Some features of the sound-signal envelope extracted by cochlear nucleus neurons in grass frog

Abstract

The responses of single neurons in the medullar auditory center of the grass frog were recorded extracellularly under the action of long tone signals of a characteristic frequency that were modulated by repeating fragments of low-frequency (0–15, 0–50 or 0–150 Hz) noise. The correlation method was used for evaluating the strength of the effect of different envelope fragments to ensure the generation of a neuron pulse discharge. While carrying out these evaluations at different time intervals, the maximum delays were assessed between a signal and a response. Two important envelope fragments were revealed. In the majority of units the most effective fragment was the time interval of the amplitude increase from the mean value to the maximum; the fragment where the amplitude fell from the maximum to the mean value was the second in the strength of the effect. This type of response was observed in the vast majority of cells in the range of the envelope frequency bands 0–150 and 0–50 Hz. These cells performed half-wave rectification of this type of envelope. However, in some neurons we observed a stronger strength of the reaction toward a time interval with a increasing amplitude, even including those where the amplitude value was smaller than the mean one. These properties were observed mainly for low-frequency (0–15 Hz) modulated signals at high modulation depth. The data show that even in the medulla oblongata specialization of neural elements of the auditory pathway occurs with respect to the time interval features of the sound stimulus. This diversity is most evident for signals with relatively slowly varying amplitudes.