Classification of the temporal discharge patterns of single auditory neurons in the frog superior olivary nucleus

Temporal discharge patterns of neurons in the superior olivary nucleus (SON) of the northern leopard frog (Rana pipiens pipiens) were studied by evaluating peri-stimulus time histograms and interspike interval histograms generated from responses to tone bursts at the neuron's characteristic frequency and at 10 dB above neuron's characteristic threshold. Four basic discharge patterns were observed, i.e., primary-like, phasic-burst, phasic, and pauser. Additionally, within each class, different neurons exhibited further subtle differences in temporal discharge patterns and thus subclasses were distinguishable. For most SON neurons, changes in signal level did not affect the discharge pattern. However, there were a few notable exceptions. The firing pattern of 12% of SON neurons changed from one class to another with a change in signal level. Two-thirds of phasic-burst neurons showed increased regularity (i.e., chopping pattern) in their discharges as the signal level was increased. The majority of SON neurons showed monotonic rate-level functions, but one-fourth gave non-monotonic rate-level functions. The presence of a fairly large number of non-monotonic units in the frog SON suggests an additional role of this nucleus in information processing beyond the simple coding of stimulus level. The discharge patterns displayed by SON neurons resembled those seen in the frog cochlear nucleus and in the mammalian superior olivary complex.