Reflections on the neural processing of echoes for distance information in echolocating bats

Multiple neural specializations in echolocating bats enable target distance determination by measurement of the delay between many frequencies in the emitted FM signal and the same frequencies in returning echoes. In the neotropical lesser bulldog bat, Noctilio albiventris, a short constant frequency (CF) component preceding the FM sweep in emitted sounds is critical to the measurement of pulse-echo delay. The author reviews behavioral experiments that have a bearing on possible neural mechanisms for this role of the CF. Other experiments that substituted a descending sequence of brief pure tones for the normal FM sweep in evoking a normal approach phase behavioral response reveal interesting differences between N. albiventris and the purely FM bat Eptesicus fuscus in several parameters: the minimum number of pure tone frequencies necessary in the “FM sweep” for distance determination, the minimum effective frequency difference between steps, and the maximum duration of pure tone steps.