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Doppler-shift compensation behavior in horseshoe bats revisited: auditory feedback controls both a decrease and an increase in call frequency

Walter Metzner1,*, Shuyi Zhang2 and Michael Smotherman1

1 Department of Biology, University of California at Riverside, Riverside, CA 92521-0427, USA
2 Institute of Zoology, Chinese Academy of Sciences, 19 Zhongguancun Road, Beijing 100080, China
* Present address: Department of Physiological Science, University of California at Los Angeles, Los Angeles, CA 90095-1606, USA



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  		Fig. 1. Behavioral audiogram of the greater horseshoe bat Rhinolophus ferrumequinum (Long and Schnitzler, 1975Go). The resting frequency (RF) is normalized to 80 kHz. The frequency ranges over which Doppler-shift-compensation behavior was tested are indicated in dark (Doppler shifts above RF) and light (Doppler shifts below RF) gray shading. Within these ranges, two examples are given for the intensity ranges tested. The numbers of playback signals analysed for this graph were 512 for positive and 537 for negative Doppler shifts. The lower and upper ends of the boxes indicate the twenty-fifth and seventy-fifth percentile, respectively, with a broken horizontal line at the median. Error bars indicate the tenth and ninetieth percentiles and squares indicate outliers. The frequency range just above RF where thresholds reach very low levels is also referred to as the `auditory fovea' (short horizontal bar beneath the abscissa) (Schuller and Pollak, 1979Go). Note that the difference between the medians of the intensity ranges tested for positive and negative Doppler shifts corresponds approximately to the difference in the hearing threshold for these frequency ranges.

 


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  		Fig. 3. Doppler-shift compensation behavior in response to stepwise changes in the frequency of echo mimics. (A) Time courses of the frequency shift in echo mimics (white circles, bottom trace) and corresponding call frequencies (black circles, top trace; only the highest frequency measured in each call is given). An initial positive shift in playback frequency causes playback frequencies to rise above the resting frequency (RF). Call frequencies are therefore lowered below RF. The subsequent negative step back to zero shift causes playbacks to return at frequencies below RF (since call frequencies are still below RF). Consequently, the bat increases its call frequencies. Each step was maintained for up to 30 s until the bat had reached its compensation frequency. At least 10 repetitions of each parameter combination were presented per bat and session. Time constants were determined by measuring the time until the call frequency had changed by 67% in response to positive and negative shifts in stimulus frequency. (B) Time courses of call frequency increases (3 kHz above RF) in response to negative steps in playback frequency to zero shift for two different attenuations (bat dsb6). Each symbol represents the maximum frequency in one call. Louder playback signals (0 dB attenuation, open circles; N=226) cause call frequency to increase faster than weaker playback signals (30 dB attenuation, filled circles; N=103). The difference between the two conditions is significant (all pairwise multiple comparison procedure, Dunn's method, P<0.05). Frequency shifts of 1.5 kHz and 4.5 kHz above RF yielded similar results (not shown). (C) Mean time constants for call frequency increases analyzed in all three bats tested in response to negative steps in playback frequency. N is the number of time constants analyzed. The numbers of calls analyzed for each condition were 366 (0 dB), 354 (10 dB), 292 (20 dB) and 169 (30 dB). Significant differences exist for 0 dB versus 20 dB and 30 dB, for 10 dB versus 20 dB and 30 dB and for 20 dB versus 0 dB, 10 dB and 30 dB (all pairwise multiple comparison procedure, Dunn's method, P<0.05). For an explanation of the box and whisker plots, see Fig. 2. (D) Mean time constants for lowering of call frequency in response to positive steps in playback frequency. Same conventions as in C. The numbers of calls analyzed for each condition were 401 (0 dB), 278 (10 dB), 332 (20 dB) and 175 (30 dB). Significant differences exist for 0 dB versus 10 dB, 20 dB and 30 dB, for 10 dB versus 0 dB and 30 dB and for 20 dB versus 0 dB and 30 dB (all pairwise multiple comparison procedure, Dunn's method, P<0.05).

 


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  		Fig. 2 . Doppler-shift compensation behavior in response to sinusoidal negative (below the resting frequency, RF) shifts in the playback frequency. The frequency of the echo mimics was shifted sinusoidally above (`normal' DSC; A) or below the bat's resting frequency (`inverse' DSC; B—D) at intensity levels approximately 60-80 dB above threshold (see Fig. 1). Each parameter combination was tested at least 10 times with at least 10 modulation cycles per bat and experimental session. All data are representative examples obtained from one bat (RF7). (A) Example of `normal' DSC behavior, i.e. lowering of call frequencies below RF (circles, bottom trace) in response to playback frequencies above RF (squares, top trace). For each vocalization (=event), the call's maximum frequency (circles) and the corresponding frequency shift introduced in the echo mimic (squares) were determined. Maximum frequency shift in the echo mimic, +3 kHz relative to RF; modulation rate, 0.1 Hz, 40 dB attenuation. (B) Example of `inverse' DSC behavior, i.e. raising of call frequencies above RF (circles, top trace) in response to playback frequencies below RF (squares, bottom trace). Same conventions as in A. Maximum frequency shift in echo mimic, -1.5 kHz relative to RF; modulation rate, 0.1 Hz, 20 dB attenuation. (C) Maximum frequency shift in echo mimic, -3 kHz relative to RF; modulation rate, 0.1 Hz, 20 dB attenuation. (D) Maximum frequencies of calls relative to RF (ordinate) plotted against the corresponding playback frequencies relative to RF (abscissa) for three different maximum frequency shifts (squares, -1.5 kHz; circles, -3 kHz; triangles, -4.5 kHz; N, number of calls analyzed for three bats). Modulation rate, 0.1 Hz, 20 dB attenuation. The three curves are the result of a non-linear regression analysis and are significantly different (Kruskal—Wallis one-way analysis of variance on ranks; P<0.001).

 


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  		Fig. 4. Effects of different audio-vocal feedback mechanisms on Doppler-shift compensation (DSC) behavior. Sensory information about different echo frequencies (B-D) is translated into motor activity that generates the corresponding call frequencies (A) using a purely inhibitory (B), all-excitatory (C) or combined excitatory/inhibitory (D) audio-vocal feedback mechanism. Note that the discussed non-linearity in the motor control system limiting call frequency increases (see Fig. 2; see also Schuller and Suga, 1976bGo; Suthers and Fattu, 1982Go) has been omitted for clarity. (A) In the motor nerve controlling the frequency of sound production by the larynx, vocalization (VOC) frequencies below resting frequency (RF) (VF1; white arrow), such as during `normal' DSC behavior (see Fig. 2A), are caused by a level of motor activity (MA1) that is lower than that generated at RF. Conversely, call frequencies above RF (VF2; black arrow), such as during `inverse' DSC behavior (see Fig. 2B-D), require a level of motor activity (MA2) that is above the resting value. (B-D) Illustrations of the three basic scenarios for how different sensory activity levels (SA1 and SA2) that are caused by echo frequencies above RF (EF1; white arrows) or below RF (EF2; black arrows) have to be integrated in a sensory-motor interface to yield the appropriate motor commands that ultimately lower and raise call frequencies (as shown in A).

 

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