A mirror trick that adds a sixth finger also changes what people think their hand can grab
With a mirror and some well-timed brushstrokes, researchers gave volunteers the sensation of a sixth finger. Afterward, people judged that their hand needed a wider opening to fit through and could grasp larger blocks.
New research published in the Journal of Experimental Psychology reveals that a brief mirror trick that creates the illusion of an additional finger can significantly alter an individual's perception of their hand's capabilities. After experiencing this illusion, participants estimated that they could grasp wider objects and required larger openings to fit their hand through.
This shift in judgment occurred even among those who did not consciously feel the extra digit, indicating that the brain may update its understanding of the hand's physical boundaries beyond conscious awareness.
The study, conducted by Denise Cadete and her colleagues at Birkbeck, University of London, involved 50 healthy adults who were divided into two conditions – one with the illusion and one as a control. During the illusion condition, participants were shown a mirror setup and experienced synchronized strokes on their hidden left pinky finger and the empty space outside their reflected right pinky finger.
After the one-minute stimulation, they judged aperture passing by estimating the smallest gap their left hand could pass through without touching the edges. They also judged grasping ability by assessing whether they could stretch their left thumb and fingers across opposite edges of various blocks.
These findings suggest that the brain can integrate the perception of an illusory finger into the overall perception of the hand, affecting how people adapt to physical actions and environmental interactions.
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