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1. The laryngeal mechanisms for the production of CF-FM orientation sounds in the horseshoe bat,Rhinolophus ferrumequinum, were studied with 3 different methods: a) denervation of laryngeal muscles, b) recordings of electrical activity of the cricothyroid muscle during vocalization and Doppler-shift compensation, and c) electrical stimulation of the cricothyroid muscles. 2. Unilateral section of the inferior laryngeal nerve (i.e. denervation of the intrinsic laryngeal muscles) had no effect on the sound pattern, the frequency of the orientation sounds and the Doppler-shift compensation. Bilateral denervation led always to suffocation. 3. Unilateral section of the superior laryngeal nerve (i.e. the denervation of the cricothyroid muscles) did not change the sound pattern, but caused a decrease in the frequency of the emitted CF by 4–6 kHz. Doppler-shift compensation was possible, but it was unstable and was not accurate. 4. Bilateral denervation of the cricothyroid muscles introduced several strong harmonics in the orientation sounds. The fundamental frequencies changed considerably between 12 and 42 kHz after surgery. The frequency pattern in each harmonic is the same as in normal orientation sounds (Fig. 1). 5. The spike number per vocalization of cricothyroid muscle fibres was proportional to the frequency of the CF component of the orientation sound in a range between the resting frequency and 5 kHz below it (Fig. 2). 6. Electrical stimulation of the cricothyroid muscles increased the frequency of the emitted CF. The maximum frequency change due to electrical stimulation was about 45 Hz per ms when measured during the CF component of the orientation sound (Fig. 3). 7. Tetanus fusion frequency of the cricothyroid muscle was about 200 Hz for continuous stimulation and 400 Hz for short stimulation applied during vocalization.

The activity of the external (motor) branch of the superior laryngeal nerve (SLN), innervating the cricothyroid muscle, was recorded in the greater horseshoe bat,Rhinolophus ferrumequinum. The bats were induced to change the frequency of the constant frequency (CF) component of their echolocation signals by presenting artificial signals for which they Doppler shift compensated. The data show that the SLN discharge rate and the frequency of the emitted CF are correlated in a linear manner.

1. The cochlea of the horseshoe bat,Rhinolophus ferrumequinum, was frequency mapped by exposing for 30 min to one or two continuous pure tones of intensities between 70 and 110 dB SPL. The evaluation was made by differentiating between normal and swollen nuclei of the outer hair cells (OHC) of the organ of Corti and by measuring the diameter of the nuclei of the OHC. 2. In control animals the radial diameter of the OHC nuclei varies systematically from a mean of 2.85 μm at the base to 3.2 um at the apex (Fig. 1). 3. All frequencies used for exposure were normalized to the resting frequency (FR), which is the frequency of the pure tone component of the orientation sound in a non-flying bat. The individual FR lay between 82.6 and 83.3 kHz. 4. For analysing the small frequencies between 83.0 to 86.0 kHz in which relevant echoes occur, 3.15 mm length of the basilar membrane is used, about the same length as for the octaves from FR/4 to FR/2 (2.85 mm) and from FR/2 to FR (3.2 mm) (Fig. Ca, b). 5. The discontinuity of the mechanical data at 4.5 mm of the length of the basilar membrane (part I of this paper) coincides with FR and the less pronounced discontinuity at 7.8 mm coincides with FR/2. 6. Location and mechanism of the auditory filter are discussed.

1. The inferior colliculus of 8 Greater Horseshoe bats (Rhinolophus ferrumequinun) was systematically sampled with electrode penetrations covering the entire volume of the nucleus. The best frequencies and intensity thresholds for pure tones (Fig. 2) were determined for 591 neurons. The locations of the electrode penetrations within the inferior colliculus were histologically verified. 2. About 50% of all neurons encountered had best frequencies (BF) in the frequency range between 78 and 88 kHz (Table 1, Fig. 1A). Within this frequency range the BFs between 83.0 and 84.5 kHz were overrepresented with 16.3% of the total population of neurons (Fig. 1B). The frequencies of the constant frequency components of the echoes fall into this frequency range. 3. The representation of BFs expressed as number of neurons per octave shows a striking correspondence to the nonuniform innervation density in the afferent innervation of the basilar membrane (Bruns and Schmieszek, in press). The high innervation density of the basilar membrane in the frequency band between 83 and 84.5 kHz coincides with the maximum of the distribution of number of neurons per octave across frequency in the inferior colliculus (Fig. 1 C). 4. The disproportionate representation of frequencies in the auditory system of the greater horseshoe bat is described as an acoustical fovea functioning in analogy to the fovea in the visual system. The functional importance of the Doppler-shift compensation for such a foveal mechanism in the auditory system of horseshoe bats is related to that of tracking eye movements in the visual system.

