O.N. Larsen

Biophysics of the ensiferan ear

Summary1.Laser vibrometry is used for observing vibrations of tympanal membranes. With this technique, the velocity and phase lag of vibrations can be measured from areas of 5–100 μm in diameter. The resolution of the apparatus is 10 Ångströms (vibration amplitude) in ‘real time’ broad-band operation (up to 100 kHz), but with averaging or selective filtering it is possible to perform more sensitive measurements.2.At ‘low’ frequencies (1–3 kHz) the sound acting upon the back of the bushcricket tympanum has almost the same numerical pressure as that acting on the front. The ear, therefore, is an ideal pressure gradient receiver.3.At ‘high’ frequencies (above 10 kHz) the sound pressure acting on the back of the tympanum is somewhat larger than that acting on the front. The membrane vibrates in its basic mode (like a piston) in the entire frequency range investigated (1–40 kHz).4.These properties are for ears with open ‘hearing trumpets’ (i.e., the horn-shaped trachea conducting sound to the back of the tympanal membranes). When the opening of the hearing trumpet is closed by blocking it with wax, the ear becomes a pressure receiver (at least at ‘low’ frequencies). It is now more sensitive to ‘low’ frequencies and less sensitive to ‘high’ frequencies.5.The vibrations of a membrane backed by a horn-shaped structure are considered theoretically. The expected variation in amplitude and phase angle of the driving force is calculated for systems in which sound is conducted through the horn-shaped structure with different amounts of gain.

Strategies for Acoustic Communication in Complex Environments

When performing his pioneering studies on the reactions of moths and other nocturnal insects to hunting bats, Kenneth D. Roeder had to get acquainted with acoustics and especially with ultrasound. He did this very well, but he never became interested in acoustics as a scientific subject. He was much more interested in the behavioural significance of the ultrasonic bat cries. His interests were always centered around the behaving animal in its natural environment, and he was well aware of the limitations of laboratory studies of the behaviour (Roeder 1970). A substantial part of Roeder’s impact on the development of neuroethology was due to his repeated demonstration that, although it is necessary to study many details in the laboratory, studies of behaviour should start and end in the field.

Temporal coding in the auditory receptor of the moth ear

SummaryTemporal coding in the moth ear was inferred from the response of the auditory receptor to acoustic stimuli with different temporal characteristics.1.Determinations of the threshold with different stimulus pulse durations showed that the moth ear behaves as an energy detector with a maximum time constant (the integration time) of 25 ms. Pulse durations beyond this value did not result in decreased thresholds (Fig. 1).2.The synchronization to amplitude modulations was determined by stimulating the moth ear with amplitude modulated (AM) tones (carrier frequency: 40 kHz) and AM white noise presented as 450 ms pulses separated by pauses of similar length. The modulation depth was constant (100%) whereas the modulation frequency,fm, was varied. The maximumfm which the auditory receptors could follow was 200 Hz (P<0.05) (Figs. 2, 3, 4).3.The relatively broad tuning of the only receptor which was functional at the relevant stimulus intensities suggested that AM detection could only be based on temporal cues. This was confirmed by the results showing the same degree of synchronization independent of carrier.4.A minimum time constant for the receptor was also determined by interrupting a 400 ms noise pulse by a gap (Figs. 5, 6). The threshold for gap detection of the moth ear was ca. 2 ms on a 2.5% significance level (one sided test).5.The temporal acuity reported here seems to be fine enough to explain the temporal resolution suggested by behavioral results from other insect species. The results are discussed in relation to acoustic communication in insects as well as in relation to temporal resolution in vertebrates.

Mechanical time resolution in some insect ears

Summary1.The mechanical time resolution is estimated in the ears of noctuid moths (Noctuidae) and locusts (Acrididae). The vibration velocity of small areas on the tympanal membrane is measured by means of laser vibrometry. The impulse response (Figs. 2B and 5 A) and the transfer function (Fig. 3) are obtained directly by stimulation with very short impulse sounds and pure tones, respectively. The transfer function is also calculated from the experimentally determined impulse response, and vice versa. Finally, the impulse response is obtained by calculation from the measured vibrations caused by noise. The directly determined and the calculated transfer functions are rather similar (Fig. 3A-B).2.The impulse response of the attachment area of the receptor cells in thenoctuid ear is a short, damped vibration with a ‘time constant’ of about 60 μs (Table 1). The attachment area of the receptor cells can thus separate impulses arriving with time intervals larger than 150–200 μs (Fig. 4).3.The ‘time constant’ of the attachment area of the d-cells in thelocust tympanum is about 90 μs (Table 1). The ‘time constant’ for other parts of the locust tympanum varies between 50 μs and 200 μs (Fig. 6).

