Bodily Self-Perception During Vestibular Galvanic Stimulation in Virtual Reality: A Replication and Extension of Karnath et al. (2019)

To navigate the world, our brains must constantly build a mental map of our own bodies. We need to know where our arms end and where our legs begin, not just to avoid bumping into things, but to understand our place in space. This internal sense of self relies on a complex team of sensors. We use our eyes to see our shape, and we use special sensors in our muscles and joints to feel our position. But there is a third, often overlooked team member: the vestibular system. Located deep inside the inner ear, this system acts as the body's built-in gyroscope, detecting head movements and orientation relative to gravity. While we know this system is vital for balance, scientists have long wondered if it also helps us decide how big our body parts feel. Does the inner ear contribute to the feeling that our arm is a certain length, or is that purely a job for our eyes and muscles?

A recent study set out to test this connection by trying to trick the inner ear and seeing if the brain would change its mind about body size. The researchers focused on a technique called galvanic vestibular stimulation, which involves placing small electrodes behind the ears to send a very mild electrical current through the skin. This current gently disturbs the signals the inner ear sends to the brain, creating a sensation of movement or tilt even when the person is perfectly still. In a previous experiment, scientists had used this method in a virtual reality setting and found that the stimulation did not change how people perceived the length of their arms, though they did notice a curious difference between the left and right sides of the body. The new study aimed to repeat that work with a larger group of people, using more powerful currents and different types of electrical signals to see if a stronger nudge to the inner ear would finally produce a change in body perception.

Twenty healthy volunteers participated in the experiment, sitting in a reclined chair while wearing a high-quality virtual reality headset. Inside the headset, they saw a digital version of themselves, an avatar that was adjusted to match their actual height and arm length. The task was simple but precise: the researchers would show the virtual arm slightly longer or shorter than the person's real arm, and the participant had to say whether it felt too long or too short. The researchers then adjusted the virtual arm until the participant felt it matched their own. This process was repeated many times under different conditions. Sometimes the participants received no electrical current at all, serving as a baseline. Other times, they received a steady, direct electrical current, or a current that flipped back and forth rapidly, at two different intensity levels. The goal was to see if the electrical stimulation would make the virtual arm feel like it needed to be longer or shorter to match their internal sense of their body.

The results were clear and consistent. No matter how strong the electrical current was, or whether it was steady or flipping back and forth, the participants did not change their perception of their arm length. The virtual arm that felt "just right" remained the same size whether the inner ear was being stimulated or not. The study also looked for a specific pattern found in the earlier research, where people tended to overestimate the length of their right arm more than their left. This time, that pattern did not appear; the left and right arms were perceived with equal accuracy. While the electrical stimulation did cause some participants to feel a mild sense of dizziness or a sensation that their head or body was moving, these physical feelings did not translate into a change in how big they thought their arms were.

The researchers concluded that, at least in this setup, the inner ear does not seem to play a direct role in determining the perceived size of our limbs. Even when the vestibular system was actively confused by strong electrical signals, the brain relied on other information, likely visual cues and the memory of how the body usually feels, to judge size. The study suggests that the influence of the inner ear on body image might be much smaller than previously thought, or that it is easily overridden by the brain's other senses in healthy people. While the experiment did not find the dramatic shift in body perception that some theories predicted, it provided a solid, detailed look at how the brain handles conflicting signals. It showed that when the inner ear sends a false message about movement, the brain does not necessarily rewrite the map of the body itself, keeping our sense of our own size steady even when our sense of motion is unsettled.