Tuesday, March 8, 2011

The nature of the split

In the previous post I described the nature of the the split between left and right brain function in chickens.  In this post I will begin to describe the nature of the split in modern humans living in modern western society.  I will draw heavily on what I have read in Master and Emissary as well as some impressions from My Stroke of Insight.  Keep in mind that this is an area of controversy and subject to the biases of the left brain that is doing much of the interpreting and communicating.

First, let's go over some of the issues that are not controversial.

It is well known that the sides of the body are lateralized to match the lateralization of the brain, with the left side of the body, including the eye, being controlled and interpreted by the right hemisphere and the right side of the body being controlled and interpreted by the left hemisphere.  This is clearly demonstrated by strokes, which typically shut down one hemisphere or the other, and much observation has been made on this point.

Most people are right handed.  This means that most of their activities are mediated by the left brain.  It is also well known that speech is mediated by Broca's area in the left hemisphere, for which there is no equivalent area in the right hemisphere.  Thus most of our interactions with the world (reaching, grasping, speaking) tend to be mediated by the left hemisphere. The left hemisphere also contains Wernicke's area which is associated with language understanding.  This led early brain researchers to designate the left hemisphere as the dominant hemisphere.

"... so only left handed people are in their right mind!"  This canard is demonstrably false.  In most left-handed people, the functions of the two hemispheres are reversed, so Broca's region, etc. are in the right hemisphere, and the normal functions of the right hemisphere are moved to the left.  (See the previous post for a possible explanation.)  Since the functions are fully reversed, everything works just as in a right brained person, but on the other side.  For ease of communication I will refer to left and right brains as if everyone was right brained.  And then there are people who are ambidextrous (I'm one of them.)  Brain lateralization has not been well studied in ambidextrous individuals.

The visual field comes from both eyes, and vision from both eyes is combined in the occipital lobe at the back of the brain.  Each hemisphere processes the optic nerve from the other side of the brain, forming a complete visual field.  It is now well known that the left brain only has access to the right visual field.   Note that this is the right visual field (created by the occipital lobe), not the right eye.  Thus, if one eye is damaged, there is still a full visual field created by the occipital lobes.  But if the right hemisphere is sufficiently damaged, only the right side of the visual field will be available to the individual.

Beyond these physiological differences, the brains are known to specialize in what activities they attend to and mediate.  I will only touch on them, because in most cases the specialization is either not well understood or otherwise controversial.  Sometimes the difference is summarized as logic vs creativity, but we shall see that understates the complex interaction between them.  The left brain is more interested in rules and linear thinking, while the right brain is better at following and solving complex problems of living like driving a car.  Have you ever wondered how thousands of us can hurtle within a few feet of one another at high speed and not have more accidents than we do now?  Thank the right brain.

There are other aspects of how our hemispheres divvy up the business of living in the world that are well known, but these are the primary ones and enough for my purposes.  In my next post I will step into more controversial aspects of the split.

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