Left Brain/Right Brain

While most of us* understand that the human brain is divided into right and left halves called hemispheres, the relationship of those hemispheres to each other and how they work together to control the body isn't understood quite as well.

*When I say that most of us understand this, I should probably mention that many of us SHOULD understand this...especially if you've taken my anatomy and physiology class. If you HAVE taken that class, and you DID NOT know that the cerebrum is divided into hemispheres separated by the logitudinal fissure and connected to each other by the corpus collosum, then maybe I really am the worst teacher in the history of the world.

Last week, a new article by Carl Zimmer was posted to Discovery Magazine's website. It's a really interesting piece about why our brain are halved, and what the physiological ramifications of that separation are. It begins with asking why we have a bi-hemispherical brain in the first place:

...Scientists have spent a lot of time pondering this very question. Their best answer has a lot to do with the form and evolutionary history of our bodies. From early in our development as embryos, humans take on a left-right symmetry that eventually gives rise to our two eyes, our two big toes, and every paired structure in between. All vertebrates are symmetrical in the same way, as are butterflies, scorpions, and a vast number of other invertebrates. This left-right structure is probably inherited from the common ancestor of all bilaterally symmetric animals, a creature that apparently emerged over 570 million years ago.

One of the greatest benefits to this type of symmetry, according to Zimmer, was that animals with a set of limbs on each side of their bodies would have been able to escape from predators more easily and quickly. And the development of a bilateral body plan effected how sense organs such as eyes and ears evolved. It makes sense, then, that the evolution of brains would follow that same pattern...and in fact we see bilateral brains in even the simplest chordates.

Even though the two hemispheres of our brains are mirror images of each other, they do not contribute equally to neurological processes. Like other organs in our bodies, the right side and the left side have evolved to perform different functions in different ways:

Of course, our bodies are not perfectly symmetrical (heart on the left, appendix on the right), and neither are our brains. Some regions are slightly bigger on one side than on the other, and these differences translate into imbalances in how the human brain works. Most people, for example, tend to favor their right hand over their left. In the mid-1800s, the French physician Paul Broca discovered a region on the left side of the brain that is essential for language; damage to Broca’s area, as it is called, leaves people unable to talk. The same region on the right side is not so vital. Another area, on the underside of the brain, is important for recognizing people’s faces. The right half of this region, known as the facial fusiform area, does most of the work of recognizing. In fact, if people view a face only through their left eye (which is linked to the brain’s right hemisphere), they will do a better job of recognizing it than if they use only their right eye.

This neural asymmetry (referred to as lateralization) has become a widely recognized phenomenon and is supported by numerous studies. The question that neurologists ask, though, is what is the purpose of lateralization...how does it benefit our species.

One hypothesis is that a lateralized brain is more powerful than one that works like a mirror image. Instead of two matching parts of the brain performing an identical task, one can take charge, leaving the other free to do something else. Lesley Rogers, a biologist at the University of New England in Australia, tested this hypothesis on chickens [and found that] the birds use their left hemisphere to peck for seeds and their right hemisphere to detect predators. Some chickens have more lateralized brains than others [which] allowed the chicks to multitask more effectively, with each eye handling a separate job.

Zimmer cites examples from studies using parrots, toads, fish, bees, and even humans to support this idea that lateralized brains evolved as a "multi-tasking" tool.**

**This is a handy trait for those times when one has to walk and chew gum at the same time...or chase their prey while aiming a spear at it.

We need to be careful, though, to understand that the two hemispheres do not operate independently of each other. They still communicate and coordinate with each other in intricate and often unknown ways. And while each of use may use one hemisphere of our brain more often or more efficiently than the other, it would be inappropriate to draw the conclusion that one side of our brain is solely responsible for our perceptions, thoughts, and behaviors.

No matter how lateralized the brain can get, though, the two sides still work together. The pop psychology notion of a left brain and a right brain doesn’t capture their intimate working relationship. The left hemisphere specializes in picking out the sounds that form words and working out the syntax of the words, for example, but it does not have a monopoly on language processing. The right hemisphere is actually more sensitive to the emotional features of language, tuning in to the slow rhythms of speech that carry intonation and stress.[...] Neuroscientists know that the hemispheres work together and that they do so by communicating through the corpus callosum. But exactly how the hemispheres cooperate is not so clear.

There is nothing in nature that comes close to rivaling the complexity of the human mind. It is, at the same time, that which connects us to the rest of nature and makes us uniquely human. As we continue to learn more about the function of this remarkable organ, we will only grow in our understanding of both who we are as a species and how we came to be.