Each cerebral hemisphere is divided into
four lobes; the frontal,
parietal, temporal, and the
The Frontal Lobe is
the most anterior lobe of the brain. Its posterior boundary
is the fissure of Rolando, or central sulcus,
which separates it from the parietal lobe. Inferiorly, it is
divided from the temporal lobe by the fissure of Sylvius which is also called the lateral fissure.
This lobe is associated with
higher level cognitive functions such as reasoning and
judgment. Most importantly for speech pathologists, the
frontal lobe contains several cortical areas involved in the
control of voluntary muscle movement, including those
necessary for the production of speech and swallowing.
Broca's Area is found on the inferior third frontal gyrus in
the hemisphere that is dominant for language. This area is
involved in the coordination or programming of motor
movements for the production of speech sounds. While it is
essential for the execution of the motor movements involved
in speech it does not directly cause movement to occur. The
firing of neurons here does not generate impulses for motor
movement; that is the function of neurons in the motor
strip. The neurons in Broca's area generate motor
programming patterns when they fire.
This area is also involved in syntax
which involves the ordering of words in speech.
Injuries to Broca's area may cause
apraxia or Broca's
The precentral gyrus,
which may also be called the primary
motor area or, most
commonly, the motor strip is immediately anterior to the
central sulcus. It controls the voluntary movements of
skeletal muscles; cell bodies of the pyramidal tract
are found on this gyrus.
The amount of tissue on the precentral
gyrus that is dedicated to the innervation of a particular
part of the body is proportional to the amount of motor
control needed by that area, not just its size. For example,
much more of the motor strip is dedicated to the control of
the articulators than to the legs.
The premotor area or
area is immediately anterior
to the motor strip. It is responsible for the programming
for motor movements. It does not, however program the motor
commands for speech as these are generated in Broca's area
which is also located in the frontal lobe.
The most anterior part of the frontal
lobe is involved in complex cognitive processes like
reasoning and judgment. Collectively, these processes may be
intelligence. A component of
biological intelligence is executive function. According to Denckla, 1996, executive function
regulates and directs cognitive processes. Decision making,
problem solving, learning, reasoning and strategic thinking
are all part of executive functioning. Some characteristics
of right hemisphere syndrome are considered problems of the
executive function. They include left side neglect where
there is a lack of awareness of the left side of the body.
The Parietal Lobe is
immediately posterior to the central sulcus. It is anterior
to the occipital lobe, from which it is not separated by any
natural boundary. Its inferior boundary is the posterior
portion of the lateral fissure which divides it from the
The parietal lobe is
associated with sensation, including the sense of touch,
kinesthesia, perception of warmth and cold, and of
vibration. It is also involved in writing and in some
aspects of reading.
The postcentral gyrus
which is also called the primary sensory area or the sensory strip is immediately posterior to
the central sulcus. This area receives sensory feedback from
joints and tendons in the body and is organized in the same
manner as the motor strip.
Like the motor strip, the sensory strip
continues down into the longitudinal cerebral fissure and so
has both a lateral and a medial aspect.
areas are located behind the
postcentral gyrus. These areas are capable of more detailed
discrimination and analysis than is the primary sensory
area. They might, for example, be involved in sensing
how hot or cold something is rather than simply
identifying it as hot or cold. Information is first
processed in the primary sensory area and is then sent to
the secondary sensory areas.
The angular gyrus lies
near the superior edge of the temporal lobe, immediately
posterior to the supramarginal gyrus. It is involved in the
recognition of visual symbols. Geschwind
described this area as "the most important cortical areas of
speech and language" and the "association cortex for
association cortices". He also claims that the angular gyrus
is not found in non-human species.
Fibers of many different types travel
through the angular gyrus, including axons associated with
hearing, vision, and meaning. The arcuate fasciculus, the
groups of fibers connecting Broca's area to Wernicke's area
in the temporal lobe connects to this area.
The following disorders may result from
damage to the angular gyrus in the hemisphere that is
dominant for speech and language: anomia, alexia with
agraphia, left-right disorientation, finger agnosia, and
Anomia is a difficulty with word-finding or naming.
Someone suffering from anomia can list the functions of an
object and explain it meaning, but cannot recall its name.
Agraphia refers to
difficulties with reading and writing.
disorientation is an
inability to distinguish right from left.
agnosia is the lack of
sensory perceptual ability to identify by touch.
Acalcula refers to difficulties with arithmetic.
The Temporal Lobe is
inferior to the lateral fissure and anterior to the
occipital lobe. It is separated from the occipital lobe by
an imaginary line rather than by any natural boundary.
The temporal lobe is
associated with auditory processing and olfaction. It is
also involved in semantics, or word meaning, as Wernicke's
area is located there.
Area is located on the
posterior portion of the superior temporal gyrus. In the
hemisphere that is dominant for language, this area plays a
critical role in the ability to understand and produce
meaningful speech. A lesion here will case Wernicke's
Gyrus, which is also known
as the anterior transverse temporal gyrus, is the
There are two secondary auditory
or auditory association
areas which make important
contributions to the comprehension of speech. They are not
completely responsible for this ability, however, as many
areas, including Wernicke's area, are involved in this
The Occipital Lobe,
which is the most posterior lobe, has no natural boundaries.
It is involved in vision.
