Localisation of function in the brain
Localisation versus holistic theory
During the 19th century, scientists such as Paul Broca and Karl Wernicke discovered
that specific areas of the brain are associated with particular physical and psychological
functions. Before these investigations and before the case of Phineas Gage - see facing
page), scientists generally supported the holistic theory of the brain - that all parts of the
brain were involved in the processing of thought and action.
In contrast, Broca and Wernicke argued for localisation of function (sometimes
referred to as cortical specialisation). This is the idea that different parts of the brain
perform different tasks and are involved with different parts of the body. It follows then,
that if a certain area of the brain becomes damaged through illness or injury, the function
associated with that area will also be affected.
Hemispheres of the brain and the cerebral cortex
The brain is divided into two symmetrical halves called left and right hemispheres. Some
of our physical and psychological functions are controlled or dominated by a particular
hemisphere - this is called lateralisation (see next spread). As a general rule, activity on
the left-hand side of the body is controlled by the right hemisphere and activity on the
right-hand side of the body by the left hemisphere. The outer layer of both hemispheres
is the cerebral cortex (or 'cortex') like a tea cosy covering the inner parts of the brain.
This 'tea-cosy' is about 3 mm thick and is what separates us from other animals because
the human cortex is much more developed. The cortex appears grey due to the location
of cell bodies (hence the phrase 'grey matter' to describe the surface appearance of the
brain).
The motor, somatosensory, visual and auditory centres
The cortex of both hemispheres is sub-divided into four lobes which are named after the
bones beneath which they lie: the frontal lobe, the parietal lobe, the occipital lobe and
the temporal lobe (see diagram on facing page). Each lobe is associated with different
functions.
At the back of the frontal lobe (in both hemispheres) is the motor area which controls
voluntary movement in the opposite side of the body. Damage to this area of the brain
may result in a loss of control over fine movements.
At the front of both parietal lobes is the somatosensory area which is separated
from the motor area by a 'valley' called the central sulcus. The somatosensory area is
where sensory information from the skin (e.g. related to touch, heat, pressure, etc.)
is represented. The amount of somatosensory area devoted to a particular body part
denotes its sensitivity, for instance, receptors for our face and hands occupy over half of
the somatosensory area.
In the occipital lobe at the back of the brain is the visual area (or visual cortex). Each
eye sends information from the right visual field to the left visual cortex and from the left
visual field to the right visual cortex. This means that damage to the left hemisphere, for
example, can produce blindness in part of the right visual field of both eyes.
Finally, the temporal lobes house the auditory area, which analyses speech-based
information. Damage may produce partial hearing loss; the more extensive the damage,
the more extensive the loss. In addition, damage to a specific area of the temporal lobe -
Wernicke's area (discussed below) - may affect the ability to comprehend language.
Localisation versus holistic theory
During the 19th century, scientists such as Paul Broca and Karl Wernicke discovered
that specific areas of the brain are associated with particular physical and psychological
functions. Before these investigations and before the case of Phineas Gage - see facing
page), scientists generally supported the holistic theory of the brain - that all parts of the
brain were involved in the processing of thought and action.
In contrast, Broca and Wernicke argued for localisation of function (sometimes
referred to as cortical specialisation). This is the idea that different parts of the brain
perform different tasks and are involved with different parts of the body. It follows then,
that if a certain area of the brain becomes damaged through illness or injury, the function
associated with that area will also be affected.
Hemispheres of the brain and the cerebral cortex
The brain is divided into two symmetrical halves called left and right hemispheres. Some
of our physical and psychological functions are controlled or dominated by a particular
hemisphere - this is called lateralisation (see next spread). As a general rule, activity on
the left-hand side of the body is controlled by the right hemisphere and activity on the
right-hand side of the body by the left hemisphere. The outer layer of both hemispheres
is the cerebral cortex (or 'cortex') like a tea cosy covering the inner parts of the brain.
This 'tea-cosy' is about 3 mm thick and is what separates us from other animals because
the human cortex is much more developed. The cortex appears grey due to the location
of cell bodies (hence the phrase 'grey matter' to describe the surface appearance of the
brain).
The motor, somatosensory, visual and auditory centres
The cortex of both hemispheres is sub-divided into four lobes which are named after the
bones beneath which they lie: the frontal lobe, the parietal lobe, the occipital lobe and
the temporal lobe (see diagram on facing page). Each lobe is associated with different
functions.
At the back of the frontal lobe (in both hemispheres) is the motor area which controls
voluntary movement in the opposite side of the body. Damage to this area of the brain
may result in a loss of control over fine movements.
At the front of both parietal lobes is the somatosensory area which is separated
from the motor area by a 'valley' called the central sulcus. The somatosensory area is
where sensory information from the skin (e.g. related to touch, heat, pressure, etc.)
is represented. The amount of somatosensory area devoted to a particular body part
denotes its sensitivity, for instance, receptors for our face and hands occupy over half of
the somatosensory area.
In the occipital lobe at the back of the brain is the visual area (or visual cortex). Each
eye sends information from the right visual field to the left visual cortex and from the left
visual field to the right visual cortex. This means that damage to the left hemisphere, for
example, can produce blindness in part of the right visual field of both eyes.
Finally, the temporal lobes house the auditory area, which analyses speech-based
information. Damage may produce partial hearing loss; the more extensive the damage,
the more extensive the loss. In addition, damage to a specific area of the temporal lobe -
Wernicke's area (discussed below) - may affect the ability to comprehend language.
