Creatures Teach Scientists About Depth Perception

Praying mantis wearing 3D glasses.
Credit: Newcastle University, UK

It’s been 66 years since the Creature from the Black Lagoon emerged from its swampy lair and crawled onto big screens of cinemas across the country. Horror movie fans who saw the 3D version of the movie enjoyed the thrilling special effect by looking through anaglyph glasses with one red and one cyan lens that the theater provided.

Now, in a sort of role reversal, scientists are outfitting some real creatures—praying mantises and cuttlefish—with tiny 3D glasses. They then showed them images in specially designed “cinemas” and observed their reactions. These experiments proved that each creature has a neural and visual system capable of depth perception, a characteristic previously thought to have existed in only mammals and reptiles.

Researchers in the U.K. who experimented with mantises found neurons in the insects’ brains can compute 3D distance and direction, a discovery that could help vision in robots. In a 2019 article published in the journal, Nature Communications, the scientists reported that mantises in a specially-designed insect cinema were fitted with 3D glasses and shown 3D movies of simulated bugs while their brain activity was monitored. When the image of the bug came into striking range for a predatory attack, scientist Dr. Ronmny Rosner was able to record the activity of individual neurons.

Dr. Rosner, a research associate at the Institute of Neuroscience at Newcastle University and lead author of the paper, was quoted in a University press releases as saying, “This helps us answer how insects achieve surprisingly complex behavior with such tiny brains and understanding this can help us develop simpler algorithms to develop better robot and machine vision.”

Praying mantises use 3D perception, scientifically known as stereopsis, for hunting. By using the disparity between the two retinas they are able to compute distances and trigger a strike of their forelegs when prey is within reach, the press release noted.

The neurons recorded were stained, revealing their shape which allowed the team to identify four classes of neuron likely to be involved in mantis stereopsis. The images captured using a powerful microscope show the dendritic tree of a nerve cell—where the nerve cell receives inputs from the rest of the brain—believed to enable this behavior.

Mantis neuron connecting both optic lobes.
Credit: Newcastle University, UK
Dr. Rosner explained, “Despite their tiny size, mantis brains contain a surprising number of neurons which seem specialized for 3D vision. This suggests that mantis depth perception is more complex than we thought. And while these neurons compute distance, we still don’t know how exactly.

“Even so, as theirs are so much smaller than our own brains, we hope mantises can help us develop simpler algorithms for machine vision.”

The Newcastle research team shared more of their learnings in this engaging video.

Scientists at the University of Minnesota conducted similar experiments with cuttlefish. The researchers built an underwater theater and equipped the cephalopods with specialized 3D glasses to investigate how cuttlefish determine the best distance to strike moving prey. Their research findings, published in the journal Science Advances revealed cuttlefish use stereopsis to perceive depth when hunting a moving target.

Cuttlefish catch a meal by deploying their tentacles and, to be successful in their strike, cuttlefish must compute depth to position themselves at the correct distance from the prey. If they are too close, the prey may be spooked and escape; too far, and the tentacles will not reach.

To test how the cuttlefish brain computes distance to an object, the team trained cuttlefish to wear 3D glasses and strike at images of two walking shrimp, each a different color displayed on a computer screen at the Marine Biological Laboratory in Woods Hole, Mass., according to a University of Minnesota press release. The images were offset, allowing for the researchers to determine if the cuttlefish were comparing images between the left and the right eyes to gather information about distance to their prey. This process—stereopsis—is also the way humans determine depth. Depending on the image offset, the cuttlefish would perceive the shrimp to be either in front of or behind the screen. The cuttlefish predictably struck too close to or too far from the screen, according to the offset.

“How the cuttlefish reacted to the disparities clearly establishes that cuttlefish use stereopsis when hunting,” said Trevor Wardill, assistant professor at the Department of Ecology, Evolution and Behavior in the College of Biological Sciences. “When only one eye could see the shrimp, meaning stereopsis was not possible, the animals took longer to position themselves correctly. When both eyes could see the shrimp, meaning they utilized stereopsis, it allowed cuttlefish to make faster decisions when attacking. This can make all the difference in catching a meal.”

Watch the video below to see how a cuttlefish hunts a virtual shrimp while wearing 3D glasses.

Through this process, the investigators also found the mechanism that underpins cuttlefish stereopsis is likely different from humans due to the cuttlefish successfully determining the distance from anti-correlated stimulus (i.e., the left and the right eye images have the same pattern, but are reversed in luminance). Humans cannot do this reliably.

“While cuttlefish have similar eyes to humans, their brains are significantly different,” said Paloma Gonzalez-Bellido, assistant professor at the Department of Ecology, Evolution and Behavior in the College of Biological Sciences. “We know that cuttlefish brains aren’t segmented like humans. They do not seem to have a single part of the brain—like our occipital lobe—dedicated to processing vision. Our research shows there must be an area in their brain that compares the images from a cuttlefish’s left and right eye and computes their differences.”

A cuttlefish wears 3D glasses while waiting for virtual prey to appear.
Credit: R. Feord

Moreover, cuttlefish have the ability to rotate their eyes to a forward-facing position, a unique trait that sets them apart from their cephalopod relatives (e.g., squid and octopus). It is possible that cuttlefish are the only cephalopods with the ability to compute and use stereopsis. Mantids are the only other invertebrate species known to use stereopsis.

If it was once thought that complex brain computations, such as stereopsis, were exclusive to higher order vertebrates, studies such as this are leading scientists to reconsider the capabilities of invertebrate brains.

“This study takes us a step further toward understanding how different nervous systems have evolved to tackle the same problem,” said Rachael Feord, PhD, the research paper’s first author. “The next step is to dissect the brain circuits required for the computation of stereopsis in cuttlefish with the aim of understanding how this might be different to what happens in our brains.”

I suspect the Creature from the Black Lagoon also used stereopsis to catch its prey, human and otherwise. If only we had been able to analyze his brain and see things through his eyes, everything might have turned out differently for him… and us.