The vertebratecerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure.
The brain can thus be described as being divided into left and right
cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter, the cerebral cortex, that is supported by an inner layer of white matter. In eutherian (placental) mammals, the hemispheres are linked by the corpus callosum, a very large bundle of nerve fibers. Smaller commissures, including the anterior commissure, the posterior commissure and the fornix,
also join the hemispheres and these are also present in other
vertebrates. These commissures transfer information between the two
hemispheres to coordinate localized functions.
Macroscopically the hemispheres are roughly mirror images of each other, with only subtle differences, such as the Yakovlevian torque seen in the human brain, which is a slight warping of the right side, bringing it just forward of the left side. On a microscopic level, the cytoarchitecture of the cerebral cortex, shows the functions of cells, quantities of neurotransmitter levels and receptor subtypes to be markedly asymmetrical between the hemispheres.
However, while some of these hemispheric distribution differences are
consistent across human beings, or even across some species, many
observable distribution differences vary from individual to individual
within a given species.
There are three poles of the cerebrum, the occipital pole, the
frontal pole, and the temporal pole. The occipital pole is the posterior
end of each occipital lobe in each hemisphere. It is more pointed than the rounder frontal pole. The frontal pole is at the frontmost part of the frontal lobe
in each hemisphere, and is more rounded than the occipital pole. The
temporal pole is located between the frontal and occipital poles, and
sits in the anterior part of middle cranial fossa in each temporal lobe.
Composition
If the upper part of either hemisphere is removed, at a level about 1.25 cm above the corpus callosum, the central white matter will be exposed as an oval-shaped area, the centrum semiovale,
surrounded by a narrow convoluted margin of gray substance, and studded
with numerous minute red dots (puncta vasculosa), produced by the
escape of blood from divided blood vessels.
If the remaining portions of the hemispheres be slightly drawn
apart a broad band of white substance, the corpus callosum, will be
observed, connecting them at the bottom of the longitudinal fissure; the margins of the hemispheres which overlap the corpus callosum are called the labia cerebri.
Each labium is part of the cingulate gyrus already described; and
the groove between it and the upper surface of the corpus callosum is
termed the callosal sulcus.
If the hemispheres are sliced off to a level with the upper
surface of the corpus callosum, the white substance of that structure
will be seen connecting the two hemispheres.
The large expanse of medullary matter now exposed, surrounded by
the convoluted margin of gray substance, is called the centrum
semiovale. The blood supply to the centrum semiovale is from the
superficial middle cerebral artery. The cortical branches of this artery descend to provide blood to the centrum semiovale.
Broad generalizations are often made in popular psychology about certain functions (e.g. logic, creativity) being lateralized,
that is, located in the right or left side of the brain. These claims
are often inaccurate, as most brain functions are actually distributed
across both hemispheres. Most scientific evidence for asymmetry relates
to low-level perceptual functions rather than the higher-level functions
popularly discussed (e.g. subconscious processing of grammar, not
"logical thinking" in general). In addition to this lateralization of some functions, the low-level representations also tend to represent the contralateral side of the body.
The best example of an established lateralization is that of Broca's and Wernicke's Areas (language)
where both are often found exclusively on the left hemisphere. These
areas frequently correspond to handedness however, meaning the
localization of these areas is regularly found on the hemisphere
opposite to the dominant hand. Function lateralization such as
semantics, prosodic, intonation, accentuation, prosody, etc. has since
been called into question and largely been found to have a neuronal
basis in both hemispheres.
Cerebral hemispheres of a human embryo at 8 weeks.
Perceptual information
is processed in both hemispheres, but is laterally partitioned:
information from each side of the body is sent to the opposite
hemisphere (visual information is partitioned somewhat differently,
but still lateralized). Similarly, motor control signals sent out to
the body also come from the hemisphere on the opposite side. Thus, hand preference (which hand someone prefers to use) is also related to hemisphere lateralization.
