and modern methods of scanning and statistical
analysis give multicoloured representations of what
is happening inside. But in every case the mystery is
obvious – how can this physical lump of stuff, with its
electrical and chemical activity, relate to conscious
experience? Whatever the answer, it is worth learn-
ing a little about the structure and function of the
human brain.
A HUMAN BRAIN
Said to be the most complex object in the known
universe, a human brain contains about 86 bil-
lion neurons connected by trillions of synapses
between them, along with billions of supporting
glial cells, some of which are also involved in sig-
nalling. Human brains are much larger, relative to
body weight, than those of any other animal, but
are organised in a roughly similar way. Sensory and
motor neurons from all over the body run into the
spinal cord and up into the brainstem at the base of
the brain. All these neurons are collectively known
as the peripheral nervous system, while the spinal
cord and brain make up the central nervous system
(CNS).
The brainstem, consisting of medulla, pons, and mid-
brain, is essential for life, not only because it carries
so many important nerve tracts but because of its
role in controlling cardiac, respiratory, and sexual
functions and arousal levels.
The reticular formation in vertebrates is involved
in the pain desensitisation pathway and, along
with its connections, forms the reticular activating
system, which activates widespread regions of
the cortex in transitions from sleep to waking or
from relaxed waking to alert attention. It has been
known since the nineteenth century that animals
with no cortex can still show normal sleep–wak-
ing cycles controlled by this system. Its function-
ing is thought necessary, but not sufficient, for
consciousness.
Behind the midbrain is the cerebellum, or ‘little brain’,
whose main function is motor control, with exten-
sive links upwards to motor cortex and downwards
through the spinocerebellar tract, which provides
feedback on body position and the effects of actions.
mAPPInG tHe BRAIn
single cell recording
Fine electrodes are inserted into living cells to
record their electrical activity. this technique
is widely used in animal studies, and more
rarely in humans.electroencephalogram (eeG)
the eeG uses electrodes on the scalp to measure
changes in electrical potential arising from the com-
bined activity of many cells in the underlying area
of the brain. the human eeG was first described
in 1929 by the German psychiatrist Hans Berger,
who showed that the resting alpha rhythm (8–12
cycles per second) is blocked by opening the eyes
or doing mental arithmetic. In the 1960s, com-
puter averaging improved the study of event-re-
lated potentials (eRPs), including evoked potentials
in response to specific stimuli, readiness potentials
that build up gradually before a response is made, and potentials
associated with unexpected events. Although the eeG has poor
spatial resolution, it is still a valuable research tool because of its
good temporal resolution.x-ray Computed tomography (Ct)
Developed in the early 1970s, Ct scans are computer-gen-
erated images of tissue density, produced by passing
x-rays through the body at many different angles and
measuring their attenuation by different tissues. the same
mathematical techniques for constructing the images are
used in newer forms of scanning.Positron emission tomography (Pet)
this is a technique for imaging the distribution of radioac-
tivity following administration of a radioactive substance.
In Pet, atoms that emit positrons are incorporated into glu-
cose or oxygen molecules, allowing brain metabolism and
blood flow to be measured directly. Radiation detectors
are arrayed on the head in several rings, allowing several
slices of brain to be studied simultaneously. Pet has good
spatial resolution but very poor temporal resolution, and
the added disadvantage of having to use radiation.C
on
C
e
P
t
4.1