Spatial hearing in Cope’s gray treefrog: II. Frequency-dependent directionality in the amplitude and phase of tympanum vibrations

Abstract

Anuran ears function as pressure difference receivers, and the amplitude and phase of tympanum vibrations are inherently directional, varying with sound incident angle. We quantified the nature of this directionality for Cope’s gray treefrog, Hyla chrysoscelis. We presented subjects with pure tones, advertisement calls, and frequency-modulated sweeps to examine the influence of frequency, signal level, lung inflation, and sex on ear directionality. Interaural differences in the amplitude of tympanum vibrations were 1–4 dB greater than sound pressure differences adjacent to the two tympana, while interaural differences in the phase of tympanum vibration were similar to or smaller than those in sound phase. Directionality in the amplitude and phase of tympanum vibration were highly dependent on sound frequency, and directionality in amplitude varied slightly with signal level. Directionality in the amplitude and phase of tone- and call-evoked responses did not differ between sexes. Lung inflation strongly affected tympanum directionality over a narrow frequency range that, in females, included call frequencies. This study provides a foundation for further work on the biomechanics and neural mechanisms of spatial hearing in H. chrysoscelis, and lends valuable perspective to behavioral studies on the use of spatial information by this species and other frogs.

Sound source localization and segregation with internally coupled ears: the treefrog model

“…In H. chrysoscelis, these ovoidal patterns of directionality are largely symmetrical about the transverse plane. As described later, this forward–rearward symmetry provides a simple biophysical explanation for the inability of females of this species to distinguish sounds coming from forward versus rearward directions in some behavioral tests of sound source localization (Caldwell and Bee 2014; Caldwell et al 2014). …”

“…These data did not reflect limitations in turning ability. Rather, as discussed above, this result was predicted by the high degree of forward–rearward symmetry in the directionality of the tympanum’s vibration amplitude (Caldwell et al 2014). …”

“…Biophysical measurements of tympanum vibrations have been undertaken using laser Doppler vibrometry in four Hyla species for which source localization and segregation have also been investigated: H. versicolor (Jørgensen and Gerhardt 1991), H. gratiosa (Jørgensen and Gerhardt 1991), H. cinerea (Michelsen et al 1986), and H. chrysoscelis (Caldwell et al. 2014).…”

“…The low-frequency peak of the tympanum’s frequency response most likely is generated by input from the lung, at least in some species. Figure 3a shows an example of tympanum transfer functions from H. chrysoscelis (Caldwell et al 2014). The bimodal spectrum has a low-frequency peak at 1.4 kHz, close to the low-frequency peak of the species’ advertisement call (1.25 kHz), and a high-frequency peak at 2.5 kHz, corresponding to the high-frequency peak of the call.…”

“… See main text for additional details and full citations Vibration amplitude difference (VAD): the difference between vibration amplitudes of the measured ear across different angles of sound incidence Interaural vibration amplitude difference (IVAD): the difference between the vibration amplitudes of the two tympana across different angles of sound incidence computed by either assuming the directionality patterns of the two ears is mirror images or by simultaneously measuring both tympana 1 Michelsen et al (1986) 2 Jørgensen and Gerhardt (1991) 3 Jørgensen and Gerhardt (1991) 4 Caldwell et al (2014) 5 Jørgensen and Gerhardt (1991) 6 Ho and Narins (2006) …”