Sunday, August 23, 2015

The Brain, Neural Circuits

Do smart people have bigger brains? Some do but not all. And that is not what makes them smart. If it is, then all adults are smarter than kids because of the brain size. That is certainly not the case. Take Albert Einstein for example. Einstein was super smart but his head was not especially large. He figured out the theory of Relativity that no other scientist could. He also needed others to tell him his home address when he got lost. That shows that smart people can be dumb too, and vice versa!

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Margaret Mead, left, with Moshe Feldenkrais
and Karl Pribram - International Feldenkrais Federation.

The size of the brain or the number of brain cells plays only a small role in intelligence. Some people with normal brain size can be much smarter than average, like the anthropologist Margaret Mead. Besides, a small-brain creature can be smarter than its large-brain cousin. A fox can always outsmart a man playing hide-and-seek. And a man is smarter than a fox when it comes to talking and using tools. They are just smart in different ways, paying attention to different things.

Comparing the brain to the Internet. From Neurons to Networks.

Can a person learn to be smart? Sure. When one has learned to play music or do science, one becomes smart. Of course, there are people who are gifted and can pick up learning much easier than others. They are naturally talented. But that does not exclude nurtured smartness. Edison says that genius is 1 percent inspiration and 99 percent perspiration. One of the goals of education is to help the students realize their intelligence. How can that be achieved? The key is the neural circuits, especially the ones within the brain. The way to get there is to nurture neural circuits to observe and to remember.

Reuniões Conjuntas - VII Semana de Valorização da Primeira Infância e Cultura da Paz (15859576516).jpg
Suzana Herculano-Houzel, the scientist who counted neurons. Reuniões Conjuntas - VII
Semana de Valorização da Primeira Infância e Cultura da Paz by Senado Federal.

Neural circuits are made of cells in the central and peripheral nervous systems. The nerve cells, brain cells included, are called neurons. A Brazilian neuroscientist Suzana Herculano-Houzel and her team did an actual investigation on the number of neurons in the human brain. They came up with an ingenious way to count them accurately from 4 donated adult brains. The result was published in 2009. They find an adult human has about 86 billion neurons on average. That is fairly close to the conventionally cited number of 100 billion neurons. The number 86 billion is about the same from birth to death. Like retinal cells, the brain cells do not regenerate like other body cells do.

A Neuron

What is a neuron like? Each neuron has dendrites, a soma (cell body where the nucleus is), and an axon. Functionally, the dendrites and the axon are the receptors and transmitter of neural impulses respectively. The axon hillock part of the soma is an impulse generator. The axon endings (terminal bulbs) connect to the dendrites or soma of other neurons. The places where they connect are the synapses.

Details of Neuron

What is special about the neurons? A neuron is a cell that can receive and transmit electrical neural impulses, also called action potentials. The circulation of impulses is of main interest here. It can have feedback loops for control and differentiation. It is a basis for a theory of the mind. Neural impulses travel from the dendrites to soma to axon and then across the synapses to the other neurons. But it is not a straight passing through. There is a threshold gating involved in the cell body. In a neuron, if the sum of all incoming impulses exceeds a threshold, then its axon hillock (the soma/axon junction) fires an impulse down the axon to the axon endings. If not over the threshold, then it won’t fire an impulse. The brain has billions of neurons and trillions of synaptic connections. It is the propagation of impulses, their patterns and dynamic transformations, that will provide clues about the mind.

Where does the number of trillions of synapses come from? Probably from estimates of various studies. One Wikipedia article on neuron places the average number of synapses to be about 7,000/neuron. Others list the number from 1,000 to 10,000 synapses per neuron. Multiply 86 billion neurons by 1,000 you get 86 trillion synapses. But the number is probably higher since 1,000 is the lowest estimate.

Neuron Connection and Disconnection

When a baby is born, the number of synapses continues to grow. The number reaches its peak, estimated at about one quadrillion (1015), when the baby is 3 years old. By the age of 60 - 70, the number goes down to 100 - 500 trillions. Synapses can connect or disconnect depending on the situation, making neural circuits transitory (or stable) as a person grows and learns and changes. The changing synaptic formations surely affect our memories, which are not always the same from month to month or even day to day unless they get “refreshed” regularly. Changes of neural circuits create changes of impulse flows, and vice versa. Which lead to changes of memory.

Neural impulses can be excitatory or inhibitory. The two opposite types are significant for they can build up a regulation mechanism. The types depend on the neurotransmitters involved. Neurotransmitters are chemicals that move from one neuron to another across the synapses, thereby allowing the electrical impulses (action potentials) to cross over as well. A lot of illicit drugs such as cocaine and marijuana work by causing changes to the delivery of neurotransmitters. These drugs work wonders, temporarily, to users who wish to experience alternative perceptions. Also, problems associated with neurotransmitters are present in mental illnesses such as depression, dementia, or Alzheimer's disease.

