1. The dorsal medullary nucleus (DMN) of frogs is the presumed homolog of the mammalian cochlear nucleus (CN). Like the CN, the DMN is the sole target of centrally projecting primary auditory-nerve fibers and the first central auditory-processing center. To study the transformation of acoustic information in the DMN, we have utilized relatively simple stimuli--tone bursts--to detail the temporal discharge patterns of DMN neurons that can be compared with those shown by auditory-nerve fibers. 2. Based on the shape of poststimulus time (PSTH) and interspike interval (ISIH) histograms, we observed six distinctive discharge patterns to tone bursts presented at the best excitatory frequency (BEF), 10 dB above threshold. Four of these (primary-like type 1-4) resembled discharge patterns seen at the level of the auditory nerve, whereas two (phasic and phasic burst) were only observed in the DMN. 3. At stimulus levels of 20-30 dB above BEF threshold several phasic neurons became tonic responders, whereas several primary-like type-2 cells gave "pauser" discharges. The response patterns of the remaining cells were intensity independent. 4. We further showed that many of the single-unit discharge patterns were related to other neuronal response properties; specifically, spontaneous firing rate, intensity-rate functions, threshold, latency, BEF, and sharpness of tuning (Q10). 5. The implications of our findings are discussed with respect to 1) the transformation of acoustic information as it is passed from the auditory nerve to the DMN, and 2) the functional organization of the DMN.
Tone and call responses of units in the auditory nerve and dorsal medullary nucleus of Xenopus laevis
