Sleight-of-hand magic trick only fools monkeys with opposable thumbs


Date:
April 4, 2023
Source:
University of Cambridge
Summary:
An illusion involving a hidden thumb confounds capuchin and squirrel
monkeys for the same reason it does humans -- it misdirects expected
outcomes of actions they can carry out. However, marmosets have
five equidistant digits, and were rarely fooled by the magician. The
research adds to evidence that animals struggle to predict movements
outside of their own 'biomechanical ability'.


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FULL STORY ==========================================================================
By performing a famous magic trick for three species of monkey with
differing hand structures, scientists have discovered that -- in order
to deceive -- a conjuror needs a similar anatomy to their audience.


========================================================================== Psychologists used a sleight-of-hand trick called the French drop, in
which an object appears to vanish when a spectator assumes it is taken
from one hand by the hidden thumb of the other hand.

The study, carried out at the University of Cambridge's Comparative
Cognition Lab, found that monkeys lacking opposable thumbs did not fall
for the assumption -- staying wise to the whereabouts of tasty treats
a magician tried to make disappear.

The research suggest that sharing a biomechanical ability may be necessary
for accurately anticipating the movements of those same limbs in other individuals.

This is true even when those apparently accurate predictions end in befuddlement at the hands of an illusionist. The study is published
today in the journal Current Biology.

"Magicians use intricate techniques to mislead the observer into
experiencing the impossible. It is a great way to study blind spots
in attention and perception," said Dr Elias Garcia-Pelegrin, who has
practiced magic for a decade, and conducted the experimental work during
his PhD at Cambridge.

"By investigating how species of primates experience magic, we can
understand more about the evolutionary roots of cognitive shortcomings
that leave us exposed to the cunning of magicians." "In this case,
whether having the manual capability to produce an action, such as
holding an item between finger and thumb, is necessary for predicting
the effects of that action in others," said Garcia-Pelegrin, recently
appointed an assistant professor at the National University of Singapore.

The French drop is often the first trick any budding magician sets out
to master.

A coin is displayed in one hand. The other hand reaches over and grabs
it. The palm of the second hand faces inwards, with the magician's thumb concealed behind fingers.

The audience knows the thumb is lurking -- ready to grip -- so assumes
the coin has been taken when it is no longer visible. Their attention
follows the second hand, only to find it empty at "the reveal." The
magician had secretly dropped the coin into the palm of the original hand.

Food morsels replaced coins for the monkeys, and were given as rewards --
but only if the animals guessed the correct hand. Scientists predicted
that monkeys with opposable thumbs would act like human audiences:
assume the hidden thumb had grabbed the item, and choose the wrong hand.

They repeatedly performed the French drop on 24 monkeys. Eight capuchins
were dazzled with peanuts, eight squirrel monkeys with dried mealworms,
and eight marmosets with marshmallows.

Capuchins are famed for dexterity, and use stone tools to crack nuts in
the wild. They can waggle each finger, and have opposable thumbs allowing "precision grip" between thumb and forefingers.

The capuchins were regularly fooled by the French drop (81% of the
time). They mostly chose the empty second hand, and experienced a paucity
of peanuts as a result.

Squirrel monkeys are much less dextrous than capuchins, with limited
thumb rotation, but can oppose their thumbs. As such, they are still
familiar with a hidden thumb interacting with fingers. However, they
cannot perform a 'precision grip' in the same way as capuchins and humans.

Yet squirrel monkeys were routinely misled by mealworms that seemingly
vanished (fooled 93% of the time). "Squirrel monkeys cannot do full
precision grips, but they were still fooled. This suggests that a monkey doesn't have to be expert in a movement in order to predict it, just
roughly able to do it," said Garcia- Pelegrin.

Marmosets do not have opposable thumbs. Their thumbs align with their
fingers to make five equidistant digits, ideal for climbing thick
tree trunks.

Marmosets were rarely taken in by magic (just 6% of the time). They
simply chose the hand in which the marshmallow was initially placed,
and stuck with it.

Previous work from the Cambridge team shows that species without hands
at all, in this case birds from the corvid family, namely Eurasian jays,
make similar choices as marmosets when confronted with the French drop.

The team also tried nullifying the tricks by actually completing the
hand-to- hand transfers, instead of misdirecting with a French drop. This
time, the capuchins and squirrel monkeys anticipated correctly and dined
out, and the marmosets missed out.

Finally, the scientists devised their own version of the French drop,
which they call the "Power drop." It utilises a hand action that all the
monkey species can perform -- essentially a full fist grab. The power
drop fooled all of the monkey species the vast majority of the time.

"There is increasing evidence that the same parts of the nervous system
used when we perform an action are also activated when we watch that
action performed by others," said Prof Nicola Clayton FRS, senior author
of the study from Cambridge's Department of Psychology.

"This mirroring in our neural motor system might explain why the French
drop worked for the capuchins and squirrel monkeys but not for marmosets." "It's about the embodiment of knowledge," added Clayton. "How one's
fingers and thumbs move helps to shape the way we think, and the
assumptions we make about the world -- as well as what others might
see, remember and anticipate, based on their expectations." "Our work
raises the intriguing possibility that an individual's inherent physical capability heavily influences their perception, their memory of what they
think they saw, and their ability to predict manual movements of those
around them." Another co-author of the study, Clive Wilkins, Artist
in Residence at Cambridge's Department of Psychology, is a professional magician and Member of the Magic Circle.

Video: https://youtu.be/bUji-1Q4UgE
* RELATED_TOPICS
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# Smoking_Addiction # Psychology # Psychedelic_Drugs
# Memory
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# Monkeys # Apes # Endangered_Animals # Animals
* RELATED_TERMS
o Illusion_of_control o Hominidae o Howler_monkey o Monkey o
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========================================================================== Story Source: Materials provided by University_of_Cambridge. The original
text of this story is licensed under a Creative_Commons_License. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Elias Garcia-Pelegrin, Rachael Miller, Clive Wilkins, Nicola
S. Clayton.

Manual action expectation and biomechanical ability in three
species of New World monkey. Current Biology, 2023 DOI:
10.1016/j.cub.2023.03.023 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/04/230404114216.htm

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