Von Frey hairs (named after the German physiologist Max von Frey, 1852–1932) have been originally produced from animal and human hairs of different diameters. Nowadays they are nylon monofilaments of different diameters, each of them mounted at right angles to the end of a plastic handle. The diameter determines the resistance of the monofilament to bending and thereby determines the amount of force applied by each filament. A filament is placed perpendicularly to the skin with slowly increasing force until it bends. It stays bent for 1–2 s and is then removed with slowly decreasing strength. Each of the filaments exerts a stimulus of a distinct force which is often expressed in grams (not correctly, since in physics “gram” is a mass unit), or more correctly in milli-Newton (in physics a measure of force). Using a series of von Frey hairs with ascending stiffness can be employed, for example, to determine sensory or neuronal thresholds.

 Cognitive-Behavior...