In some aspects, the hemispheres are asymmetrical; the right side is slightly bigger. There are higher levels of the neurotransmitternorepinephrine on the right and higher levels of dopamine on the left. There is more white matter (longer axons) on the right and more grey matter (cell bodies) on the left.
Linearreasoning functions of language such as grammar and word production are often lateralized to the left hemisphere of the brain. In contrast, holisticreasoning functions of language
such as intonation and emphasis are often lateralized to the right
hemisphere of the brain. Other integrative functions such as intuitive
or heuristic arithmetic, binaural sound localization, etc. seem to be more bilaterally controlled.
As a treatment for epilepsy the corpus callosum may be severed to cut the major connection between the hemispheres in a procedure known as a corpus callosotomy.
A hemispherectomy is the removal or disabling of one of the hemispheres of the brain. This is a rare procedure used in some extreme cases of seizures which are unresponsive to other treatments.
Broca's area, or the Broca area , is a region in the frontal lobe of the dominant hemisphere, usually the left, of the brain with functions linked to speech production.
Functional magnetic resonance imaging has shown language processing to also involve the third part of the inferior frontal gyrus the pars orbitalis, as well as the ventral part of BA6 and these are now often included in a larger area called Broca's region.
Studies of chronic aphasia have implicated an essential role of Broca's area in various speech and language functions. Further, fMRI
studies have also identified activation patterns in Broca's area
associated with various language tasks. However, slow destruction of the
Broca's area by brain tumors can leave speech relatively intact, suggesting its functions can shift to nearby areas in the brain.
Structure
Broca's area is often identified by visual inspection of the topography of the brain either by macrostructural landmarks such as sulci or by the specification of coordinates in a particular reference space. The currently used Talairach and Tournoux atlas projects Brodmann'scytoarchitectonic map onto a template brain. Because Brodmann's parcelation
was based on subjective visual inspection of cytoarchitectonic borders
and also Brodmann analyzed only one hemisphere of one brain, the result
is imprecise. Further, because of considerable variability across brains
in terms of shape, size, and position relative to sulcal and gyral
structure, a resulting localization precision is limited.
The differences between area 45 and 44 in cytoarchitecture and in
connectivity suggest that these areas might perform different
functions. Indeed, recent neuroimaging
studies have shown that the PTr and Pop, corresponding to areas 45 and
44, respectively, play different functional roles in the human with
respect to language comprehension and action recognition/understanding.
Functions
Language comprehension
For
a long time, it was assumed that the role of Broca's area was more
devoted to language production than language comprehension. However,
there is evidence to demonstrate that Broca's area also plays a
significant role in language comprehension. Patients with lesions
in Broca's area who exhibit agrammatical speech production also show
inability to use syntactic information to determine the meaning of
sentences. Also, a number of neuroimaging studies have implicated an involvement of Broca's area, particularly of the pars opercularis of the left inferior frontal gyrus, during the processing of complex sentences. Further, it has recently been found in functional magnetic resonance imaging (fMRI) experiments involving highly ambiguous sentences result in a more activated inferior frontal gyrus.
Therefore, the activity level in the inferior frontal gyrus and the
level of lexical ambiguity are directly proportional to each other,
because of the increased retrieval demands associated with highly
ambiguous content.
There is also specialisation for particular aspects of comprehension within Broca's area. Work by Devlin et al. (2003) showed in a repetitive transcranial magnetic stimulation (rTMS) study that there was an increase in reaction times when performing a semantic task under rTMS aimed at the pars triangularis
(situated in the anterior part of Broca's area). The increase in
reaction times is indicative that that particular area is responsible
for processing that cognitive function. Disrupting these areas via TMS
disrupts computations performed in the areas leading to an increase in
time needed to perform the computations (reflected in reaction times).
