Illustrations by Thomas Fuchs March 2022, ScientificAmerican.com 15NEUROSCIENCEPhantom Finger
An unusual illusion points to philosophy and robotics
Brains contain maps of the bodies they
inhabit, with neurons dedicated to the per-
ception or control of particular parts—and
research suggests there may be quite a bit of
wiggle room in that representation. In 2016
scientists set up an experiment that made
subjects fleetingly feel like they had a sixth
finger on one hand (one subject yelled,
“Witchcraft!”), and in 2020 another research
group extended that sensation indefinitely.
The second group went further in its latest
work, published in Cognition, to make partici-
pants feel as if they had a sixth finger—and to
control the invisible digit’s perceived length.
To experience the mental illusion, the
participants placed their hands on a table
with a vertical mirror between them, posi-
tioned to show the reflection of the right
hand where the left hand should be. Starting
with the thumb, experimenters stroked the
top of each finger up and down twice, strok-ing the right and left hand
simultaneously. When they got
to the pinky, the experimenters stroked
the top of the right pinky and the inner side
of the left pinky. Finally, they made 20 double
strokes on the table next to the right pinky
while stroking the outer side of the left pinky,
creating a self-reported feeling of an invisible
sixth finger on the left hand.
“It’s quite honestly scary,” says Denise
Cadete, a neuroscience graduate student at
Birkbeck, University of London, and lead
author on the new paper. “Even if we under-
stand all that’s happening, the illusion
doesn’t go away—it’s a very striking feeling.”
In her group’s latest study, the strokes on
the table were sometimes half or double the
length of a typical pinkie. Twenty right-
handed participants then used a mechanical
slider to indicate how long the new finger
felt—reporting it felt on average about 1.centimeters shorter or
about three centimeters
longer than their real-life
pinky, respectively. These
differences suggest that
the extra finger was per-
ceived not merely as a duplicate
pinky but as its own entity.
Cadete says that beyond its intriguing
philosophical implications about humans’
sense of self, this research could also be
useful for people with robotic limbs. She
says a mechanical appendage might trans-
fer sensation to a nearby body part via
these brain illusions, even for limbs with
complex, “Swiss Army–like” features.
The experiment is well done, says Eti-
enne Burdet, a roboticist at Imperial College
London who has studied the one-handed
capabilities of people born with six fingers
and was not involved in the research. Future
studies could explore whether the phenom-
enon applies beyond an extra pinky; if he
wanted to start a company to make robotic
body parts, Burdet says, “I would start with
an arm.” — Matthew Hutson
© 2022 Scientific AmericanNEUROSCIENCEPhantom Finger
An unusual illusion points to philosophy and robotics
Brains contain maps of the bodies they
inhabit, with neurons dedicated to the per-
ception or control of particular parts—and
research suggests there may be quite a bit of
wiggle room in that representation. In 2016
scientists set up an experiment that made
subjects fl eetingly feel like they had a sixth
fi nger on one hand (one subject yelled,
“Witchcraft!”), and in 2020 another research
group extended that sensation indefi nitely.
The second group went further in its latest
work, published in Cognition, to make partici-
pants feel as if they had a sixth fi nger—and to
control the invisible digit’s perceived length.
To experience the mental illusion, the
participants placed their hands on a table
with a vertical mirror between them, posi-
tioned to show the refl ection of the right
hand where the left hand should be. Starting
with the thumb, experimenters stroked the
top of each fi nger up and down twice, strok-ing the right and left hand
simultaneously. When they got
to the pinky, the experimenters stroked
the top of the right pinky and the inner side
of the left pinky. Finally, they made 20 double
strokes on the table next to the right pinky
while stroking the outer side of the left pinky,
creating a self-reported feeling of an invisible
sixth fi nger on the left hand.
“It’s quite honestly scary,” says Denise
Cadete, a neuroscience graduate student at
Birkbeck, University of London, and lead
author on the new paper. “Even if we under-
stand all that’s happening, the illusion
doesn’t go away—it’s a very striking feeling.”
In her group’s latest study, the strokes on
the table were sometimes half or double the
length of a typical pinkie. Twenty right-
handed participants then used a mechanical
slider to indicate how long the new fi nger
felt—reporting it felt on average about 1.centimeters shorter or
about three centimeters
longer than their real-life
pinky, respectively. These
diff erences suggest that
the extra fi nger was per-
ceived not merely as a duplicate
pinky but as its own entity.
Cadete says that beyond its intriguing
philosophical implications about humans’
sense of self, this research could also be
useful for people with robotic limbs. She
says a mechanical appendage might trans-
fer sensation to a nearby body part via
these brain illusions, even for limbs with
complex, “Swiss Army–like” features.
The experiment is well done, says Eti-
enne Burdet, a roboticist at Imperial College
London who has studied the one-handed
capabilities of people born with six fi ngers
and was not involved in the research. Future
studies could explore whether the phenom-
enon applies beyond an extra pinky; if he
wanted to start a company to make robotic
body parts, Burdet says, “I would start with
an arm.” — Matthew Hutsonsad0322Adva3p.indd 15sad0322Adva3p.indd 15 1/20/22 4:39 PM1/20/22 4:39 PM
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