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Some features of background firing in the auditory brainstem neurons of the amphibian brain


N.G. Bibikov - Dr.Sc. (Biol.), Chief Research Scientist, JSC N.N. Andreev Acoustic Institute (Moscow)
I.V. Makushevich - Research Scientist, JSC N.N. Andreev Acoustic Institute (Moscow)
S.V. Nizamov - Research Scientist, JSC N.N. Andreev Acoustic Institute (Moscow)

The neural activity consists of a sequence of short identical impulses (spikes), and therefore it can be interpreted as a point stochastic random process. An analysis of this activity in the absence of controlled external influences can be useful for un-derstanding and modeling the principles of the functioning of individual nerve cells and their ensembles. We have analyzed the background activity of a cell located in the superior olive nucleus of a grass frog. A number of programs developed in the laboratory made it possible to obtain several functions characterizing the point process under investigation. They include: a) interpulse interval distribution; b) autocorrelation function; c) hazard function; d) dependence of each interpulse interval on the previous one; e) the dependence of the ratio of the variance to the average value of pulse number (Fano factor) on the duration of the analysis; f) the dependence of the local variations of the interpulse intervals (Allan factor) on the duration of the analysis; g) the dependence of the difference between the shortest and the longest inter-pulse interval on the duration of the analysis (Hurst index). Registration of the sequence of spikes generated by this neuron in the absence of external stimulation was performed for 500 s, and during this time, 9565 spikes were recorded. In the autocorrelation function of the impulse process studied, it was possible to note the initial minimum corresponding to the refractory period and subsequent local maxima. The interdependence of neighboring intervals was negative for small intervals (less than 30 ms), but positive for the whole set of intervals. Consequently, the process did not correspond to the renewal process both for large and small intervals. The values of the Fano factor were close to 1 for the smallest analysis sites (<10 ms) and were significantly less than unity for the 0,02-1,0 s sites. However, with further increase in the duration of the analyzed period the Fano factor increased in accordance with the power law with an exponent of 0,81. This result also indicated the existence of chaotic changes in the background firing frequency of the cell under investigation. Specifically, the dynamics of changes in the characteristics of the process under investigation could be traced by analyzing the changes in the Hurst index over time. The analysis was carried out for sufficiently representative areas of impulses (3000 spikes). It was found that during the whole duration of the presentation the process could be characterized either as a trend or as a stochastic one, but close to a trend one. In some rather long periods, the trend was expressed quite clearly (the Hurst index value was > 0,6), while on others the value of this index did not reach the criterion corresponding to the description of the process as a trend. These data allow us to suggest that the background firing of the auditory neurons of the brain shows variability in its very variability. This feature can be significant in modeling of neuronal plasticity and learning.
This work was financially supported by RFBR in grant № 16-04-01066.

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