patients with prenatal and early postnatal brain injury located in either the right or left hemisphere and drew several conclusions. First, if the injury occurs very early, language ability may survive even after left hemisphere brain damage. Second, they found that an appropriation of language regions by the right hemisphere is responsible for the survival of these abilities, but because of this there is a tendency for visuo-spatial ability to be diminished. Third, right hemisphere lesions have the same effect in prenatal and early postnatal patients as they do in adults (Kolb and Whishaw, 2009). Brenda Milner and Ted Rasmussen used the Wada test to determine that early brain injury can cause either left, right or bilateral speech dominance, and that those who retained left hemisphere dominance had damage that was not in either the anterior (Broca’s) or posterior (Wernicke’s) speech zone (Kolb and Whishaw, 2009). Those whose dominance shifted to the right hemisphere most often had damage to these areas. Milner and Rasmussen also found that brain damage which occurs after the age of 5 does not cause a shift in lateralization but rather reorganizes within the hemisphere, potentially employing surrounding areas to take responsibility for some aspects of speech (Kolb and Whishaw, 2009).
In patients who have had hemispherectomy of the left hemisphere, the right hemisphere can often gain considerable language ability. When performed in adulthood, speech comprehension is usually retained (though speech production suffers severe deficits); reading capability is small, and there is usually no writing capability at all (Kolb and Whishaw, 2009).
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