Also known as a "moving itch", knismesis is a mildly annoying sensation caused by a light movement on the skin, such as from a crawling insect. Its underlying mechanism rely on spatial summation, i.e. amplification of the signal due to summation of stimulated fields, like it is for dynamic allodynia in neuropathic pain. This physiological study aims at determining the physical factors of the cutaneous stimulation inducing knismesis as well as the effect of confounding factors. The main area of exploration will be the back, because of its size and the possibility to blind the participant. The information collected, along with animal data in surrogate models, would help to better understand neuropathic pain in the future.

This is a physiological proof-of-concept, also aiming to validate a functional exploration model in healthy volunteers. Each participant will undergo a battery if psychometric then psychophysics tests, presented sequentially. However, most of the main study tests (knismesis) will be admistered in a random order (crossover). The primary endpoint is to characterise, in the healthy human, the physical determinants of the cutaneous stimulation inducing knismesis: length of the pathway, speed, direction, intensity, continuity break of the contact, hairiness, body site, or distraction by other cutaneous stimuli. The secondary endpoint is to study the relationship between the individual sensitivity to spatial summation (knismesis) and other individual sensitivities or profiles.

Inclusion Criteria:

- Ability to answer French questionnaires.

· BMI between 19 and 30 kg/m2 at inclusion.

Exclusion Criteria:

• Pregnancy or breastfeeding.
• Natural intolerance to tickling.
• Any cutaneous, ocular or neurological disease contraindicating the psychophysical tests.
• Any disease or medical history that could expose the subject to unacceptable risk during the study.
• Any medication intake within 7 days before testing.
• History of drug or alcohol abuse.
• Involvement in another concomitant research protocol.
• Major dorsal hairiness.
• Inability to undergo the tests.
• Legal protection.
• Concomitant involvement in another clinical research.
• Refusal to participate.