Breaking News: Fish Confused by Optical Illusions

Despite the sarcastic title, this work is pretty neat. In a recent Scientific Reports paper (open access, yay!), researchers from the University of Padua in Italy found that fish pretty much see the world as we do, as least when talking about motion illusions. If you’ve spent time as a child, you’re probably familiar with optical illusions (personally, I was obsessed with Magic Eye books; maybe I shouldn’t say was). Motion illusions are a type of optical illusion that make the brain perceive motion from a static image (see picture below).

illusion

Their version of the classic Rotating Snakes illusion, abbreviated RSI in the paper because all academic papers need more abbreviations.

Why fish? It turns out that fish don’t have a visual cortex like humans and other mammals. We know fish can see (they need to to hunt and escape predators) but we don’t know exactly what they see. We do know they see changes in light, but can they see texture and contrast and form? In mammals, this additional sight comes from our visual cortex. If fish do get additional visual information, then they must do so in a manner completely distinct from us. That’s why fish were chosen: to see if they perceive an illusion that arises in mammals from our visual cortex.

To find out this interesting piece of scientific information, they crammed a fish tank between two computer monitors. On one monitor was the RSI (the allure of abbreviations has not yet left me). The other monitor had a static version of the image, only subtly different, without the motion illusion. The fish were trained to spot motion to get a food reward (tasty, tasty brine shrimps).

After all was said and done, 18 out of 24 fish were confused (that’s 75%). They thought the illusion was real and tried to get their food reward (their… just desserts). This compares fairly well with the percentage of humans who can see the illusion (that’s 84%).

The experiment didn’t explain how fish, with their lack of visual cortex, saw the motion. If anything it threw more questions into the mix, which I think is a good thing. The object of a good scientific paper shouldn’t be to answer all the questions but to ask more… unless you’re trying for a Theory of Everything (the answer to it all, the mack daddy of theories, the big ToE).

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