Later work by Nixon et al. (2004)
showed that when the pars opercularis (situated in the posterior part
of Broca's area) was stimulated under rTMS there was an increase in
reaction times in a phonological task. Gough et al. (2005)
performed an experiment combining elements of these previous works in
which both phonological and semantic tasks were performed with rTMS
stimulation directed at either the anterior or the posterior part of
Broca's area. The results from this experiment conclusively
distinguished anatomical specialisation within Broca's area for
different components of language comprehension. Here the results showed
that under rTMS stimulation:
Semantic
tasks only showed a decrease in reaction times when stimulation was
aimed at the anterior part of Broca's area (where a decrease of 10%
(50ms) was seen compared to a no-TMS control group)
Phonological
tasks showed a decrease in reaction times when stimulation was aimed at
the posterior part of Broca's area (where a decrease of 6% (30ms) was
seen compared to control)
To summarise, the work above shows anatomical specialisation in
Broca's area for language comprehension, with the anterior part of
Broca's area responsible for understanding the meaning of words
(semantics) and the posterior part of Broca's area responsible for
understanding how words sound (phonology).
Action recognition and production
Recent
experiments have indicated that Broca's area is involved in various
cognitive and perceptual tasks. One important contribution of Brodmann's
area 44 is also found in the motor-related processes. Observation of
meaningful hand shadows resembling moving animals activates frontal
language area, demonstrating that Broca's area indeed plays a role in
interpreting action of others. An activation of BA 44 was also reported during execution of grasping and manipulation.
Speech-associated gestures
It
has been speculated that because speech-associated gestures could
possibly reduce lexical or sentential ambiguity, comprehension should
improve in the presence of speech-associated gestures. As a result of
improved comprehension, the involvement of Broca's area should be
reduced.
Many neuroimaging studies have also shown activation of Broca's
area when representing meaningful arm gestures. A recent study has shown
evidence that word and gesture are related at the level of translation
of particular gesture aspects such as its motor goal and intention. This finding helps explain why, when this area is defective, those who use sign language also suffer from language deficits.
This finding that aspects of gestures are translated in words within
Broca's area also explains language development in terms of evolution.
Indeed, many authors have proposed that speech evolved from a primitive
communication that arose from gestures.
Speaking without Broca's area
Damage
to Broca's area is commonly associated with telegraphic speech made up
of content vocabulary. For example, a person with Broca's aphasia may
say something like, "Drive, store. Mom." meaning to say, "My mom drove
me to the store today." Therefore, the content of the information is
correct, but the grammar and fluidity of the sentence is missing.
The essential role of the Broca's area in speech production has
been questioned since it can be destroyed while leaving language nearly
intact. In one case of a computer engineer, a slow-growing glioma tumor was removed. The tumor and the surgery destroyed the left inferior and middle frontal gyrus, the head of the caudate nucleus, the anterior limb of the internal capsule, and the anterior insula.
However, there were minimal language problems three months after
removal and the individual returned to his professional work. These
minor problems include the inability to create syntactically complex
sentences including more than two subjects, multiple causal conjunctions, or reported speech. These were explained by researchers as due to working memory problems. They also attributed his lack of problems to extensive compensatory mechanisms enabled by neural plasticity in the nearby cerebral cortex and a shift of some functions to the homologous area in the right hemisphere.
Aphasia
is an acquired language disorder affecting all modalities such as
writing, reading, speaking, and listening and results from brain damage.
It is often a chronic condition that creates changes in all areas of
one's life.
Expressive aphasia vs. other aphasias
Patients with expressive aphasia, also known as Broca's aphasia, are individuals who know "what they want to say, they just cannot get it out".
They are typically able to comprehend words, and sentences with a
simple syntactic structure (see above), but are more or less unable to
generate fluent speech. Other symptoms that may be present include
problems with fluency, articulation, word-finding, word repetition, and producing and comprehending complex grammatical sentences, both orally and in writing.
