Inherent Directionality Determines Spatial Release from Masking at the Tympanum in a Vertebrate with Internally Coupled Ears

Abstract

In contrast to humans and other mammals, many animals have internally coupled ears that function as inherently directional pressure-gradient receivers. Two important but unanswered questions are to what extent and how do animals with such ears exploit spatial cues in the perceptual analysis of noisy and complex acoustic scenes? This study of Cope's gray treefrog (Hyla chrysoscelis) investigated how the inherent directionality of internally coupled ears contributes to spatial release from masking. We used laser vibrometry and signal detection theory to determine the threshold signal-to-noise ratio at which the tympanum's response to vocalizations could be reliably detected in noise. Thresholds were determined as a function of signal location, noise location, and signal-noise separation. Vocalizations were broadcast from one of three azimuthal locations: frontal (0 °), to the right (+90 °), and to the left (-90 °). Masking noise was broadcast from each of 12 azimuthal angles around the frog (0 to 330 °, 30 ° separation). Variation in the position of the noise source resulted in, on average, 4 dB of spatial release from masking relative to co-located conditions. However, detection thresholds could be up to 9 dB lower in the "best ear for listening" compared to the other ear. The pattern and magnitude of spatial release from masking were well predicted by the tympanum's inherent directionality. We discuss how the magnitude of masking release observed in the tympanum's response to spatially separated signals and noise relates to that observed in previous behavioral and neurophysiological studies of frog hearing and communication.

Keywords: acoustic communication; cocktail party problem; pressure-gradient receiver; sound source segregation; spatial unmasking; vocal communication.

FIG. 1

Measuring signal detection thresholds for the tympanum’s response to advertisement calls presented in chorus-like noise. A Waveforms depicting a natural advertisement call of Hyla chrysoscelis (top) and the synthetic call (bottom) used in the present study. The amplitudes of both calls have been normalized to the same value; scale bars indicate time. B Schematic illustration of the spatial arrangements of signals and noise in azimuth relative to a subject and the measurement laser. Signals were broadcast from one of three positions relative to the subject’s snout (0, +90, and −90 °). Noise was presented from 12 locations around the animal separated by 30 ° (e.g., 0, +30, +60 °, etc.). The configuration illustrated here includes a signal location of +90 ° and a noise location of −60 ° (+90/−60 °). The laser for measurement was positioned at +115 °. Objects in this schematic are not drawn to scale. C Data from one animal (mhch041) showing computed d _a values with fitted exponential fits as a function of signal level for a co-located condition (open circles and dashed line, +90/+90 °) and a separated condition (closed circles and solid line, +90/−60 °). The solid horizontal line depicts the threshold criterion (d _a = 2), and the vertical dashed lines indicate the signal levels at which the two fitted exponential curves cross the threshold criterion. The threshold difference between the two vertical lines corresponds to the magnitude of spatial release from masking (SRM).

FIG. 2

Polar plots showing mean signal detection thresholds (SDT) or threshold differences (ΔSDT). All plots show data measured from the right tympanum for the absolute signal and noise locations indicated in azimuth around a subject with its snout oriented toward 0 ° (frontal). Distances along the radial axes correspond to thresholds or threshold differences measured in dB. A Mean signal detection thresholds in response to calls broadcast from three signal locations (0, +90, and −90 °; see text) in the presence of chorus-like noise broadcast from 12 different azimuthal angles (0 to 330 °, 30 ° steps; plotted on the angular axis). B SRM depicted as the mean differences in signal detection thresholds as functions of signal location and noise location computed relative to the three co-located conditions (signal/noise, 0/0 °; +90/+90 °; and −90/−90 °). C Mean differences in signal detection thresholds as functions of signal location and noise location computed relative to the frontal noise presented at 0 ° (signal/noise, 0/0 °; +90/0 °; and −90/0 °). Also plotted are the amplitude of tympanum vibrations and sound pressure level adjacent to the right tympanum in response to noise alone after also standardizing these values to their magnitude at the 0 ° noise location. In B and C, positive values indicate masking release and negative values indicate greater masking relative to the co-located conditions (in B) and the frontal noise condition (in C). D Mean best-ear advantage as functions of signal location and noise location. Values were calculated from measurements taken at the right tympanum, assuming bilateral symmetry in the mechanical response of the two tympana.

FIG. 3

Relationship between body size and signal detection thresholds. The scatterplot shows the best-fit regression line for the relationship between the individual means of signal detection thresholds (averaged across signal and noise locations within each individual) and individual scores on the first principal component from a principal component analysis of three body size measures (mass, length, and interaural distance).