Archerfish are capable of some pretty incredible feats: from underwater, they can pick out an insect flying or hanging on a leaf above the surface, accurately shoot it with a stream of water, and analyze the prey’s falling trajectory to determine the spot in the water where it will land. As if that weren’t impressive enough, a new study in PNAS finds that archerfish share a complex visual processing ability with humans, despite lacking the particular area of the brain that we rely on for advanced visual tasks.

Humans exhibit orientation-based saliency, which essentially means that objects oriented differently than their backgrounds tend to “pop out” at us. This ability, which helps us quickly identify important things in our visual field, comes from an area of our brain called the visual cortex. Archerfish completely lack visual cortices, but their astounding visual accuracy led a team of Israeli researchers to hypothesize that these fish might exhibit orientation saliency anyway.

Using LCD screens positioned above the water, the researchers exposed captive archerfish to various images of two bars against a patterned background, simulating prey items in the habitat. The bars were oriented in one of three ways: either both in the same direction as the background pattern, both in the opposite direction, or one in each direction. The archerfish were shown the image for two seconds, and any time they shot water at an image, the researchers would determine which bar, or prey item, they detected and aimed at.

Overwhelmingly, the archerfish shot most often at the bars that were oriented in the opposite direction as the background pattern, indicating that these bars were more visually obvious to them. When tested with the same stimuli, human subjects were also much more adept at picking out the bars oriented opposite the background pattern.

These findings suggest that, like humans, archerfish experience a “pop out” effect when the orientation of objects contrasts that of the background. However, since these fish have no visual cortex, the study raises questions about the mechanism behind orientation saliency in archerfish: is the neural mechanism similar to that in humans, but located in another area of the brain, or is this effect caused by another mechanism altogether in archerfish?

Evolutionarily, there are two potential scenarios. First, it’s possible that archerfish and humans have an ancient common ancestor with the capacity; this scenario suggests that orientation saliency is so useful that it has been conserved over millions of years, and no better alternative has evolved during that time. On the other hand, it’s also possible that orientation saliency has evolved independently in each species.

Both scenarios suggest that orientation saliency may be a more effective and widespread mechanism than previously assumed.

PNAS, 2010. DOI: 10.1073/pnas.1005446107 (About DOIs).

Listing image by University of Michigan