This specific group of symptoms distinguishes those who have
expressive aphasia from individuals with other types of aphasia. There
are several distinct "types" of aphasia, and each type is characterized
by a different set of language deficits. Although those who have
expressive aphasia tend to retain good spoken language comprehension,
other types of aphasia can render patients completely unable to
understand any language at all, unable to understand any spoken language
(auditory verbal agnosia),
whereas still other types preserve language comprehension, but with
deficits. People with expressive aphasia may struggle less with reading
and writing than those with other types of aphasia.
Although individuals with expressive aphasia tend to have a good
ability to self-monitor their language output (they "hear what they say"
and make corrections), other types of aphasics can seem entirely
unaware of their language deficits.
In the classical sense, expressive aphasia is the result of
injury to Broca's area; it is often the case that lesions in specific
brain areas cause specific, dissociable symptoms, although case studies show there is not always a one-to-one mapping between lesion location and aphasic symptoms.
The correlation between damage to certain specific brain areas (usually
in the left hemisphere) and the development of specific types of
aphasia makes it possible to deduce (albeit very roughly) the location
of a suspected brain lesion based only on the presence (and severity) of
a certain type of aphasia, though this is complicated by the
possibility that a patient may have damage to a number of brain areas
and may exhibit symptoms of more than one type of aphasia. The
examination of lesion data in order to deduce which brain areas are
essential in the normal functioning of certain aspects of cognition is
called the deficit-lesion method; this method is especially important in
the branch of neuroscience known as aphasiology. Cognitive science - to be specific, cognitive neuropsychology - are branches of neuroscience that also make extensive use of the deficit-lesion method.
Major characteristics of different types of acute aphasia
Newer implications related to lesions in Broca's area
Since studies carried out in the late 1970's it has been understood that the relationship between Broca's area and Broca's aphasia is not as consistent as once thought.
Lesions to Broca's area alone don't result in a Broca's aphasia, nor do
Broca's aphasic patients necessarily have lesions in Broca's area.
Lesions to Broca's area alone are known to produce just a transient
mutism that resolves inside 3–6 weeks. This discovery suggests that
Broca's area may be included in some aspect of verbalization or
articulation; however, it does not address its part in sentence
comprehension. Still, Broca's area frequently emerges in functional
imaging studies of sentence processing. However, it also becomes activated in word-level tasks. This suggests that Broca’s area is not dedicated to sentence processing
but supports a function common to both. In fact, Broca's area can show
activation in such non-linguistic tasks as imagery of motion.
Considering the hypothesis that Broca's area may be most involved
in articulation, its activation in all of these tasks may be due to
subjects' covert articulation while formulating a response. Despite this
caveat, a consensus seems to be forming that whatever role Broca's area
may play, it may relate to known working memory functions of the
frontal areas. (There is a wide distribution of Talairach coordinates
reported in the functional imaging literature that are referred to as
part of Broca's area.) The processing of a passive voice sentence, for
example, may require working memory to assist in the temporary retention
of information while other relevant parts of the sentence are being
manipulated (i.e. to resolve the assignment of thematic roles to
arguments). Miyake, Carpenter, and Just have proposed that sentence
processing relies on such general verbal working memory mechanisms while
Caplan and Waters consider Broca’s area to be involved in working
memory specifically for syntactic processing. Friederici (2002) breaks
Broca's area into its component regions and suggests that Brodmann's
area 44 is involved in working memory for both phonological
and syntactic structure. This area becomes active first for phonology
and later for syntax as the time course for the comprehension process
unfolds. Brodmann's area 45 together with Brodmann's area 47 is viewed
as being specifically involved in working memory for semantic features
and thematic structure where processes of syntactic reanalysis and
repair are required. These areas come online after Brodmann's area 44
has finished its processing role and where comprehension of complex
sentences must rely on general memory resources. All of these theories
indicate a move towards a view that syntactic comprehension problems
arise from a computational rather than a conceptual deficit. Newer
theories are taking a more dynamic view of how the brain integrates
different linguistic and cognitive components and are examining the time
course of these operations.
