By Guillaume Riesen
Humans are r--lly g--d at filling in th- bl-nks... but can cuttlefish do the same? It’s very common for objects to be partially hidden from view, but we perceive them clearly even when only bits and pieces are actually visible. If a rock is partially covered in sand, it’s easy for us to see that the different bits poking out all belong to a single bigger rock. In perception research, this is called ‘filling-in’. Scientists have shown that other animals can do this too - including rodents, birds, fish and even bees! A study by Zylinski et al asked whether cuttlefish could.
Cuttlefish are small molluscs (about the size of your hand) related to octopi and squid. Despite being invertebrates, they have very complex eyes and brains which they use to change colors and camouflage themselves against their environments. When in sandy areas, they tend to use a ‘mottled’ body pattern with subtle speckling. When surrounded by larger objects like rocks, they take on a ‘disruptive’ body pattern with high-contrast blotchy features. Zylinski and her colleagues had the clever idea of using this behavior to figure out how the cuttlefish saw the world around them. When presented with little pieces of larger objects, would they go with a speckled or blotchy body pattern? Observing their behavior allowed a window into their perception of the environment without the need for special training or invasive techniques.
The cuttlefish were put into testing tanks with five different wallpapers covering the floors and walls. 18 cuttlefish were tested, with each one shown a different pattern for half an hour at a time. Their pictures were taken every five minutes during testing and then rated by two judges to determine how blotchy or speckled they were. Two of the five patterns tested were controls - a uniform gray background and one with clear polka-dot outlines. As expected, the cuttlefish took on speckled patterns in the gray wallpaper condition and blotchy patterns in the condition with circles. The remaining three conditions were variations of the circles. The first simply cut them into four pieces each, effectively producing dotted-line circles. The next rotated these circle segments randomly so that they no longer lined up right, and the last scattered them around after rotating. The cuttlefish responded to the dotted-line circle condition with blotchy patterns similar to the original circle condition, but went with more speckled-looking patterns when shown the rotated and scattered segments. The authors conclude that the cuttlefish see the dotted-line circles as full circles despite their fragmentation. This would allow them to see that different pieces of a sand-covered rock belong to the same object in their natural environments. The other two conditions show that they aren’t simply responding to all circle fragments with a blotchy pattern. It’s the specific locations and orientations of the fragments which cause the cuttlefish to interpret them as belonging to larger, partially-hidden objects. When the bits are farther apart or don’t line up, they’re seen more like separate pebbles best matched by a speckled pattern. The cuttlefish responses to these carefully-designed wallpapers show conclusively that they are capable of filling-in - showing this ability in animals for the first time without special training!
Zylinski and her colleagues set out to test whether cuttlefish could perceive whole objects from their parts, and found that they could! They showed that the cuttlefish chose the same camouflage patterns in response to wallpapers with complete and fragmented circles, but only if those fragments lined up correctly. Despite being invertebrates who evolved brains separately from us, these little critters can solve some of the same perceptual problems as humans. As our understanding of their brains improves, it will be fascinating to see whether or not they’re using the same kinds of circuits to approach this problem. Have we come around to similar biological solutions, or are there many ways to see the sea?