Fine Control of Call Frequency by Horseshoe Bats

The auditory system of horseshoe bats is narrowly tuned to the sound of their own echoes. During flight these bats continuously adjust the frequency of their echolocation calls to compensate for Doppler-effects in the returning echo. Horseshoe bats can accurately compensate for changes in echo frequency up to 5 kHz, but they do so through a sequence of small, temporally-independent, step changes in call frequency. The relationship between an echo's frequency and its subsequent impact on the frequency of the very next call is fundamental to how Doppler-shift compensation behavior works. We analyzed how horseshoe bats control call frequency by measuring the changes occurring between many successive pairs of calls during Doppler-shift compensation and relating the magnitude of these changes to the frequency of each intervening echo. The results indicate that Doppler-shift compensation is mediated by a pair of (echo)frequency-specific sigmoidal functions characterized by a threshold, a slope, and an upper limit to the maximum change in frequency that may occur between successive calls. The exact values of these parameters necessarily reflect properties of the underlying neural circuitry of Doppler-shift compensation and the motor control of vocalization, and provide insight into how neural feedback can accommodate the need for speed without sacrificing stability.