1. Single-unit recordings were made in the auditory midbrain, the torus semicircularis (TS) of the northern leopard frog, to independently characterize the processing of different temporal attributes (signal duration, rise-fall time, and rate of amplitude modulation) of natural sounds and to investigate how these temporal variables interact to produce the observed responses to complex amplitude-modulated (AM) signals. Response functions, on the basis of mean spike count, were derived and categorized to describe the unit's temporal response characteristics to each of the variables. 2. To characterize the duration response functions, tone bursts of different durations (stimuli repeated at a constant repetition rate) at the unit's characteristic frequency (CF) and 10 dB above minimum threshold at CF (MT) were presented monaurally to the contralateral ear. The duration response function of a TS neuron was often related to the temporal discharge characteristics of the neuron. Increases in stimulus duration elicited an increase in spike counts (therefore, long-pass response function) from most neurons (74%) in the TS; 91% of these neurons showed tonic discharge patterns. Phasic-burst (PB) cells that were rapidly adapting showed long-pass duration response functions that were highly nonlinear, having peaks and notches embedded within the functions. On the other hand, one-third of phasic neurons tended to be insensitive to stimulus duration, giving similar spike counts in response to stimuli of greatly different durations (i.e., all pass). In the TS, some neurons (9%) only responded to a limited range of durations (i.e., band-duration pass), and still others showed a preference for shorter durations (9%;i.e., short pass); these were exhibited primarily by phasic and PB neurons. 3. To characterize the rise-fall time response functions, tone bursts having different rise-fall times were presented. The rise-fall time response functions of TS neurons had two distinct characteristics. The majority of tonic cells (91%), as well as some PB (38%) and phasic (29%) neurons, gave essentially invariant spike counts for all stimulus rise-fall times (i.e., all pass; 73% of neurons). Despite the relatively stable spike counts of neurons showing all-pass functions, the peristimulus time histograms (PSTHs) deriving from responses to slower rise-fall time stimuli exhibited a longer and somewhat more variable onset latency. About one-fourth (27%) of TS neurons, mostly phasic and PB neurons, showed higher spike counts for signals with rapid rise-fall times. 4. The sensitivity of TS neurons to AM rate was determined primarily with sinusoidally amplitude-modulated (SAM) and pulsed amplitude-modulated (PAM) signals. Neurons in the TS generally showed poor time locking to the envelope waveform. Synchronous firing, at best, was observed at very low AM rates; the synchronization coefficient dropped precipitously beyond AM rates of 10-20 Hz for all TS neurons, regardless of the type of AM stimuli. 5. The modulation transfer function (MTF) based on spike count was calculated for responses to the different AM stimuli. More than one-third of TS neurons (35%) showed essentially invariant spike counts (all pass) to a series of stimuli with different SAM rates; these MTFs were exhibited primarily by tonic cells. About one-fourth (24%) of TS neurons displayed band-pass MTFs that showed a peak response to a limited range of AM rates, with lower and higher SAM rates eliciting much reduced spike counts. TS neurons also displayed low-pass (19%), high-pass (14%), and band-suppression (8%) MTFs. 6. In response to PAM stimuli, the majority of neurons (64%) gave greater spike counts as the AM rate was increased, yielding high-pass MTFs; these response characteristics were displayed primarily by tonic and PB neurons. About one-fifth (21%) of TS neurons also showed band-pass MTFs. The remaining TS neurons exhibited low-pass, all-pass, and band-suppression MTFs. 7. We evaluated the influence of different temporal attributes on the response to complex sounds by comparing the MTFs of a single neuron to two or three different AM stimuli with the independently derived, duration and rise-fall time response functions. In many cells it was the duration along with the AM rate, and in other cells it was the rise-fall time plus the AM rate, that influenced the shape of the MTF for different AM stimuli; a few neurons exhibited an interaction of all three temporal variables. This effect of duration and rise-fall time was most evident at lower AM rates (< 30-40 Hz). 8. In summary, the processing of temporally complex sounds, such as AM signals, is complex and is influenced by a number of temporal attributes that covary with AM rate.
