The brain can sometimes trick itself into feeling like it has an extra appendage or that the body extends farther into space than it actually does. This can happen when signals in the brain get crossed due to a mix of visual and tactile cues in the environment. This phenomenon has been the subject of several studies. Now, a study published in the Journal of Experimental Psychology: Human Perception and Performance goes a step beyond making the brain think it has an extra part to testing if this also reshapes the perception of the body’s capabilities.
The sixth finger
Earlier experiments showed that changing a hand’s apparent size or adding a physical extension can alter perceived action limits. Some experiments demonstrated that a person watching one hand get touched through a mirror can create the illusion of an extra finger with its own size, shape or orientation in the other hand. One experiment even caused people to feel as if a sixth finger stuck out of their hand 180 degrees away from the palm.
The team involved in the new study built upon this sixth-finger illusion in another study where they modified the way the hand was touched. In their experiment, the participant watches the reflection of their right hand in a mirror while their left hand is hidden on the other side of the mirror.
The experimenter then strokes the fingers of both hands back and forth at the same time. In the last stroke, the outer edge of the hidden pinky finger is stroked at the same time as empty space next to the right hand, which the participant sees reflected in the mirror. This induces an illusion of having another finger there.
The authors of the new study write, “Supernumerary fingers can be embodied with shapes, sizes and orientations independent of the actual fingers. Yet it remains unclear whether such experiences alter the perception of the hand as a whole, particularly its overall size, which would require the illusory finger to be integrated into the body representation. It is also unknown whether feeling a sixth finger modifies our perception of action abilities and boundaries.”
Estimating the abilities of an illusion
To test what participants felt their altered hand with an illusory finger was capable of, the team had them estimate the smallest opening their hand could pass through and the largest object they could grasp after inducing the sixth-finger illusion.
The study included 50 healthy adults who underwent both an illusion condition and a matched control condition. All but one participant was right-handed.
After participants underwent the illusion, the team presented them with a series of apertures with openings ranging from 2.7 to 18 cm (1.1 to 7.1 inches) and asked at which point they thought their hand would fit while the hand was hidden to prevent them from visually estimating the fit. They were also asked how large a square foam board they thought they could grasp.
The boards ranged from 4 to 25 cm (1.6 to 9.8 inches) and increased in increments of 1.4 cm (0.6 inches). Actual fits and grasps were tested after the participants made their estimates.
On average, after the sixth-finger illusion, participants judged that their hand needed a larger opening to pass through than they did after the control procedure. They also felt they could grasp a bigger block when the sixth-finger illusion was induced, although the effect was smaller.
The participants also completed a questionnaire about whether they perceived another finger, a larger little finger or even an extra hand during the experiments, and even those who claimed they did not feel a sixth finger felt that they needed a bigger aperture to accommodate their hand. The researchers say this suggests that conscious experience and action judgments can diverge.
Updated perceptions
After the illusion is performed on a participant, the team posits that their brain almost automatically remaps its boundaries. Some research has suggested that the brain does this when a person is working with a tool, such as a blind person with a cane. However, this study indicates that the brain creates new boundaries even without any real feedback. And while the participants did not demonstrate any real-life improvements in hand performance, their perceptions were at least temporarily altered.
The study authors write, “This study shows that people can instantly update their sense of what their hands can do when experiencing the illusion of having a sixth finger.
“Even without real movement, the brain incorporates an illusory and invisible extra digit into action judgments, altering the perceived abilities of what the hand can grasp or pass through. This flexibility has direct implications for the design of prosthetics and augmentation robotics and broadens our understanding in cognitive science and psychology of how perceiving extra body parts alters action perception.”
Written for you by our author Krystal Kasal, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
If this reporting matters to you, please consider a donation (especially monthly). You’ll get an ad-free account as a thank-you.
More information
Denise Cadete et al., Beyond five: How the experience of a supernumerary sixth finger alters action perception., Journal of Experimental Psychology: Human Perception and Performance (2026). DOI: 10.1037/xhp0001446
Clinical categories
© 2026 Science X Network
Citation:
Study participants tricked into feeling an extra finger experience altered perceptions of their hand’s abilities (2026, October 30)
retrieved 30 October 2026
from https://medicalxpress.com/news/2026-10-extra-finger-perceptions-abilities.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.
