Paul D. MacLean one of the early research directors for NIMH, postulated a theory that the human brain is actually the product of three evolutionary changes. As the brain is very dynamic and therefore does not operate via autonomous and separate areas, what MacLean proposed was a kind of general evolutionary map. His theory was cored in the notion that what constituted the human brain was really for the most part comprised of three separate brains which had evolved over very long periods of time.
MacLean proposed that the base brain which was instantiated within the human brain at its base was a variation in the elaboration of the reptilian brain. He termed it the R-complex. The limbic system within humans was an elaboration of the new mammalian brain from the Jurassic period. He termed it the mid brain or the neo-mammalian brain (new mammal). As humans are mammals this is fairly straightforward. Finally he termed the large cerebral cortex which encompasses our logic centers, the neo-cortex (new cortex). The following two charts show how this looks from a general function perspective.
The following chart is a geologic timeline which encompasses the timeframe when all three elements within the human brain in MacLean's triune brain theory came about.
From the imaging studies done over the past two decades using new technologies such as PET, SPECT and fMRI we are gaining increasing insight into how the limbic system functions within our brain functions. When information is taken into your brain from sight, sound, smell and touch it goes directly into the hippocampus. The hippocampus is the receptacle for short term memory where your brain parks data till it has decided what to do it. Since the brain is massively parallel we don't know the exact sequence yet and therefore I will simply address the main two branches which your brain takes with every piece of input data.
The amygdala is paired with the hippocampus and if the data is interesting and not alarming it is packed off to the neo-cortex for comparison with prior experiences and filed into long term memory as the mind rationalizes the data. It could be new thinking or reinforcement of currently held thinking. However if the brain is alarmed by the data then your flight or fight responses react setting off a chain reaction in responses. The following image is of the amygdala and hippocampus in the brain.
If alarmed the hypothalamus sends out chemicals which heighten your five senses and dramatically decreases your analysis engines blood flow. The brain pumps up your oxygen in order to increase chances for survival. Images taken of individuals exposed to long term trauma, dramatic hippocampal shrinkage were seen when compared with individuals who had not suffered any significant traumas in their lives. You may also have experienced events which scared you to the degree that you simply couldn't speak even though you very much wanted to. This was your brain running in the completely non-verbal fight or flight mode.
There are those who feel that this response is the working reaction of the three brains which MacLean postulated in his triune brain theory. Since two of these brains are non verbal the mechanical process of decreasing blood flow to verbal centers is logical as the older limbic system and R-complex are in effect given governance of your mind during perceived crises.
The example given is very general and overly simplistic however if you apply the general principal to the model then what you see in evolution of worldview is to some degree validated. Early humans were hunted and living under constant threat. Therefore even though their neo-cortex was growing the output was as a ratio not producing new thinking relative to the increase in size of the neo-cortex. This is because it is very likely that they were suffering from hippocampal shrinkage and therefore were mentally driven for the most part within their non-verbal limbic and R-complex regions. This is complete speculation on my part.
Dr. Eric Kandel and the neuroscientists following in his work have now proven that the older parts of the brain circuits are several times faster than the analytical components of the brain. What humanity is in effect doing through the updates to worldview is changing the thickness of the cerebral connections to enable increasing analytical response to the world which we inhabit. How this will specifically will manifest itself is hard to say but I can say that in looking at my children manage a dozen instant messaging conversations on their computers, you can see the techno-economic causes for what can only be described as the brain being retrained. They are training their hippocampal regions to sort ever increasing real time data handling which is typically beyond the capability of people who are my age or older. The example of instant messaging is microcosm of how humans have used their common cognitive wiring to extend their economy and then synchronize their worldview inside of that economy and all the pertinent thinking of the day.