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Post Synaptic Potential Summation by OpenStax College
EPSP - Excitatory PostSynaptic Potential, IPSP: Inhibitory PostSynaptic Potential.

Electrically, excitatory neurotransmitters increase action potentials while inhibitory ones decrease them. Glutamate is an excitatory neurotransmitter. Mono sodium glutamate (MSG) used in Chinese food is a hyper tasty seasoning. But it leaves a feeling of dull headache to some people. One possible explanation for this is that MSG acts like artificial neurotransmitters that overheat the neural taste circuitry. Bones are full of glutamates. And soups made of bones are naturally tasty.

There are many other neurotransmitters, most of them are inhibitory. Acetylcholine and GABA are ones, as are serotonin and dopamine. The variety of inhibitory neurotransmitters suggest that their roles in many neural circuits are to reduce the flow of impulses, like friction does to slow down a motion. This is a natural and necessary occurrence as decreed by Chatelier’s Principle. It says that a reaction always acts to counter the changes introduced by the instigating action.

Factoid: excitatory impulses go across synapses that exist between axon endings and dendrites, whereas inhibitory impulses go across synapses between axon endings and soma.

Yin-Yang by Nevit Dilmen

Why are there two opposite types of impulses? What can they do together that one alone cannot? One answer is that they make negative feedback control possible. The body engages in many homeostatic feedback controls: heartbeats and body temperature. To regulate certain actions or variables it is always necessary to have two opposite agents, one contracting and the other expanding, a yin and a yang. For example, driving a car requires two opposite actions - acceleration by gas and deceleration by brakes - to control the speed of the car. Likewise, excitatory and inhibitory action potentials are both needed to push and pull, so as to regulate the flow of neural impulses.

Looking at the big picture, the brain and the nervous system are parts of a circuit that connects a person to his environment. The five senses are the connectors that pick up outside information by resonances and transmit that to the inside. They convert optical, auditory, olfactory, gustatory, and tactile vibrations into nerve impulses. These impulses then move along the nerves cells to the brain. The brain itself is again a city of circuits where the impulses continue to transform and propagate. From the point of view of circulation, what is inside and outside of a body, the I and not-I, are connected circuits where information flows. A body does not sever the loop where information flows. And sense organs are like synapses where information cross over from one side to another. That corresponds to the Buddhist concept that all things are interconnected (informationally). It is just that in Buddha’s time there was no word for "information". The lack of concept for information caused the bodhisattvas at that time great difficulties in understanding the Buddha’s teaching about the mind.

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Donald Olding Hebb.

What is amazing is that since 1949 it was already known some of the neural circuits in the brain are dynamically formed, especially in the cerebral cortex. This was discovered by a Canadian psychologist Donald O. Hebb. He noticed that neural wiring could be directly affected by timing, by whether the firing of impulses in neurons are relatively synchronous or not. Basically, if they are nearly synchronous then synaptic connections may establish. If not, then the connections may dissolve. Specifically, If the firings of neuron A happen repeatedly just before the firings of neuron B, then A will likely connect to B if they are not connected before. Conversely, if the firings of A are not repeatedly happening just before the firings of B, then A will likely disconnect itself from B if they are connected before. This gives rise to the phrase “cells that fire together, wire together,” or “cells that do not fire together, do not wire together.”

Hebb’s finding becomes a foundation for the theory of neural circuits. It is called the Hebb’s Rule or Neuroplasticity. This rule describes a basic mechanism of how neurons connect together, and how the connections can bundle up into circuits. But how do neural circuits and impulse transmissions relate to memories, thoughts, dreams, fear, and love? So far it is still a mystery.

There is a hypothesis that may help to unveil this mystery. It is a hypothesis to answer the question why does the formation of neural circuits conform to Hebb’s rule? The answer seems to be economics. Economy exists in all interactive systems and is as powerful as feedbacks. Hebb’s rule is an outcome of economic forces at play. For example, It takes less energy for two neurons to fire impulses if they are wired together, because their total number of impulse inputs can be reduced. So by economic bonding the synaptic connections take place. Also, neural circulation of impulses can bring in economic or other benefits. The mind rewards the body with getting and absorbing food more efficiently. And it makes the body adjusting quickly to environmental stresses such as predators, temperature variations, shortage of resources, etc. These benefits in turn promotes further neural circuit formations for the impulses to circulate.

