A Brief Tour of the Brain
by Simon Catterall
Introduction
We start off with a brief history of the study of the brain. Then we
describe some of the more important large-scale components and their
specific functions (for example: memory and vision). We then discuss the
brain's fundamental building blocks - the brain cells or the neurons.
The mechanism of operation for a single neuron is outlined, and how that
contributes when many neurons are networked together. We will come to the
conclusion that the brain represents an incredibly complicated
system with very sophisticated abilities. It is one of the aims of
research in Artificial Intelligence to comprehend these processes in order
to build intelligent machines.
History of the Study of the Brain
The ancient Greeks were divided as to the respective roles of the heart
and the brain. Some philosophers, such as Homer and Aristotle, believed
that the heart was the seat in which intelligence resided. This idea
persisted for many years - even in the seventeenth century, Descartes felt
that the flow of blood from the heart to the brain served the purpose of
producing "animal spirits" which animated the body.
Galileo, in 1623, put forward the view that science should only be
concerned with primary qualities, those of the external world that
could be measured or weighed. So-called secondary qualities, such
as love, beauty, meaning and value, were said to lie outside the realm of
science. Descartes, himself, supported this idea and proposed two
categories: mind and matter. The matter category related to
physical or extended substance, the mind category to thinking substance -
that which is unextended and indivisible.
These philosophers and scientists thus distinguished the physical
operation of the brain from the thought process. While the former was
thought to be amenable to scientific study, consciousness was excluded
from the scientific world-view. It is only recently that researchers have
begun to challenge this mind/matter split with evidence that many human
qualities traditionally associated with the mind, such as personality,
are, at least in part, determined by biochemistry. Some researchers
believe that consciousness itself may emerge as a by-product of the
complex workings of the human brain.
A major impetus to the study of the physical workings of the brain came
in 1791, when Galvani showed that electricity existed as a force within
the body - in fact, inside the brain cells. He showed in a sequence of
experiments that it was possible to control the motor nerves of frogs
using electrical currents.
(The novel "Frankenstein" by Mary Shelley is testament
to the flurry of public interest in this new research.) However, Galvani
did not have the technology to measure the currents involved in the body;
they were too small. His experiments were later confirmed by Du Bois-Reymond
in 1850, who found that neurons emit pulses of electricity that travel at
around 200 mph. Purkinje, in 1838, found that nerve cells consist of two
parts: a nucleus similar to other cells and a set of fibers which emanated
out from the nucleus - these were later identified as the axons and
dendrites. In 1870, Golgi made the observation that there were literally
billions of neurons making up the central nervous system and established
that the neurons in the brain sent information to the motor nerves and
that information from the sensory nerves was sent to the brain for
analysis.
In the early 1900's, Adrian, Gasser and Erianger found that the
electrical pulses within the neurons caused chemicals to be released, the
function of which was to send a message to other neurons using the
connections between them and that it took one-thousandth of a second for
the neuron to recharge after this firing process had taken place.
These initial discoveries paved the way for modern neuroscience which
in recent years has yielded enormous amounts of information about the
physical functions of the brain.
Large Scale Features
The brain is probably the most complex structure in the known universe;
complex enough to coordinate the fingers of a concert pianist or to create
a three-dimensional landscape from light that falls on a two-dimensional
retina. While it is the product of many millions of years of evolution,
some of the structures unique to the human species have only appeared
relatively recently.
For example, only 100,000 years ago, the ancestors of modern man had a
brain weighing only about one pound - roughly a third of the weight of the
current version. Most of this increased weight is associated with the most
striking feature of the human brain - the cortex - the two roughly
symmetrical, corrugated and folded hemispheres which sit astride the
central core.
Almost all the tasks that seem hard or difficult for human beings but
that the present generation of computers can easily perform are associated
with processing in parts of the relatively new cortex. Conversely, tasks
that humans normally find easy but that are difficult for computers
typically have a much longer evolutionary history. Although playing chess,
doing higher mathematics and trouble-shooting electronic circuits may seem
intellectually challenging for humans, current computers can cope very
straightforwardly. However, a modern computer (even after much careful
programming) is typically very poor at such simple tasks as sensing its
environment or coordinating movements. A simple operation like recognizing
someone's face, which we find rather straightforward, is a formidable
problem for a computer. Indeed, a 2-year-old child will perform much
better at these tasks! This observation is not so surprising, though, when
one considers that the child is using multiple levels of processing that
have evolved over many hundreds of thousands of years.
In evolutionary terms, all brains are extensions of the spinal cord.
