Stretching the skin can alter how we perceive our fingers

When moving around in their surroundings, humans heavily rely on what is known as proprioception, sometimes referred to as the “sixth sense.” This is the body’s subconscious ability to sense its own position, movements and location in space, via many tiny receptors that respond to mechanical strain located in the muscles, tendons and skin.

Researchers at University of Pisa, the Italian Institute of Technology (IIT), and University of Rome Tor Vergata recently carried out a study investigating how artificially stretching the skin while people are moving their fingers influences proprioception. Their findings, published in the Journal of the Royal Society Interface, suggest that precisely deforming the skin while people are flexing their fingers alters how they perceive their hand and finger postures.

“According to common belief, humans have only five senses: vision, hearing, taste, smell, and touch; yet physiology tells us there are more,” Eleonora Fontana, first author of the paper, told Medical Xpress. “An essential, often overlooked ‘sixth sense’ is proprioception—our body’s ability to perceive limb position and movement in space. While much of this information is processed subconsciously, it is central to guiding our everyday movements; in fact, individuals with proprioceptive deficits suffer from severe movement impairments.”

Exploring how skin sensations affect body awareness

Past psychology studies have consistently documented sensory illusions, instances in which people’s senses can be deceived. Fontana and her colleagues set out to investigate the possibility that artificially deforming the skin under specific conditions could alter how people perceive their own body.

“Using a custom wearable haptic interface developed by our research group (called TWIST—Tactile Wearable Interface for Skin sTretch), we were able to artificially augment natural skin deformation over the finger joint,” Fontana explained. “This systematically altered the subjects’ active perception, making them feel as though their fingers were more flexed than they actually were while grasping an object.”

This study built on earlier research focusing on proprioception. Its main goal was to explore the extent to which the skin’s deformation contributes to how people perceive their fingers’ position as they are moving them.

“Previous studies showed that skin stretching can influence proprioceptive perception, but most of them relied on passive stimulation or experimental conditions that did not reflect how we normally move and sense our bodies,” said Fontana. “We wanted to explore how skin-based signals interact with the proprioceptive information arising from muscles and joints during everyday movement, when all sensory and motor pathways are naturally engaged.”

A wearable device that amplifies skin stretching

For the purpose of their study, the researchers designed a wearable, noninvasive tactile device that can stretch the skin around the middle joint of the index finger. Using this device, they selectively increased how much the skin of participants stretched while they were completing a task that required them to move their fingers.

“We first needed a way to precisely augment the skin deformation that naturally occurs around a finger joint during movement, so we designed a wearable device capable of applying controlled skin stretch while participants freely moved their fingers,” Fontana explained. “To measure the perceptual effects of this stimulation, we used a hand-matching task, a commonly adopted method in proprioception research.”

During each experimental trial, participants were asked to reproduce the posture they thought one hand was in using their other hand. Meanwhile, the researchers used their device to systematically change how much their skin stretched. They then compared the participants’ responses under different stretch conditions to determine whether different levels of deformation affected people’s perceived finger position.

“This approach allowed us to study the contribution of skin deformation under conditions that more closely resemble natural voluntary movement than many previous experiments,” said Fontana. “We found that selectively augmenting the skin deformation associated with finger movement can systematically bias the perception of finger posture during natural voluntary motion.”

The idea that skin deformations play a role in how people perceive their own posture and movements was already introduced in earlier studies. However, this is among the first studies to confirm this hypothesis under realistic movement conditions.

“We found that increasing skin stretch led participants to behave as if their finger were more flexed than it actually was,” Fontana explained. “As a result, they unconsciously compensated for this altered perception by keeping their finger slightly more extended, providing direct evidence that skin deformation is actively incorporated by the nervous system when estimating body posture.”

Fueling research and the development of new technologies

The results of this study offer new insight into how the human nervous system combines tactile and proprioceptive information to construct an overall representation of the body at any given moment. In the future, they could potentially inspire the development of new wearable technologies designed to enhance people’s proprioception or compensate for specific sensory impairments.

“For example, similar approaches could be used to help stroke patients during rehabilitation, provide more intuitive sensory feedback to users of robotic prostheses, or enhance immersion in virtual reality by making finger movements feel more natural,” said Fontana. “Our findings could also be applied in teleoperation and human-robot interaction, where conveying information about limb posture is often as important as conveying touch or force.”

As part of their next studies, Fontana and her colleagues plan to explore the interactions between artificial skin stretching and other sensory perceptions, for instance during tasks that require people to grasp and manipulate objects. In addition, they would like to refine the wearable device they developed, as well as other technologies introduced by their lab, to make them more compact and versatile. This could facilitate their use in real-world settings while also making them more comfortable for wearers.

“We also plan to expand our understanding of skin-based contributions to proprioception beyond a single finger joint and toward more complex, multi-joint hand configurations and movements,” said Fontana. “Building on previous work from our group showing that skin strain patterns across different finger joints carry highly informative signals about overall hand posture, we are particularly interested in developing a next-generation prototype that can be worn across multiple finger joints. This would allow us to systematically investigate how distributed patterns of skin deformation contribute to the perception of full-hand configuration, rather than isolated joint angles.”

Who’s behind this story?

Ingrid Fadelli

Freelance journalist with BSc Psychology and MA International Journalism. Covers AI, robotics, neuroscience, and astrophysics since 2018.

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Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

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