Temporal coding in the frog auditory midbrain: the influence of duration and rise-fall time on the processing of complex amplitude-modulated stimuli.

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.(ABSTRACT TRUNCATED AT 400 WORDS)