Cortical Sensory Functions

The cortical sensory functions are tests of the brain’s ability to interpret sensory information, not tests of whether sensation has reached it. Two-point discrimination, stereognosis, graphesthesia, tactile localization, and extinction all depend on intact primary sensation as a prerequisite. They then probe what the parietal cortex makes of that information. A patient who cannot perceive a pin will fail every cortical sensory test for trivial reasons; the cortical exam is meaningful only when the patient can perceive the stimulus but cannot integrate or interpret it.

This page covers the bedside cortical sensory exam: how to test it, what each test reveals, and how to interpret a positive finding. The functions covered are localizing to the contralateral parietal cortex and adjacent structures. Their preservation in the setting of profound peripheral sensory loss tells you the cortex is intact; their loss with preserved primary modalities tells you the parietal cortex is not.

The Anatomical Framework

The primary somatosensory cortex sits in the postcentral gyrus, organized topographically: face inferiorly and laterally, hand laterally, leg and foot superiorly and medially with the foot folded over onto the medial surface of the hemisphere (the paracentral lobule). The somatosensory homunculus magnifies the hand, lips, and tongue out of proportion to the rest of the body, reflecting the density of cortical neurons devoted to those regions.

The cortical sensory functions are not all in the primary somatosensory cortex. Most involve the secondary somatosensory cortex and the parietal association areas in the superior parietal lobule and inferior parietal lobule. The angular and supramarginal gyri play roles in higher-order sensory integration. The arrangement explains why a small primary somatosensory cortex lesion produces a discrete deficit (loss of cortical sensation in a hand), while a larger parietal lesion produces a broader pattern of cortical sensory loss with potentially severe functional consequences (astereognosis, neglect, tactile agnosia).

Two-Point Discrimination

How to Test

Use two-point discriminator (a calibrated tool) or, at the bedside, the two points of a paperclip bent into the standard tool shape. Touch the patient’s skin with one or two points and ask whether they perceive one or two stimuli. Begin with a separation of about 5 mm and adjust based on the patient’s response. Compare sides.

Normal two-point discrimination varies enormously by body site, reflecting the density of cortical representation:

• Tongue tip: 1 mm
• Fingertip: 2-3 mm
• Palm: 8-12 mm
• Lower forearm: 30-40 mm
• Upper arm: 40-50 mm
• Lower leg: 30-40 mm
• Back: 60-70 mm

Because absolute values vary so widely, side-to-side comparison is the most useful measure. An asymmetry of more than about 50% — the right finger can resolve 3 mm but the left needs 6 mm — is pathologic. The test is most sensitive at the fingertips because the resolution is finest there.

What Reduced Two-Point Discrimination Means

Asymmetric impairment of two-point discrimination localizes to:

• Contralateral primary somatosensory cortex: the postcentral gyrus.
• Contralateral parietal white matter: deep lesions disconnecting cortex from thalamus.
• Contralateral thalamic VPL: thalamic strokes typically produce more global sensory loss but can present with selective cortical sensory features.

Symmetric impairment is harder to interpret and is more likely to reflect peripheral nerve disease at the testing site (digital sensory neuropathy from carpal tunnel, for example).

Stereognosis

How to Test

The patient closes their eyes. Place a familiar object — a key, a coin, a paperclip, a button — in one hand and ask them to identify it. The patient should be able to do this within a few seconds for each hand, using one hand at a time. Use common objects, and start with the most distinct shapes.

Three potential outcomes:

• Normal stereognosis: the patient identifies the object quickly and correctly.
• Astereognosis with intact primary sensation: the patient can describe the object’s features (round, metallic, with markings around the edge) but cannot say what it is. This is the cortical sensory finding. The test confirms the dissociation between perception and recognition. Confirm primary sensation is intact by testing pinprick, vibration, and joint position at the site first.
• Astereognosis with impaired primary sensation: the patient cannot identify the object because they cannot feel it well enough. This is a peripheral or pathway problem, not a cortical one.

What Astereognosis Means

True astereognosis — failure of object recognition by touch with intact primary sensation — localizes to the contralateral parietal cortex. It is most common with lesions of the superior parietal lobule and the postcentral gyrus. Pure cases (without associated motor deficit) are uncommon, but the finding can be very useful in defining the extent of parietal involvement in a stroke or tumor and in distinguishing parietal cortex disease from subcortical disease.

The dissociation between visual recognition (seeing the key and identifying it) and tactile recognition (handling the key and failing to identify it) is the cardinal feature. A patient with associative visual agnosia shows the opposite dissociation: they can identify objects by touch but not by sight.

Graphesthesia

How to Test

The patient closes their eyes. Trace a single-digit number on the palm of the hand (or sometimes the fingertip) with a blunt object. Use medium-sized numbers — about an inch tall — and trace once with reasonable speed. Ask the patient to identify the number. Repeat on the other hand.

Most adults can identify single-digit numbers traced on the palm reliably. A few practical points: the numbers should be drawn from the patient’s perspective (so that they are right-side up to the patient looking at their own palm), not from the examiner’s. Mix the numbers up so the patient cannot anticipate. Start with familiar digits before testing letters.

What Impaired Graphesthesia Means

Impaired graphesthesia (agraphesthesia) with preserved primary sensation localizes to the contralateral parietal cortex. The test is sensitive to the same region as stereognosis but is sometimes preserved when stereognosis is lost, and vice versa. Both should be tested in any patient with suspected parietal disease.