The distant ancestor of the human brain originated in the primordial seas
some 500,000,000 years ago. Life and survival in those seas was relatively
simple and in consequence these early brains consisted of just a few
hundred nerve cells. As these initial sea-creatures evolved and became
more complex, so too did the brain. A major change occurred when these
early fish crawled out of the seas and onto the land. The enhanced
difficulties of survival on land led to the creation of the
"reptilian brain". This brain design is still visible in all
modern reptiles and mammals and is a powerful clue to our common
evolutionary ancestry.
The next major addition occurred with the mammalian brain in which a
new structure emerged - the cerebrum or forebrain along with
its covering, the cortex. By now, the brain consisted of literally
hundreds of millions of nerve cells organized into separate regions of the
brain and associated with different tasks. About 5,000,000 years ago,
another type of cortex appeared in a new species - early man. In
this brain, the surface of the cortex was organized into separate columnar
regions less than one millimeter wide but each containing many millions of
nerve cells or neurons. This new structure allowed much more complex
processing to take place. Finally, about 100,000 years ago, this new
cortex underwent rapid expansion with the advent of modern man. The
present day cortex contains something like two-thirds of all neurons and
weighs about three pounds - almost triple its weight only one hundred
thousand years ago!
Thus the human brain consists of roughly three separate parts.
1. The first segment in the lower section, sometimes called the brain stem,
consisting of structures such as the medulla (controlling
breathing, heart rate and digestion) and the cerebellum
(coordinating senses and muscle movement). Much of these features are
inherited "as is" from the reptilian brain.
2. The second segment appears as a slight swelling in lower vertebrates
and enlarges in the higher primates and ourselves into the midbrain.
The structures contained here link the lower brain stem to the thalamus
(for information relay) and to the hypothalamus (which is
instrumental in regulating drives and actions). The latter is part of the limbic
system.
The limbic system, essentially alike in all mammals, lies above
the brain stem and under the cortex and consists of a number of
interconnected structures. Researchers have linked these structures to
hormones, drives, temperature control, emotion, and one part, the hippocampus
to memory formation. Neurons affecting heart rate and respiration appear
concentrated in the hypothalamus and direct most of the
physiological changes that accompany strong emotion. Aggressive behavior
is linked to the action of the amygdala, which lies next to the
hippocampus. The latter plays a crucial role in processing various forms
of information as part of our long term memory. Damage to the hippocampus
will produce global retrograde amnesia, or the inability to lay
down new stores of information.
As we have seen, much of the lower and mid brain are relatively simple
systems which are capable of registering experiences and regulating
behavior largely outside of any conscious awareness (we don't have to
think to remember to breathe!). In a sense, the human brain is like an
archeological site with the outer layer composed of the most recent brain
structure, and the deeper layers consisting of structures from our shared
evolutionary history with the reptiles and mammals.
3. Finally, the third section, the forebrain appears as a mere bump in
the brain of the frog but balloons into the cerebrum of higher life
forms and covers the brain stem like the head of a mushroom. It has
further evolved in humans into the walnut-like configuration of left and
right hemispheres. The highly convoluted surface of the hemispheres - the cortex
- is about two millimeters thick and has a total surface area of about 1.5
square-meters (the size of a desktop).
The structure of the cortex is extremely complicated. It is here that
most of the "high-level" functions associated to the mind are
implemented. Some of its regions are highly specialized - for example, the
occipital lobes located near the rear of the brain are associated
with the visual system. The motor cortex helps coordinate all
voluntary muscle movements.
The parietal lobes positioned in an arch over the center of the
cortex contain a detailed map of whole body surface.
More neurons may be dedicated to certain regions of the body than
others - for example, the fingers have many more nerve endings than the
toes.
There is an approximate symmetry between left and right hemispheres -
for example, there are two occipital lobes, two parietal lobes and there
are two frontal lobes.. However this symmetry is not exact - for example,
the area associated with language appears only on the left hemisphere.
The frontal lobes occupy the front part of the brain behind the
forehead and compose the portion of the brain most closely associated with
"control" of responses to input from the rest of the system.
They are most closely linked with making decisions and judgements.
In most people, the left hemisphere is dominant over the right
in deciding which response to make. Since the frontal lobes occupy 29
percent of the cortex in our species (as opposed to 3.5 percent in rats
and 17 percent in chimpanzees), they are often regarded as an index of our
evolutionary development. In individuals with normal hemispheric
dominance, the left hemisphere, which manages the right side of the body,
controls language and general cognitive functions. The right hemisphere,
controlling the left half of the body, manages nonverbal processes, such
as attention, pattern recognition, line orientation and the detection of
complex auditory tones. Although the two hemispheres are in continual
communication with each other, each acting as independent parallel
processors with complementary functions, the dominant left-hemisphere
appears most closely associated with a conscious self.
