Left-Handers Have Prettier Brains

Biology concepts – evolution, internal bilateral asymmetry, lateralization of function, brain, neural plasticity

This MRI start at one ear and shows slices through the head until it gets to the other ear. It looks as though the loop reverses itself halfway through, because we are bilaterally symmetric – supposedly. But the brain has some very specific asymmetries. Can you see the differences from side to side?

Brains are amazing things – and we know next to nothing about them. For instance, every once in a while a seemingly normal person will show up at a doctor’s office or hospital for a headache or perhaps a tingling in an extremity. When they have an X-ray or an MRI of their head, low and behold they’re missing some large part of their brain! Sometimes, an entire hemisphere just isn’t there.

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.

The corpus callosum is just the largest connection (commissure) between the different hemispheres, but there are others. The anterior may be involved in color perception while the posterior is for the pupillary light reflex. The optic chiasm crosses some visual signals to the opposite optic nerve, but the functions for the middle and habenular are unknown.

His right half wasn’t speaking his to his left half (like many extended families), yet he functioned just perfectly. The reason is that he was most likely born that way, and his brain had ample time to build up the lesser communications between the hemispheres; this is plasticity. And yes, there are other ways for the two hemispheres to talk to one another, though most people believe it occurs only through the corpus callosum (see image to left).

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.

This very crude map shows the motor strip, where your muscle movements begin. Just behind it is the somatosensory strip, where sensations from you body come in. Broca’s area is for language understanding and Wernicke’s in the temporal lobe is for speech making. Notice they show the left side – it’s bigger in right- handed people.

Each hemisphere has its own version of these lobes and deep structures, although sometimes the functions carried by the same lobe might be different. This is called lateralization of function; these are functional asymmetries, but they can have effects on the structure of the brain as well.

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.

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.

The number of right and left-handed people has always been about the same. They can tell from the brush strokes and hand prints in cave paintings. Interestingly, new measurements of finger lengths indicate that about 75% of cave paintings were done by women.

Handedness also affects the size of the motor strip in the frontal lobe (see picture above). Righties have a larger left frontal lobe motor strip, but the opposite is not true with lefties. Their right frontal lobe motor strip might be larger, but is not as increased in size as the left motor strip is for righties – go figure.

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 movement.

The brainstemis the oldest part of the brain and connects our higher functioning areas to the spinal cord. The brainstem has specific jobs in maintaining the basic functions of life; sleep/wake, breathing, cardiovascular control, and pain. In addition, all the neurons that take signals toad n from the higher parts of the brain have to pass through the brainstem to the spinal cord.

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.

The brain has grey and white matter. The white neurons are covered with myelin; this helps the signals travel faster, but doesn’t allow for connections. Grey matter is where all the connections are made between neurons. The finger here is pointing out the corpus callosum of a pig. It is white because these are just signals that need to get from one side to the other; no processing is done in the corpus callosum.

But integration isn’t the CC’s only job. There are many inhibitory signals that pass through the corpus callosum as well. It is thought that these signals depress neural function in one hemisphere or the other, and this is the basis for lateralization of function in each half of the cerebrum. This is the hypothesis, but until now it has mostly been investigated in animal models of CC function. A new study (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?

Small birds need to watch to find food to peck up and eat, but also to pay attention for predators or angry vegetarians. These things are done by different sides of the brain and it is lateralization of function that lets us do multiple things at the same time.

Evolution has resulted in many animals having specialization of functions in separate halves of the brain, so it must convey some advantage. It seems that multitasking is the answer. Lateralization of function allows for simultaneous brain function on different tasks, or at least makes it a lot easier. 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.