Sunday, July 29, 2012

Nature's Brains, Part 3

July 8, 2012
Bob Fiske

Nature’s Brains, Part 3

(Note: I invite you to read Part 2 before you dive into this part.)

As nature continued to tinker with brain designs, sooner or later some sophisticated features were bound to arise.  In larger, brained animals, such as dinosaurs, the reptilian or “old brain” did little more than regulate basic bodily processes.  These brains sent “down” nerve impulses for modifying respiration, digestion, heart rate, and perhaps even body temperature.  The old brain could also chain together primitive movements known as reflexes.  Reflexes are simple and are coded in the spinal cord.  Through the dominance of the brain these simple movements could be orchestrated into more complex behavioral sequences, sort of like composing words from the letters of the alphabet.

The complex behavioral sequences could accomplish tasks such as hunting, mating, building a nest, walking, running, fighting, and so on.  How did the old brain come to encode the complex behaviors?  Through trial-and-error.  In other words, species went extinct or found a survival advantage based upon the behavior sets that were genetically hard-wired into their members’ brains.  These behaviors were determined by the DNA code in that species’ genes and were passed from generation to generation.  Learning of the sort that we take for granted had not yet been invented as a brain design feature.

Even today we are able to see in the “advanced” mammalian brain vestiges of hard-wired, genetically coded behavior.  One example of this is the newborn foal.  Within minutes of being born, baby horses struggle to their feet and begin to walk.  Seeing a fully developed behavior of this sort is fairly unusual in the mammalian brain because the innovations it has acquired generally impose a long development period on the young brain.

One of the premier innovations that enabled mammals to survive was the brain’s ability to learn.  This is anything but trivial (even though we take it for granted).  In order to learn, the brain needed to have a memory that could be loaded with new patterns.  But, for that to happen, the brain required an exquisitely complex coding mechanism that could replicate, in a “neural form”, qualities of the real world, a virtual model, so to speak.  This required more and more neurons in the brain.  The result is the “new” brain, a larger accessory that physically sits above and around the old brain.  All this new neural tissue was crammed into a larger skull in a folded and wrinkled fashion.  Scientists call this the cortex.

Parts of the cortex could more richly record auditory information or visual information.  Also, parts of the cortex were dedicated to producing complex movement sequences in various muscle groups such as the limbs, the mouth, the tail, the vocal chords, etc.

By the way, mammalian brains exerted pressure on other species to keep up.  So, we see that many birds (the descendants of the dinosaurs) also innovated their brain designs in similar fashion.  Maybe we mammals are not so special, after all, just lucky to come out in front of the race for survival.

Of course all this ability to encode a rich a faithful inner world model or command exquisitely complex movements would be better utilized if the brain were endowed with an equally rich storage system, that is, a memory.  The memory would allow multiple experiences in an individual’s life to be compared.  This is essential to learning (and survival), for it enables the search for cause-and-effect relationships to be found.

Here’s a simple example.  I am travelling with a herd of my companion mammals over an area of dry, parched earth.  Yet my eyes and visual cortex are able to discern a distant spot of green and brown as a concentration of things known as plants.  The brain, commanding the eyes to look more closely, enables the visual cortex to spy that this is a rich and dense collection of vegetation.  And the brain’s memory yields up a “conclusion” that other dense collections of vegetation have proved to be a source for water.  Even without the benefit of language that can name things, my mammalian brain has recorded (learned) the causal meaning of an oasis.

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