Neurocognitive studies have already implicated frontal areas
adjacent to Broca's area as important for working memory in
non-linguistic as well as linguistic tasks.
Cabeza and Nyberg's analysis of imaging studies of working memory
supports the view that BA45/47 is recruited for selecting or comparing
information, while BA9/46 might be more involved in the manipulation of
information in working memory. Since large lesions are typically
required to produce a Broca's aphasia, it is likely that these regions
may also become compromised in some patients and may contribute to their
comprehension deficits for complex morphosyntactic structures.
Broca's area: A key center in the linking phonemic sequences
Broca's
area has been previously associated with a variety of processes,
including phonological segmentation, syntactic processing, and
unification, all of which involve segmenting and linking different types
of linguistic information.
Although repeating and reading single words do not engage semantic and
syntactic processing, they do require an operation linking phonemic
sequences with motor gestures. Findings indicate that this linkage is
coordinated by Broca's area through reciprocal interactions with
temporal and frontal cortices responsible for phonemic and articulatory
representations, respectively, including interactions with motor cortex
before the actual act of speech. Based on these unique findings, it has
been proposed
that Broca's area is not the seat of articulation per se, but rather is
a key node in manipulating and forwarding neural information across
large-scale cortical networks responsible for key components of speech
production.
History
In a
study published in 2007, the preserved brains of both Leborgne and
Lelong (patients of Broca) were reinspected using high-resolution
volumetric MRI.
The purpose of this study was to scan the brains in three dimensions
and to identify the extent of both cortical and subcortical lesions in
more detail. The study also sought to locate the exact site of the
lesion in the frontal lobe in relation to what is now called Broca's
area with the extent of subcortical involvement.
Broca's patients
Louis Victor Leborgne (Tan)
Leborgne was a patient of Broca's. At 30 years old, he was almost completely unable to produce any words or phrases. He was able to repetitively produce only the word tan. After his death, a neurosyphilitic lesion was discovered on the surface of his left frontal lobe.
Lelong
Lelong
was another patient of Broca's. He also exhibited reduced productive
speech. He could only say five words, 'yes', 'no', 'three', 'always',
and 'lelo' (a mispronunciation of his own name). A lesion within the
lateral frontal lobe was discovered during Lelong's autopsy. Broca's
previous patient, Leborgne, had this lesion in the same area of his
frontal lobe. These two cases led Broca to believe that speech was
localized to this particular area.
MRI findings
Examination of the brains of Broca's two historic patients with high-resolution MRI
has produced several interesting findings. First, the MRI findings
suggest that other areas besides Broca's area may also have contributed
to the patients' reduced productive speech. This finding is significant
because it has been found that, though lesions
to Broca's area alone can possibly cause temporary speech disruption,
they do not result in severe speech arrest. Therefore, there is a
possibility that the aphasia denoted by Broca as an absence of productive speech also could have been influenced by the lesions in the other region.
Another finding is that the region, which was once considered to be
critical for speech by Broca, is not precisely the same region as what
is now known as Broca's area. This study provides further evidence to
support the claim that language and cognition are far more complicated
than once thought and involve various networks of brain regions.
Evolution of language
The pursuit of a satisfying theory that addresses the origin of language
in humans has led to the consideration of a number of evolutionary
"models". These models attempt to show how modern language might have
evolved, and a common feature of many of these theories is the idea that
vocal communication was initially used to complement a far more dominant mode of communication through gesture. Human language might have evolved
as the "evolutionary refinement of an implicit communication system
already present in lower primates, based on a set of hand/mouth
goal-directed action representations."
