Tricking the brain into feeling an invisible sixth finger can quickly change how people judge what their hands are physically capable of doing, according to new research published in Journal of Experimental Psychology: Human Perception and Performance. After experiencing a brief mirror-based illusion of having an extra digit beside their pinky, volunteers estimated that they could grasp wider blocks and needed larger openings to fit their hand through. The shift in opening-size judgments appeared even among participants who did not consciously feel the extra finger, suggesting that the brain may update its sense of the hand’s physical boundaries partly outside conscious awareness.
Whenever a person reaches through a narrow gap or picks up a wide glass with one hand, the brain automatically calculates how the physical dimensions of the body match up with the surrounding environment. Psychologists refer to these perceived possibilities for action as affordances. Rather than measuring spaces in inches or centimeters, people use their own bodies as a built-in ruler, and this internal gauge adapts rapidly when physical dimensions change. For instance, a 2008 study on movement decisions found that when adults wore a prosthetic that widened their hand by one centimeter, they recalibrated their judgments and selected wider openings as the smallest gap their hand could pass through.
Beyond adapting to physical prosthetics or tools, the brain can also adopt nonexistent extra body parts, known as supernumerary limbs or digits, when visual and touch signals are manipulated in unison. In a 2016 paper describing what is known as the Anne Boleyn illusion, researchers showed that sitting at a mirror and receiving synchronized strokes on both hands alongside a stroke in empty space could trick volunteers into feeling a sixth finger beside their pinky. Building on this technique, a 2025 study on extra-finger illusions indicated that people can experience this phantom sixth digit even when it is angled at biologically impossible postures up to 180 degrees away from the palm, while still feeling their real pinky in its normal spot.
Knowing that an illusory sixth finger can feel independent from the rest of the hand raised an unresolved question: does the brain treat this invisible digit as an isolated sensory quirk, or does it integrate the extra finger into the whole hand when sizing up physical actions? To test whether an illusory sixth finger alters perceived action boundaries, Denise Cadete and her colleagues designed an experiment combining the mirror-box setup with tests of hand width and grasping span. Cadete is an associate lecturer in the School of Psychological Sciences at Birkbeck, University of London, a researcher in the university’s Body Representation Lab and Vestibular×Space Neuroscience Lab, and the lead author of the 2025 posture study. The team preregistered the study’s design, hypotheses, and main analyses before collecting data.
“I first wanted to find a way of having an implicit measure for the experience of having a sixth finger, to overcome the limitations of having self-reported data,” Cadete told PsyPost. “I was interested in the illusory perception of an extra finger beyond the conscious interpretation of that phenomenon. Later, I was very intrigued by using an action perception task that would unravel whether the illusion of having an extra finger would update action representations too.”
The researchers recruited 50 healthy adults, including 32 women, with an average age of 31.4 years. All but one of the participants were right-handed, and every volunteer completed the experiment using their left hand as the target hand across both an illusion condition and a control condition in a counterbalanced order. During each session, participants sat at a table with an upright mirror aligned down the center of their body, placing their left hand out of sight behind the mirror and their right hand in front of it. Looking into the glass created the visual impression that the reflection of their right hand was actually their hidden left hand.
To induce the illusion, a researcher stroked the matching fingers of both hands at the exact same time, brushing back and forth twice from the thumbs to the little fingers. After touching the inner side of the hidden left pinky alongside the top of the visible right pinky, the researcher delivered 20 synchronized strokes to the outer edge of the hidden left pinky while simultaneously stroking the empty table space just outside the reflected right pinky. Because participants saw the stroke land on empty space at the moment they felt touch on the side of their hidden hand, many experienced the sensation of a sixth finger. In the control condition, the researcher followed the same one-minute routine, except the final 20 strokes were applied directly onto both pinky fingers rather than empty air.
Immediately after the one-minute stimulation, the researcher covered the participant’s hands with a box to block visual inspection, and participants kept their hands still so that muscle and joint feedback, known as proprioception, would not disrupt the illusion. Across eight trials per condition, participants first judged aperture passing by watching a wooden frame fitted with black plastic triangles that opened and closed like a camera shutter (from 2.7 to 18 centimeters), calling out when the opening matched the smallest gap their closed left hand could pass through without touching the edges.
Across another eight trials per condition, they judged grasping ability by looking at 16 square foam-board blocks (ranging from 4 to 25 centimeters wide in 1.4-centimeter steps) and stating whether they could stretch their left thumb and fingers across opposite edges to hold each block. Brief top-up strokes were delivered after every two estimates, and at the end of each block, participants rated their conscious sensations on a 0-to-10 scale. At the end of the session, participants physically placed their left hand inside the shutter and grasped the foam blocks so the researchers could calculate an accuracy ratio dividing each person’s estimated size by their actual physical limit, where 1.00 is an exact match, above 1.00 is an overestimation, and below 1.00 is an underestimation.
