Coding of concurrent vocal signals by the auditory midbrain: effects of duration

Neural selectivity to signal duration within the auditory midbrain has been observed in several species and is thought to play a role in signal recogn ition. Here we examine the effects of signal duration on the coding of indi vidual and concurrent vocal signals in a teleost fish with exceptionally lo ng duration vocalizations, the plainfin midshipman. Porichthys notatus, Nes ting mates produce long-duration, multi-harmonic signals known as hums to a ttract females to their nests; overlapping hums produce acoustic beats at t he difference frequency, of their spectral components. Our data show that a ll midbrain neurons have sustained responses to long-duration hum-like tone s and beats. Overall spike counts increase linearly with signal duration, a lthough spike A rates decrease dramatically. Neurons show varying degrees o f spike rate decline and hence, differential changes in spike rate across t he neuron population may code signal duration. Spike synchronization to bea t difference frequency progressively increases throughout long-duration bea ts such that significant difference frequency coding is maintained in most neurons. The significance level of difference frequency synchronization cod ing increases by an order of magnitude when integrated over the entirety of long-duration signals. Thus, spike synchronization remains a reliable diff erence frequency, code and improves with integration over longer time spans .