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.

Fig. 1

Directionality in TVA, TVP, and the amplitude and phase of sound pressure adjacent to the tympanum during tone presentations. a Mean TVAs from 12 H. chrysoscelis in response to tones presented from 12 azimuthal angles and 5 frequencies. For playbacks at each tone frequency, results across all signal levels have been standardized to velocity measured at 0° (frontal direction). Ipsilateral directions are positive, contralateral directions are negative. b Amplitude of sound recorded adjacent to the tympanum. c Mean differences between TVA and sound amplitude measured at the tympanum, calculated from data presented in a and b. d TVPs in response to tones. Negative phase shifts indicate lags relative to response at 0°. e Phase of sound pressure adjacent to the tympanum. f Mean differences between TVP sound phase measured at the tympanum, calculated from data in d and e. Error bars represent s.e.m. In some cases, errors are obstructed by data markers

Fig. 2

Interaural comparisons of the amplitude (IVAD) and phase (IVPD) of tympanum vibrations and sound pressure adjacent to the tympanum. a Mean IVADs in response to tones presented to 12 H. chrysoscelis at 12 azimuthal angles and 5 frequencies. The black circle towards the center of the plot indicates no difference between response of the ipsilateral and contralateral ears, and the outermost circle indicates 5 dB of difference. Positive sound presentation angles occur on the side of the frog ipsilateral to the focal tympanum. Plot values extending radially at ipsilateral angles indicate higher amplitude response in the ipsilateral versus the contralateral tympanum at ipsilateral sound presentation angles. b IADs during tone presentations. c IVPDs during tone presentations. Plot values extending radially in the ipsilateral direction indicate a phase lead at the ipsilateral ear, Values extending in the contralateral direction indicate a phase lag at the ipsilataeral ear. d IPDs during tone presentations. e IVTDs during tone presentations. f ITDs during tone presentations. The function of the two ears is assumed to be symmetrical in the calculation of interaural differences

Fig. 3

Signal level and frequency dependence of directionality in TVA. Shown are mean and s.e.m for TVAs from 12 H. chrysoscelis recorded in response to tones presented from 12 azimuthal angles and 3 signal levels across 5 frequencies: a 600 Hz, b 1,250 Hz, c 1,625 Hz, d 2,500 Hz, and e 3,200 Hz. All responses are standardized to the velocity of tympanum vibration at a sound presentation angle of 0°

Fig. 4

Directionality in amplitude and phase in response to calls. a Mean ± s.e.m. amplitude of tympanum vibrations (TVA, solid line) and sound pressure adjacent to the tympanum (dashed line) in 10 H. chrysoscelis in response to calls from 16 azimuthal angles. Results across all signal levels have been standardized to velocity measured at 0°. b Mean phase shift for tympanum vibrations (TVP, solid line) and sound pressure adjacent to the tympanum (dashed line) relative to phase when sound presentation angle was 0°. Shown are shifts for the lower (1,250 Hz) and upper (2,500 Hz) call peaks. Negative phase shifts indicate that measurements at the contralateral ear lagged behind those measured at 0°. c IVADs (solid line) and IADs (dashed line). Plot values extending towards positive sound presentation angles indicate that amplitude was greater on the ipsilateral side of the frog. d IVPDs (solid line) and IPDs (dashed line) for the lower and upper call peaks. Plot values extending towards ipsilateral angles indicate a lead in phase relative to 0°

Fig. 5

Directionality in TVA for male and female H. chrysoscelis. Mean TVA spectra (peak standardized) of a males (n = 5) and b females (n = 5) in response to FM sweeps presented at 12 azimuthal angles

Fig. 6

The effects of lung inflation on directionality in TVA. Tympanum response to FM sweeps presented from 12 azimuthal angles in a typical male H. chrysoscelis with a normally inflated lungs, b deflated lungs, and c lungs that were refilled with air. Note that the spectral peak at 1,450 Hz (“peak”, red arrow) and strong directionality between 1,600 and 1,900 Hz (“trough”, black arrow) disappears when the lungs are deflated

Fig. 7

Lung effects and directionality in H. chrysoscelis. Shown is the TVA spectrum of a typical male H. chrysoscelis presented with FM sweeps from a sound presentation angle of 90°. Plots indicate spectra measured when lungs of this subject were either in a natural state of inflation, deflated, or re-inflated. Also shown is the spectrum of body wall vibrations recorded in response to the same frequency sweeps

Fig. 8

Frequencies of the lower and upper peaks in the tympanum response for male and female H. chrysoscelis. Shown are lower (open symbols) and upper (closed symbols) peaks for the five males (squares) and five females (circles) presented with FM sweeps, along with the mass of these individuals. Overlapping data markers have been offset