This project examines how bats use sonar to perceive the location and shape of targeted objects, like prey, and simultaneously avoid interference caused by clutter, the echoes from untargeted objects. Bat sonar broadcasts are composed of a wide range of ultrasonic frequencies. Experiments reveal that small differences in the frequency content between broadcasts and their echoes have disproportionate effects on the perception of objects. The echoes from targets, located in front of a bat, contain all of the frequencies in the broadcast except for those few that are removed by the target itself and bats perceive both their shape and location. In contrast, bats perceive only the location, not the shape, of objects located to the side or farther away. More frequencies are removed from echoes that arrive from these locations. The bat's failure to perceive shape and, at the same time, perceive the location of untargeted objects allows the bat to avoid obstacles and orient to a target because the clutter itself disappears from the part of the perceived scene concerned with the identification of targets. This novel process is a defocusing method that deemphasizes background objects in the bat's images and it has technological significance for man-made sonar designs. Experiments using electronically-controlled echoes will determine the effects of removing or displacing selected frequencies on the perception of echo delay, used to determine target location, and the echo spectrum, used to determine target shape. Experiments with bats carrying a miniature radio microphone (Telemike) while they fly through a dense obstacle array composed of rows of chains hanging from the ceiling will determine whether bats make changes in the frequencies of successive sonar sounds to avoid interference caused by clutter. The broader impacts of this proposal include training of a postdoctoral fellow and a graduate student and the results of this study are likely to be incorporated into human-devised guidance devices.