These structural features of the brain have been known for some time.
In the Building Blocks section we will explore the nature of the cells
themselves and later in Organization try to understand how this set of
intercommunicating complex structures we have described can possibly arise
from the function and organization of the neurons themselves.
Building Blocks
Nerve cells, called neurons, are the fundamental elements of the
central nervous system. The central nervous system is made up of
about 100 billion neurons (10 to the power 11). Neurons are much like
other cells of the body in their general organization and their
biochemical systems. However they also possess unique features which are
crucial to the functioning of the central nervous system. In essence, a
given neuron may both receive and send out signals to neighboring neurons
in the form of electrical pulses. A neuron is built up of three parts: the
cell body, the dendrites and the axon.
The body of the cell contains the nucleus of the cell and
carries the biochemical transformations necessary to synthesize
enzymes and other molecules necessary to the life of the neuron. It is
roughly spherical or pyramidal in shape - the precise shape depending on
position and function in the brain. It is typically several microns in
diameter (a micron is a millionth of a meter).
Each neuron has a hair-like structure surrounding it - these are the dendrites.
Dendrites are some tens of microns in length. They branch out into a
tree-like form around the cell body. The dendrites are like electrical
cables which serve to conduct incoming signals to the cell. The axon
or nerve fiber is the outgoing connection for signals emitted by the
neuron. It differs from the dendrites in its shape and by the properties
of its external membrane. It is usually much longer than the dendrites,
varying from a millimeter (one thousandth of a meter) to one meter. At its
end it branches into smaller structures which communicate with other
neurons. The branching of the dendrites, in contrast, takes place much
closer to the cell body. Neurons are connected together at these
extremities in a complex spatial arrangement. Typically a given neuron is
connected to about ten thousand other neurons. The specific point of
contact between the axon of one cell and a dendrite of another is called a
synapse.
Organization
A simplified description of the operation of a neuron is that it
processes the electric currents which arrive on its dendrites and
transmits the resulting electrical currents to other connected neurons
using its axon. A simple explanation of the processing step is
that the cell sums up the incoming signals and produces an output
signal only if this sum exceeds some threshold; i.e. only if the total
input signal is big enough will the cell 'fire' an output signal to its
neighbors.
Once the cell fires, an electrical signal travels down the axon at a
speed of around 100 meters per second (200 mph). These currents are very
small by usual standards. The typical voltage difference between the
outside and inside of a nerve cell is 70 millivolts (one millivolt is one
thousandth of a volt). This is to be compared with the voltage at a power
socket in your home of 110 volts - a thousand times bigger. This signal is
passed onto other neurons at the synapse points in the following way.
The pulse reaches the end of the axon branch and causes the release of
certain chemicals called neurotransmitters. These diffuse across
the synaptic gap (a distance of some one-hundredth of a micron) to
be picked up by so-called receptors on the ends of the dendrites of
a neighbor neuron. This absorption process on the new cell changes its
electrical state. If there are sufficient incoming signals to this
neighbor neuron, this change in its electrical state can be big enough to
generate a new pulse in the neuron. Thus the process repeats in this new
cell. The movie sequence illustrates this firing sequence between neurons.
This cell is itself connected to many others and in this way a wave of
electrical activity can be set up. Different types of brain activity
correspond to different patterns of firings.
While we are born with a complete set of neurons, the connections
between them are determined in major part by a learning process;
external stimuli coming in the form of electrical currents from the
sensory cells cause patterns of nerve impulses to be set up. These
impulses can alter the strength of the coupling between different neurons.
While the overall program for determining which neurons should be
connected together is under genetic control, it is external stimuli which
are crucially important in determining what network connections are made.
Indeed, to some extent our brains are continually rewiring themselves to
cope with passing experience. This is particularly true for small children
who are born with a full complement of neurons but a relatively primitive
set of connections - a useful set of network connections must be learnt
during the early years.
It is also true that the levels of various neurotransmitters are, in
part, determined by early experiences. The overall functioning of the
brain is strongly influenced by such chemical balances. For example, the
neurotransmitter serotonin plays a role in regulating aggression; a
lack of dopamine, another such chemical, reduces frontal lobe
activity and has been associated with schizophrenia. Endorphins
play a role in the system which produces sensations of pain and pleasure.
