Tone and call responses of units in the auditory nerve and dorsal medullary nucleus of Xenopus laevis

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Abstract
The clawed frog Xenopus laevis produces vocalizations consisting of distinct patterns of clicks. This study provides the first description of spontaneous, pure-tone and communication-signal evoked discharge properties of auditory nerve (n.VIII) fibers and dorsal medullary nucleus (DMN) cells in an obligatorily aquatic anuran. Responses of 297 n.VIII and 253 DMN units are analyzed for spontaneous rates (SR), frequency tuning, rate-intensity functions, and firing rate adaptation, with a view to how these basic characteristics shape responses to recorded call stimuli. Response properties generally resemble those in partially terrestrial anurans. Broad tuning exists across characteristic frequencies (CFs). Threshold minima are −101 dB re 1 mm/s at 675 Hz; −87 dB at 1,600 Hz; and −61 dB at 3,000 Hz (−90, −77, and −44 dB re 1 Pa, respectively), paralleling the peak frequency of vocalizations at 1.2–1.6 kHz with ∼500 Hz in 3 dB bandwidth. SRs range from 0 to 80 (n.VIII) and 0 to 73 spikes/s (DMN). Nerve and DMN units of all CFs follow click rates in natural calls, ≤67 clicks/s and faster. Units encode clicks with a single spike, double spikes, or bursts. Spike times correlate closely with click envelopes. No temporal filtering for communicative click rates occurs in either n.VIII or the DMN.