In human neuroanatomy, brain asymmetry can refer to at least two quite distinct findings:
- Neuroanatomical differences between the left and right sides of the brain
- Lateralized functional differences: lateralization of brain function
A stereotypical image of brain lateralisation - demonstrated to be false in neuroscientific research.
Neuroanatomical differences themselves exist on different scales, from neuronal densities, to the size of regions such as the planum temporale, to—at the largest scale—the torsion or "wind" in the human brain, reflected shape of the skull, which reflects a backward (posterior) protrusion of the left occipital bone and a forward (anterior) protrusion of the right frontal bone.
In addition to gross size differences, both neurochemical and
structural differences have been found between the hemispheres.
Asymmetries appear in the spacing of cortical columns, as well as
dendritic structure and complexity. Larger cell sizes are also found in
layer III of Broca's area.
The human brain has an overall leftward posterior and rightward
anterior asymmetry (or brain torque). There are particularly large
asymmetries in the frontal, temporal and occipital lobes, which increase
in asymmetry in the antero-posterior direction beginning at the central
region. Leftward asymmetry can be seen in the Heschl gyrus, parietal operculum,
Silvian fissure, left cingulate gyrus, temporo-parietal region and
planum temporale. Rightward asymmetry can be seen in the right central
sulcus (potentially suggesting increased connectivity between motor and
somatosensory cortices in the left side of the brain), lateral
ventricle, entorhinal cortex, amygdala and temporo-parieto-occipital
area. Sex-dependent brain asymmetries
are also common. For example, human male brains are more asymmetrically
lateralized than those of females. However, gene expression studies
done by Hawrylycz and colleagues and Pletikos and colleagues, were not
able to detect asymmetry between the hemispheres on the population
level.
History
In the
mid-19th century scientists first began to make discoveries regarding
lateralization of the brain, or differences in anatomy and corresponding
function between the brain's two hemispheres. Franz Gall,
a German anatomist, was the first to describe what is now known as the
Doctrine of Cerebral Localization. Gall believed that, rather than the
brain operating as a single, whole entity, different mental functions
could be attributed to different parts of the brain. He was also the
first to suggest language processing happened in the frontal lobes.
However, Gall's theories were controversial among many scientists at
the time. Others were convinced by experiments such as those conducted
by Marie-Jean-Pierre Flourens, in which he demonstrated lesions to bird brains caused irreparable damage to vital functions.
Flourens's methods, however, were not precise; the crude methodology
employed in his experiments actually caused damage to several areas of
the tiny brains of the avian models.
Paul
Broca was among the first to offer compelling evidence for localization
of function when he identified an area of the brain related to speech.
In 1861 surgeon Paul Broca
provided evidence that supported Gall's theories. Broca discovered that
two of his patients who had suffered from speech loss had similar
lesions in the same area of the left frontal lobe.
While this was compelling evidence for localization of function, the
connection to “sidedness” was not made immediately. As Broca continued
to study similar patients, he made the connection that all of the cases
involved damage to the left hemisphere, and in 1864 noted the
significance of these findings—that this must be a specialized region.
He also—incorrectly—proposed theories about the relationship of speech
areas to “handedness”.
Accordingly, some of the most famous early studies on brain asymmetry involved speech processing. Asymmetry in the Sylvian fissure
(also known as the lateral sulcus), which separates the frontal and
parietal lobes from the temporal lobe, was one of the first
incongruencies to be discovered. Its anatomical variances are related to
the size and location of two areas of the human brain that are
important for language processing, Broca's area and Wernicke's area, both in the left hemisphere.
Around the same time that Broca and Wernicke made their discoveries, neurologist Hughlings Jackson
suggested the idea of a “leading hemisphere”—or, one side of the brain
that played a more significant role in overall function—which would
eventually pave the way for understanding hemispheric “dominance” for
various processes. Several years later, in the mid-20th century,
critical understanding of hemispheric lateralization for visuospatial,
attention and perception, auditory, linguistic and emotional processing
came from patients who underwent split-brain procedures to treat disorders such as epilepsy. In split-brain patients, the corpus callosum
is cut, severing the main structure for communication between the two
hemispheres. The first modern split-brain patient was a war veteran
known as Patient W.J., whose case contributed to further understanding of asymmetry.
Brain asymmetry is not unique to humans. In addition to studies
on human patients with various diseases of the brain, much of what is
understood today about asymmetries and lateralization of function has
been learned through both invertebrate and vertebrate animal models,
including zebrafish, pigeons, rats, and many others. For example, more
recent studies revealing sexual dimorphism in brain asymmetries in the cerebral cortex and hypothalamus
of rats show that sex differences emerging from hormonal signaling can
be an important influence on brain structure and function. Work with zebrafish
has been especially informative because this species provides the best
model for directly linking asymmetric gene expression with asymmetric
morphology, and for behavioral analyses.