It is becoming increasingly clear that certain traits of personality may
be determined in major part by biochemistry. This opens up the possibility
of a "chemically improved" society and all the profound
implications that implies.
The fact that our neurons can rewire themselves "on the fly"
has the consequence that our brains are amazingly robust - if a given
neuron dies (which will have happened to something like 20 percent of our
original neurons by the time we die!), our brain automatically undergoes a
rewiring process in which new connections are made to circumvent the
defunct neuron. It is also the origin of the amazing diversity in peoples'
types and abilities. Intelligence is determined partly by genetics (the
program that governs the overall structure of what connections should be
set up) and partly by our experience which can influence very strongly the
nature and quality of our neural networks.
Two neurons are not merely joined or not - the nature of the
synaptic connection between them determines whether one neuron firing has
a strong or weak effect on the other - we talk of the strength of a
connection between them. A strong connection between two neurons means
that it is more likely that one of the neurons firing will stimulate the
other to fire - with a weak connection it may only happen occasionally
depending on the state of very many more neurons for example. Of course,
this connection strength feature has much to do with the presence or
absence of certain neurotransmitters since they are crucial in determining
the size of electrical signal which can pass between neurons.
This notion of dynamically changing connection strengths is thought to
be important for memory function - new memories are stored not on
individual neurons but by adjusting the strengths of connections between
neurons. A simple rule appears to govern this process: the
connection between two neurons will strengthen if more often than not the
two neurons fire together. This is often called the Hebb rule.
Thus the operation of the cell depends on both electrical and chemical
properties - those of the neurotransmitter molecules. There are a number
of types of these neurotransmitters (research has identified at least
fifty distinct types of neurotransmitter). Some, referred to as exciter
neurotransmitters act to trigger the receiving neuron, while others
called inhibitors act to damp out signals in the neighbor neuron.
For example, one particular such inhibitor chemical called GABA acts to
prevent abnormal or parasitic muscle movements. The degeneration of
certain synaptic sites rich in GABA provokes an illness called
Huntington's Chorea, whose symptoms are almost incessant involuntary
movements. The famous folk singer Woodie Guthrie succumbed to this genetic
disease.
Much use has been made of the way that these neurotransmitter chemicals
are used by neurons. Tranquilizers act by modifying the natural
chemicals in the synaptic gap and drugs such as LSD by altering the
balance of various neurotransmitters. This can cause very dramatic
effects; for example, sounds can be perceived as colors. The new drug
Prozac also acts directly to remedy chemical imbalances in the brain.
We may ask the question: what is it about the structure of
neurons and their organization which determines the amazing computational
power of the brain? Certainly it is not the raw processing power
of a single neuron - it takes about one-thousandth of a second for a cell
to return to a normal state after firing. This is the minimum time before
the neuron can process another incoming signal. While this seems quite a
short time it is ridiculously slow compared to even a modest home computer
whose silicon chip can perform operations in the incredibly short time of
one-hundred-millionth of a second.
The secret lies in the very number of neurons - many tens of
billions as we have said. If these neurons can be made to work efficiently
and simultaneously on a given task it is clear that the effective
power of the brain is very much larger than current computers. The hint
that such a scenario may indeed be realized lies in the detailed structure
of the brain in terms of the connections between neurons. It is clear that
in order for neurons to cooperate in performing some function, they must
be able to talk to each other. We know that each neuron has many tens of
thousands of connections to other neurons that function as communication
channels. These connections have an incredibly complicated structure -
different portions of the brain have different types of connection
pattern, while these different sectors of the brain are themselves linked
together by further specialized networking. We still have only the crudest
understanding of why these neural pathways are connected up the way they
are. But it is surely the very detailed way in which these connections are
made that is at the heart of the power of the brain as a thinking
machine.
The very complexity of these neural networks poses a formidable barrier
to understanding. Nobody knows in detail how the individual firings of
neurons coupled to their interconnections can lead to all of the features
observed - short and long term memory, complex pattern recognition,
logical reasoning, emotion and consciousness. Indeed, it is not known how
even the lower level unconscious functions such as those which regulate
breathing and heart rate emerge out of the complicated mutual interaction
of millions of neurons. Furthermore, it is generally believed that at
least a partial understanding will be necessary in order to build truly
intelligent machines.
Nevertheless, we are making rapid progress in understanding some of the
simpler aspects of these systems in part through the study of computers
whose architecture resembles that of the brain. These computers go under
the name of artificial neural networks.
~~~~~~~~~~
About the author - Simon Catterall
is Assistant Professor of Physics at Syracuse University.
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