"Hand/mouth goal-directed action representations" is another way
of saying "gestural communication", "gestural language", or
"communication through body language". The recent finding that Broca's
area is active when people are observing others engaged in meaningful
action is evidence in support of this idea. It was hypothesized that a
precursor to the modern Broca's area was involved in translating
gestures into abstract ideas by interpreting the movements of others as
meaningful action with an intelligent purpose. It is argued that over
time the ability to predict the intended outcome and purpose of a set of
movements eventually gave this area the capability to deal with truly
abstract ideas, and therefore (eventually) became capable of associating
sounds (words) with abstract meanings. The observation that frontal
language areas are activated when people observe Hand Shadows
is further evidence that human language may have evolved from existing
neural substrates that evolved for the purpose of gesture recognition.
The study, therefore, claims that Broca's area is the "motor center for
speech", which assembles and decodes speech sounds in the same way it
interprets body language and gestures. Consistent with this idea is that
the neural substrate that regulated motor control in the common
ancestor of apes and humans was most likely modified to enhance
cognitive and linguistic ability. Studies of speakers of American Sign Language
and English suggest that the human brain recruited systems that had
evolved to perform more basic functions much earlier; these various
brain circuits, according to the authors, were tapped to work together
in creating language.
Another recent finding has showed significant areas of activation
in subcortical and neocortical areas during the production of
communicative manual gestures and vocal signals in chimpanzees. Further, the data indicating that chimpanzees
intentionally produce manual gestures as well as vocal signals to
communicate with humans suggests that the precursors to human language
are present at both the behavioral and neuronanatomical levels. More
recently, the neocortical distribution of activity-dependent gene
expression in marmosets provided direct evidence that the ventrolateral
prefrontal cortex, which comprises Broca's area in humans and has been
associated with auditory processing of species-specific vocalizations
and orofacial control in macaques, is engaged during vocal output in a New World monkey.
These findings putatively set the origin of vocalization-related
neocortical circuits to at least 35 million years ago, when the Old and
New World monkey lineages split.
Wernicke's area (/ˈvɛərnɪkə/ or /ˈvɛərnɪki/; German: [ˈvɛʁnɪkə]), also called Wernicke's speech area, is one of the two parts of the cerebral cortex that are linked to speech (the other is Broca's area).
It is involved in the comprehension of written and spoken language (in
contrast to Broca's area that is involved in the production of
language). It is traditionally thought to be in Brodmann area 39,40, which is located in the superior temporal lobe
in the dominant cerebral hemisphere (which is the left hemisphere in
about 95% of right handed individuals and 60% of left handed
individuals).
Damage caused to Wernicke's area results in receptive, fluent aphasia.
This means that the person with aphasia will be able to fluently
connect words, but the phrases will lack meaning. This is unlike non-fluent aphasia, in which the person will use meaningful words, but in a non-fluent, telegraphic manner.
However, there is an absence of consistent definitions as to the location.
Some identify it with the unimodal auditory association in the superior
temporal gyrus anterior to the primary auditory cortex (the anterior
part of BA 22). This is the site most consistently implicated in auditory word recognition by functional brain imaging experiments. Others include also adjacent parts of the heteromodal cortex in BA 39 and BA40 in the parietal lobe.
While previously thought to connect Wernicke's area and Broca's area, new research demonstrates that the arcuate fasciculus instead connects to posterior receptive areas with premotor/motor areas, and not to Broca's area. Consistent with the word recognition site identified in brain imaging, the uncinate fasciculus connects anterior superior temporal regions with Broca's area.
Function
Right homologous area
Research using Transcranial magnetic stimulation
suggests that the area corresponding to the Wernicke’s area in the
non-dominant cerebral hemisphere has a role in processing and resolution
of subordinate meanings of ambiguous words—such as ‘‘river’’ when given
the ambiguous word "bank." In contrast, the Wernicke's area in the
dominant hemisphere processes dominant word meanings (‘‘teller’’ given
‘‘bank’’).
Modern views
Neuroimaging suggests the functions earlier attributed to Wernicke's area occur more broadly in the temporal lobe and indeed happen also in Broca's area.
