Oct. 13, 1998 PROVIDENCE, R.I. -- A bat's brain can resolve sonar images up to three times more sharply than biologists had previously thought and much better than man-made equipment. The findings, described in the current issue of the journal Proceedings of the National Academy of Sciences, may lead to improved sonar for naval and other uses.
The new view of bats comes from research at Brown University that suggests the mammal's sonar images are of a higher quality and are suited to a wider variety of orientation tasks than just catching insects. The processing of sonar echoes in the bat's brain is correspondingly more sophisticated than had been suspected.
In experiments, bats separately perceived and processed overlapping echo delays arriving as little as two microseconds - two millionths of a second - apart, an ability roughly three times keener than scientists had believed was possible in the mammals.
This fine-tuned capability, based in the bat's nervous system, allowed the animals to resolve echo-reflecting points on an object as close together as three-tenths of a millimeter, about the width of a pen line on paper. Such image resolution is significantly better than any man-made sonar, say the study authors.
"Using the same sounds as the bat, the best man-made sonar equipment can only process echo delays arriving five to 10 microseconds apart," said study leader James Simmons, professor of neuroscience in the Brown University School of Medicine. "The experiments showed that a bat's sonar resolved echoes that arrived two microseconds apart as easily and routinely as if there were 10 microseconds between them."
Simmons and colleagues have also begun experiments to record neural responses in bat brains that may account for the superior sonar resolution. The scientists are using the new resolution information to construct a computer model that mimics those neural responses and produces sonar images.
The research is of particular importance to the Navy, which relies on both man-made sonar and the sonar of trained dolphins to detect distant or obscure objects, particularly mines. "The idea is to use the results to build a sonar system that would find these mines without the need for dolphins or divers," Simmons said. Dolphins and bats have similar high-quality sonar, but bats are easier and more practical to work with in the lab, he said.
Bats primarily "view" their environment through sonar, sending out high-frequency sound waves and registering returning sounds - echoes - from surrounding objects ranging from buildings to bugs. Depending on its size, an object returns a varying spectrum of reflected echoes. The bat uses this spectrum to achieve fine time resolution.
The resolving power of a sonar imaging system is expressed as the minimum spacing of two objects which can be detected separately. According to Simmons, bats process returning sound waves in two different ways in order to form a single, high-definition image. One set of brain-based responses measures the timing of returning echoes and determines distance to a structure or target. The other set of neural responses gauges the spectrum of the echoes to flesh out a three-dimensional image of the object.
The experiments involved big brown bats collected from New England homes. The bats sat on a Y-shaped elevated platform and broadcast sonar sounds into microphones to their left and right. The signals were delayed, then replayed back as altered artificial echoes from loudspeakers as single or double sounds with various two-point spacings. A bat's task was to decide whether the electronic echoes varied in delay or were stationary in delay from one broadcast to the next.
The study's other authors are Michael Ferragamo, former doctoral researcher at Brown, now an assistant professor of biology at Gustavas Adolphus College, and Cynthia Moss, former doctoral student and post-doctoral researcher at Brown, now associate professor of psychology at the University of Maryland.
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