Masked thresholds of auditory midbrain neurons in the frog Rana ridibunda.

Single-unit recordings from neurons in the torus semicircularis of Rana ridibunda were analyzed to determine the degree to which these neurons can detect acoustic stimuli superimposed on continuous, broad-band noise. In the first series of trials the stimuli were tone pulses 25 ms long, 5 dB above threshold, and at the characteristic frequency of the neuron. Most cells responded to the stimulus for signal-to-noise ratios of 25-35 dB (tone intensity minus spectral noise density); the lowest values were about 20 dB. The same signal-to-noise ratio was obtained with tones 20 dB above threshold. In the second series, conspecific mating calls at an intensity of 60 dB were tested. Here the signal-to-noise ratio (peak intensity of the call minus noise level) was usually 45-60 dB, in 5 of 27 cells it was 40 dB. Increasing the call intensity to 80 dB raised the SNR by an average of 6 dB. Noise-tolerant units more commonly gave synchronous responses to the pulse groups in the call, and are better tuned to its frequency range, than noise-sensitive units.