Cerebral Sensory Functions

Cerebral sensory functions
are those which 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, 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 (CNS)
lesion. A patient with severe carpal tunnel syndrome and numb fingers
may not be able to identify a small object by feel; this finding is not
astereognosis anymore than the inability to recognize sound from a deaf
ear is auditory agnosia or the inability to recognize objects visually
from a blind eye is visual agnosia.

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 fiberboard,
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. 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.

Graphesthesia (traced figure discrimination) is the
ability to recognize letters or numbers written on the skin with a
pencil, dull pin, or similar object. It is a fine, discriminative
variety of cutaneous sensation. Testing is often done over the finger
pads, palms, or dorsum of the feet. Letters or numbers about 1 cm in
height are written on the finger pads, larger elsewhere. Easily
identifiable, dissimilar numbers should be used (e.g., 3 and 4 rather
than 3 and 8). It really does not seem to matter whether the numbers
are written as the patient would “read” them or “upside down”; and,
despite the temptation, it is not necessary to “erase” between stimuli.
Loss of this sensory ability is known as agraphesthesia or
graphanesthesia. Even minimal impairment of primary sensory modalities
may cause agraphesthesia. A related function is the ability to tell the
direction of movement of a light scratch stimulus drawn for 2 cm to 3
cm across the skin (tactile movement sense, directional cutaneous
kinesthesia), which may be a sensitive indicator of function of the
posterior columns and primary somatosensory cortex. Loss of
graphesthesia or the sense of tactile movement with intact peripheral
sensation implies a cortical lesion, particularly when the loss is
unilateral.

Two-point, or spatial, discrimination is the ability to
differentiate, eyes closed, cutaneous stimulation by one point from
stimulation by two points. The best instrument for testing is a
two-point discriminator designed for the purpose. Commonly used
substitutes are electrocardiogram calipers, a compass, or a paper clip
bent into a “V,” adjusting the two points to different distances. There
are two types of two-point discrimination: static and moving. To test
static two-point, the test instrument is held in place for a few
seconds on the site to be tested. Either one-point or two-point stimuli
are delivered randomly, and the minimal distance that can be discerned
as two points is determined. Accurate instructions are vital. It is
best to start with a two-point stimulus, points relatively far apart
(“this is two points”), then a single point (“this is one point”), and
then two points close together (“this is two so close it feels like
one”). Then one- and two-point stimuli are varied randomly, bringing
the points closer and closer until the patient begins to make errors.
The result is taken as the minimum distance between two points that can
be consistently felt separately. This distance varies considerably in
different parts of the body. Normal two-point discrimination is about 1
mm on the tip of the tongue, 2 mm to 3 mm on the lips, 2 mm to 4 mm on
the fingertips, 4 mm to 6 mm on the dorsum of the fingers, 8 mm to 12
mm on the palm, 20 mm to 30 mm on the back of the hand, and 30 mm to 40
mm on the dorsum of the foot. Greater separation is necessary for
differentiation on the forearm, upper arm, torso, thigh, and leg. The
findings on the two sides of the body must always be compared. For
moving two-point, the technique is the same except the

instrument
is drawn slowly across the test area. To test moving two-point on a
finger pad, the discriminator would be pulled from the crease of the
distal interphalangeal joint toward the tip of the finger over several
seconds. Discrimination for two moving points is slightly better than
for two stationary points. Moving two-point tests the rapidly adapting
mechanoreceptors, and may have some advantages in the management of
patients with peripheral nerve injuries.

Two-point discrimination requires keen tactile
sensibility. The pathway is mainly through the posterior columns and
medial lemniscus. Loss of two-point discrimination with preservation of
other discriminatory tactile and proprioceptive sensation may be the
most subtle sign of a lesion of the opposite parietal lobe.

Sensory extinction or inattention is loss of the ability
to perceive two simultaneous sensory stimuli. Testing for tactile
extinction uses double simultaneous light touch stimuli at homologous
sites on the two sides of the body. Extinction occurs when one of the
stimuli is not felt. Extinction can also be done on one side, touching
the face and hand simultaneously. In general the more rostral area is
the dominant one; when face and hand are stimulated, there is
extinction of the hand percept (the face-hand test). It may be normal
to extinguish the hand stimulus. The most subtle abnormality is for a
hand stimulus on the normal side to extinguish a face stimulus on the
abnormal side, but such testing pushes the limit of usefulness of the
technique. The severity of extinction can be approximately quantitated
by increasing the intensity of the stimulus on the abnormal side. Using
one fingertip on the normal side, a patient with mild extinction will
extinguish a two-fingertip stimulus on the abnormal side, but a
one-fingertip/three-fingertip set will be felt as bilateral stimuli.
With severe extinction, it may require a whole hand stimulus or even a
firm squeeze on the abnormal side for the patient to appreciate that
the stimulation was bilateral. Similar testing can be done with
pinprick.

Tactile extinction is most likely to occur with a lesion
of the parietal lobe, but has been reported with lesions involving the
thalamus or sensory radiations. Double simultaneous stimulation above
and below the presumed level of a spinal cord lesion in which there is
relative but not absolute sensory loss may aid in demonstrating the
level of the lesion. If only the upper stimulus is perceived, the lower
is moved more rostrally until the intensity of both is equal; this may
indicate the segmental level of the lesion.

Autotopagnosia (somatotopagnosia, body-image agnosia) is
inability to identify body parts, orient the body, or understand the
relation of individual parts—a defect in the body scheme. The patient
may have complete loss of personal identification of one limb or one
half of the body. He may drop his hand from the table onto his lap and
believe that some other object has fallen, or feel an arm next to his
body and not be aware that it is his own. Lack of awareness of one-half
of the body is referred to as agnosia of the body half. Finger agnosia
is an inability to name or recognize fingers. Finger agnosia occurs
most commonly as part of Gerstmann syndrome (finger agnosia, agraphia,
acalculia, and right-left disorientation). Anosognosia is an absence of
awareness, or denial of the existence, of disease. It is often used
more or less synonymously with somatotopagnosia to refer to patients
who deny the existence of hemiplegia or fail to recognize the paralyzed
body parts as their own. Anosognosia is most often found in lesions of
the right parietal lobe. These disorders are discussed in more detail
in Chapter 7.