Perceptual Distortions in Acute Neck Pain

In the saline injection condition, acute neck pain will be induced via 0.5 ml hypertonic (5% NaCl) saline bolus injection which is a commonly used model of acute pain. Saline will be injected into the right (or left) trapezius muscle at the level of C7 cervical level. Injections will be performed by a physician under ultrasound imaging guidance to ensure that each single injection will be equally placed at the same depth. The side for the pain induction will be randomised across subjects. The place of needle insertion and corresponding point on the opposite side of the body will be covered by placing a small piece of adhesive plaster to ensure blinding.

In the sham-injection condition (nocebo comparator), a real needle will be shown to the participants to imitate and produce the anticipation of a pain experience. A pinprick sensation will be produced by a weighted stimulus applied perpendicularly to the skin without piercing the skin. A stimulus of 512 millinewtons (mN) will be used to produce a pinprick sensation and activation of cutaneous nociceptors. Adhesive tapes covering stimulation points will also be provided. The stimuli will be applied in exactly the same spot as the real injection in previous experimental condition.