Basics of hearing theory and noise in cochlear implants

Stochastic behavior in spiking of the electrically stimulated bipolar afferent cochlear neuron of man and cat is analyzed with a compartment model that includes the first unmyelinated part, every node and internode in the peripheral and central axon, and the soma. Following a suggestion of Rubinstein, ion current fluctuations across the neural membrane are assumed to be proportional to the square root of the number of sodium channels in every compartment [Rubinstein JT. Biophys J 1995;68:779–85]. Intracellular conductance and capacitance effects disturb a direct relation between voltage and current fluctuations in a single compartment. Paradoxically, the unmyelinated human soma is the compartment with the highest number of sodium channels and the smallest voltage noise. Voltage fluctuates with maximum amplitudes in the peripheral axon, followed by the thicker central axon. The model demonstrates that a train of biphasic stimulating pulses can generate a pattern in the cochlear nerve with temporal structures which are fine enough to discriminate speech signals. However, several important differences between the electrically and acoustically generated spiking patterns demand for sophisticated strategies to obtain better speech understanding for cochlear implant users.