Cuttlefish are masters at altering their appearance to blend into their surroundings. But the cephalopods can no longer hide their inner thoughts, thanks to a technique that infers a cuttlefish’s brain activity by tracking the ever-changing patterns on its skin. The findings, published in Nature on 17 October1, could help researchers to better understand how the brain controls behaviour.
The cuttlefish (Sepia officinalis) camouflages itself by contracting the muscles around tiny, coloured skin cells called chromatophores. The cells come in several colours and act as pixels across the cuttlefish’s body, changing their size to alter the pattern on the animal’s skin.
The cuttlefish doesn’t always conjure up an exact match for its background. It can also blanket itself in stripes, rings, mottles or other complex patterns to make itself less noticeable to predators. “On any background, especially a coral reef, it can’t look like a thousand things,” says Roger Hanlon, a cephalopod biologist at the Marine Biological Laboratory in Woods Hole, Massachusetts. “Camouflage is about deceiving the visual system.”
Mix and match
To better understand how cuttlefish create these patterns across their bodies, neuroscientist Gilles Laurent at the Max Planck Institute for Brain Research in Frankfurt, Germany, and his collaborators built a system of 20 video cameras to film cuttlefish at 60 frames per second as they swam around their enclosures. The cameras captured the cuttlefish changing colour as they passed by backgrounds such as gravel or printed images that the researchers placed in the tanks.
The recording began soon after the cuttlefish hatched, and continued for weeks. Laurent’s team developed video-processing techniques to identify tens of thousands of individual chromatophores on each cuttlefish, including cells that emerged as the animal grew larger over time. The team used statistical tools to determine how different chromatophores act in synchrony to change the animal’s overall skin patterns.
Previous studies have shown that each chromatophore is controlled by multiple motor neurons that reach from the brain to muscles in the skin, and that each motor neuron controls several chromatophores. These in turn group together into larger motor systems that create patterns across the cuttlefish’s body.
The latest study maps how the animal links chromatophores together in different ways to create a pattern that mimics the geometry of its surroundings. The findings should allow the researchers to work backwards from the skin patterns to determine the pathways through which neurons in the cuttlefish's brain control its camouflage.
The imaging technique “gives you amazing neural data by proxy”, Laurent says. “It’s just an amazing thing to work on animals that are so different from us and about which we have very little intuition about what makes them tick.”
Masters of disguise
The ability to see the inner workings of the cuttlefish’s brain reflected on its skin — without cutting the animal open, attaching electrodes to it or training it to behave in a certain way — could also help researchers to understand the links between brain activity and behaviour. Right now, Laurent says, the link between what the cuttlefish sees and what it sends to the motor neurons is a mystery.
The answer probably lies in the brain, which processes both input from the eyes and output to the chromatophores. It creates a geometrical pattern that resembles the cuttlefish's surroundings, instead of an exact copy. “There’s got to be a neurobiological shortcut,” says Hanlon, who was not involved in the study. ”There’s so much visual information available that it would take a supercomputer to manage it.”
Working out that computational shortcut could provide inspiration to researchers creating artificial neural networks with computers, Laurent says. These include programs that attempt to fill in a missing part of an image using information from pixels around it.
Laure Bonnaud-Ponticelli, a biologist at the National Museum of Natural History in Paris, is impressed by the researchers’ statistical analyses of the chromatophore data. She suspects that other biological mechanisms, such as light-sensing proteins on the cuttlefish’s skin, could help the brain to form these complex patterns. “It is the beginning of another story,” she says.