Finding a mate at a cocktail party: Spatial release from masking improves acoustic mate recognition in grey treefrogs

Abstract

The 'cocktail party problem' refers to the difficulty that humans have in recognizing speech in noisy social environments. Many non-human animals also communicate acoustically in noisy social aggregations, and thus also encounter - and solve - cocktail-party-like problems. Relatively few studies, however, have investigated the processes by which non-human animals solve sound source segregation problems in the behaviourally relevant context of acoustic communication. In humans, 'spatial release from masking' contributes to sound source segregation by improving the ability of listeners to recognize speech that is spatially separated from other sources of speech or 'speech-shaped' masking noise. Using a phonotaxis paradigm, I tested the hypothesis that spatial release from masking improves the ability of female grey treefrogs, Hyla chrysoscelis, to discriminate between conspecific and heterospecific calls that were spatially separated from two sources of 'chorus-shaped' masking noise by either 15° or 90°. As the signal-to-noise ratio (SNR) was decreased from +3 dB to -15 dB (by decreasing the signal level in 6-dB steps), fewer females made a choice and the likelihood of a female choosing the heterospecific call also increased. At a SNR of -3 dB, females oriented toward and chose the conspecific call in the 90° separation condition, but not when signals and maskers were separated by 15°. These results support the hypothesis that a well-known solution to the cocktail party problem in humans - spatial release from masking - also plays a role in acoustic signal recognition in animals that communicate in biological equivalents of cocktail-party-like environments.

Figure 1

Schematic diagram illustrating the circular test arena divided into 15° bins along the perimeter and the positions of speakers broadcasting signals and chorus-shaped noises in (a) the grouped condition and (b) the separated condition. Note that the frog and speakers are not drawn to scale.

Figure 2

(a–b) Oscillograms of the (a) conspecific (H. chrysoscelis) signal and (b) the heterospecific (H. versicolor) signal depicting a temporal window of 1.5 s around each call. Insets in (a) and (b) depict a single pulse from the corresponding call. The pulses in the H. chrysoscelis call (a) were 11 ms in duration and separated by 11 ms inter-pulse intervals (pulse period = 22 ms; pulse duty cycle = 50%) and were shaped with onsets (4 ms) and offsets (7 ms) that had half-amplitude times that were 43% and 53% of the duration of the onset and offset times, respectively. The pulses in the H. versicolor call (b) were 30 ms in duration and separated by 30 ms inter-pulse intervals (pulse period = 60 ms; pulse duty cycle = 50%) and were shaped with onsets (20 ms) and offsets (10 ms) that had half-amplitude times that were both 55% of the duration of the respective envelope ramp. Note that this H. versicolor call was effective at eliciting phonotaxis from females of H. versicolor (M. A. Bee unpublished data). (c) Power spectrum of the chorus-shaped noise.

Figure 3

Histograms showing the proportion of responsive females making a choice that chose the conspecific call as a function of SNR. Results are shown separately for the grouped (grey bars) and separated (white bars) conditions. The numbers associated with each bar depict the ratio of the number of females that chose the conspecific call to the number of females that actually made a choice in that condition; N represents the total number of females that were tested in that condition. The error bars depict either the upper or lower 95% confidence interval around the proportion of females choosing and the horizontal dashed lines depict the expected proportion according to the null hypothesis that females would choose the conspecific and heterospecific calls in a 50:50 ratio. Asterisks indicate that significantly more females than expected by chance chose the conspecific call (one-tailed binomial P < 0.05).