The human brain's hemispheres, while appearing similar, possess specialized functions (cortical lateralization), with the left typically handling language and logic, and the right managing spatial and holistic processing, though both hemispheres work together for most tasks.
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although the left and right side to the
human cortex look almost identical the
hemispheres have specialized functions
you might have heard that the left brain
is analytical and logical while the
right is more intuitive and creative the
idea that people are either left-brained
or right-brained is very popular but it
turns out thankfully that we use our
whole brain we just recruit to the left
and right hemispheres for different
tasks a phenomenon known as cortical
lateralization let's explore this in
this woman is facing us so her left is
on our right and vice versa the primary
motor cortex which controls voluntary
movement is located roughly here
we know from work of German physiologist
Edward insig in the 1860s that this part
of the brain has a contralateral
connection to the body parts it controls
which means that it is connected to the
opposite side hit seek first noticed
this while caring for wounded soldiers
he found that touching the surface of a
specific strip of cortex caused movement
on the opposite side of the body the
various primary sensory areas also have
this contralateral organization such
that incoming sensory information from
the body is sent to the cortex on the
opposite side the primary somatosensory
cortex is located just behind the motor
strip which is responsible for the
perception of touch sensations it
receives its information from sensory
neurons whose axons cross over so that
the right side of the body talked to the
left somatosensory strip and the left
side of the body tops to the right the
connections of these primary sensory
motor areas of the cortex are also
symmetrical and that the left and right
sides of the body have equal
representation in their respective hemispheres
let's consider the organization of
information from the visual system to
the cortex
unlike the somatosensory system which
sends signals separately from each side
of the body the visual system is a
little different because the left and
right eyes capture images from both the
left and right side to visual space as
shown in this figure
for example the outside part of the
right eye and the inside part of the
left eye detect stimuli in the left
visual field shown in red the pathway
from the inside part of the left eye
crosses over at the optic chiasm and the
signals then get sent to the right
hemisphere the pathway from the outside
part of the right eye stays on the same
side so ultimately the signals from the
left visual world all end up in the
right hemisphere if you follow the green
lines you can see that this is the same
arrangement for the right visual field
as well in other words information from
left and right visual space is sent to
the primary visual cortex from the
opposite side in a way that conserves
unlike the primary sensory motor areas
the association cortex is asymmetrical
because it's functions differ between
the left and right hemispheres the left
hemisphere appears to have a stronger
role in processing language and logical
based thought while the right is
involved in more holistic spatial type
tasks most people about 95% of
right-handers and 85% of left-handers
show this bias although a small
proportion of people have the opposite arrangement
regardless of whether the lateralization
is typical or reversed ordinarily our
hemispheres communicate with each other
via a large bundle of axons called the
corpus callosum located here in the
middle of the brain this highway allows
for the exchange of information between
the hemispheres but how do we know about
Association cortex asymmetry or the role
of the corpus callosum
the asymmetrical distribution of
function in the association cortex is
called cortical lateralization a lot of
what we know about lateralization comes
from observations of people with split
brain syndrome who have had the corpus
callosum cut believe it or not this is a
condition caused by surgeons in extreme
cases as a last resort treatment for
epilepsy epilepsy is characterized by
excessive electrical activity in the
brain which can spread across large
areas severing the corpus callosum can
prevent the spread of troublesome
electrical activity and confine the
seizure to one area one consequence of
split brain surgery is that stimuli from
the various sensory systems can be sent
separately to each hemisphere recall how
sensory information is conserved in a
contralateral and symmetrical way so in
the visual system the bits of the eye
that capture left visual space are sent
to the primary visual cortex in the
right hemisphere and vice versa because
the corpus callosum is cut in split
brain patient the hemispheres can't
transfer information to each other
so what happens to people who have this
procedure as it turns out people
generally function just fine under
everyday conditions
you can notice the difference in
information processing under controlled
laboratory conditions with the split
brain patient keeps their head
stationary and the researcher presents
visual and tactile stimuli separately to
each hemisphere what research has shown
is that people with split brain syndrome
can easily verbally identify something
presented in their right visual field
since this information travels to the
language intensive left hemisphere
however when something appears in the
left visual field which travels to the
spatial nonverbal right hemisphere they
cannot name it
if instead they are given a spatial
identification task that is confined to
the right hemisphere such as identifying
an object with their left hand by touch
they can easily retrieve it even though
they can't name what they just saw this
demonstrates that the left and right
hemispheres if they can't talk to each
other are very limited in what they can do
another source of information of a
cortical lateralization and asymmetry
comes from studies of people who have
sustained injury to particular regions
of the Association cortex we can't cover
all of the brain damage studies there
are simply too many but we can look at
some areas in particular in the left and
right hemispheres whose damage results
in specific types of impairments two
examples we'll consider our aphasias
which are impairments and language
comprehension or production usually as a
result of damage to the left association
cortex and egg noses which are
perceptual impairments resulting in
difficulty in recognizing objects or
people from their sensory features and
are usually result of damage to the
Broca's aphasia first described by the
French physician Paul Broca is
characterized by difficulty producing
speech although speech comprehension is
mostly unaffected Broca examined these
patients brains after they died and
discovered damage to an area in the left
frontal lobe located near the primary
motor cortex which became known as
Broca's area Bernanke's aphasia first
described by German physician Carl
Wernicke is characterized by few
problems producing the movements to
speak but speech doesn't make sense in
patients have difficulty understanding
others upon examination of their brains
they were found to have damage to an
area in the left temporal lobe quite
near the primary auditory cortex
involved in hearing this area became
known as Wernicke's area whereas
aphasias are usually associated with
damage to different parts the left
association cortex often through injury
or stroke agnosia are most commonly
found in people with damage to parts of
ignores our very specific perceptual
issues unrelated to any problem with the
eyes or a vision in general but rather
in dealing with processing incoming
visual information and making spatial
sense out of it contralateral neglect is
an unusual condition where people ignore
the left side of their world dressing
only the right side of the body or
eating from only the right side of one's
plate a common symptoms keep in mind
that the right Association cortex is
specialized in spatial tasks and this
disorder is linked to damage in the
right parietal cortex
prosopagnosia is an even more unusual
condition also known as face blindness
people with prosopagnosia have
difficulty identifying specific faces
even though they have no trouble seeing
faces face recognition is a very
specialized spatial task think about it
it's difficult to describe someone using
our words everyone has two eyes a nose
and a couple of lips it's the
differences in size shape and position
of these features that allows us to tell
the difference between our best friend
and a stranger has the same coloring and
general features this disorder is
associated with damage to an area of
Association cortex that spans the
occipital and temporal lobes called the
fusiform area and is tucked in behind
so Association Ares and the left and
right cortices often show a bias toward
particular types of functions before the
development of brain imaging
technologies discoveries of a cortical
lateralization were made possible by
careful observations of people with
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