PHASE-LOCKED RESPONSE CHARACTERISTICS OF SINGLE NEURONS IN THE FROG COCHLEAR NUCLEUS TO STEADY-STATE AND SINUSOIDAL-AMPLITUDE-MODULATED TONES

1. We made extracellular recordings from 164 single neurons in the fro g dorsal medullary nucleus (DMN), a homologue of the cochlear nucleus. Phase-locked responses to tones at the unit's characteristic frequenc y (CF) and to off-CF tones were evaluated. We also stimulated units wi th tones at CF that were amplitude modulated sinusoidally between 5 an d 1,000 Hz and examined responses to these stimuli. 2. Results showed that single neurons in the frog DMN displayed phase-locked discharges to tones at frequencies less than or equal to 800 Hz. Phase-locking wa s robust at low frequencies(<400 Hz) and became poorer at higher frequ encies; the variation of the synchronization coefficient (SC) with fre quency typically showed a low-pass characteristic. 3. The capacity of phase-locking to tones was correlated with the functional classificati on of a DMN neuron and the firing rate of its CF response. Primarylike neurons exhibited various degrees of phase-locked discharges to tones at off-CF frequencies. The average upper cutoff frequency, i.e., the frequency at which the SC dropped to 0.5 of maximum value, differed fo r the three classes of primarylike neurons. The average cutoff frequen cy was respectively 183, 325, and 536 Hz for primarylike neurons that displayed low (PL-I), intermediate (PL-2), and high (PL-3) steady-stat e firing rates to CF stimulation. The phasic neurons showed poor phase -locking capacities at all tone frequencies. 4. The frequency range of phase-locking to amplitude-modulated stimuli was also different for t he different cell types, as evidenced by the units' modulation transfe r functions (MTFs). The primarylike neurons exhibited mostly all-pass or low-pass sync-based MTFs. The mean upper cutoff frequencies for pri marylike neurons having low-pass MTFs were 155 Hz for PL-1 neurons, 17 6 Hz for PL-2 neurons, and 218 Hz for PL-3 neurons. Pauser, chopper, p hasic, and phasic-burst neurons gave mostly low-pass MTFs having a mea n upper cutoff frequency of 219, 235, 242, and 251 Hz, respectively. 5 . The phase-locking ability of DMN neurons to tones and to amplitude-m odulated stimuli are compared with those of frog's primary afferent fi bers and with those of avian and mammalian cochlear nucleus neurons. T he significance of results in terms of sound localization and sound pa ttern recognition is discussed.