Combining Hebb’s rule and theory of feedbacks, we may guess that cyclic circulation of impulses can create and stabilize circuits of neural pathways. The excitatory and inhibitory impulses can set up negative feedback loops to stabilize the flow of impulses. Then the stabilized impulse flows can in turn mold the formation of neural circuits since the relative timing of impulse firings becomes more definitive. The formation of stabilized neural circuits attracts further regulated incoming impulses. This is like the evolution of any economy. It becomes a cycle of reinforcement in a positive feedback loop.

One example of positive reinforcement is capitalism. A capitalist uses money to invest in companies to make money further. In such a positive feedback loop, an unchecked capitalism applied in a society will differentiate the rich and the poor more and more by the ongoing economic reinforcement.

Neural circuits may produce smart or dumb results. And habit is like a kind of neural reinforcement that can harden these circuits. For example, a person forgets where he has left his keys, a dumb result. He lets that continue and becomes dumber and dumber in recalling where his keys are. His mind is set to forget the placement of this particular set of keys. How can he correct that? Only by somehow rewiring his neural circuits that are “forgetful”. That rewiring is what learning is.

Nature always finds its way to counter-balance. Positive reinforcements cannot go on forever. It will either break down or be stopped. That corresponds to a tenet of Buddhism: everything is impermanent. Take the 5,000-year Chinese history for example. It is full of revolutions or invasions that punctuate its dynasties and territories. Are these revolutions not a natural reaction to break up the economic and political reinforcements of the prior regime? A person’s life journey is full of surprising experiences that may punctuate his career. Are these experiences not a natural intervention to break up the mental reinforcements of his previous beliefs and choices?

Hebb’s rule indicates that the wiring of some neural circuits are a matter of nurture, induced by actions of the environment. How to apply Hebb’s rule to develop neural circuits that are intelligent or happy is of great interest to many. Some of that can be done already. This has been amply demonstrated by the psychological exercise of positive thinking and by the religious exercise of keeping faith. People who think positively or can keep faith are happier and more peaceful. It just works.

There are also synaptic connections that are a matter of nature, laid out by the expressions of DNA. These neural circuits are naturally hardwired to regulate heartbeats, sweating, reflexes, and other autonomic functions. They are not learned but innate. But even some of that can be “mastered” by cultivating overlay neural circuits. For example, some actors can voluntarily make facial tics or shed tears at will as if they are genuine involuntary reactions. More amazingly, Tibetan meditators who practice Tummo can control their body temperature. Their feats are mind boggling. Presumably this is done by nurturing secondary neural circuits on top of the primary neural circuits to achieve the desired physiological “miracle”.

Iceman Wim demos body temperature control.

Neural circuits can be transitory or persistent. The transitory ones correspond to short term memory. And the persistent ones correspond to long term memory. In the brain, stable and persistent neural circuits are what Richard Semon called engrams. Engrams are assemblies of connected neurons where the circulation of impulses is cyclic and therefore self-stabilizing. Ecosystems such as coral reef, tropical forest, and wetland share something similar to engrams. They all have living creatures circulating about and molding the habitat, making the structure stable. However, neuroscientists have a hard time locating engrams in the brain by experiment. They remove the brain tissues of some lab rats to see how that affect their memory. This mystery is that no brain region corresponds directly to any particular memory.

This problem is probably due to the notion that memory consists of bits and pieces stored somewhere. Although that is true for computer memory, it is certainly not so for human memory. Memory storage space is simply absent in our brain. To understand memory, we will instead consider both engrams and impulse circulation patterns together, not just engrams alone. It is like trying to understand a coral reef ecosystem by examining the corals and the movements of plankton, krill, and fish around the corals. Both the stationary corals and the moving sea creatures change and affect each other. The two of them together, like text and context, will provide a better picture for understanding.

The first human EEG recording obtained by Hans Berger in 1924.
The upper tracing is EEG, and the lower is a 10 Hz timing signal.

How good is such analogy? We can use the brain’s electrical activities as data for comparison. Sea creatures can migrate from one coral reef to another. Such migration is like the circulation of neural impulses from one engram to another. We can tap that data using EEG. EEG (Electroencephalography) is recordings of impulse voltages from electrodes attached the scalp. The EEG traces show that the voltage modulations around the head are roughly rhythmic but can also include transients. These modulations correspond to the marine creatures’ seasonal migration (rhythmic movement) and habitat dwelling/feeding/mating (transient movement). So the analogy is not too far-fetched.

With the vocabulary of engrams and impulse circulations, and the grammar of feedbacks, we are now close to having a language that can describe the human memory.