Temporally selective processing of communication signals by auditory midbrain 1 neurons 2 3 4

36 Perception of the temporal structure of acoustic signals contributes critically to vocal 37 signaling. In the aquatic clawed frog Xenopus laevis, calls differ primarily in the 38 temporal parameter of click rate, which conveys sexual identity and reproductive state. 39 We show here that an ensemble of auditory neurons in the laminar nucleus of the torus 40 semicircularis (TS) of X. laevis specialize in encoding vocalization click rates. We 41 recorded single TS units while pure tones, natural calls, and synthetic clicks were 42 presented directly to the tympanum via a vibration stimulation probe. Synthesized click 43 rates ranged from 4 to 50 Hz, the rate at which the clicks begin to overlap. Frequency 44 selectivity and temporal processing were characterized using response-intensity curves, 45 temporal discharge patterns, and autocorrelations of reduplicated responses to click 46 trains. Characteristic frequencies ranged from 140 to 3250 Hz, with minimum thresholds 47 of -90 dB re 1 mm/s at 500 Hz and -76 dB at 1100 Hz near the dominant frequency of 48 female clicks. Unlike units in the auditory nerve and dorsal medullary nucleus, most 49 toral units respond selectively to the behaviorally relevant temporal feature of the rate of 50 clicks in calls. The majority of neurons (85%) were selective for click rates and this 51 selectivity remained unchanged over sound levels 10 to 20 dB above threshold. Selective 52 neurons give phasic, tonic, or adapting responses to tone bursts and click trains. Some 53 algorithms that could compute temporally selective receptive fields are described. 54