TARGET FLUTTER RATE DISCRIMINATION BY BATS USING FREQUENCY-MODULATED SONAR SOUNDS - BEHAVIOR AND SIGNAL-PROCESSING MODELS

This study utilized psychophysical data and acoustical measurements of sonar echoes from artificial fluttering targets to develop insights t o the information used by FM bats to discriminate the wingbeat rate of flying insects. Fluttering targets were produced by rotating blades t hat moved towards the bat, and the animal learned to discriminate betw een two rates of movement, a reference rate (30 or 50 Hz) and a slower , variable rate. Threshold discrimination performance depended on the rotation rate of the reference target, with a difference value of 9 Hz for the reference rate of 30 Hz and 14 Hz for the reference rate of 5 0 Hz. Control experiments demonstrated that the bats used sonar echoes from the moving targets to perform the discrimination task. Acoustica l measurements showed that the moving target produced a Doppler shift in the echo and a concomitant change in the arrival time of each frequ ency in the linear period FM sweep. The difference in delay between ec hoes from moving and stationary parts varied linearly with flutter rat e and depended on the characteristics of the bat's sonar sounds. Simul ations also showed a reduction in average echo bandwidth with increasi ng flutter rate, which may account for a higher delay discrimination t hreshold using the 50-Hz reference rate. This work suggests that Doppl er-induced changes in echo delays produced by fluttering targets may c ontribute to the FM bat's perception of flying insect prey. (C) 1998 A coustical Society of America.