Study of the Optimal Number of Repetitions to Investigate the Cerebral Activity by fNIRS During a Voluntary Movement and an Illusory Movement Induced by Vibration

Functional near-infrared spectroscopy (fNIRS) is a non-invasive method that measures cortical hemodynamic responses by measuring variations in oxyhemoglobin (HbO) and deoxyhemoglobin (HbR). Thanks to its portability, ease of use, and low cost, fNIRS is particularly well suited to the study of neurological populations such as stroke patients.

To obtain a robust hemodynamic response, fNIRS studies use a block-design experimental paradigm, alternating between periods of task performance and rest. However, there is considerable heterogeneity in the number of blocks used in the literature, making it difficult to construct an optimal paradigm. Among the methodological issues, knowledge of the minimum number of blocks for which the cerebral response is maximal appears to be a key point.

It is generally acknowledged that increasing the number of blocks reduces intra-individual variability and improves the detection of task-specific responses. However, increasing the number of blocks increases the acquisition time, which can lead to fatigue or discomfort, particularly in elderly participants or participants with neurological disorders. A compromise must be found between methodological precision and clinical tolerance in the construction of future experimental paradigms, particularly those involving a motor task in a neurological rehabilitation context.

To date, the literature does not recommend a specific number of blocks based on the actual motor task (voluntary movement) or illusory task (induced by tendon vibration stimulation (TVS)), as both modalities are used in rehabilitation. Improving our understanding of the relationship between the number of blocks and the hemodynamic response measured by fNIRS depending on the type of task would enable us to move towards standardizing experimental paradigms.

The aim of this study is to identify the minimum number of blocks required to obtain a maximal cerebral hemodynamic response (variations in HbO and HbR), measured by fNIRS during a unilateral hand motor task and an illusory movement induced by TVS in healthy participants. Ultimately, these results will help optimize fNIRS experimental paradigms by ensuring reliable capture of task-related brain activity in a minimum of time, compatible with a pathological population.

The protocol comprises two different modalities: Voluntary movement and illusory movement of the right upper limb induced by TVS. This experiment will be conducted on young, healthy participants aged 18 to 40.

The experiment will begin and end with a one-minute rest period. Participants will be asked to open and close their right hand 20 times in 10 seconds. Similarly, they will experience 10 seconds of TVS-induced illusory movement 20 times. Each task epoch will be preceded by a 12-20 second resting period.

Both conditions will be randomized and counterbalanced. Participants can start with either the voluntary movement task or the illusory.

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

The vibration tool is a VibraMoov vibrator from Technoconcept in Mane, France.

Inclusion criteria

• Healthy volunteers aged 18 to 40
• Right-handed: Edinburgh laterality test short format QL > 60 (Veale et al., 2014).
• Sufficient command of the French language to understand instructions
• No known neurological medical history

Exclusion criteria

• Allergy to neoprene (fNIRS cap material)
• Hair volume preventing the cap from fitting and/or access to the scalp
• Injury to right upper limb causing pain during mobilization
• Protected person (under guardianship or trusteeship)
• Person under court protection
• Persons deprived of liberty
• Persons not affiliated to a social security scheme
• Pregnant or breast-feeding women
• Volunteers opposed to the study