Tactile Localization

Touch the patient at a defined point — the back of the wrist, for example — and ask the patient (with eyes closed) to point to the spot you touched. The patient with normal tactile localization points within a centimeter or two of the actual site. Marked inaccuracy with preserved perception of the touch points to parietal cortex dysfunction on the contralateral side. The test is rarely performed in isolation but adds confirmation when other cortical sensory tests are abnormal.

Extinction (Double Simultaneous Stimulation)

How to Test

Touch the patient on one side, then the other, then both sides simultaneously. The patient with normal cortical attention reports the single touches correctly on each side and reports two touches when both sides are stimulated. The patient with extinction reports only one touch — typically on the side opposite the lesion (usually the right) — when both are stimulated.

Extinction can be tested in three modalities:

• Tactile: touches on the hands or face. Most commonly tested.
• Visual: finger movements in both visual hemifields simultaneously. The patient detects each separately but extinguishes the left when both are presented.
• Auditory: a snap or whisper near each ear, simultaneously. The patient detects only the right.

What Extinction Means

Extinction is a sign of hemispatial neglect — most commonly from right parietal disease, producing left-sided extinction. The lesion can be cortical (especially right inferior parietal lobule), subcortical (right thalamus, right striatum), or sometimes right frontal. Extinction is a specifically right-hemisphere finding because the right hemisphere appears to mediate attention bilaterally, while the left hemisphere attends mostly to the right side. Loss of right-hemisphere function therefore leaves left-side attention without a substrate.

The bedside finding of extinction is one of the most sensitive tests for mild neglect. Patients may have entirely normal hemifield testing and normal point-localized sensation on each side, but extinguish on double simultaneous stimulation. Detection requires specifically testing for it; routine sensory testing does not reveal extinction.

Tactile Agnosia

Severe astereognosis — inability to recognize any object placed in the hand despite intact primary sensation — is sometimes given the name tactile agnosia. The lesion is typically a large parietal stroke or tumor. In its most severe form, the patient cannot recognize a coin, a key, a pen, or a cellphone in the affected hand; placing the same object in the other hand or letting the patient see it produces immediate recognition. The deficit is genuinely modality-specific, and the dissociation is striking when demonstrated at the bedside.

Cortical Sensory Patterns by Lesion Site

🔍 Did You Know?

The “useless hand of Oppenheim” — first described in patients with parietal lobe lesions, classically multiple sclerosis — is a striking dissociation: the hand has normal strength, normal reflexes, and normal primary sensation, but the patient cannot use it for skilled tasks. They cannot button a shirt, write fluidly, or pick up small objects without looking. The deficit is in the cortical processing of sensory feedback during movement. The patient’s hand is, in a real sense, blind to itself.

How Cortical Sensory Testing Fits the Overall Exam

Cortical sensory testing should be performed only when primary sensation has been documented to be at least relatively preserved. The order at the bedside should be:

1. Test primary modalities (pinprick, temperature, vibration, joint position, light touch) and identify whether any are impaired.
2. If primary modalities are preserved or only mildly impaired, test cortical functions in the relevant territory.
3. Always test for extinction with double simultaneous stimulation, especially in any patient with suspected right hemisphere disease.
4. Document the pattern: which functions are impaired, which are preserved, and on which side.

The cortical sensory exam takes about three minutes. It is one of the most rewarding parts of the examination because positive findings are highly localizing and often reveal disease that would not be detected on routine testing.

Pitfalls and Pearls

• Cortical sensory testing requires intact primary sensation. A patient with severe pinprick loss cannot meaningfully be tested for two-point discrimination at the same site.
• Side-to-side comparison is the most useful single measure for cortical sensation. Absolute values vary widely; asymmetry is what matters.
• Test both hands in stereognosis, separately. Comparison reveals the asymmetric deficit.
• Trace numbers from the patient’s perspective, not the examiner’s. A “6” drawn from your side reads as “9” from theirs.
• Always test for extinction in any patient with suspected right hemisphere disease. Routine single-stimulus testing misses it.
• The useless hand — full strength but cannot perform skilled tasks — is a parietal cortex lesion. Look for it in any patient with vague “weakness” of a hand that does not match a peripheral pattern.
• Cortical sensory loss without primary loss is specifically parietal cortex disease. Use it to confirm cortical localization when other findings are ambiguous.
• The thalamic pain syndrome can present with predominantly cortical sensory loss initially. The hyperalgesic phase develops weeks to months later.
• Pseudoneglect — a small, normal tendency to bisect a line slightly to the left of true center — is a finding in healthy right-handers and reflects the right hemisphere’s slight dominance for spatial attention. Mark the difference between mild pseudoneglect and pathologic neglect; the latter is asymmetric far beyond the normal physiological deviation.

References

1. Campbell WW. DeJong’s The Neurologic Examination. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2013. Chapter 35.
2. Mesulam MM. Principles of Behavioral and Cognitive Neurology. 2nd ed. Oxford University Press; 2000.
3. Caselli RJ. Bilateral impairment of somesthetically mediated object recognition in humans. Mayo Clin Proc. 1991;66(4):357-364.
4. Vallar G. Spatial neglect, Balint-Holmes’ and Gerstmann’s syndrome, and other spatial disorders. CNS Spectr. 2007;12(7):527-536.
5. Heilman KM, Valenstein E, eds. Clinical Neuropsychology. 5th ed. Oxford University Press; 2012.
6. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Philadelphia: Wolters Kluwer; 2017.