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.
The brainstem is the oldest part of the brain. Most of the
time it can’t be seen because the cerebral hemispheres
cover it up. The cerebellum sticks off the brainstem
and has two hemispheres. It coordinates muscle
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 –