“
There
are some suggestions that middle and inferior temporal gyri and basal
temporal cortex reflect lexical processing ... there is consensus that
the STG from rostral to caudal fields and the STS
constitute the neural tissue in which many of the critical computations
for speech recognition are executed ... aspects of Broca’s area
(Brodmann areas 44 and 45) are also regularly implicated in speech
processing.
... the range of areas implicated in speech processing go well beyond
the classical language areas typically mentioned for speech; the vast
majority of textbooks still state that this aspect of perception and
language processing occurs in Wernicke’s area (the posterior third of
the STG).
”
Support for a broad range of speech processing areas was furthered by a recent study caried out at the University of Rochester in which American Sign Language native speakers were subject to MRI
while interpreting sentences that identified a relationship using
either syntax (relationship is determined by the word order) or
inflection (relationship is determined by physical motion of "moving
hands through space or signing on one side of the body"). Distinct
areas of the brain were activated with the frontal cortex (associated
with ability to put information into sequences) being more active in the
syntax condition and the temporal lobes (associated with dividing
information into its constituent parts) being more active in the
inflection condition. However, these areas are not mutually exclusive
and show a large amount of overlap. These findings imply that while
speech processing is a very complex process, the brain may be using
fairly basic, preexisting computational methods.
Clinical significance
Human brain with Wernicke's area highlighted in red
Aphasia
Wernicke's area is named after Carl Wernicke, a Germanneurologist and psychiatrist
who, in 1874, hypothesized a link between the left posterior section of
the superior temporal gyrus and the reflexive mimicking of words and
their syllables that associated the sensory and motor images of spoken
words. He did this on the basis of the location of brain injuries that caused aphasia. Receptive aphasia in which such abilities are preserved is also known as Wernicke's aphasia.
In this condition there is a major impairment of language
comprehension, while speech retains a natural-sounding rhythm and a
relatively normal syntax. Language as a result is largely meaningless (a condition sometimes called fluent or jargon aphasia).
While neuroimaging and lesion evidence generally support the idea
that malfunction of or damage to Wernicke's area is common in people
with receptive aphasia, this is not always so. Some people may use the
right hemisphere for language, and isolated damage of Wernicke's area
cortex (sparing white matter and other areas) may not cause severe
receptive aphasia. Even when patients with Wernicke's area lesions have comprehension deficits, these are usually not restricted to language processing
alone. For example, one study found that patients with posterior
lesions also had trouble understanding nonverbal sounds like animal and
machine noises. In fact, for Wernicke's area, the impairments in nonverbal sounds were statistically stronger than for verbal sounds.
The
human brain is divided into two hemispheres–left and right. Scientists
continue to explore how some cognitive functions tend to be dominated by
one side or the other; that is, how they are lateralized.
The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or 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 appears to be almost
identical, different composition of neuronal networks allows for
specialized function that is different in each hemisphere.
Lateralization of brain structures is based on general trends expressed
in healthy patients; however, there are numerous counterexamples to each
generalization. Each human's brain develops differently leading to
unique lateralization in individuals. This is different from
specialization as lateralization refers only to the function of one
structure divided between two hemispheres. Specialization is much easier
to observe as a trend since it has a stronger anthropological history. The best example of an established lateralization is that of Broca's and Wernicke's areas where both are often found exclusively on the left hemisphere. These areas frequently correspond to handedness,
however, meaning that the localization of these areas is regularly
found on the hemisphere corresponding to the dominant hand (anatomically
on the opposite side). Function lateralization, such as semantics, intonation, accentuation, and prosody, has since been called into question and largely been found to have a neuronal basis in both hemispheres. Another example is that each hemisphere in the brain tends to represent one side of the body. In the cerebellum this is the same bodyside, but in the forebrain this is predominantly the contralateral side.
Lateralized functions
Language
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-handers, it is more bilateral, or even right-lateralized, in
approximately 50% of left-handers.
Broca's area and Wernicke's area areas associated with the production of speech and comprehension of speech, respectively, are located in the left cerebral hemisphere for about 95% of right-handers, but about 70% of left-handers.
