Chronic pain (CP) is disabling for people triggering important costs for society. A deficit of diffuse noxious inhibitory controls (DNIC) is one of the CP mechanisms. DNICs are evaluated in research setting using a CPM protocol (conditioned pain modulation). There is a lack of reference values on the effectiveness of DNICs. Wider research on DNIC will help to understand CP and to develop a clinical screening test evaluating DNICs.

This study aims:

1. To establish baseline values of DNICs using CPM protocol
2. To identify the variables that will be integrated in the algorithm of the clinical screening test (clinical decision rule).

First the target population will be healthy volunteers, male and female, stratified by age. The reference values will be established via a non-parametric method for a standard CPM protocol in which two different pain stimuli are applied. Two "stimuli tests" of the same intensity and nature (heat) will be applied before and after the application of another "conditioning stimulus" (cold water bath). The perceived pain difference between the 1st and 2nd stimuli tests will reflect the intensity of the DNICs.

Secondly, these results will be compared to those from volunteers suffering of chronic pain. The clinical decision rule will result from clinical and paraclinical elements correlating with the amplitude of the efficacy of CPM (serum noradrenaline, intensity of pain, heart rate and blood pressure measurements, psychometric questionnaires assessing anxiety, depressive feelings and pain catastrophizing). Logistic regression analysis will determine the best predictors of a CPM deficit.

Inclusion Criteria:

• 18-79 years old
• Able to provide consent

Exclusion Criteria:

• cardiovascular disease (arrhythmia, cerebrovascular accident, infarction...)
• Raynaud syndrome
• severe psychiatric disease (dementia, schizophrenia, psychosis, major depression)
• injuries or loss sensitivity to their forearms or hands
• pregnant women or in post-partum period (<1 year)

Healthy male and female volunteers (with no pain) will provide reference values to further study volunteers with chronic pain.

People with chronic pain will constitute a second group to associate the conditioned pain modulation to their pain (chronic pain is defined by any regular pain for more than 6 months except pain caused by cancer or migraine)