The activity of the recurrent laryngeal nerve (RLN) was recorded in the greater horseshoe bat,Rhinolophus ferrumequinum. Respiration, vocalization and nerve discharges were monitored while vocalizations were elicted by stimulation of the central gray matter. This stimulation evoked either expiration or expiration plus vocalization depending on the stimulus strength. When vocalization occurred it always took place during expiration. Recordings from the RLN during respiration showed activity during the inspiration phase, but when vocalization occurred there was activity during inspiration and expiration. These results are consistent with the view that the RLN innervates muscles which control the opening and closing of the glottis. During vocalization the vocal folds are closely approximated and the discharge patterns of the nerve suggests that it controls the muscles which start and end each pulse.

1. Recordings were made from 135 single inferior colliculus neurons of the Greater horseshoe bat,Rhinolophus ferrumequinum. These bats emit sonar signals which are characterized by a long constant frequency (CF-) component and a terminal frequency modulated (FM-) component. The responses of the units to CF- and FM-stimuli with different modulation heights, durations and directions were studied. 2. The majority of single units studied had minimum thresholds that were either equal for CF- and FM-signals of the same duration or lower for the CF-stimulus. Only rarely was the sensitivity for FM-signals higher than for a CF-signal. 3. With FM-signals the response properties of units with best frequencies between 65–81 kHz, i.e. in the frequency range of the FM-component of the echolocation call, did not differ significantly from units with best frequencies in lower frequency ranges. Relatively many units with best frequencies in the filter region of the audiogram (81–88 kHz) required FM-signals with particular modulation heights and rates to elicit excitatory responses. 4. Response patterns and spike count functions were found to differ with CF- vs. FM-signals. The most common difference was a smaller number of impulses per stimulus and a temporally more restricted (‘phasic’) discharge pattern to FM-signals than to CF-signals. Greater discharge activity to FM-signals occurred in only a few units. ‘Latency constant’ and ‘FM-specialized’ neurons, as reported for other species were also found. 5. Inhibition was frequently observed with FM-stimuli in spontaneously active neurons. An accurate prediction of a neuron's response to FM-signals from knowledge of the excitatory and inhibitory response areas to CF-signals was not always possible. 6. The responses of ‘FM-specialized’ neurons to the FM-component of a CF-FM-stimulus was not significantly altered by the presence of a CF-component in the filter region of the audiogram. 7. Responses of single units to signal and noise combinations are described. 8. Data are discussed in relation to results on the processing of FM-signals in other bats.

The directionality of sound emission by a horseshoe bat (Rhinolophus ferrumequinum) has been determined for the constant frequency component of its orientation sounds. The bat was fixed in the center of an acoustic perimeter and the SPL of the orientation sounds measured with a scanning microphone at different angles compared with the SPL measured by another microphone located in the direction perpendicular to the plane of the horseshoe-like structure of the nose-leaf. The maximum SPL was always found in this direction which also corresponds to the flight direction of a bat in horizontal flight. Above and lateral to this direction the SPL decreases steadily with -6 dB-points at 24 ‡ above and 23 ‡ lateral. Below the flight direction we found a prominent side lobe with a -6 dB-point at 64 ‡.

1. When catching flying prey under laboratory conditionsRhinolophus ferrumequinum typically emit FM-CF-FM signals (Fig. 2). Except for the last two sounds during approach and final buzz the FM-parts are fainter than the CF-component by a factor of 0.76+-14 (Fig. 1). The final FM-part was undetectable in some signals emitted during approach (Fig. 3). 2. In obstacle avoidance flights both, preceding and final FM-parts are prominent and louder than the CF-part by a factor of 1.14 to 1.63 (Fig. 1 and 3). Bandwidths of the FM-components increased from ca. 3.5 to 12 kHz for the starting and from 12 to 20 kHz on the average for the final FM-sweep (Table 1). 3. In the open field during cruising flightsNyctalus noctula emits pure tones of 22.5 to 25.0 kHz without any frequency modulated components and a duration of 10 to 50 ms (Fig. 4). Brief frequency modulated signals sweeping from ca. 50 to 20 kHz in about 1–2 ms are emitted during pursuit of prey (Fig. 4). 4. Under laboratory conditionsNyctalus noctula does not emit pure tones and is not able to catch flying prey in a flight chamber 10.5×3.5×2.15 m in size. During flights towards a landing platformNyctalus noctula invariably emits brief frequency modulated pulses. During an individual flight the structure is not changed (Fig. 5). 5. InNyctalus noctula specific features of the echolocation pulses, e.g. frequency range swept through, presence of harmonics and double pulses (Fig. 6) are maintained during an individual flight. These specific characteristics of the signal may be used to identify echoes belonging to its own emitted echolocation pulse.

The greater horseshoe bat (Rhinolophus ferrumequinum) emits echolocation sounds consisting of a long constant-frequency (CF) component preceeded and followed by a short frequency-modulated (FM) component. When an echo returns with an upward Doppler-shift, the bat compensates for the frequency-shift by lowering the emitted frequency in the subsequent orientation sounds and stabilizes the echo image. The bat can accurately store frequency-shift information during silent periods of at least several minutes. The stored frequency-shift information is not affected by tone bursts delivered during silent periods without an overlap with an emitted orientation sound. The system for storage of Doppler-shift information has properties similar to a sample and hold circuit with sampling at vocalization time and with a rather flat slewing rate for the stored frequency information.