Avian species differences in susceptibility to noise exposure.

Previous studies of hair cell regeneration and hearing recovery in birds after acoustic overstimulation have involved relatively few species. Studies of the effects of acoustic overexposure typically report high variability. Though it is impossible to tell, the data so far also suggest there may be considerable species differences in the degree of damage and the time course and extent of recovery. To examine this issue, we exposed four species of birds (quail, budgerigars, canaries, and zebra finches) to identical conditions of acoustic overstimulation and systematically analyzed changes in hearing sensitivity, basilar papilla morphology, and hair cell number. Quail and budgerigars showed the greatest susceptibility to threshold shift and hair cell loss after overstimulation with either pure tone or bandpass noise, while identical types of overstimulation in canaries and zebra finches resulted in much less of a threshold shift and a smaller, more diffuse hair cell loss. All four species showed some recovery of threshold sensitivity and hair cell number over time. Canary and zebra finch hearing and hair cell number recovered to within normal limits while quail and budgerigars continued to have an approximately 20 dB threshold shift and incomplete recovery of hair cell number. In a final experiment, birds were exposed to identical wide-band noise overstimulation under conditions of artificial middle ear ventilation. Hair cell loss was substantially increased in both budgerigars and canaries suggesting that middle ear air pressure regulation and correlated changes in middle ear transfer function are one factor influencing susceptibility to acoustic overstimulation in small birds.

Tympanal membrane motion is necessary for hearing in crickets

Summary1.Sound guided through the tracheal tube to the internal tracheal spaces in the region of the cricket ear is capable of eliciting auditory neural responses in the prothoracic ganglion if the tympanal membrane is allowed to vibrate freely. If the tympanal membrane motion is prevented mechanically neural responses are abolished (Fig. 3) whereas the sound pressure in the tracheal air spaces behind the tympanum is increased.2.If the motion of the tympanum, as measured with laser vibrometry, is prevented by adjusting the internal and external sound pressure, then neural responses cease simultaneously (Fig. 5).3.These findings demonstrate that motion of the large tympanum is a necessary requisite in the sound transduction process of the cricket ear.

Directionality of the cricket ear: A property of the tympanal membrane

Heavy Metals in the Environment, Heidelberg 1983, p. 888 2. Schwedhelm, E., Irion, G. : ibid., p. 1037; Miiller, G., Irion, G.: Mitt. Geol.-Palfiont. Inst. Univ. Hamburg 56, 423 (1984); Schwedhelm, E., Irion, G. : Courir Forschungsinst. Senckenberg (in Vorbereitung) 3. Reineck, H.-E. : Natur u. Museum 83, 102 (1963) 4. Irion, G., Wunderlich, F. : Senckenbergiana Maritima (in Vorbereitung) 5. Anonymos: Dtsch. Hydrogr. Inst. 2500, 97 (1982) 6. Mfiller, G., et al.: Naturwissenschaften 67, 595 (1980) 7. Bundesanstalt ffir Gewfisserkunde, Bericht BfG-0075 (I 982)

Auditory Processing of Temporal Cues in Insect Songs: Frequency Domain or Time Domain?

This contribution is an attempt to answer the questions: 1) Is it possible from the present knowledge of peripheral and central auditory processing to account for the behavioural preferences shown by listening insects? 2) Do we have evidence elucidating whether the processing of rapid changes in the sound signals is done in the time domain, in the frequency domain, or in both domains?

Acoustic Equipment and Sound Field Calibration

High-quality microphones make it possible to select loudspeakers with appropriate specifications and to test the sound field. Shortcomings in the loudspeakers’ frequency responses may be overcome by using digital signal processing techniques. Only under free-field conditions do sounds reach the experimental subject from the loudspeaker alone. Free-field conditions rarely exist even in anechoic rooms, since echoes from the setup create an inhomogeneous sound field, but sound reflecting objects can be located by measuring the acoustic impulse response of the setup, and their disturbance of the field can be minimized by reducing their dimensions or covering them with sound absorbing material. Outside the sonic range special problems prevail, especially at very low frequencies.