Spatial release from masking in a free-field source identification task by gray treefrogs

Abstract

Humans and other animals often communicate acoustically in noisy social groups, in which the background noise generated by other individuals can mask signals of interest. When listening to speech in the presence of speech-like noise, humans experience a release from auditory masking when target and masker are spatially separated. We investigated spatial release from masking (SRM) in a free-field call recognition task in Cope's gray treefrog (Hyla chrysoscelis). In this species, reproduction requires that females successfully detect, recognize, and localize a conspecific male in the noisy social environment of a breeding chorus. Using no-choice phonotaxis assays, we measured females' signal recognition thresholds in response to a target signal (an advertisement call) in the presence and absence of chorus-shaped noise. Females experienced about 3 dB of masking release, compared with a co-localized condition, when the masker was displaced 90° in azimuth from the target. The magnitude of masking release was independent of the spectral composition of the target (carriers of 1.3 kHz, 2.6 kHz, or both). Our results indicate that frogs experience a modest degree of spatial unmasking when performing a call recognition task in the free-field, and suggest that variation in signal spectral content has small effects on both source identification and spatial unmasking. We discuss these results in the context of spatial unmasking in vertebrates and call recognition in frogs.

Fig. 1

Natural and artificial choruses and signals. The left column depicts spectrograms (top traces) and waveforms (bottom traces) of a segment of a natural gray treefrog chorus (top left panel) and a natural gray treefrog advertisement call (bottom left panel). The right column depicts spectrograms (top trace) and waveforms (bottom trace) of an exemplar of chorus-shaped noise used in the masking experiment (top right panel) and the bimodal target signal (bottom right panel).

Fig. 2

Spectra of the three target signals in relation to the spectrum of chorus-shaped noise. Each plot shows the spectrum of a single target signal (shaded gray) and that of an exemplar of chorus-shaped noise used as a masker (area under the dashed line). The bimodal target signal is shown in the top panel and the two unimodal signals with carrier frequencies of 1.3 kHz and 2.6 kHz are shown in the middle and bottom panels, respectively.

Fig. 3

Test apparatus and signal-masker configurations. A) The schematic diagram on the left illustrates the 2-m diameter circular test arena in a sound chamber and shows the positions of speakers, the central release point, and the infrared (IR) sensitive camera. On the right is shown an actual screen-shot from an IR video recording of a gray treefrog responding in a phonotaxis test. The floor or the arena was dark gray and the walls black, though the latter reflect as gray under IR illumination. B) In the co-localized configuration, signals and maskers were broadcast from the same speaker. C) In the separated conditions, signals and maskers were broadcast from two speakers positioned 90° apart around the perimeter of the circular arena. Note: frogs, speakers, and arenas are not drawn to scale in the schematic representations.

Fig. 4

Signal recognition thresholds. Depicted here are the mean (points), ± s.e.m. (boxes), and ±95% confidence intervals (whiskers) for signal recognition thresholds in quiet conditions (gray) and for masked signal recognition thresholds determined in the co-localized (black) and 90° separated (white) spatial configurations.

Fig. 5

Variability in masked signal recognition thresholds and spatial unmasking across noise exemplars. Plots A–C depict data for those combinations of target signal and noise exemplar that yielded relatively lower signal recognition thresholds in the separated configuration compared with the co-localized configuration. Plots D–F show equivalent data for the remaining combinations of target signal and noise exemplar for which thresholds in the separated configuration were the same as or higher than those in the co-localized condition. Each point depicts the average of 1–3 individuals tested with a given target signal and noise exemplar in the specified signal-masker configurations; pairs of points connected by a line indicate the averages of the group of individuals tested within-subjects using the same target signal and noise exemplar in both the co-localized and separated configurations.