1. Single-unit responses to different temporal acoustic parameters were characterized in the dorsal medullary nucleus (DMN) o the Northern leopard frog, Rana pipiens pipiens. Our goal was to provide both a quantitative and a qualitative assessment of the neural representation of behaviorally relevant temporal acoustic patterns in the frog's DMN. 2. Acoustic stimuli included tone bursts having different durations, rise times, or rates of amplitude modulation (AM). Several metrics were used to compute temporal response functions for each of these, including mean spike count, average firing rate, and/or peak firing rate. Synchronization coefficients were also used to characterize responses to stimuli presented at different AM rates. 3. On the basis of mean spike count, the temporal response functions of DMN neurons with respect to signal rise time could be characterized as 1) all-pass, in which the mean spike count was largely independent of rise time, or 2) fast-pass, in which the mean spike count decreased with increasing rise time. Fast-pass response functions were of two types, those that decayed rapidly and those that decayed gradually from their peak values. 4. The minimum threshold varied with signal rise time for cells showing fast-pass but not all-pass response functions. Minimum response thresholds for fast-pass neurons were typically higher with slower signal rise time. 5. The filtering characteristics of cells displaying fast-pass rise time response functions were dependent on signal level, becoming all-pass when signal levels exceeded 30-40 dB above the minimum threshold. 6. Approximately 44% of DMN neurons exhibiting fast-pass response functions for signal rise time showed all-pass filtering characteristics when broadband noise rather than best frequency tones were used, thereby signifying an influence of signal spectrum on the pass-band characteristics of these cells. 7. All DMN neurons, regardless of discharge pattern, showed maximal instantaneous firing rates to signals having short (< 25 ms) rise times. Response functions based on instantaneous firing rate were, therefore, fast-pass in nature. These responses were independent of signal level and spectrum. 8. There was an ordinal relationship between signal duration and the duration of tonic but not phasic unit discharges. This relationship was not intensity dependent. 9. On the basis of mean spike count, the temporal response functions of DMN neurons with respect to signal duration were characterized as 1) all-pass, in which the mean spike count was largely independent of signal duration, or 2) long-pass, in which the mean spike count increased with increasing signal duration. The rate at which long-pass response functions reached their peak values was graded and varied among the sample population. 10. On the basis of mean firing rate, the duration response functions of DMN neurons were characterized as 1) all-pass, in which the mean firing rate was independent of signal duration; 2) short-pass, in which the mean firing rate decreased with increasing signal duration; or 3) band-pass, in which the mean firing rate was greatest over a narrow range of signal durations. Most (92%) band-pass units had "best durations" between 5 and 25 ms, indicating a pronounced preference for signals of short duration. In all cases, the filtering characteristics of the response functions were relatively independent of signal level. 11. On the basis of mean spike count, the temporal response functions of DMN neurons with respect to AM rate could be characterized as 1) all-pass, in which the mean spike count was independent of AM rate; 2) high-pass, in which the mean spike count increased with increasing AM rate; or 3) band-pass, in which the mean spike count was greatest over a narrow range of AM rates. The rate at which high-pass response functions reached peak value was graded and varied among the sample population. The majority (76%) of band-pass units preferred AM rates between 10 and 40 Hz. The temporal filtering characteristics of some high-pass and all band-pass response functions were dependent on signal level, whereas the filtering characteristics of two band-pass units were also dependent on signal spectrum. 12. On the basis of synchronization coefficient, the temporal response functions of DMN neurons with respect to AM rate exhibited all-pass, low-pass, and band-pass temporal filtering characteristics. Most (66%) of the cells exhibited low-pass response functions; i.e., their ability to time lock to the modulating waveform decreased with increasing AM rate. The best AM rates for cells having band-pass response functions were distributed over a narrow range, 30-55 Hz. The filtering characteristics of cells showing low-pass and band-pass but not all-pass response functions based on synchronization coefficient were intensity dependent. 13. It is concluded that the temporal pattern of complex signals is well represented in the discharges of DMN neurons. Moreover, there is a significant transformation with respect to the neural representation of signal rise time, signal duration, and AM rate in the DMN compared with that occurring at the level of the auditory nerve. Further, the temporal processing that does occurs in the DMN is surprisingly complex and in many respects resembles that taking place in higher auditory centers.
