The archer fish, a skilled marksmen that shoots insects down from trees by spiting streams of water, spots prey that aren't in line with what's behind them, an ability once thought to be found only in mammals, according to a linkurl:study;http://www.pnas.org/cgi/doi/10.1073/pnas.1005446107 published in Proceedings of the National Academy of Sciences (PNAS) today (13th September). The results suggest that the ability to see objects oriented differently than their backgrounds is "a fundamental
The archer fish, a skilled marksmen that shoots insects down from trees by spiting streams of water, spots prey that aren't in line with what's behind them, an ability once thought to be found only in mammals, according to a linkurl:study;http://www.pnas.org/cgi/doi/10.1073/pnas.1005446107 published in Proceedings of the National Academy of Sciences (PNAS) today (13th September). The results suggest that the ability to see objects oriented differently than their backgrounds is "a fundamental building block of vision," said linkurl:Susanna Martinez-Conde,;http://smc.neuralcorrelate.com/ a neurobiologist at the Barrow Neurological Institute in Phoenix, Arizona, who was not involved in the research. "The fact that both humans and fish, which are very distant in terms of evolution, process orientation in similar ways [suggests] that orientation is a critical aspect of visual information," agreed linkurl:Ohad Ben-Shahar,;http://www.cs.bgu.ac.il/faculty/person/obs.html a computer scientist at Ben Gurion University of the Negev in Israel and lead author of the study. Up till now, researchers believed that the ability to see objects positioned out of line with their backgrounds, known as orientation saliency, was only found in mammals, which have specialized neurons in their brain that allow the animal to perceive these off-kilter objects. But recent anecdotal evidence suggested that archer fish are also capable of picking out askew objects, such as insect prey, from complex visual backgrounds. Curious to see if the same processes were at work in both taxa, Ben-Shahar and his colleagues conducted a series of experiments to test the archer fish's aptitude for orientation saliency, and compared their performance with that of human test subjects. The researchers first trained five archer fish to spit at images of insects on an LCD screen above their tanks. Then, they replaced the insects with black bars positioned in line with or perpendicular to textured backgrounds and counted how many times the fish accurately hit the different targets. Overall, the fish hit targets that were askew significantly more often than those aligned with the background, suggesting that the fish are able to see the orientation of their prey and that this ability can help them find their next meal.
One of Ben-Shahar's archer fish in the experimental arena Video courtesy of Ohad Ben-Shahar
"Obviously evolution manages to bring out phenomena when they are useful for a species," said linkurl:Hermann Wagner,;http://www.bio2.rwth-aachen.de/users/wagner/html/hermann.htm a neuroethologist at RWTH Aachen University in Germany, who was not involved in the study. "It's possible that [orientation saliency] is a specific adaptation in these fish because they need it for survival, in order to be able to accurately shoot a flying insect above [the water]." Archer fish are "a really good model for studying the role of orientation because they have the perceptual abilities of an accomplished marksmen, with a simpler brain," said Martinez-Conde. It appears that "excellent visual performance and the ability to discriminate between orientations go hand and hand." The question now, she added, is what neural pathways of the fish brain are used to comprehend differing orientations. Ben-Shahar agreed. "In mammals the saliency of orientation is associated with [specialized neurons] that we know to exist in the visual cortex," he said. If fish, who don't have a visual cortex, can exhibit the same behavior, then researchers need to reestablish their understanding of how these processes work. A. Mokeichev et al., "Orientation saliency without visual cortex and target selection in archer fish," PNAS, doi:10.1073/pnas.1005446107 **__Related stories:__***linkurl:Frankenfish;http://www.the-scientist.com/2009/03/1/26/1/ [1st March 2009]*linkurl:How do we look?;http://www.the-scientist.com/2005/11/07/18/1/ [7th November 2005]*linkurl:Evolution of mammalian brains;http://www.the-scientist.com/article/display/19649/ [11th May 2001]
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