Temporal modulation transfer functions for single neurons in the auditory midbrain of the leopard frog. Intensity and carrier-frequency dependence.

The sensitivity for amplitude modulation was investigated for 77 neurons from the auditory midbrain of the leopard frog. The results show that tuning to modulation frequencies occurs in about one-third of the units but is quite varied in its appearance. Two slightly differing characterizations for this tuning have been used; the overall response or rate-Modulation Transfer Function and the synchronized response or temporal-MTF (tMTF). The relation between the two characterizations is given by the vector-strength. Only one-third of the units showed a vector-strength that was significantly different from zero. Another synchronization measure, the synchronization factor which is based on the auto-coincidence function, was significantly different from zero in about 3/4 of the units. The Best Modulation Frequency (BMF) and tuning band-width were found to be independent of both stimulus intensity and carrier frequency, although the average BMF for band-pass units was slightly higher for the amphibian papilla range of carrier frequencies than for the basilar papilla range (66 Hz vs. 49 Hz). The most frequent BMF for band-pass units was around 55 Hz, this does not correspond with the dominant modulation frequency of the mating call which is around 20 Hz. The synchronization measures were negatively correlated with intensity and independent of carrier frequency. The phase response of the tMTF was used to calculate the group delay. In contrast to the latency of the units the group delay was independent of stimulus intensity.