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The coding of auditory signals in the torus semicircularis of the fire-bellied toad and the grass frog: Responses to simple stimuli and to conspecific calls

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The function of the anuran midbrain auditory nucleus was studied by recording the responses of single cells in the torus semicircularis of the grass frog (Rana t. temporaria L.) and the fire-bellied toad (Bombina bombina (L.)) to single tone pulses, combinations of two tones, white noise, and tape-recorded conspecific calls.

About 50% of the auditory neurons in the midbrain of the fire-bellied toad, and 60–70% of those in the grass frog, are spontaneously active. In winter the proportion of spontaneously active cells is lower. The frequencies of spontaneous discharge are between 0.1 and 15.3 impulses/s.

The basic tonic and phasic responses, either excitation or inhibition, are frequently combined to form response patterns.

The distribution of characteristic frequencies among the units studied matches the frequency spectrum of the calls. Sharpness of tuning varied widely, Q10dB ranging from 0.29 to 4.5 with one exception (7.5 in one neuron).

The threshold curves are usually V-shaped but can also have the shape of a W. In some cases the response region is confined to a continuous area on the frequency-intensity surface; in others there may be separate areas, within which opposite responses are elicited.

The intensity characteristics can be classified into two main types with roughly equal frequency of occurrence — teadily rising or saturating curves, and curves decaying from a maximum on either side of the most effective SPL.

Latencies were very short (5–10 ms) in some cases, but responses could occur with latencies as long as 300 ms.

The optimal repetition rates for pulses at the best frequency are consistent with the temporal structure of the conspecific calls.

About 50% of the units studied in the toad and about 75% of those in the frog responded to the conspecific calls. Some of the neurons gave different responses to the different calls, ranging from gradation in magnitude to opposite reactions. A few units responded exclusively to the release calls.

In most cases the responses to the calls were consistent with those to the simple artificial stimuli. Magnitude gradation, opposite responses to different calls and selective responses could be ascribed primarily to the tuning characteristics of the units. In some cases, however, the temporal structure of the call was the critical parameter.

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best excitatory frequency


best inhibitory frequency


clasping call


mating call

RC1, RC2, RC3:

first-, second-, third-order release call


territorial call


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I am grateful to Prof. H. Schneider for discussions during the planning of these experiments and for his generous support throughout. I also thank Prof. R.R. Capranica, Cornell University, Ithaca NY, USA, who kindly provided me with blueprints for the construction of the earphone housing and Dr. J. Rheinlaender, Ruhr-University, Bochum, for reading the manuscript.

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Walkowiak, W. The coding of auditory signals in the torus semicircularis of the fire-bellied toad and the grass frog: Responses to simple stimuli and to conspecific calls. J. Comp. Physiol. 138, 131–148 (1980).

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  • Temporal Structure
  • Opposite Response
  • Threshold Curve
  • Auditory Neuron
  • Selective Response