Questionnaire ratings confirmed that the mirror procedure worked as intended: on the 0-to-10 scale, average scores for feeling six fingers rose from 1.13 in the control condition to 4.36 in the illusion condition. Participants also gave higher ratings during the illusion for feeling two little fingers (3.96 versus 1.24), touch in an unusual place (4.92 versus 1.86), touch off the body (3.55 versus 0.93), and a larger little finger (3.06 versus 1.80). By contrast, a control prompt about feeling an entire extra hand showed no statistically significant difference between the illusion (1.35) and control (1.10) conditions. Ratings across the full group averaged below the scale’s midpoint, in part because 22 of the 50 participants reported little or no sixth finger.
“The illusion is not imaginary, is not a weak sensation or a delusion; it is a vivid sensation of having a sixth finger, created by a trick using visual-tactile stimulation that will map a real touch onto empty space next to the hand,” Cadete explained.
In the action-judgment tasks, experiencing the sixth-finger illusion shifted how participants sized up both their hand width and their grip reach. In the aperture task, the average ratio of estimated-to-actual opening size increased from 0.92 in the control condition to 0.97 after the sixth-finger stimulation, reducing the proportion of participants who underestimated their needed space from 72 percent (36 of 50 people) in the control session to 58 percent (29 of 50 people) during the illusion.
Similarly, though more modestly, in the grasping task, the average ratio of estimated-to-actual grasp width rose from 0.98 in the control condition to 1.01 in the illusion condition, indicating that participants believed they could span wider blocks after the illusion. Participants who showed a larger increase between the control and illusion sessions on the aperture task also tended to show a larger increase on the grasping task.
“These findings point to a flexibility of the mind, not only in perceiving extra body parts that they never had, but also in integrating them immediately into their action perception,” Cadete said. “People change the perception of the action abilities of their hand when experiencing an invisible and illusory sixth finger, as if their hand had indeed six fingers to move and grasp objects.”
Unexpectedly, statistical analyses showed no significant correlation between how strongly participants reported feeling a sixth finger on the questionnaire and how much they adjusted their aperture or grasping estimates. In an exploratory analysis that was not preregistered, the researchers split the sample into 28 participants who consciously felt a sixth finger (scoring between 4 and 10) and 22 participants who did not (scoring between 0 and 2), finding that both subgroups still selected wider openings in the aperture task after the sixth-finger stimulation.
The average aperture ratio climbed from 0.90 to 0.97 among those who consciously felt the extra digit and rose from 0.95 to 0.98 among those who did not, though the shift was roughly twice as large for those who felt the illusion (an average change of 0.07 versus 0.03). In the grasping task, the within-group shifts from 0.97 to 1.00 among those who felt the finger and from 1.00 to 1.01 among those who did not were not statistically significant, possibly because splitting the sample into smaller subgroups reduced statistical power.
The aperture result among participants who did not feel the extra finger surprised Cadete. “This shows that the illusion is updating the perception of action abilities of the hand, regardless of its conscious experience of it,” she noted.
The authors interpret this disconnect between conscious reports and spatial judgments as a sign that the illusion works on two partly distinct levels. In their view, synchronized touch and visual cues beside the hand may update the body schema, the brain’s unconscious working map of body size and posture used to plan actions, even when a person does not consciously experience a sixth finger.
As with all research, there are some caveats to consider when interpreting these results. Participants estimated their physical limits by looking at the apparatus and imagining the actions while their hands stayed motionless under a cover, rather than physically reaching out during the illusion. Because imagining a movement and physically executing it rely on partly separate neural pathways, actually moving the hand might produce different results than imagined estimates. In addition, whereas people in prior aperture studies typically overestimated the space their hidden hand needed, participants here underestimated their required opening in both conditions (ratios below 1.00), which the authors suggest may stem from participants watching their hand in the mirror during the stroking routine.
Because spatial judgments shifted even among volunteers who reported little or no sensation of an extra digit, the changes in action perception may reflect a broader effect of mapping touch onto nearby empty space rather than the specific mental image of a sixth finger. Future work will need to examine how this sensory remapping affects real-time physical movement and whether the findings generalize beyond healthy adults in laboratory settings. Even so, the results suggest that passive touch and vision alone can begin reshaping perceived hand capabilities in just 60 seconds, without requiring active movement or mechanical control.
“These findings can help design supernumerary robotics and prosthetics, in a way that updates their body perception but also their action perception,” Cadete added. “At the same time, this shows the flexibility of our mind in the way our bodies are perceived, allowing for additions and including them into a new and extended representation of the acting body.”
“I wish to understand the mechanisms for representing extra body parts and why they seem to have different constraints from real body parts,” she continued. “Ultimately, I want to explore why would humans have this ability, and extend this research into supernumerary robotics, so that they can be better integrated into one’s body representation, overcoming motor limitations due to injury or neurological disorders.”
The study, “Beyond Five: How the Experience of a Supernumerary Sixth Finger Alters Action Perception,” was authored by Denise Cadete, Harriet Morris, Megan Rose Readman, Sally A. Linkenauger, Elisa R. Ferrè, and Matthew R. Longo.
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