The purpose of this study is to investigate the cerebral activation of healthy and stroke participants in 4 or 2 different conditions (repeated 2 times) of vibration-induced illusion of movement respectively, resulting in 8 or 4 vibration blocks with 3 vibrations per block. The frequency of the vibration being 80 Hz.

Healthy participants:

• Right arm, eyes opened
• Right arm, eyes closed
• Left arm, eyes opened
• Left arm, eyes closed

Stroke participants:

• Deficient side, eyes opened
• Deficient side, eyes closed

The aim is to compare the subjective sensation of movement score and cerebral activations of healthy/stroke participants depending on the condition.

Perceived sensations of limb and movements (kinesthesia) allow us to situate ourselves without visual feedback and not dedicate all of our attention to everyday tasks such as walking or taking and holding an object. These perceived conscious sensations come from receptors called proprioceptors which are located in the skin, joints and muscles. When a limb is moving and a muscle is stretched, the main sensation of limb stretching comes from the stimulation of a proprioceptor called the muscle spindle, more specifically from its primary ending called the 1a fibber. Artificially, 1a fibbers can be (optimally) stimulated by vibration of the muscle around 80 Hz. When stimulating 1a fibbers of the triceps, a signal of stretching is produced and an illusion of flexion can be felt as long as their no other contrary feedback involved such as vibration of the antagonist muscle or visual feedback of the immobile limb as it provides a more reliable information to the brain. As of now, neural correlates to muscle spindle vibration and illusion of movement have not been fully characterize in healthy participants and are not known in stroke participants.

To investigate the effect of vibration-induced illusion of movement on cerebral activations in healthy and stroke participants, the investigators propose to use an innovative tool of neuroimaging, the functional near infrared spectroscopy (fNIRS).

fNIRS allows to investigate cortical cerebral activations by measuring cerebral haemodynamic variations. Indeed, just like fMRI, it relies on the haemodynamic response function (HRF) caused by neurovascular coupling. Its portability and non-invasiveness make it a great tool to study brain activations in a more ecological environment (in a standing or sitting position for example).

This research aims to study and compare the subjective sensation of movement and the cortical cerebral areas involved in muscle spindle vibration and illusion of movement in healthy and stroke participants.

The protocol consists of 4 or 2 different conditions (repeated 2 times) of vibration-induced illusion of movement in healthy and stroke participants respectively, resulting in 8 or 4 vibration blocks with 3 vibrations per block:

Healthy participants, 2 times each (24 vibrations):

• Right arm, eyes opened (no-illusion condition; RO)
• Right arm, eyes closed (illusion condition; RC)
• Left arm, eyes opened (no-illusion condition; LO)
• Left arm, eyes closed (illusion condition; LC)

Stroke participants, 2 times each (12 vibrations):

• Deficient side, eyes opened (no-illusion condition; DO)
• Deficient side, eyes closed (illusion condition; DC)

Conditions order is pseudorandomized, participants can start by the left or the right arm with eyes opened or closed. When starting with either the left or right arm, all conditions are completed before moving on to the other arm (for healthy participants). In total, the number of participants starting with right or left vibration should be the same.

The fNIRS devices that will be used are the Brite MKII and Brite MKIII (Artinis Medical system, Netherlands).

The vibration tool that will be used is a vibrator from VibraMoov. The subjective sensation of movement will be analyzed using the Standardized Kinesthetic Illusion Procedure (SKIP) scale. Participants will have to report their sensation of movement after each condition (3 vibrations block) from 0 to 3, 0 being no sensation of movement and 3 being a clear and precise sensation of movement. Participants will also have to describe the direction of movement, a score of 1 will be attributed if the movement described correspond to the one expected and a score of 0 will be attributed for every other movements.

Inclusion Criteria:

• 18-85 years old
• Able to sign a consent form
• Sufficient command of the French language to understand instructions
• Affiliated to a social security scheme

Healthy participants:

• Right handed
• No neurological disease or motor deficit

Stroke participants:

• Stroke volunteers in acute phase (before day 14 post-stroke)
• Not aphasic
• Able to maintain sitting position without difficulty
• Deficit moteur
• Able to sign a consent form

Exclusion Criteria:

• Person under tutorship or curatorship
• Known allergy to of the fNIRS cap components: neoprene
• No vibrations feeling on the deficient side
• Vigilance-modifying drugs (high-dose psychotropics, antispastic drugs)