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 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. 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.
Graphesthesia (traced figure discrimination, number writing) 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. 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 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, with 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. 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.
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 to 3 mm on the lips to 4 mm on the fingertips to 6 mm on the dorsum of the fingers to 12 mm on the palm to 30 mm on the back of the hand, and 30 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. 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. Loss of two-point discrimination limited to the distribution of a peripheral nerve or root is helpful in diagnosis and management. Two-point discrimination may also be used to demonstrate a sensory level on the trunk in myelopathy.
Sensory extinction, inattention, or neglect is loss of the ability to perceive two simultaneous sensory stimuli. It is a test of sensory attentional mechanisms rather than somatosensory function. It may occur in isolation with parietal lobe lesions or in company with other deficits of attention to hemispace with more extensive lesions. At its most extreme, there is inattention to all of contralateral hemispace (anosognosia, Chapter 10).
Testing for tactile extinction uses double simultaneous stimuli at homologous sites on the two sides of the body. Light touch is most often used. Extinction occurs when one of the stimuli is not felt. If using pinprick (with equally sharp pins), the stimulus on the abnormal side may feel blunt compared to the normal side. 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.
Sensory extinction may occur as the only manifestation of a lesion. 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.
The ability to localize sensory stimuli also depends on the parietal lobes. To test this function, touch the patient on one side and ask him to point with the opposite index finger to the point touched by the examiner. When testing a hand, the patient should be able to localize the point touched precisely; with other body regions, the accuracy of localization may vary as occurs with two-point discrimination. A right parietal lesion interferes with touch localization on the left side of the body; a left parietal lesion causes localization deficits bilaterally.
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’s 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 10.
Bender MB, Stacy C, Cohen J. Agraphesthesia. A disorder of directional cutaneous kinesthesia or a disorientation in cutaneous space. J Neurol Sci 1982;53:531–555.
Dellon AL. The moving two-point discrimination test: clinical evaluation of the quickly adapting fiber/receptor system. J Hand Surg [Am] 1978;3:474–481.
Fuller G. Neurological Examination Made Easy. 5th ed. Edinburgh: Churchill Livingstone, 2013.
Gardner EP, Sklar BF. Discrimination of the direction of motion on the human hand: a psychophysical study of stimulation parameters. J Neurophysiol 1994;71:2414–2429.
Gilman S. Clinical Examination of the Nervous System. New York: McGraw-Hill, 2000.
Gilman S, Newman SW. Manter and Gatz’s Essentials of Clinical Neuroanatomy and Neurophysiology. 10th ed. Philadelphia: FA Davis, 2003.
Hankey GJ, Edis RH. The utility of testing tactile perception of direction of scratch as a sensitive clinical sign of posterior column dysfunction in spinal cord disorders. J Neurol Neurosurg Psychiatry 1989;52:395–398.
Hermann RP, Novak CB, Mackinnon SE. Establishing normal values of moving two-point discrimination in children and adolescents. Dev Med Child Neurol 1996;38:255–261.
Kandel ER. Principles of Neural Science. 5th ed. New York: McGraw-Hill Medical, 2013.
Lundborg G, Rosén B. The two-point discrimination test—time for a re-appraisal? J Hand Surg Br 2004;29:418–422.
Massey EW, Pleet AB, Scherokman BJ. Diagnostic Tests in Neurology: A Photographic Guide to Bedside Techniques. Chicago: Year Book Medical Publishers, Inc., 1985.
Robertson SL, Jones LA. Tactile sensory impairments and prehensile function in subjects with left-hemisphere cerebral lesions. Arch Phys Med Rehabil 1994;75:1108–1117.
Ross RT. How to Examine the Nervous System. 4th ed. Totowa: Humana Press, 2006.
Schwartzman RJ. Neurologic Examination. 1st ed. Malden: Blackwell Publishing, 2006.
Standring S, ed. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. New York: Elsevier Limited, 2016.
van Nes SI, Faber CG, Hamers RM, et al. Revising two-point discrimination assessment in normal aging and in patients with polyneuropathies. J Neurol Neurosurg Psychiatry 2008; 79:832–834.
Weibers DO, Dale AJD, Kokmen E, et al., eds. Mayo Clinic Examinations in Neurology. 7th ed. St. Louis: Mosby, 1998.
Wolf JK. Segmental Neurology. Baltimore: University Park Press, 1981.
Wolny T, Linek P, Michalski P. Inter-rater reliability of two-point discrimination in acute stroke patients. Neurorehabilitation 2017;41:127–134.