July 7, 1999 How can a brightly colored and striped tiger be so hard to see moving in jungle shadows?
Part of the explanation may be found in new research from the University of Southern California and the University of California, Irvine.
According to USC experimental psychologist Zhong-Lin Lu, research in neurophysiology has discovered two independent brain pathways for visual information in the brain. One pathway is sensitive to color but not sensitive to motion; the other is sensitive to motion but not sensitive to color.
Integration of color and motion information is therefore vital to detect the camouflage of the moving tiger. Humans can, in fact, perceive motion from "isoluminant" images, which contain different colors with the same brightness, like those of a big cat moving through dappled shadows. But they only do so by a less sensitive mechanism than is used detecting motion using brightness or texture cues.
Dr. Lu says his team's new set of studies has unraveled how this less sensitive system works, and what its limitations are in perceiving motion by isoluminant objects. An account of their experiments appears in the July 6 edition of the Proceedings of the National Academy of Science of USA.
According to Lu, an assistant professor of psychology in the USC College of Letters, Arts and Sciences, researchers have found that human motion perception is served by three parallel brain mechanisms. These include an extremely sensitive "first order" system that depends on cues from varying brightness, a slightly less sensitive "second order" system that depends on cues from texture, and a highly versatile but considerably less sensitive "third order" system that depends on figure-ground contrast.
The series of recent experiments confirms that motion is detected in pure, isoluminant color only by the third-order, figure-ground system.
The experimental team, which also included George Sperling, Ph.D., of the University of California, Irvine, department of cognitive science, and USC graduate student Luis A. Lesmes, began with computer displays in which alternating green and red stripes moved across a screen. The team then systematically purified the displays, removing brightness differences from the green and red stripes, making them "isoluminant."
Using complex displays with special mathematical properties, the researchers discovered that, unlike first- and second-order motion, color motion depends critically on the location of the dominant features in the display but doesn't use information about which eye is providing the information.
The team also definitively confirmed that third-order, figure-ground perception is the only way isoluminant color motion can be perceived. They did so by designing and creating a striking illusion of "motion standstill."
The moving objects were not tiger stripes but a series of computer-generated moving red and green color bars. After appropriate adjustment of the saturation of the green bars relative to the red bars, the display appeared stationary to 53 out of 53 observers, even though the bars continued to move.
"The strategy we used in these studies is similar to that used in physics," commented Lu, who was originally trained as a physicist. "Ordinarily, moving objects stimulate all three motion systems. In the laboratory, we can create a 'pure stimulus' that only stimulates one particular motion system. By studying a 'pure stimulus', like an isolated atom, we can reveal the secrets of the human brain."
According to Lu, the methodology and results from this study will provide a basis for a broad class of neurophysiological investigations on high-order brain functions. The computational algorithms will be incorporated into models of human visual motion perception.
Eventually, the computer models might be replicated for robotic devices that could mimic motion perception similar to that of a human for various purposes -- a robotic device as vulnerable as a human would be to a stalking tiger.
The research was funded by the U.S. Air Force Office of Scientific Research.
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