Monday, March 7, 2011

Two brains

(I finished The Master and His Emissary, so now I can continue my original intention.)

If you're reading this blog, you probably have some appreciation for the split between our left and right brains.  Or perhaps you are a skeptic.  In either case I will try to make the extent of the split clear, which might make you appreciate even more how well they work together.

As a point of poetic license I will refer to each of the two hemispheres of a single brain as a "brain".  I will refer to the left brain and the right brain, even though they are really integral parts of a unified brain.  I believe this poetic license allows us to understand the issues better by overstating the separation.  Later I will speak about the incredibly intimate way in which they interact to make a living being.

Much of this post is based on the Scientific American (SA) article "Origins of the Left & Right Brain" (July 2009) by Peter F. MacNeilage, Lesley J. Rogers, and Giorgio Vallortigara.  I encourage you to read the article.

It is important to understand that the left/right split in animal brains is ancient.  Nature has evidently found it valuable to provide largely unimpeded parallel processing for some critical functions.  Of course, the brain is split in several dimensions.  There are layers (brain stem, limbic system, cerebellum, cerebrum) that seem to have been added with time, each apparently providing some new capability.  For example, the cerebrum is only found in mammals.

But even before animals had a cerebrum there was brain lateralization in the cerebellum.  The SA authors contend that hemispheric specialization "was already present in its basic form when vertebrates emerged about 500 million years ago."  The left brain "was originally specialized for the control of well-established patterns of behavior under ordinary and familiar circumstances".  The right brain "was at first specialized for detecting and responding to unexpected stimuli in the environment."

They give examples.  Chickens have two ongoing higher level functions, survival and eating, and these appear to be lateralized in chicken brains (and other birds).  Typically, the right eye (left brain) specializes in finding food.  In that case the left eye (right brain) specializes in detecting threats.  The authors suggest that by specializing two hemispheres, chickens can more safely eat, perhaps since the security function can't be compromised to help look for food.  The authors report in a sidebar an experiment in which chicks were 'de-lateralized', in which case they were unable to do both critical functions simultaneously.

An interesting note here is that about 15 percent of chickens are reversed, that is, their right eye specializes in detecting threats and the left eye specializes in feeding.  The authors suggest that this is  so that predators don't easily learn the nature of their specialization and take advantage of it.  Note that this is about the same percentage of left-handed humans.

Like most physiological functions, the actual assignment of function to the two brains has varied over time and species, depending on the nature and level of the critical functions for each animal.  Neither I nor the SA authors are suggesting that the lateralization of our brains evolved directly from the specifics of the lateralization of chickens.  There are likely interesting paths of evolution that lead to both chicken and human lateralization.

I hope you now believe how ancient lateralization is and how extensive it can and has been.  I believe the separation of function in our two brains is no less substantial than that of chickens, and is no less critical to our own survival.

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