We always know the position of our hands and fingers, even without looking at them or with our eyes closed. This ability is known as proprioception: the subconscious sense that allows us to perceive the position and movement of our body in space, often described as our “sixth sense”, to which receptors in muscles and tendons make a fundamental contribution.
A new study, published in the Journal of the Royal Society Interface, now demonstrates the important role played by the stretching of the skin across joints during active voluntary movement in our perception of limb position. Previous studies had investigated this phenomenon under non-physiological conditions, such as anaesthesia or passive stimulation. Researchers from the Centro Piaggio and the Department of Information Engineering at the University of Pisa, the Italian Institute of Technology (IIT) and the University of Rome Tor Vergata have now addressed the issue under conditions much closer to real voluntary movement. To do so, the team developed TWIST (Tactile Wearable Interface for Skin sTretch), a non-invasive wearable device that amplifies the natural stretching of the skin around the proximal interphalangeal (PIP) joint, the middle joint of the index finger.
During the experiment, participants were asked to reproduce with one hand the position they believed the other hand was in, first without the device and then while wearing it.
“Increasing skin stretch caused participants to behave as though their finger was more flexed than it actually was,” explains Eleonora Fontana, engineer at the University of Pisa and one of the study’s authors. “As a result, they compensated for this altered perception by keeping the finger slightly more extended. This provides direct evidence that the nervous system actively incorporates skin deformation when estimating body posture.”
A further methodological detail strengthens the findings: no significant difference was observed between the bare-hand condition and the condition in which the device was worn but switched off. The effect therefore results from the amplified skin stretch rather than from simply wearing the device.
“The main implications of our research concern wearable technologies, which are at the heart of the PERCEIVING project funded by the Italian Science Fund (FIS),” continues Matteo Bianchi, Professor of Engineering at the University of Pisa. “Similar approaches could one day support rehabilitation after stroke, provide more intuitive sensory feedback for users of robotic prostheses, or make interaction in virtual reality and teleoperation more natural, where conveying information about limb posture is just as important as conveying touch or force.”
Among the group’s future developments is a wearable prototype covering multiple joints, enabling researchers to investigate how distributed patterns of skin deformation contribute to the perception of the hand’s overall configuration.
“For several years we have been studying how our skin — in particular the receptors responsible for our sense of touch — contributes to the control of body movement,” says Alessandro Moscatelli, Associate Professor of Physiology at the Department of Systems Medicine of the University of Rome Tor Vergata. “Every time we open and close our fingers to grasp an object, the skin stretches and deforms in a characteristic way. Our brain uses this information to control fine hand movements. By exploiting this principle, we can deform the skin to create perceptual illusions of movement. Just as a magnifying glass makes objects appear larger than they really are, the TWIST device tricks our proprioception, making finger movements appear greater than they are.”
“Understanding the physical and neural principles that govern proprioception is not only a scientific objective,” concludes Professor Antonio Bicchi of the University of Pisa. “It is also an enabling condition for the next generation of machines capable of perceiving and controlling their own bodies in the world. From wearable devices to humanoid robots, any system that aspires to move and interact with the efficiency of the biological body requires engineering solutions grounded in this knowledge.”
