Frequency selectivity in the anuran auditory midbrain: Single unit responses to single and multiple tone stimulation

Summary

• 1. The excitatory response properties of 130 single units in the torus semicircularis of the northern leopard frog (Rana p, pipiens) were recorded. 86% responded maximally to single tones. 14% responded maximally only to specific combinations of 2 or more tones, and were either completely unresponsive or weakly responsive (<20% maximal response) to single tones over a wide range of intensities.

• 2. Single tone responders were placed in 6 classes on the basis of configurations of their excitatory tuning curves. Neurons in each class also tended to show similar inhibitory response properties, particularly in regard to exhibiting partial or total two-tone inhibition (TTI). Neurons with symmetrical excitatory tuning curves generally exhibited only partial TTI, while those with more asymmetrical curves exhibited total TTI.

• 3. Inhibitory response properties were determined for 78 single tone responders. 69% could be totally inhibited by a second tone; the remainder showed only partial two-tone inhibition (TTI). Most neurons exhibiting total TTI (74%) had best inhibitory frequencies (BIFs) which were lower than the best excitatory frequencies (BEFs). Only 26% had BIFs > BEFs.

• 4. Absolute thresholds for total TTI by frequencies lower than the BEF were sometimes independent of the intensity of the excitatory tone. The property was not observed in the absolute thresholds for total TTI by higher frequencies, in which case all inhibitory thresholds were elevated upon increasing the intensity of the excitatory tone.

• 5. A small number of neurons were either unresponsive or weakly responsive (<20% maximal excitation) to broad-band stimulation. This response property was associated with total TTI in which the relative inhibitory threshold was lower than the intensity of the excitatory tone.

• 6. Neurons requiring 2 or more tones for maximal excitation all had restricted excitatory frequency bands from which the tone combinations could be drawn. These neurons were placed in two categories: those requiring narrowly separated (<200 Hz) tones and which had a single excitatory frequency band, and those which required more widely separated (>500 Hz) tones and had two excitatory bands. The latter category required the presence of one tone from each band to be excited.

• 7. All multiple tone responders could be totally inhibited by the addition of a single tone to the excitatory tone combination. Also, all multiple tone responders had non-monotonic intensity-rate functions, with excitatory intensity ranges as narrow as 25 dB and as broad as 50 dB.

• 8. The frequency selectivities of all multiple tone responders matched the spectral features of the species mating call. Their possible role in the selective detection of this call is discussed.