Peripheral encoding of behaviorally relevant acoustic signals in a vocal fish: harmonic and beat stimuli.

Nesting male midshipman fish, Porichthys notatus, emit simple, long-duration sounds known as hums, which are attractive to gravid females. While hums share the multi-harmonic structure typical of many vertebrate communication sounds, their lack of amplitude modulation gives individual hums unusually simple temporal envelopes. However, hums often overlap, producing beats in the summed acoustic waveform. This study presents responses of individual saccular afferent fibers to two-tone harmonic and beat stimuli presented via an underwater loudspeaker. Spike activity was quantified as vector strength of synchronization and average spike rate. Responses to harmonic stimuli depended on harmonic phase; these effects apparently resulted primarily from variation in waveform fine temporal structure rather than auditory non-linearities. At most phases, addition of the harmonic enhanced afferent synchronization compared to the fundamental alone. Two-tone beat stimuli evoked stronger synchronization to the component frequencies than to the beat modulation rate. Vector strength tended to be higher to the lower frequency component, and this pattern appeared to derive from afferent tuning. Midshipman saccular afferents encoded both the temporal envelope and waveform fine structure of these naturalistic signals, information that may be important in conspecific communication.