Midbrain combinatorial code for temporal and spectral information in concurrent acoustic signals

All Vocal species, including humans, often encounter simultaneous (concurre nt) vocal signals from conspecifics. To segregate concurrent signals, the a uditory system must extract information regarding the individual signals fr om their summed waveforms. During the breeding season, nesting male midship man fish (Porichthys notatus) congregate in localized regions of the intert idal zone and produce long-duration (>1 min), multi-harmonic signals ("hums ") during courtship of females. The hums of neighboring males often overlap , resulting in acoustic beats with amplitude and phase modulations at the d ifference frequencies (dFs) between their fundamental frequencies (F(0)s) a nd harmonic components. Behavioral studies also show that midshipman can lo calize a single hum-like tone when presented with a choice between two conc urrent tones that originate from separate speakers. A previous study of the neural mechanisms underlying the segregation of concurrent signals demonst rated that midbrain neurons temporally encode a beat's dF through spike syn chronization; however, spectral information about at least one of the beat' s components is also required for signal segregation Here we examine the en coding of spectral differences in beat signals by midbrain neurons. The res ults show that, although the spike rate responses of many neurons are sensi tive to the spectral composition of a beat, virtually all midbrain units ca n encode information about differences in the spectral composition of beat stimuli via their interspike intervals (ISIs) with an equal distribution of ISI spectral sensitivity across the behaviorally relevant dFs. Together, t emporal encoding in the midbrain of dF information through spike synchroniz ation and of spectral information through ISI could permit the segregation of concurrent vocal signals.