A minimum perceptible pressure exists

What it is

There is a floor below which skin cannot detect pressure — no matter how long a force below that floor is applied, it goes unnoticed. This floor is the absolute tactile detection threshold, typically expressed as force per unit area (mN/mm² or an equivalent gram-force). The fingertip has the lowest threshold in the body, on the order of a few tenths to a few millinewtons. This entry is about the intensity question — how small a force can be detected — which is a different matter from the spatial-resolution question of whether two points can be told apart (covered in the two-point discrimination entries); the two should not be conflated.

Why it happens

The low threshold comes from the receptors' own transduction sensitivity to tiny deformation — applying pressure to skin produces local deformation, and only once that deformation exceeds a certain physical floor do mechanically gated ion channels open to produce a sufficient afferent signal. Below that floor, the deformation is too small to trigger transduction at all — it is not that "a weak signal exists but goes unnoticed," but that no conducting signal is produced in the first place. The fingertip's low threshold partly reflects that region's high receptor density and higher per-receptor transduction efficiency, and signals from multiple receptors may also converge centrally, boosting the detectability of a faint stimulus.

Studying it

The classic measurement tool is the von Frey filament set — a series of nylon filaments of different diameters with known buckling force — applied to the skin in ascending force order, recording the force at which the participant first reports "feeling it," which is the threshold. Modern studies more often use a programmable servo-controlled indenter instead of manual filaments, allowing more precise control over the rate and duration of force application, paired with forced-choice psychophysics to reduce subjective bias.

Where it stops holding

• The measurement method itself affects the result: point application versus distributed application (a larger contact area) produces different measured thresholds — a larger contact area usually raises the total threshold force but can lower the threshold pressure per unit area, so the contact condition must be confirmed before citing a number.
• The threshold is not a fixed physiological constant; it varies systematically with site, temperature, and skin condition (detailed in the other two entries in this group), so citing a single "fingertip threshold" figure requires stating the measurement conditions.

Applying it

• When designing a touchscreen, haptic button, or other interaction that needs "reliable triggering," set the drive or feedback force well above the target site's absolute threshold rather than designing right at it — the threshold marks the point of "detectable on average," and individual variation plus environmental noise mean some users will have unstable detection right around it.
• For tactile cues aimed at sites other than the finger (a forearm-worn device, say), don't reuse fingertip threshold figures directly — that site's threshold can be an order of magnitude higher, and needs its own measurement or reference to site-specific data.
• How to check: measure the threshold on the target population and target site using an ascending-force forced-choice test ("felt it / didn't feel it," with catch trials to control guess rate) rather than citing the classic fingertip figure from the literature.

Related

• Same group: A4.04.2 Threshold varies with site, temperature, and skin condition · A4.04.3 Gloves or clothing significantly raise the threshold
• Nearby: A4.02 Two-point discrimination threshold and body-site differences · A4.01 Types of cutaneous mechanoreceptors
• Search terms: von Frey filament · absolute detection threshold · tactile force threshold