The primary visual area receives input from the optic tract via the
The secondary visual areas integrate visual information, giving meaning to
what is seen by relating the current stimulus to past
experiences and knowledge. A lot of memory is stored here.
These areas are superior to the primary visual cortex.
Damage to the primary visual area causes
blind spots in the visual field, or total blindness,
depending on the extent of the injury. Damage to the
secondary visual areas could cause visual agnosia.
People with this condition can see visual stimuli, but
cannot associate them with any meaning or identify their
function. This represents a problem with meaning, as
compared to anomia, which involves a problem with naming, or
Insula is a cortical area
which lies below the fissure of Sylvius and is considered by
some anatomists to be the fifth lobe of the cerebrum. It can
only be seen by splitting the lateral fissure. Little is
known about the connections of this area, but it may be
linked to the viscera. Drunkers, 1996 feels that it may be
involved in programming for speech for speech sounds.
It is important to remember that while
some functions can be localized to very specific parts of
the brain, others cannot be classified in this way because
many areas are involved in their performance. Word-finding,
for example, is associated with several different areas.
Also, we cannot say that all higher level cognitive
functioning is associated with the frontal lobe; the
processing of word meaning carried out by Wernicke's
certainly involves a sophisticated type of cognition. Also,
right hemisphere lesions often result in
The cortex is about four
millimeters thick and is composed of six layers. Listed from
most superior to most inferior, these layers are; the
the internal granular
layer, the ganglionic layer,
and the fusiform or
The molecular layer is
the most superior layer of the cortex. It contains the cell
bodies of neuroglial cells.
The external granular layer is very dense and contains small granular cells
and small pyramidal cells.
The external or
layer contains pyramidal
cells arranged in row formation. The cell bodies of some
association fibers are found here.
The internal granular layer is thin, but its cell structure is the same as
that of the external granular layer.
The ganglionic layer
contains small granular cells, large pyramidal cells as well
as the cell bodies of some association fibers. The
association fibers that originate here form two large
tracts: The Bands of
Baillarger and Kaes Bechterew.
The fusiform layer is
also known as the multiform
layer; its axons enter white
matter, it function is unknown.
All layers are present in all parts of
the cortex. However, they do not have the same relative
density in all areas. Depending upon the function of a
particular area, some of these layers will be thicker than
others in that location.
The cortex wraps around the brain,
covering its inferior surface and lining the gap between the
right and left cerebral hemispheres, which is called the
The part of the cortex covering the sides
of the hemispheres is called the lateral cortex while the
part covering the sides of the hemispheres that lie within
the longitudinal cerebral fissure is called
Studies done by Brodmann in the early
part of the twentieth century generated a map of the cortex
covering the lobes of each hemisphere. These studies
involved electrical probing of the cortices of epileptic
patients during surgery. Brodmann numbered the areas that he
studies in each lobe and recorded the psychological and
behavioral events that accompanied their stimulation.
The Frontal Lobe
contains areas that Brodmann identified as involved in
cognitive functioning and in speech and language.
4 corresponds to the
precentral gyrus or primary motor area.
Area 6 is the premotor or supplemental motor area.
Area 8 is anterior of the premotor cortex. It
facilitates eye movements and is involved in visual reflexes
as well as pupil dilation and constriction.
Areas 9, 10, and 11 are
anterior to area 8. They are involved in cognitive processes
like reasoning and judgment which may be collectively
called biological intelligence.
Area 44 is Broca's area.
Areas in the Parietal Lobe play
a role in somatosensory processes.
3, 2, and
1 are located on the primary sensory strip, with
area 3 being superior to the other two. These are
somastosthetic areas, meaning that they are the primary
sensory areas for touch and kinesthesia.
7, and 40 are found
posterior to the primary sensory strip and correspond to the
presensory to sensory association areas.
Area 39 is the angular gyrus.
Areas involved in the processing of
auditory information and semantics as well as the
appreciation of smell are found in the Temporal Lobe.
41 is Heschl's gyrus, or the
primary auditory area.
Area 42 immediately inferior to area 41 and is also
involved in the detection and recognition of speech. The
processing done in this area of the cortex provides a more
detailed analysis than that done in area 41.
Areas 21 and 22 are the
auditory association areas. Both areas are divided into two
parts; one half of each area lies on either side of area
Area 37 is found on the posterior-inferior part of the
temporal lobe. Lesions here will cause anomia.
The Occipital Lobe
contains areas that process visual stimuli.
17 is the primary visual
Areas 18 and 19 are the
secondary visual areas.
A pedagogical device called the
literally means "little man," is often used to explain the
organization of the motor strip and to demonstrate that
specific areas of this gyrus are responsible for sending
commands to specific parts of the body. The body is
represented on the motor strip in an upside-down fashion.
The lower parts of the body, like the feet and the legs,
receive motor movement commands from the superior part of
the precentral gyrus. Parts of the face, on the other hand
are innervated by the inferior part of the motor strip.
The motor strip extends down
some distance into the longitudinal cerebral fissure. The
portion inside this fissure is its medial aspect. The
part on the lateral surface of the hemisphere is called its
aspect. The medial cortex
controls the movements of the body from the hips on down
while the lateral aspect sends commands to the upper body
including the larynx, face, hands, shoulders, and trunk.
The medial and lateral aspects of the
motor strip have different blood supplies. Blood comes to
the medial area from the anterior cerebral artery while the lateral portion is supported by the