| Natural auditory scenes, like frog choruses, comprise multiple sound sources like individual vocalizations and background noise that overlap in the time- and frequency domains. Detection of sound in such a background is challenging, and depends on the bandwidth and modulation depth of the background noise. A process that facilitates target detection in noisy environments is comodulation masking release (CMR), whereby the detectability improves when the noise is coherently modulated across a wide frequency range. Here, we examined how noise attributes, such as bandwidth and modulation, influence the detection threshold for single neurons in response to a synthetic advertisement call. Additionally, seasonal effects on the response properties of neurons were also studied. Extracellular recordings were made from single neurons in the auditory midbrain (torus semicircularis) of the frog (Rana pipiens pipiens). In response to the synthetic advertisement call, broadcast via a loudspeaker, torus neurons displayed differences in frequency tuning and temporal response properties in the different seasons. In winter, response latency increased and time locking decreased, while no significant differences in threshold and response rates were observed. To determine whether there is a neural basis for CMR in the frog central auditory system, noise was added to the target. It was found that both unmodulated and modulated noise generally increased the signal detection threshold, however, sinusoidally modulated noise elevated this threshold to a lesser extent. When the noise bandwidth was increased, the probe detection threshold decreased for modulated noise for 15% of the TS neurons. These results suggest that CMR is present in the frog. The temporal properties of the responses observed in 11% of the cells showed response only to tone pulses placed in the troughs of modulated noise; coincidence with peaks in the noise suppressed the probe response. This supports the dip-listening hypothesis, which presumes that the presence of gaps in the noise across frequency bands is used to enhance signal detectability. However, suppression of the response to the noise by the probe was also observed, supporting a noise-suppression mechanism. The neural response properties suggest that frogs may exploit across channel modulation to improve signal detection in noise. |
