Biology concepts – evolution, internal bilateral asymmetry,
lateralization of function, brain, neural plasticity
In 2009, a 10 year old girl in Britain was found to have
depth of vision and a full visual field even though she was completely missing
her right cerebral hemisphere. As you might already know, some visual signals
from each eye cross the midline and are processed by the alternate side of the
brain, but her brain had reprogrammed itself to process all visual signals
through the left hemisphere.
This cerebral
hypoplasia often brings some functional defects, but not always. Defects
might occur because the brain is functionally asymmetric; there are
lateralization of functions (more on this below).
However, the brain is quite plastic in early life. Many
possible defects can be worked around, especially if the problem is congenital
or comes from a trauma that occurred early in life. Rerouting of functions is more likely if
the brain is in the process of
lateralizing function, rather than if the brain is set in its ways.
Take for example the case of an 88 year old man who came to
the hospital in a bit of confusion with a tingling in one foot. A subsequent
MRI showed that his brain was completely missing a structure called the corpus callosum (Latin = tough body).
This is the main connection between the right and left hemispheres of the
cerebrum.
We have filled several posts this year discussing the
asymmetry of some animals and plants (see this, this, this, this, this, and
this post). In each case, we have been talking about external asymmetries, but
that isn’t the only kind. Our bodies, and those of many animals, also have
internal asymmetries.
Most animals have significant internal asymmetry, but there’s no better place to start than the brain. It is both structural and
functionally asymmetric. Here are a couple of stories of the brain structures/functions
and the asymmetries that are built into them.
The human brain has many parts; each of which has varied
functions, although most parts coordinate together. There are usually two cerebral hemispheres which
make up the cerebrum (Latin for
brain… well, duh). These hemispheres make up 80% of the volume of the human
brain. It is here that our more advance thinking takes place – language,
thought, attention, decision making, emotion, and consciousness just to name a
few.
Each of the main parts of the brain can be broken down into
many subparts, each with unique or coordinated functions. For example, the
cerebrum can be broken down into lobes; frontal temporal, parietal, and
occipital. Deep inside these are the more primitive structures, like the
hypothalamus, amygdala, and thalamus that have their own function in emotion
and control.
For instance, which hand you normally use can be reflected
in the size of your brain in the planum temporale, persylvian region, and other
parts of the frontal, parietal, and temporal lobes. You know that most stimuli cross to
the opposite hemisphere where they are then converted into responses. If you’re
right handed, your left hemisphere will be in control of your right hand.
These areas of the brain normally have asymmetries anyway,
since they are the areas that process language and speech. Both understanding
speech and making speech are lateralized to the left hemisphere (well, there
are exceptions, a few people process language in the right hemisphere or
equally in both hemispheres). Areas such as Broca and Wernicke (see picture above)
are larger on the left hemisphere because language is crucially important for
humans and has therefore developed to take more area.
(Even though we aren’t going into the subject here, I just
want to say that the whole thing about people being right brained or left
brained is a myth. We’ll tackle it another time.)
But handedness does play a small role. About 95% of
right-handers have a left dominance for language processing, but only about 80%
of lefties are left hemisphere speech dominant. So in some cases, the hand you
use is reflected in asymmetries of your brain.
And it isn’t just the motor strip; the speech and language
area of lefties isn’t as big as it is in righties. All in all, left-handed
people tend to have much more symmetric brains, in terms of shape and
size. I wonder if this makes them more attractive
(see this post). “You have a lovely brain. Are you left handed?”
The second largest structure is the cerebellum (Latin = little brain, I see a pattern). This part of
the brain is much older, evolutionarily, and is responsible for posture and
coordinating muscle movement to give balance. It’s nice to know that primitive
animals are capable of good posture – why aren’t teenagers?
The cerebellum comes in two hemispheres, just like the
cerebrum, but they are smaller and located below the posterior part of the cerebrum.
Similar to our examples above, some people only have one cerebellar hemisphere
too. This is called unilateral
cerebellar hypoplasia.
But that isn’t to say that the brainstem in humans is just
as it was in early evolution and is now in lower animals. A 2014 study was the
first to look at asymmetry in the halves of the brainstem (it has a right and
left half even though there is just one brainstem).
The structural asymmetries in the cerebral hemispheres were
recapitulated in some of the structures of the brainstem (inferior olive and
dentate nucleus), suggesting that the evolution of higher functions and lateralization
of those functions has brought about a lateralization and structural asymmetry
in the old brain as well. You can teach an old brain new tricks.
The corpus callosum
(CC) is the main commissure between the cerebral hemispheres as we outlined
above. It is thought that one of the functions of the CC is to integrate signals
processed in each of the hemispheres. There are millions of messages buzzing
back and forth from one hemisphere to the other every second, like a giant
highway with 125 million lanes in each direction.
The result of all this traffic is a coordination of
responses; each hemisphere doing its lateralized job plus both doing the jobs
they share (and there are many). The result is a smooth integration of thought,
sensation, and action.
2015)
shows that patients with agenesis of the CC (AgCC) indeed have more hemispheric
autonomy.
Many functions are usually more acute in one hemisphere, like
hearing and repeating one speech while listening to two simultaneously. This is
one of the dichotic listening tests and usually shows a right-eared advantage
(the right ear is better at separating out the two voices), but the patients
with AgCC have no ear advantage for this.
And it wasn’t just “earedness;” the AgCC patients were also
much more like to be ambidextrous, showing no predilection for right or left
hand in fine motor functions. Together, these findings suggest that one of the
functions of the CC is to suppress some functions in each hemisphere to give
lateralization. This brings up an important question. What’s the advantage to a functionally (and therefore structurally) asymmetric
brain?
A 2004 study in baby chicks showed this.
Some chicks were lateralized in the shell (one hemisphere will do all the visual processing if you expose the shells to light 3 days before
hatching). As compared to normal chicks, the lateralized chicks could find food
just fine, but couldn’t pay attention to predatory birds while they pecked for corn. The two functions, which are normally
controlled by different hemispheres could be carried out by the control
chicks, but couldn't be done when both functions were experimentally forced into
the same hemisphere.
Next week, we’ll see how sex hormones play a role in
asymmetric development of the brain structure and function. And there are individual
differences (fluctuating asymmetries) in our brains as well.
For more information or
classroom activities, see:
Brain structures –
Lateralization of function –
salt likit
ReplyDeletesalt likit
dr mood likit
big boss likit
dl likit
dark likit
TZVN