EFFECTS OF CEREBRAL LESIONS

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.

 

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.

 

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”.

 

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.

 

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.

 

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.
 

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
 

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.

 

Dyslexia is an incomplete alexia, and characterized by an inability to read more than a few lines with understanding.