Dynamic properties of the compensation system for doppler shifts in the bat,Rhinolophus ferrumequinum

Abstract

1. Echolocation sounds of the batRhinolophus ferrumequinum were played back to the bat as artificial echoes shifted in frequency by a few kHz (0–4 kHz) without distortion of the amplitude and frequency pattern of the sound. The simulated Doppler shifts in the echoes were modulated either sinusoidally or in step functions. 2. The bats did not compensate for frequency shifts within one sound, but in the sounds subsequent to the introduction of the shift. 3. Responses to symmetrical sinusoidal modulations of the echo from −2 kHz to +2kHz demonstrate that in the dynamic situation the bat compensates for positive frequency shifts and ignores negative frequency shifts (Fig. 2). Asymmetrical sinusoidal modulations between 0 Hz and different positive frequency shifts (1, 2, 3, and 4 kHz) yielded amplitude and phase response curves for modulation frequencies between 0.01 and 2 Hz (Fig. 5, 6). The response to step functions gave the time course for positive and negative changes of the frequency shift (Fig. 7). 4. The Doppler compensation system shows nonlinearities: The response of the system depends on the sign of the frequency shift change. Positive changes of the frequency shift in sinusoidal modulations are optimally followed by the system, when it is considered as a sampling system, in which the pulse repetition rate is the sampling rate. The response to negative frequency shift changes is much slower (Fig. 6). With an increase of the modulation frequency the response is DC-shifted. The amplitude and phase response curves depend on the amplitude of the frequency shifts presented in the modulation (Table 1, 2). 5. The maximum compensation between two succeeding bat orientation sounds was 1400 Hz, giving a frequency change of about 10 Hz/msec. This value is about the same as the minimum frequency modulation of the constant frequency portion of the sound detected by the bat. 6. The dynamic properties of the Doppler shift compensation system are discussed in the context to neurophysiological findings on the hearing system of the bat.