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 frog dorsal medullary nucleus (DMN), a homologue of the cochlear nucleus. Phase-locked responses to tones at the unit's characteristic frequency (CF) and to off-CF tones were evaluated. We also stimulated units with tones at CF that were amplitude modulated sinusoidally between 5 and 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 was robust at low frequencies(<400 Hz) and became poorer at higher frequencies; the variation of the synchronization coefficient (SC) with frequency typically showed a low-pass characteristic. 3. The capacity of phase-locking to tones was correlated with the functional classification 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 for the three classes of primarylike neurons. The average cutoff frequency was respectively 183, 325, and 536 Hz for primarylike neurons that displayed low (PL-I), intermediate (PL-2), and high (PL-3) steady-state 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 the different cell types, as evidenced by the units' modulation transfer functions (MTFs). The primarylike neurons exhibited mostly all-pass or low-pass sync-based MTFs. The mean upper cutoff frequencies for primarylike neurons having low-pass MTFs were 155 Hz for PL-1 neurons, 176 Hz for PL-2 neurons, and 218 Hz for PL-3 neurons. Pauser, chopper, phasic, and phasic-burst neurons gave mostly low-pass MTFs having a mean upper cutoff frequency of 219, 235, 242, and 251 Hz, respectively. 5. The phase-locking ability of DMN neurons to tones and to amplitude-modulated stimuli are compared with those of frog's primary afferent fibers and with those of avian and mammalian cochlear nucleus neurons. The significance of results in terms of sound localization and sound pattern recognition is discussed.