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 s ystem must encode the spectral and temporal features of the fused waveforms such that at least one signal can be individually detected. In the plainfi n midshipman fish (Porichthys notatus), the overlapping mate calls of neigh boring 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 cod e of the temporal and spectral characteristics of beats via synchronization of spike bursts to dF and changes in spike rate and interspike intervals w ith changes in spectral composition. In the present study we examine the ef fects of changes in signal parameters of beats (overall intensity level and depth of modulation) on the spike train outputs of midbrain neurons. The o bserved changes in spike train parameters further support the hypothesis th at midbrain neurons provide a combinatorial code of the spectral and tempor al features of concurrent vocal signals. (C) 2001 Acoustical Society of Ame rica.