Desert Scientists Have Epiphany About How Fish Fish

Ben-Gurion University brain researchers now suspect all vertebrates, not just mammals, can identify targets against a busy background, they're just using different parts of the brain.

Dani Machlis, BGU

Of all places to discover this, scientists at the desert-baked Ben-Gurion University of the Negev have discovered that archer fish perform visual searching for prey much the way you do. Their resultant speculation: "Pop-out" target searching mechanisms in the brain, that had been thought to be unique to mammals, exist in all vertebrates.

When scanning the environment, say for a tasty fly, or our specific car in a parking lot, we look around us and our brains can isolate the target from the busy background, or as the scientists put it, the target "pops out."

Or, think of yourself trying to find the kid on a playground: you scan, and then when you see the child, it "pops out" against the background, which recedes into an uninteresting blur.

We do this by identifying unique visual features of our target – fly, car, offspring, the crocodile lurking in the water that wants to eat us. And the sooner the better.

It had been known that in mammals, the desirability of fast visual search led to the development of a capability, to scan the scene in a "parallel fashion" – our brains process the entire visual scene, rather than processing each object one at a time.

Brain scientists call this ability "pop-out visual search," but hadn't realizes that it evidently began to evolve way before the first mammal quaked at the dinosaurs – since it has now been found in archer fish.

The belief is that pop-out search, or the ability to quickly identify a target, originates in an area in the cortex that identifies the location that differs the most from neighboring ones, explain the scientists in a statement following publication of their paper, "Pop-out in visual search of moving targets in the archer fish", which published this week in Nature Communications. Since it differs, based on our "search criteria," the observer’s attention is drawn towards this location.

Now, the cortex is the most recently evolved part of the mammalian brain, they say, and the version found in the archer fish is a primitive thing. Yet archer fish show a wonderful ability to target and "shoot down" bugs hovering in the air, by spitting water at them. How do the simple-minded fishies do that?

And how does the archer fish learn – and it does - to distinguish between artificial targets presented on a computer monitor in an experimental setting?

The fish, the team at BGU led by Ronen Segev showed, uses pop-out visual search to find its moving targets and aim its watery firepower at them. What it sees is an object moving faster than others on the screen, say the scientists – an ability not affected by the number of objects in the display, which is an indicator for pop-out ability.

And thus the researchers concluded that pop-out search ability isn't just an artifact of advanced apes such as we, but of all vertebrates, including the less advanced ones such as fish.

By the way, the fish's popping-out ability did not stem from the cortex but from the optic tectum, which is the main visual area of the brain. It is also one of the largest areas in the archer fish brain, they explain.

Primates have optic tecta too but while the fish's one is very large proportional to the rest of the brain, the ape's is very small. Anyway, since the scientists found similar activity patterns in the archer fish brain and in the mammalian visual cortex, they now suspect that the visual cortex may not be the crucial component in generating a map of unique locations in the visual field after all.

The team included Prof. Ronen Segev and PhD student Mor Ben-Tov, from the Life Sciences Department, Prof. Opher Donchin from the Biomedical Engineering Department and Prof. Ohad Ben-Shahar from the Computer Science Department.