What happens when you rest a chopped ping pong ball on your finger and look at it from above? Experimental psychologists from KU Leuven, Belgium, have shown that our visual system fills in the bottom part of the ball, even though we know it’s missing. This makes our finger feel unusually short, as if to compensate for the ‘complete’ ball. The findings indicate that the completion is due to our visual system, not our imagination.
Getting people to pick a particular card, diverting attention from sleight-of-hand moves, creating illusions: magic tricks reveal a lot about how our minds work. That is why the science of magic holds great attraction for psychologists studying our perception and beliefs.
Experimental psychologist Vebjørn Ekroll from KU Leuven is one of them. In 2013, he used magic tricks to show that our eyes fill in the back of a round shape, even if it’s missing. This perceptual illusion underlies the classic ‘multiplying balls’ routine . Using semi-spherical shells, the magician makes his audience believe that he can make several little balls magically appear between his fingers. In reality, the audience sees shells but visually completes them into complete balls.
In their latest study, Ekroll and his colleagues show that this illusion persists when observers are allowed to touch the inside of the shell with their fingertip. What is more, touching the shell produces a second illusion: observers report that their finger somehow feels shorter, as if to compensate for the illusory bottom volume of the ball.
Magic Multiplying Balls (White)
The ‘shrunken finger illusion’ provides strong evidence for a counterintuitive idea about how our brain works. “We already know that our mind completes what we don’t see”, Ekroll explains. “Our study shows that our visual system is behind the illusion, not our conscious mind. Rationally, we know that our finger is not actually shorter, but the illusion persists nonetheless.”
About this cisual illustions research
Source: Vebjørn Ekroll – KU Leuven Image Source: The image is credited to Current Biology. Video Source: The video is credited to Vanishing Inc. Magic. Original Research:Abstract for “Illusory Visual Completion of an Object’s Invisible Backside Can Make Your Finger Feel Shorter” by Vebjørn Ekroll, Bilge Sayim, Ruth Van der Hallen, and Johan Wagemans in Current Biology. Published online March 31 2016 doi:10.1016/j.cub.2016.02.001
Illusory Visual Completion of an Object’s Invisible Backside Can Make Your Finger Feel Shorter
Highlights •The experience of the hidden backsides of things acts as a real percept •These percepts have causal powers, although they do not correspond to real objects •They can evoke a bizarre illusion in which the observer’s own finger feels shrunken •The perceptual representation of body shape is highly malleable
Summary In a well-known magic trick known as multiplying balls, conjurers fool their audience with the use of a semi-spherical shell, which the audience perceives as a complete ball. Here, we report that this illusion persists even when observers touch the inside of the shell with their own finger. Even more intriguingly, this also produces an illusion of bodily self-awareness in which the finger feels shorter, as if to make space for the purely illusory volume of the visually completed ball. This observation provides strong evidence for the controversial and counterintuitive idea that our experience of the hidden backsides of objects is shaped by genuine perceptual representations rather than mere cognitive guesswork or imagery.
“Illusory Visual Completion of an Object’s Invisible Backside Can Make Your Finger Feel Shorter” by Vebjørn Ekroll, Bilge Sayim, Ruth Van der Hallen, and Johan Wagemans in Current Biology. Published online March 31 2016 doi:10.1016/j.cub.2016.02.001