Minds and Computers : An Introduction to the Philosophy of Artificial Intelligence

particles and which are punctuated with pauses.
The parietal lobe is thought to play a role in integrating inform-
ation from the sensory modalities. It contains the primary sensory
cortex, otherwise known as the somatosensory strip. The somatosen-
sory strip, like the adjacent motor strip in the frontal lobe, is
topologically organised. This means that certain parts of the sensory
cortex are correlated with certain parts of the body, with larger parts
of the cortex devoted to those parts of the body which are more sen-
sitive (have more nerve endings). So large parts of the somatosensory
strip are devoted to the lips, fingers and genitalia, but comparatively
little is devoted to less sensitive areas.
The temporal lobe is thought to be implicated in certain memory
functions. It contains the primary auditory cortex and, immediately
adjacent, the other speech area of the brain – Wernicke’s area.
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Wernicke’s area – named after Karl Wernicke – also gives rise to a dis-
tinctive aphasia when damaged – Wernicke’s aphasia. Wernicke’s
aphasia is characterised by fluent but meaningless speech. Su
fferers
typically evidence poor linguistic comprehension and little awareness
of their deficit.
The extraordinary biological adaptation subserving our linguistic
capacity which I mentioned earlier is the arcuate fasciculus. The
arcuate fasciculus is a thick strand of neural fibres which connects
Broca’s area directly to Wernicke’s area. Damage to the arcuate fasci-
culus gives rise to conduction aphasia, one of the distinctive symptoms
of which is di
fficulty with repeating an utterance back to an inter-
locutor.
Finally, the occipital lobe is mostly involved with the processing of
visual information. It contains the primary visual cortex at the very
rear of the brain, which accounts for why a blow to the back of the
head can cause one to ‘see stars’. Damage to the occipital lobe can
result in blindness, even when the visual sensory apparatus remain
intact and functional.
Although there is considerable localisation of function in the
brain, it does exhibit a certain degree of neural plasticity, particularly
in younger brains. This means that if a certain part of the brain is
damaged, other parts of the brain may be able to take up its function
to some extent. This is generally more so with the functions imple-
mented in the cerebral hemispheres. Damage to the brain stem is
usually irreversible and will typically quickly lead to death since these
areas regulate vital functions.
4.2 MICRO-NEUROANATOMY
The final aim of this chapter is to briefly describe the operations of
neurons. Neurons are individual nerve cells which conduct electrical
impulses and the brain consists of a very large number of them.
There are roughly ten billion neurons in the brain, each of which is
connected, on average, to about ten thousand other neurons. That
makes brains astonishingly complex. Imagine taking a country the
size of India – which has a population of about a billion – and giving
every man, woman and child a thousand pieces of string with instruc-
tions to find a thousand distinct people to hold the other end of each
piece of string. When the whole country is connected up like this, with
every person connected to a thousand other people by pieces of
string, multiply the whole system in complexity by an order of mag-
nitude and that’s how complex your brain is.
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There are a number of quite distinct types of neurons, but that
needn’t concern us here. We’re going to describe the structural fea-
tures and operations of a paradigm neuron.
Neurons have a cell body or soma which contains the nucleus of the
cell. This is connected via an axon hillock to the axon, which is a pro-
tuberance which can extend as long as roughly a metre. These axons
are coated with a myelin sheath which helps electrical signals flow
more quickly and aids in insulation. At the end of the axon are axon
branches which terminate at axon terminals.
Axons are efferent connections – they carry signals away from the
soma and along the axon towards the axon terminals. Incoming, or
afferent, signals are carried towards the soma along the dendrites of
the neuron. Dendrites are organised in a dendritic tree and there may
be very many of them.
When an axon terminal is in close proximity to a dendrite, a
synapse will form (see Figure 4.5). These synaptic connections conduct
signals from one neuron to another (strictly speaking, a neuron can
form a synaptic connection with pretty much any part of a neigh-
bouring neuron, but we’re aiming to keep things as simple as possi-
ble).
The operations of neurons are electrochemical in nature. An electri-
cal signal flows along an axon to a presynaptic axon terminal where it
is transduced into a chemical signal. This chemical signal is then carried
across the synaptic cleft by neurotransmitters in synaptic vesicles.
Once these synaptic vesicles reach the postsynaptic structure, the
chemical signal encoded by the neurotransmitters is transduced back
to an electrical signal which propagates along the postsynaptic den-
drite and into the body of its neuron.
The cell body of a neuron has a certain electrical resting potential.
As it receives a
fferent electrical signals along its dendrites, the
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