Coding of concurrent vocal signals by the auditory midbrain: effects of stimulus level and depth of modulation.

The segregation of concurrent vocal signals is an auditory processing task faced by all vocal species. To segregate concurrent signals, the auditory system must encode the spectral and temporal features of the fused waveforms such that at least one signal can be individually detected. In the plainfin midshipman fish (Porichthys notatus), the overlapping mate calls of neighboring males produce acoustic beats with amplitude and phase modulations at the difference frequencies (dF) between spectral components. Prior studies in midshipman have shown that midbrain neurons provide a combinatorial code of the temporal and spectral characteristics of beats via synchronization of spike bursts to dF and changes in spike rate and interspike intervals with changes in spectral composition. In the present study we examine the effects of changes in signal parameters of beats (overall intensity level and depth of modulation) on the spike train outputs of midbrain neurons. The observed changes in spike train parameters further support the hypothesis that midbrain neurons provide a combinatorial code of the spectral and temporal features of concurrent vocal signals.