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
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
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
Thus the human brain consists
of roughly three separate parts.
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.
- 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.
- 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.
These structural features of the brain have been known for some
time. In the section Building
Blocks 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.
- 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. We can use this to construct a distorted map of the
body which shows the emphasis given to certain regions of the body's
There is an approximate symmetry between left and right hemispheres -
for example, there are two occipital lobes, two parietal lobes and
there are two two frontal lobes.. However this symmetry is not exact
- for example, the area associated with language appears only on the
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.