Frequency selectivity in the anuran medulla: Excitatory and inhibitory tuning properties of single neurons in the dorsal medullary and superior olivary nuclei

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1. The excitatory and inhibitory tuning properties of 70 dorsal medullary nucleus (DMN) and 108 superior olivary nucleus (SON) neurons were examined in the medulla of the northern leopard frog (Rana p. pipiens). Inhibitory tuning curves were determined using the two-tone inhibition (TTI) paradigm. 2. The tuning properties of DMN neurons were very similar to those of the VIIIth nerve fibers. (1) Excitatory tuning curves were unimodal, (2) only low frequency (80–500 Hz) tuned neurons exhibited TTI, (3) only higher frequencies were inhibitory, and (4) relative inhibitory thresholds were dependent upon the intensity of the excitatory tone (i.e., as the excitatory tone intensity was elevated, so was the inhibitory threshold). Also like VIIIth nerve fibers, most DMN neurons responded tonically to tonal stimulation. Unlike VIIIth nerve fibers, a small percentage (11%) of DMN neurons elicited a phasic-on response to tonal stimulation. 3. Most SON neurons also exhibited excitatory and inhibitory tuning properties similar to those of the VIIIth nerve. However, 19% exhibited properties previously reported only at the midbrain level of the auditory system. These latter properties include (1) bimodal, recurved and closed excitatory tuning curve configurations; (2) all neurons, not just those tuned to low frequencies, were susceptible to TTI; (3) TTI was exerted by lower, as well as higher, frequencies, and (4) TTI thresholds could be independent, as well as dependent, upon the intensity of the excitatory tone. Also like midbrain neurons, a small percentage (4%) of SON neurons exhibited a phasic-off response to tonal stimulation. 4. In both the DMN and SON, some neurons exhibited the ability to respond consecutively to tone cycles, trill pulses and beats generated by 2 tones at repetition rates up to 280 Hz. The maximum following rate of a given neuron was similar for each type of repeating event. Neurons tuned to all frequency ranges exhibited this property, and all acted as low-pass temporal filters.