Synchronization Index of Neural Spike Trains in Response to Simulated Vowel Signal Stimuli in the Presence of a Pseudo-spontaneous Activity

This article presents a statistical analysis of neural spike trains in an auditory nerve fiber (ANF) model stimulated extracellularly by simulated vowel electric stimuli under the case where a high-rate pulsatile waveform is presented as a conditioner for increasing the across-fiber-independency, i.e., desynchronization. In the computer simulation, stimulus current waveforms were presented repeatedly to a stimulating electrode located 1 mm above the 26th node of Ranvier, in an ANF axon model having 50 nodes of Ranvier, each consisting of stochastic sodium and potassium channels. From spike firing times recorded at the 36th node of Ranvier, the raster plots were depicted to explore the temporal precision and reliability of spike trains. Then the period histograms were generated to obtain the synchronization index defined using Shannon's entropy as a distance between the period histogram and the vowel electric stimuli. In the present article, it is shown that at a specific amplitude of simulated vowel waveforms, the possibility to encode the vowel signals with various amplitudes became greater, as well as the synchronization index was found to be maximized. It was implied that setting the amplitude of vowel signals to the specific values which maximize the synchronization index might contribute to efficiently encoding information on vowel formants under the high-rate pulsatile stimulation in cochlear prostheses.