Optical illusions show how animals perceive the world



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Visual illusions remind us that we are not passive decoders of reality but active interpreters. Our eyes pick up information from the environment, but our brains can play tricks on us. Perception does not always correspond to reality.

Scientists have used illusions for decades to explore the psychological and cognitive processes that underlie human visual perception. More recently, evidence is emerging suggesting that many animals, like us, can perceive and create a range of visual illusions.

Understanding where these illusions arise in different brains could help us not only learn more about how we perceive our world, but also how other animals perceive theirs.

In an August study in Proceedings of the National Academy of Sciences, for example, Yale researchers have shown that fruit flies, like humans, can be fooled into seeing movement in an image where there is none, like the well-known rotating snake illusion neuroscientists and psychologists. Additionally, by tracking and manipulating neurons in the visual processing areas of the flies’ brains as they watched the illusion, they were able to determine that the illusion resulted from small imbalances in the contributions of different types of neurons from the flies. motion detection.

It’s possible that the same neural mechanism is at work when humans and other species also see illusory movement, according to lead author Damon Clark, professor of molecular, cellular and developmental biology at Yale University.

“The last common ancestor of flies and humans lived half a billion years ago, but both species developed similar strategies for perceiving movement,” Clark said. “Understanding these shared strategies can help us better understand the human visual system.”

There are many examples showing that not only can many animals perceive the same illusions as humans, but they can also create and use illusions to deceive others.

Moving images

And it’s not just fruit flies and humans – research has shown that monkeys, cats and fish can all be tricked into seeing movement where there is none.

Some researchers are studying whether illusionary movement can be used to enrich the lives of zoo animals. In 2019, Italian researchers presented captive lions with the rotating serpent illusion. Two in three lions interacted with the illusion as if it were moving prey, biting it and dragging it into their enclosure. The researchers also reported improvements in well-being, including more prosocial behaviors and less stereotypical behaviors in lionesses.

Researchers say they want to try this with more captive animals to see if the illusions can help reduce stress and improve their well-being.

Avian illusionists

Every spring in Australia, the large tree bird males build and maintain structures called bowers to impress the females. The barrel consists of a tunnel of twigs with a thatched roof that opens onto a courtyard which the male decorates with bones, shells and stones. When a potential mate enters the avenue, the male stands in the yard and shows him his items, one by one. Females will visit a handful of barrels before choosing a mate based on the attractiveness of their barrel.

Male bowerbirds are very particular in the arrangement of their special items and rely on an illusion known as forced perspective to give them a boost. They order the objects on the ground so that they increase in size as the distance from the tunnel increases: “It doesn’t make sense until you consider that during courtship the female is standing in. a predetermined position in the middle of the yard, ”says Laura Kelley, a biologist at the University of Exeter.

From the point of view of the female bird, therefore, the positive size-distance gradient makes all objects on the court appear to be the same size. Therefore, the female may perceive the court as being smaller than it actually is and the male as taller. (Learn more about bowers and forced perspective.)

Humans have used forced perspective in art and architecture for centuries. Take Cinderella’s Castle in Disney World or Sleeping Beauty’s Castle in Disneyland, both of which use forced perspective techniques. (The Walt Disney Company is the majority owner of National Geographic Partners.)

“The bricks and windows get smaller as the building gets bigger, so when you’re at the bottom your brain is tricked into thinking that the building is much bigger than it is,” she says.

the the trick seems to work for bowerbirds – males who create better quality forced perspective illusions get more mates.

Fool the dragons

Scientists are finding that more species of animals than previously known can perceive the same visual illusions as we do. The trick is to find a way to ask them what they see.

Christian Agrillo, a psychologist at the University of Padua, Italy, decided to search for evidence of visual illusions in reptiles, a group of animals under-studied when it comes to the perception of illusions. In his team’s first experiment, they investigated whether bearded dragons perceive Delboeuf’s illusion. In this famous illusion, a full circle seems to be larger or smaller depending on the size of the circle that surrounds it. (In a real life example of this, people who eat from small plates tend to think their portions of food are larger than they really are.)

In testing whether lizards also fall into this illusion, Agrillo says he played on his subjects’ strengths – the love of food.

“You don’t have to train the animal. You just observe his spontaneous preference for the greater amount of food, ”he says. “If they are fooled by the illusion, they should prefer the food presented in a smaller dish to the same amount of food in a larger dish.

This is exactly what the bearded dragons did, suggesting a sensitivity to delusion.

Researchers have since tested the sensitivity of bearded dragons to other illusions that also deal with the perception of relative size, such as the Müller-Lyer illusion, in which two lines of the same length appear to have different lengths depending on the orientation of the arrows at the ends and the horizontal-vertical illusion, in which a vertical line appears longer than a bisecting horizontal line of the same length. So far, they have found that reptiles and humans seem to have a similar perception.

Agrillo says that if two species such as bearded dragons and humans perceive the same illusions, it is likely that they share a similar perception mechanism – either inherited from a common ancestor, or independently evolved to solve similar problems in their environments.

Cunning cuttlefish

Some animals create their own optical illusions. Think about camouflage. While one type of camouflage helps an animal adapt to its surroundings – like an arctic fox in snow – another type called disruptive coloring masks the shape and contour of the body itself, the ecologist of the l ‘University of Exeter, Martin Stevens.

“The disruptive coloring breaks up the shape and destroys the outline of telltale features of the animal’s body, like the wings or limbs, and makes it harder to detect,” he says. Zebra stripes and leopard spots are both good examples – the various dark and light spots help them blend into intricate backgrounds.

Stevens measured the disruptive coloring in shore crabs, which come in a wide range of colors and live in many different habitats. He found that crabs living in tidal pools – a visually complex environment – have higher contrast marks than crabs living in more homogeneous looking mudflats. More complex habitat means crabs need more extreme disruptive coloring to break up the outline of their body.

Cuttlefish, like this large club cuttlefish off the island of Cebu in the Philippines, can change the color and pattern of its skin to fool potential predators and prey.

The master of disruptive coloring may be the cuttlefish, which can change the color and pattern of its skin. It can create disruptive, high contrast patterns, even adopting an off-white black and white pattern seconds after being placed on a checkerboard in a lab.

Research into when cuttlefish produce these marks has shown the importance of visual markers such as object area, contrast, and edges.

The more scientists look – in the lab and in nature – the more similarities they find between the way humans and animals view the world. It’s a reminder that even though we’re as different as homo sapiens and fruit flies, our subjective worlds may be more similar than previously thought.

Mary bates is a Boston-based science writer who focuses on the brain and behavior of humans and other animals. Follow her on Twitter.



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