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Two-point discrimination
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There is an office test that looks like a toy and tells you a great deal. Two points touch the skin at the same time, moving closer and closer together, until the person stops feeling two contacts and feels only one. The shortest distance at which they can still tell the two apart is their two-point discrimination, and on the fingertips normal is less than 5 millimeters.
What is interesting is what that number depends on. Each receptor in the skin watches over a small patch of surface, and that patch is called its receptive field: it is the area where a stimulus can reach that receptor. If the receptive fields are large, two nearby points fall inside the same one and the brain gets a single signal.
That is why areas with many small receptive fields, like the fingertips and the lips, have better spatial resolution. It is the same principle behind reading Braille with your fingertips and not with your elbow.
In brief
Two-point discrimination measures the smallest distance at which someone still feels two separate touches.
On the fingertips, normal is less than 5 millimeters, or 6 in older adults.
Behind it is the receptive field: the area of the body where a stimulus can affect one sensory receptor.
Where receptive fields are smaller and more numerous, resolution can be better, as on the fingertips.
Two close points feel like two only where the receptive fields are small.
Sources (3)
- National Center for Biotechnology Information, StatPearls PublishingInferior Alveolar Nerve and Lingual Nerve Injury, StatPearlsPublished
- National Center for Biotechnology Information, StatPearls PublishingDistal Radius Fractures, StatPearlsPublished
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Sensory Receptors, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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