Reduced recruitment potential for study participants.

The most common form of dementia is Alzheimer's disease, with 900,000 people affected in France in 2015 and a forecast of 1.3 million in 2020. As a consequence of their advanced age, dementia patients often suffer from pain, mainly musculoskeletal or neuropathic pain. However, the exact prevalence of pain in dementia is underestimated. Indeed, several studies indicate that people suffering from dementia report less pain. This phenomenon is all the more true as the stage of dementia is advanced. In addition, people with dementia receive less pain medication than people without cognitive impairment in similarly painful conditions. Hetero-evaluation alone also seems insufficient, with the result that pain is under-treated compared to patients without cognitive impairment. Better pain screening is a major challenge and self-assessment tools should be favoured as a first line of treatment, even for patients with cognitive impairment. suffering from dementia.

The investigators propose in this work to evaluate the variation of vegetative parameters that accompany a painful stimulus. These variations can be recorded at the cardiac, vascular, pupillary or skin conductance level.

It will therefore be a question of inducing experimental pain in patients with Alzheimer's disease, in the early stages of the disease and to check whether pain-related changes in the autonomic nervous system via the Nociception Level index (NoL® system) differ from a control group of subjects matched in age, gender and education level. To induce pain, the investigators will use the "cold pressor test" technique, which consists of leaving the arm immersed in ice water for as long as the pain is bearable. The investigators will also record before, during and after the immersion, the variations of several parameters derived from the autonomic nervous system during the painful stimulation via the NoL® system. During this experience, the participants will be filmed in their entirety to observe the facial emotions related to pain.

Inclusion Criteria for patients with alzheimer's disease :

• Diagnosis of the disease by a hospital doctor after an etiological assessment including at least a cerebral MRI, a neuropsychological assessment and, if possible, dementia markers in the cerebrospinal fluid.
• Mini Mental State Score > 23

Inclusion Criteria for control subject :

- Matching by age (± 5 years), gender, and grade level

Exclusion Criteria for 2 groups :

• presence of pain,
• Presence of mood and/or psychiatric disorders (HAD A ou A scores >7).
• Presence of cardiovascular risk factors and beta-blocker and/or calcium channel blocker therapy.
• Patients under guardianship or curatorship