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article imageArcher Fish Solves Complex Visual Problem with Simple Brain

By Stephen Push     Sep 22, 2010 in Science
Beersheba - The archer fish, which spits a stream of water at its prey, can see its target pop out of the background even though it has no visual cortex, the complex brain region thought to enable humans to perform the same perceptual feat, according to a new study.
People have the ability to spot objects that differ from the background in such features as color, shape or orientation. This skill allows us to locate food, avoid danger and find Waldo in the Where's Waldo? series of children's books. The new study from Ben-Gurion University of the Negev shows humans and archer fish can use the same orientation cues to find visual targets, a process the investigators call "orientation saliency."
The study suggests "orientation saliency is a universal building block of visual information processing" in humans and other animals, Dr. Ohad Ben-Shahar of the university's computer science department said in an email message. Scientists may have to reevaluate their understanding of orientation saliency and related brain processes because "it may be that visual cortex circuitry are not necessary to achieve them," he said.
Found in mangrove swamps in Southeast Asia and Australia, archer fish spit a stream of water up to six feet (two meters), with an accuracy any marksman would envy, to knock terrestrial insects and other small prey into the water. They correct their aim to account for light diffraction at the water's surface. They calculate where the prey will fall, lest other fish beat the archer to its prize. And they can even shoot down flying insects, although the new study tested their ability to detect stationary targets.
In the study, published in this week's issue of the U.S.-based Proceedings of the National Academy of Sciences, researchers trained five archer fish to shoot at images on a computer screen suspended above the surface of the water. In each trial the researchers presented two bars, oriented at right angles to each other, on textured backgrounds for two seconds. The positions of the bars varied, as did the background texture's orientation, which was vertical, horizontal or uniform. The fish were rewarded with food for hitting either of the bars.
An 1895 drawing of an archer fish shooting at a flying insect.
An 1895 drawing of an archer fish shooting at a flying insect.
1895 Edition of Funk and Wagnalls Dictionary
In hundreds of trials with the vertical and horizontal backgrounds, the fish were far more likely to hit a bar when its orientation was different from that of the background. With the uniform background, individual fish showed some bias for either vertical or horizontal bars. But after the researchers subtracted the effect of this bias, the data still showed that the fish had a strong, statistically significant preference for bars oriented differently from the vertical or horizontal backgrounds.
Scientists studying visual attention in humans have used different stimuli than the researchers used in the archer fish study. To insure that their results were comparable with results obtained in people, Ben-Shahar and his coauthors conducted a similar experiment with human volunteers. (No spitting was involved!) The people were shown two, one or no bars on each of the textured backgrounds for an instant and asked to say how many bars they saw. As with the fish, the people were far more likely to notice a bar if its orientation differed from the background.
With its relatively simple brain, the archer fish is "a promising animal model for the study of visual attention in general," concluded Ben-Shahar and his coauthors, Mr. Alik Mokeichev, a graduate student in computer science, and Dr. Ronen Segev of the university's life science department.
More about Fish, Vision, Psychology, Animal behavior, Brain
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