Cerebral Sensory Functions

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Cerebral sensory functions are those that involve the primary sensory areas of the cortex to perceive the stimulus and the sensory association areas to interpret the meaning of the stimulus and place it in context. These functions are also referred to as secondary or cortical modalities. The term combined sensation describes perception that involves integration of information from more than one of the primary modalities for the recognition of the stimulus. Cortical sensory processing is primarily a function of the parietal lobes. The parietal lobe functions to analyze and synthesize the individual varieties of sensation and to correlate the perception of the stimulus with memory of past stimuli that were identical or similar and with knowledge about related stimuli to interpret the stimulus and aid in discrimination and recognition.

The parietal cortex receives, correlates, synthesizes, and refines the primary sensory information. It is not concerned with the cruder sensations, such as recognition of pain and temperature, which are subserved by the thalamus. The cortex is important in the discrimination of the finer or more critical grades of sensation, such as the recognition of intensity, the appreciation of similarities and differences, and the evaluation of the gnostic, or perceiving and recognizing, aspects of sensation. It is also important in localization, in the recognition of spatial relationships and postural sense, in the appreciation of passive movement, and in the recognition of differences in form and weight and of two-dimensional qualities. These elements of sensation are more than simple perceptions, and their recognition requires integration of the various stimuli into concrete concepts as well as calling forth engrams.

Cortical sensory functions are perceptual and discriminative rather than the simple appreciation of information from the stimulation of primary sensory nerve endings. The cortical modalities of greatest clinical relevance include stereognosis, graphesthesia, two-point discrimination, sensory attention, and other gnostic or recognition functions. The loss of these varieties of combined sensation may be considered a variety of agnosia, or the loss of the power to recognize the meaning of sensory stimuli. The primary modalities must be relatively preserved before concluding that a deficit in combined sensation is due to a parietal lobe lesion. Only when the primary sensory modalities are normal can the unilateral failure to identify an object by feel be termed astereognosis and be attributed to a central nervous system lesion. Impairment of primary modalities too slight to account for the recognition difficulty can also properly be termed astereognosis; making this judgment requires experience.

Stereognosis is the perception, understanding, recognition, and identification of the form and nature of objects by touch. Inability to do this is astereognosis. Astereognosis can be diagnosed only if cutaneous and proprioceptive sensations are intact; if these are significantly impaired, the primary impulses cannot reach consciousness for interpretation. There are several steps in object recognition. First, the size is perceived, followed by appreciation of shape in two dimensions, form in three dimensions, and finally identification of the object. These steps may be analyzed individually. Size perception is tested by using objects of the same shape but different sizes, shape perception with objects of simple shape (circle, square, triangle), cut out of stiff paper or plastic, and form perception by using solid geometric objects (cube, pyramid, ball). Finally, recognition is evaluated by having the patient identify only by feel simple objects placed in his hand (e.g., key, button, coin, comb, pencil, safety pin, paper clip). For more refined testing, the patient may be asked to differentiate coins, identify letters carved from wood or fiber-board, or count the number of dots on a domino.

Obviously, stereognosis can be tested only in the hands. If weakness or incoordination prevents the patient from handling the test object, the examiner may rub the patient’s fingers over the object. It is striking confirmation of the restricted nature of the deficit in pure motor stroke to demonstrate exquisitely preserved stereognosis in a paralyzed hand. When stereognosis is impaired, there may be a delay in identification or a decrease in the normal exploring movements as the patient manipulates the unknown object. Stereognosis testing normally compares the two hands, and any deficit will be unilateral. Inability to recognize objects by feel with either hand, if the primary modalities are intact, is tactile agnosia. Recognition of texture is a related type of combined sensation in which the patient tries to recognize similarities and differences between objects of varying textures, such as cotton, silk, wool, wood, glass, and metal. Astereognosis is usually accompanied by agraphesthesia and other cortical deficits; it may occur in isolation as the earliest sign of parietal lobe dysfunction.

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