Researchers at the University of Minnesota recently constructed a miniature underwater movie theater, outfitted a group of cuttlefish with 3D glasses, and proceeded to show them short movies of shrimp—all to see if humans and cuttlefish have more in common than we previously thought.
Cuttlefish, squid-like cephalopods with an internal shell, ensnare prey with one swift snatch of their tentacles. If they under- or over-estimate their distance from whatever unsuspecting marine animal they’re eyeing, however, they’ll fail to grasp their prey and give away their position, too.
To find out how cuttlefish estimate distance so accurately, Trevor Wardill, assistant professor in the University of Minnesota’s Department of Ecology, Evolution, and Behavior, and his team devised an innovative study, published in the journal Science Advances. After placing 3D glasses over a cuttlefish’s eyes, they set it in front of a screen that showed offset images of two different-colored shrimp on a leisurely walk.
If you’ve ever briefly taken off your 3D glasses during a movie, you’ve seen the offset—or partially overlapped—images that filmmakers use to create the illusion of depth. The process by which we perceive depth is called stereopsis, where our brain receives different images from our left and right eyes and combines that information to help us understand when some objects are closer to us than others. When you’re watching a 3D movie, your brain is combining the offset images, as seen differently by your left and right eyes, to make you think that flat images have depth, and some are closer than others.
And, as demonstrated in the experiment, the same thing happens with cuttlefish. The researchers varied the positioning of the offset images so the cuttlefish would either perceive the shrimp to be in front of or behind the screen. When the cuttlefish then struck out at their would-be prey, their tentacles ended up grasping at empty water (if they thought the shrimp was in front of the screen) or colliding with the screen (if they thought the shrimp was behind it). In other words, stereopsis allowed them to interpret how far away the shrimp was, just like humans would have done.
"How the cuttlefish reacted to the disparities clearly establishes that cuttlefish use stereopsis when hunting," Wardill said in a statement. "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."
But cuttlefish brains aren’t as similar to ours as their depth perception skills might imply.
“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,” Wardill’s colleague Paloma Gonzalez-Bellido said in the press release. “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.”
Unlike squids, octopuses, and other cephalopods, cuttlefish can rotate their eyes to look directly forward, so the experiment isn’t suggesting that all cephalopods can use stereopsis. It is, however, suggesting that we may have underestimated invertebrates’ capacity for what we consider complex brain computations—and overestimated how unique humans actually are.