Methodological Study on the Minimum Number of Blocks in fNIRS: Brain Activity During Voluntary Movement and During Illusory Movement Induced by Vibratory Tendon Stimulation of the Right Distal Upper Limb in Healthy Right-handed Participants

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.