Sensory processing
The
processing of basic sensory information is lateralized by being divided
into left and right sides of the body or the space around the body.
In vision, about half the neurons of the optic nerve
from each eye cross to project to the opposite hemisphere and about
half do not cross to project to the hemisphere on the same side. This means that the left side of the visual field is processed largely by the visual cortex of the right hemisphere and vice versa for the right side of the visual field.
Because of this functional division of the left and right sides
of the body and of the space that surrounds it, the processing of
information in the sensory cortices is essentially identical. That is,
the processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally. 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.
Value systems
Rather
than just being a series of places where different brain modules occur,
there are running similarities in the kind of function seen in each
side, for instance how right-side impairment of drawing ability making
patients draw the parts of the subject matter with wholly incoherent
relationships, or where the kind of left-side damage seen in language
impairment not damaging the patient's ability to catch the significance
of intonation in speech. This has led Iain McGilchrist to say that the two hemispheres as having different value systems,
where the left hemisphere tends to reduce complex matters such as
ethics to rules and measures, where the right hemisphere is disposed to
the holistic and metaphorical.
Clinical significance
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.
Hemisphere damage
Damage
to either the right or left hemisphere, and its resulting deficits
provide insight into the function of the damaged area. Left 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 grave
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, people with right hemisphere damage often exhibit
discourse that is 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 affect visual perceptual spatial
resolution. People with left hemisphere damage may have impaired
perception of high resolution, or detailed, aspects of an image. People
with right hemisphere damage may have impaired perception of low
resolution, or big picture, aspects of an image.
Plasticity
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
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 most commonly in the left inferior frontal
hemisphere. Thus, the aphasia that develops from the lack of functioning
of the Broca's area is an expressive 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
Wernicke's aphasia
is the result of damage to the area of the brain that is commonly in
the left hemisphere above the sylvian fissure. Damage to this area
causes primarily a deficit in language comprehension. While the ability
to speak fluently with normal melodic intonation is spared, the language produced by a person with Wernicke's aphasia is riddled with semantic
errors, and may sound nonsensical to the listener. Wernicke's aphasia
is characterized by phonemic paraphasias, neologism or jargon. Another
characteristic of a person with Wernicke's aphasia is that they are
unconcerned by the mistakes that they are making.
Society and culture
Misapplication
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.
Pop psychology
The
oversimplification of lateralization in pop psychology. This belief was
widely held even in the scientific community for some years.
Some popularizations oversimplify the science about lateralization,
by presenting the functional differences between hemispheres as being
more absolute than is actually the case.
Sex differences
In the 19th century and to a lesser extent the 20th, it was thought
that each side of the brain was associated with a specific gender: the
left corresponding with masculinity and the right with femininity and
each half could function independently.
The right side of the brain was seen as the inferior and thought to be
prominent in women, savages, children, criminals, and the insane. A
prime example of this in fictional literature can be seen in Robert Louis Stevenson's Strange Case of Dr. Jekyll and Mr. Hyde.
Evolutionary advantage
The
widespread lateralization of many vertebrate animals indicates an
evolutionary advantage associated with the specialization of each
hemisphere.
History
Broca
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.
Wernicke
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 temporalgyrus (Wernicke's area) caused language comprehension deficits rather than speech production deficits, a syndrome known as receptive aphasia.
Imaging
These
seminal works on hemispheric specialization were done on patients 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.
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 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 also studied such patients and found some evidence for the right hemisphere having at least some syntactic ability.
Language is primarily localized in the left hemisphere. One of
the experiments carried out by Gazzaniga involved a split-brain male
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 visual field, and thus the left or
right brain, respectively. It was observed that if the patient was
presented with an image to his left visual field (right brain), he would
report not seeing anything. 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 whereas the right brain
does not have this capability, and when the corpus callosum is cut, the
two hemispheres cannot communicate in order for situation-pertinent
speech to be produced.