Spatial release from masking improves sound pattern discrimination along a biologically relevant pulse-rate continuum in gray treefrogs

Abstract

Many frogs form large choruses during their mating season in which males produce loud advertisement calls to attract females and repel rival males. High background noise levels in these social aggregations can impair vocal perception. In humans, spatial release from masking contributes to our ability to understand speech in noisy social groups. Here, we tested the hypothesis that spatial separation between target signals and 'chorus-shaped noise' improves the ability of female gray treefrogs (Hyla chrysoscelis) to perform a behavioral discrimination task based on perceiving differences in the pulsatile structure of advertisement calls. We used two-stimulus choice tests to measure phonotaxis (approach toward sound) in response to calls differing in pulse rate along a biologically relevant continuum between conspecific (50 pulses s(-1)) and heterospecific (20 pulses s(-1)) calls. Signals were presented in quiet, in colocated noise, and in spatially separated noise. In quiet conditions, females exhibited robust preferences for calls with relatively faster pulse rates more typical of conspecific calls. Behavioral discrimination between calls differing in pulse rate was impaired in the presence of colocated noise but similar between quiet and spatially separated noise conditions. Our results indicate that spatial release from energetic masking facilitates a biologically important temporal discrimination task in frogs. We discuss these results in light of previous work on spatial release from masking in frogs and other animals.

Keywords: ANOVA; FET; FFT; Fisher's exact test; GEE; HSD; IC; ILD; ITD; RMS; SNR; SPL; analysis of variance; fast Fourier transform; generalized estimating equation; honestly significant difference; inferior colliculus; interaural level differences; interaural time difference; root mean square; s.d.; s.e.m.; signal-to-noise ratio; sound pressure level; standard deviation; standard error of the mean.

Fig. 1

Real and synthetic signals and ‘chorus-shaped’ noise. A) Oscillograms of natural conspecific (H. chrysoscelis; 50 pulses s⁻¹ [50 pps]) and heterospecific (H. versicolor; 20 pulses s⁻¹ [20 pps]) calls are shown on the left; synthetic calls with pulse rates spanning a continuum between 50 pulses s⁻¹ [50 pps] and 20 pulses s⁻¹ [20 pps] are shown on the right. B) Frequency spectra of the signals (shaded gray area) and chorus-shaped noise (solid black line). C) A 10-s segment of the waveform of a chorus-shaped noise.

Fig. 2

Experimental set-up. A) Schematic diagram of the 2-m diameter test arena enclosed in a hemi-anechoic sound chamber. Signals and noise were broadcast from speakers positioned on the floor around the perimeter of the arena. Subjects were placed in the release cage at the start of a test. B) Schematic diagram of the 2-m diameter test arena floor showing the two speaker pairs on opposite sides of the arena. In the quiet condition, alternating signals were broadcast in the absence of noise. In the colocated condition, alternating signals and continuous chorus-shaped noise were broadcast from the same pair of speakers. In the separated condition, alternating signals and continuous chorus-shaped noise were broadcast from opposite pairs of speakers.

Fig. 3

Subject responses in six choice tests conducted in three acoustic conditions and at two signal levels. Points depict the proportions (±95% exact binomial confidence intervals) of subjects that responded or chose the alternative with the relatively faster pulse rate in quiet conditions (white circles), in the presence of spatially separated chorus-shaped noise (gray circles), and when signals and masking noise were colocated (black circles). A & B) The proportions of all subjects that chose one of the two calls in choice tests [P(responsive); N = 40]. C & D) The proportions of all subjects that chose calls with faster pulse rates [P(faster); N = 40]. E & F) The proportions of responsive subjects that chose calls with faster pulse rates [P(faster|response)]. Numbers below the circles in E and F indicate the total number of responsive subjects in each choice test. In panels A, C, E, signals were broadcast at 85 dB and chorus-shaped noise was broadcast at 82 dB SPL (+3 dB SNR). In panels B, D, F, both signals and chorus-shaped noise were broadcast at 82 dB SPL (0 dB SNR). The dashed lines in E and F indicate the proportion of responsive subjects expected to choose the faster pulse rate by chance in a two-stimulus choice test (50%).

Fig. 4

Responses latencies in six choice tests conducted in three acoustic conditions and at two signal levels. Points and error bars depict means ± s.e.m. White and black circles indicate response latencies, respectively, at signal levels of 85 dB (+3 dB SNR in noise; white circles) and 82 dB (0 dB SNR in noise; black circles). Response latencies were not assigned to subjects that failed to respond; therefore, sample sizes varied for each combination of acoustic condition, signal level, and choice stimuli (16 ≤ N ≤ 40).

Fig. 5

Characterization of the sound field in quiet, colocated, and separated conditions. Measurements of the sound field during playbacks of 50 pulses s⁻¹ signals alone, chorus-shaped noise alone, and signals and noise together in both colocated and separated conditions are depicted as sound maps over the surface of the floor of the test arena. Prior to taking measurements, signals were calibrated to 85 dB SPL or 82 dB SPL and noise was calibrated to 82 dB SPL at the center of the arena. A) Signal levels across the arena floor in quiet (in dB SPL). B) Noise levels (in dB SPL) and SNRs (in dB) across the arena floor in the colocated condition. C) Noise levels (in dB SPL) and SNRs (in dB) across the arena floor in the separated condition. Data for target signals broadcast from the left and the right signal speakers are shown separately. Letters above speaker icons indicate broadcasted sounds; S1 = signal broadcast from speaker 1, S2 = signal broadcast from speaker 2; N = chorus shaped noise.