The moment before this unsuspecting squirrel monkey discovers its treat is not where it thought. (University of Cambridge)
Monkeys appear to be more easily fooled by a famous magic trick if they possess opposable thumbs, new research shows – revealing just how much our anatomy can impact our ability to perceive the world around us.
"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," explains psychologist Nicola Clayton from the University of Cambridge in the UK.
Watching others do a task can activate the same brain regions that light up if we were doing the task ourselves. This happens within our mirror neuron system, and the more familiar we are with a task the more closely our brains will mirror it.
Ballet dancers have stronger activation of mirror neurons while watching ballet rather than capoeira dances, for instance. These actions are robustly encoded in our brains as a sequence of cause and effect to save you the effort of predicting common movements.
Magicians can exploit our reliance on this mental map to mess with our perceptions. For example if a magician shows you a coin in one hand, and performs the steps of taking it with the other hand, you would be forgiven for assuming the coin has transferred from one hand to the other.
This is true even when the coin is obscured and the magician leaves the coin in the initial hand. This sleight of hand is called a French drop, as seen in the first demonstration below.
As a practicing magician with a decade of experience under his belt, comparative psychologist Elias Garcia-Pelegrin was curious to know if an ability to performing a set of actions personally is necessary for mirror neurons to map the same sequence of steps.
A researcher at Cambridge University at the time of the study, Garcia-Pelegrin and a team of his colleagues tested 24 monkeys representing three species, performing the French drop for the adorable audience. First they showed their primate audience a favored treat. They then either pretended to transfer the treat in a French drop, or actually transferred it to the other hand.
Each test relied on a precision grip involving the thumb to transfer the treat, regardless of whether the transfer was real or fake.
They also did some real transfers of the treat from one hand to the other, the second demonstration in the video above. Each test appears as if the magician is using a precision grip involving a thumb, to transfer the treat, regardless of if the transfer was real or fake.
Marmosets were only fooled by a fake transfer of a marshmallows 6 percent of the time, possibly thanks to the fact their tiny thumbs aren't opposable enough to take the treat in pinched fingers.
Relatively dexterous capuchins, on the other hand, were fooled 81 percent of the time. Poor baffled squirrel monkeys were faked-out a whopping 93 percent of the time. Given their ability to thumb-grip, it's possible their brains were already primed to anticipate the movement of an object in a pinch.
Not all sleight of hand tricks require the use of a thumb, as demonstrated in the third and fourth examples in the video above. Although the monkeys might not be capable of performing the variations personally, nothing in their anatomy would preclude them from giving it a go.
In another round of testing, the researchers used a fist to grab (or pretend to grab) the treat instead of a pincer grip, which seemed to fool all three species of monkey most 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," explains Garcia-Pelegrin.
Previous work by the team shows that woodland bird, the Eurasian jay (Garrulus glandarius), has a similar response to marmosets when shown the French drop, they don't fall for the trick that needs a thumb to perform.
"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," says Clayton.
The psychologist goes on to say, "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."
This research was published in Current Biology.
