The results, published Tuesday in the Nature journal Scientific Reports, could help us understand how humans got so good at telling each other apart. Or how most people got to be good at that, anyway. I'm terrible at it.
It's generally accepted that the fusiform gyrus, a brain structure located in the neocortex, allows humans to tell one another apart with a speed and accuracy that other species can't manage. But there's some debate over whether human faces are so innately complex — and that distinguishing them is more difficult than other tricks of memory or pattern recognition — that this region of the brain is a necessary facilitator of the skill that evolved especially for it. Birds, which have been shown to distinguish humans from one another, have the same structure. But some researchers still think that facial recognition might be something that humans learn — it's not an innate skill — and that the fusiform gyrus is just the spot where we happen to process all the necessary information.
That's where fish come in: They don't have anything like this structure within their (relatively) simple brains. But they've been trained to spit at particular shapes or colors before, so Cait Newport, Marie Curie research fellow in the department of zoology at Oxford University, wanted to see whether faces posed a particular challenge for fish brains.
And based on the spits she monitored, they don't.
"It's very similar to training a pet dog," said Newport, who conducted the research along with scientists from the University of Queensland. First she taught archerfish that spitting at a cursor she moved around on a screen would result in food pellets. Then she taught them to spit at pictures of human faces — and then she taught them to spit at a particular face, even when given options. In two experiments — each using four fish — the subjects were presented with a random sequence of 44 faces in addition to the one they'd been trained to pick. In both cases (including an experiment where the researchers made the faces gray-scale) the fish made better than random guesses, obtaining average accuracies of 81 percent and 86 percent.
"Obviously the first takeaway is that they could do it. They were distinguishing something really complicated," Newport said. This also shows that the fish have surprisingly good memories. "It certainly challenges the whole idea of a fish with a 30-second memory," she joked.
There's no doubt that whatever mechanism humans use for recognizing one another is much more sophisticated — and more accurate — than the recognition Newport saw in archerfish. "We may even find that fish aren't able to do it in other conditions," she said, while humans are able to recognize faces at different angles and under different light, at least up to a point. She plans on testing the limits of archerfish abilities in future studies.
"But this does start to suggest that there's nothing special about human faces, and that they can be treated as any other object and still be recognized," she said, which further suggests that facial recognition is learned and not innate. Again, this doesn't mean the fusiform gyrus isn't important — irregularities there have been linked to a condition called face blindness, which prevents humans from recognizing individuals — but it could mean that the human ability boils down to more than just some unique brain matter we've evolved to carry.
"This is just the scratching of the surface," Newport said. "It's hard to say what it might mean."
But for now, be kind to any spitting fish you meet. Because they might remember you down the road.