Brain Asymmetry in Humans
Lateralized Functional Differences and Significant regions in each side of the brain and their function
The
left and right hemispheres operate the contralateral sides of the body.
Each hemisphere contains sections of all 4 lobes: the frontal lobe,
parietal lobe, temporal lobe, and occipital lobe. The two hemispheres
are separated along the mediated longitudinal fissure and are connected
by the corpus callosum which allows for communication and coordination of stimuli and information.
The corpus callosum is the largest collective pathway of white matter
tissue in the body that is made of more than 200 million nerve fibers.
The left and right hemispheres are associated with different functions
and specialize in interpreting the same data in different ways, referred
to as lateralization of the brain. The left hemisphere is associated
with language and calculations, while the right hemisphere is more
closely associated with visual-spatial recognition and facial
recognition. This lateralization of brain function
results in some specialized regions being only present in a certain
hemisphere or being dominant in one hemisphere versus the other. Some of
the significant regions included in each hemisphere are listed below.
Left Hemisphere
Broca's area is located in the left hemisphere prefrontal cortex above the cingulate gyrus in the third frontal convolution.
Broca's area was discovered by Paul Broca in 1865. This area handles
speech production. Damage to this area would result in Broca aphasia
which causes the patient to become unable to formulate coherent
appropriate sentences.
Wernicke's area was discovered in 1976 by Carl Wernicke and was
found to be the site of language comprehension. Wernicke's area is also
found in the left hemisphere in the temporal lobe. Damage to this area
of the brain results in the individual losing the ability to understand
language. However, they are still able to produce sounds, words, and
sentence although they are not used in the appropriate context.
Right Hemisphere
The Fusiform Face Area (FFA) is an area that has been studied to
be highly active when faces are being attended to in the visual field. A
FFA is found to be present in both hemispheres, however, studies have
found that the FFA is predominantly lateralized in the right hemisphere
where a more in-depth cognitive processing of faces is conducted. The left hemisphere FFA is associated with rapid processing of faces and their features.
Other Regions and Associated Diseases
Some
significant regions that can present as asymmetrical in the brain can
result in either of the hemispheres due to factors such as genetics. An
example would include handedness. Handedness can result from asymmetry
in the motor cortex of one hemisphere. For right handed individuals,
since the brain operates the contralateral side of the body, they could
have a more induced motor cortex in the left hemisphere.
Several diseases have been found to exacerbate brain asymmetries
that are already present in the brain. Researchers are starting to look
into the effect and relationship of brain asymmetries to diseases such
as schizophrenia and dyslexia.
Schizophrenia is a complex long-term mental disorder that causes
hallucinations, delusions and a lack of concentration, thinking, and
motivation in an individual. Studies have found that individuals with
schizophrenia have a lack in brain asymmetry thus reducing the
functional efficiency of affected regions such as the frontal lobe.
Conditions include leftward functional hemispheric lateralization,
loss of laterality for language comprehension, a reduction in
gyrification, brain torsion etc.
As study earlier, language is usually dominant in the left
hemisphere. Developmental language disorders, such as dyslexia, have
been researched using brain imaging techniques to understand the
neuronal or structural changes associated with the disorder. Past
research has exhibited that hemispheric asymmetries that are usually
found in healthy adults such as the size of the temporal lobe is not
present in adult patients with dyslexia. In conjunction, past research
has exhibited that patients with dyslexia lack a lateralization of
language in their brain compared to healthy patients. Instead patients
with dyslexia showed to have a bilateral hemispheric dominance for
language.
Current Research on Brain Asymmetry
Lateralization
of function and asymmetry in the human brain continues to propel a
popular branch of neuroscientific and psychological inquiry.
Technological advancements for brain mapping have enabled researchers to
see more parts of the brain more clearly, which has illuminated
previously undetected lateralization differences that occur during
different life stages.
As more information emerges, researchers are finding insights into how
and why early human brains may have evolved the way that they did to
adapt to social, environmental and pathological changes. This
information provides clues regarding plasticity, or how different parts
of the brain can sometimes be recruited for different functions.
Continued study of brain asymmetry also contributes to the
understanding and treatment of complex diseases. Neuroimaging in
patients with Alzheimer's disease,
for example, shows significant deterioration in the left hemisphere,
along with a rightward hemispheric dominance—which could relate to
recruitment of resources to that side of the brain in the face of damage
to the left. These hemispheric changes have been connected to performance on memory tasks.
As has been the case in the past, studies on language processing
and the implications of left- and right- handedness also dominate
current research on brain asymmetry.
