Somatosensory stimulation biases hand selection in motor tasks

Hand choice is an unconscious decision frequently made in daily life, whether it's reaching for an object such as a cup or performing any other task. This decision is influenced by target-related information, such as the location, shape, and orientation of the object. However, the selection probability for each hand reaches equilibrium when the target-related factors are similar for the left and right hands. Recent findings suggest that hand choice in such ambiguous situations is biased by prior information before the target presentation. One such factor is prior somatosensory stimulation on one wrist, which likely affects brain activity, enhancing the likelihood of choosing the stimulated hand. This phenomenon highlights an intriguing connection between sensory input and motor decisions.

A research team led by Dr. Kento Hirayama from Waseda University and the University of Southern California, along with Dr. Rieko Osu from Waseda University and Dr. Toru Takahashi from Waseda University and the Laureate Institute for Brain Research, have now explored how somatosensory stimulation on the wrist influences hand-choice decision. The study demonstrates that applying sensory stimuli to the median and ulnar nerves of the wrist can bias subsequent motor decisions, increasing the likelihood of using the stimulated hand in tasks that require choosing one hand as quickly as possible to reach the target. The sensation of stimulation appears to influence the brain's motor decision system, subtly guiding individuals toward selecting the stimulated hand. The study was published in the journal of Scientific Reports on 30 September 2024.

The research team conducted a series of experiments where healthy participants were asked to perform hand-choice tasks while receiving unilateral wrist somatosensory stimulation at 0, 300, or 600 ms before the target presentation. The results revealed a clear pattern where participants were significantly more likely to use the stimulated hand for targets around the center area, where the choice is difficult to determine based on target information. In the peripheral area where the target information was effective, the ipsilateral hand to the target tended to be selected regardless of the stimulation. These findings suggest that peripheral sensory input can bias motor decisions particularly on the ambiguous situation for hand selection, revealing a previously underappreciated role of somatosensation in guiding hand choice. Furthermore, the electrical stimulation of the unilateral wrist led to faster reaction time than both bilateral wrist stimulation and no-stimulation conditions, suggesting that targeted sensory inputs can facilitate decision-making process.

"This research could potentially open new therapeutic avenues to facilitate the use of the paretic hand, improving function in individuals with motor impairments, such as stroke survivors," says Hirayama. "By applying controlled somatosensory stimulation, it may be possible to bias motor decisions in ways that encourage more effective use of the affected hand, which could support recovery and rehabilitation."

In addition to its clinical implications, the study also contributes to our understanding of the fundamental processes involved in motor decision-making. By showing that the brain integrates sensory input from the body to guide action, the research adds a new layer to our knowledge of how the brain controls movement and responds to environmental cues.

As the field of neurorehabilitation continues to evolve, understanding how peripheral sensory signals can influence motor choices could lead to more effective, individualized treatments for patients with motor disabilities.

"Overall, our study lays the foundation for development of a compact, lightweight, affordable, and easily accessible rehabilitation device that can be integrated with conventional rehabilitation methods to enhance motor function recovery," concludes Hirayama.