EFFECTS
OF CEREBRAL LESIONS |
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The
sensory association cortex |
It receives
fibres from somesthetic area. Data pertaining to the general senses
are integrated, permitting a comprehensive assessment of the characteristics
of an object held in the hand and its identification without visual
aid.
Lesions of this area without involvement of the somesthetic cortex leaves
intact the general awareness of the general senses, but the significance
of this information on the basis of previous experience is elusive.
This condition is called agnosia.
There are several types of agnosia depending on the sense most affected.
Damage to these areas also causes asteriognosia
, in which there is lack of recognition of the opposite side
of the body. It is characterized by cortical neglect in which the patient
ignores and even denies the existence of one side of the body. This
is sometimes called hemineglect. |
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The
visual area |
This is more
extensive than seen on the surface. It is also called striate
cortex because of the lines of Gennari.
The lateral geniculate body forms the main course of input to this area.
Visual
association cortex: They receive fibres from the area,
the lateral geniculate nucleus and have reciprocal connections with
other cortical areas and with the pulvinar of the thalamus.
These areas function mainly to process complex aspects of vision including
the relating of present to past visual experiences, with recognition
of what is seen and appreciation of its significance. Lesions of the
visual association cortex causes visual
agnosia.
Bilateral lesions involving the superior parts of area
19, cause disorientation, loss of coordination
of the eye movement and inability to carry out visually directed movements
of the hands.
Inferior temporal and lateral occipitotemporal gyri also form visual
association cortex. Stimulation of these areas causes visual hallucinations
of scenes from the past, indicating a role of this cortex in the storage
or recall of visual memories.
Bilateral lesions lead to prosopagnosia
- impaired recognition of previously known familiar faces.
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The total
exposure of the association
cortex in the parietal lobe is responsible (along with
association cortex in the frontal lobe) for many of the unique qualities
of the human brain. Engrams or memory traces are laid down over the
years, possibly as macromolecular changes in neurons throughout the
cortex. These form the basis for learning at intellectual level.
Recently acquired information is not consolidated into long-term memory
when the forebrain is extensively damaged, as in Alzheimer's
diseases, or bilateral lesions of the limbic system.
Neocortex of the frontal lobe has a special role in motor activities
in the attributes of judgment and foresight, and in determining mode
or “feeling tone”. |
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The
body representation is inverted in the primary
motor area and when stimulated causes contractions
of muscles that make up a functional group. Lesions of this area cause
voluntary paresis.
The
secondary motor area
is on the medial
surface of area 6, and therefore a part of the pre-motor
area with special properties.
Loss of function of the sensory area causes spasticity
of muscles paralyzed as a result of an upper motor
neuron lesion. In humans, there is an increased blood flow in the supplementary
motor area during the mental processes that precede the execution of
a movement.
Bilateral
lesion of this area causes profound paralysis, as well as mutism. |
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The pre-motor
area (area 6) receives input
from the anterior ventral lateral thalamic nuclei, which in turn receives
input from the corpus striatum. This area contributes to motor functions
in two ways:
- Direct contribution to the pyramidal system.
- Direct influences on primary motor cortex.
Functionally, it elaborates programs for motor routines
necessary for skilled voluntary action, both when a new program is established
and when a previous established program is altered.
In general, the primary motor cortex is the area through which commands
are channeled for the execution of movements. In contrast, the pre-motor
area programs skilled motor activity and thus directs the motor area
in its execution.
The term apraxia
refers to results of a cerebral lesion characterized by impairment
in the performance of learned movement, in the absence of paralysis.
When the impairment involves writing, it is called agraphia. |
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The prefrontal
eye field comprises the lower
part of the area 8. It controls voluntary conjugate
movement of the eyes. Lesions cause deviation of the eyes to the opposite
side.
The prefrontal area (9,
10, 11, and 12) is well developed in primates and in
man. Its functions include:-
- Determination of effective reactions on the basis
of past experience.
- Monitoring behavior and exercising control based
on such higher mental facilities as judgment and foresight.
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The use of
language is a peculiarly human accomplishment, requiring special neural
mechanism in association areas of the cortex. Two cortical areas have
specialized language functions located in the dominant hemisphere (usually
the left, with a few exceptions).
Sensory language areas
- auditory association cortex of “Wernicke's” occupying part
of the posterior region of the superior temporal gyrus and part of the
parietal lobe around the ascending posterior part of the lateral sulcus.
It is also called receptive language area .
Motor
speech (Broca's) area on the postero-inferior part
of the frontal lobe, just anterior to the pre-motor area. It is also
called the expressive speech area.
These two areas are connected through the
superior longitudinal fasciculus.
Lesions involving the language areas or their connections
cause aphasia
. There are several types of aphasia depending on the
area affected:
- Receptive aphasia
- Expressive aphasia
- Global aphasia
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Alexia
is loss of ability to read and occurs with or without other aspects
of aphasia. Pure alexia may result from lesions involving the white
of the occipital lobe on the dominant side and splenium of the corpus
callosum, severing the connections between visual cortices and unilaterally
situated language areas.
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Dyslexia
is an incomplete alexia, and characterized by an inability
to read more than a few lines with understanding. |
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