Behavioral measures of signal recognition thresholds in frogs in the presence and absence of chorus-shaped noise

Abstract

Anuran amphibians are superb animal models for investigating the mechanisms underlying acoustic signal perception amid high levels of background noise generated by large social aggregations of vocalizing individuals. Yet there are not well-established methods for quantifying a number of key measures of auditory perception in frogs, in part, because frogs are notoriously difficult subjects for traditional psychoacoustic experiments based on classical or operant conditioning. A common experimental approach for studying frog hearing and acoustic communication involves behavioral phonotaxis experiments, in which patterns of movement directed toward sound sources indicate the subjects' perceptual experiences. In this study, three different phonotaxis experiments were conducted using the same target signals and noise maskers to compare different experimental methods and analytical tools for deriving estimates of signal recognition thresholds in the presence or absence of "chorus-shaped noise" (i.e., artificial noise with a spectrum similar to that of real breeding choruses). Estimates of recognition thresholds based on measures of angular orientation, response probabilities, and response latencies were quite similar in both two-choice and no-choice phonotaxis tests. These results establish important baselines for comparing different methods of estimating signal recognition thresholds in frogs tested in various masking noise conditions.

Figure 1

Target signal and chorus-shaped noise. (a) Waveforms of the synthetic H. chrysoscelis call comprising the target signal (top) and a segment of chorus-shaped noise (bottom). (b) Power spectra showing the spectral profile of the target signal (black line) relative to that of the chorus-shaped noise (gray area).

Figure 2

Response probabilities and phonotaxis scores based on two-choice tests in Experiment 1 (left) and no-choice tests in Experiment 2 (right). [(a) and (e)] Response probabilities in the no-noise groups showing the numbers of individuals (total N=20) that exhibited the two types of responses, correct responses (1.0) and incorrect or no responses (0.0). For each response type, the relative sizes of the two paired points at each signal level depict the numbers of individuals exhibiting that response type, with the largest point corresponding to 20 individuals, and the smallest point corresponding to 1 individual (the absence of a point corresponds to zero individuals). The smooth curve represents the best-fit logistic regression function fitted to the response probabilities depicted in the figure. [(b) and (f)] Response probabilities and logistic regression functions (as in panels a and e) for the noise groups. [(c) and (g)] Comparison of the fitted logistic regression functions for the no-noise groups (solid lines) and the noise groups (dashed lines) for each experiment. [(d) and (h)] Mean (±s.e.m.) phonotaxis scores with best-fit sigmoid curves from Eq. 2 for responses in the no-noise groups (circles) and the noise groups (squares) for each experiment. ^*P<0.05 in a Bonferroni post hoc test comparing phonotaxis scores in the no-noise and noise groups at each signal level.

Figure 3

Individual recognition thresholds based on no-choice tests in the no-noise and noise groups of Experiment 3. Depicted here are the median (point), inter-quartile range (box), and range (whiskers).