Peripheral encoding of behaviorally relevant acoustic signals in a vocal fish: single tones.

The midshipman fish, Porichthys notatus, generates acoustic signals for intraspecific communication. Nesting males produce long-duration "hums" which attract gravid females and can be effectively mimicked by pure tones. In this study we examine the encoding of tonal signals by the midshipman peripheral auditory system. Single-unit recordings were made from afferents innervating the sacculus while presenting sounds via an underwater loudspeaker. Units were characterized by iso-intensity spike rate and vector strength of synchronization curves, as well as by peristimulus time histograms. Additionally, response-intensity curves and responses to long-duration (up to 10 s) stimuli were obtained. As has been seen in other teleosts, afferents had highly variable activity profiles. Excitatory frequencies ranged from 60 to over 300 Hz with most units responding best around 70 or 140 Hz. Thresholds at 90 Hz ranged from 95 to 145 dB re 1 microPa. Strong synchronization provided a robust temporal code of frequency, comparable to that described for goldfish. Spike rate showed varying degrees of adaptation but high rates were generally maintained even for 10-s stimuli. The midshipman peripheral auditory system is well suited to encoding conspecific communication signals, but nonetheless shares many response patterns with the auditory system of other teleosts.