Statistical characteristics of background firing in the midbrain auditory units of grass frog ( Rana t. temporaria ) located in torus semicircular (TS) were investigated. Only about 5% of the cells demonstrated prominent spontaneous firing. For such units the following characteristics were obtained: the distribution of interpulse intervals, the autocorrelation functions (ACF) for the real firing process and for the process with shuffled intervals, the hazard function (HF) and the joint distribution of adjacent interpulse intervals. The burstiness of firing was also estimated. In the absolute majority of the cells, the background firing demonstrated considerable deviation from the renewal process. There was weak but significant positive correlation between adjacent interpulse intervals. The burstiness of firing in the midbrain auditory units was moderate but higher than reported for medullary auditory neurons. The value of burstiness did not decrease after interval shuffling. Along with the reduction in excitability (generalized refractoriness) in many neurons observed post-spike facilitation effects were observed. Comparing background activity in medullary and midbrain nucleus suggests that there is an increase in complexity of the information processing along the auditory pathway. Highlights ? In midbrain auditory center of the frog only a few units revealed any spontaneous firing. ? In medullar auditory center of the frog spontaneous activity is far more common. ? In spite of the rarity of spontaneous firing in the midbrain, it is far more variable. ? In some torus semicircularis units the long and strong refractoriness was observed. ? Other units revealed some tendency to form bursts.
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