Cross-fiber interspike interval probability distribution in acoustic stimulation: a computer modeling study

Abstract

Electrical stimulation strategies for cochlear implants may be improved by studying temporal frequency coding in single auditory fibers and across fibers in acoustic stimulation (Clark et al, this suppl, section 5). In single nerve fibers, phase locking between action potentials and the acoustic stimulus can only be maintained at frequencies below about 600 Hz. At these frequencies, the time interval between successive action potentials, called the interspike interval (lSI), is distributed around the period of the stimulus, and it can therefore be used to code frequency within single fibers. At higher frequencies, the phase locking of individual nerve fibers diminishes, but it may still be possible to retain phase-locking properties by combining the action potentials in an ensemble of nerve fibers. In an ensemble of fibers, the lSI in each nerve is affected by factors such as the spectral shape of the stimulus, the characteristic frequency, and the firing characteristics of the nerve. The lSI between the fibers, however, is further affected by the propagation or phase delay of the traveling wave. It is therefore uncertain how these factors would affect frequency coding across fibers.

It is possible that the propagation delay between the fibers may lower the phase locking in an ensemble of nerves -because the probability that the majority of nerves in an ensemble will fire simultaneously may be low. It is also possible that the combined firing statistics of the fibers in an ensemble may result in a higher degree of synchrony such that the predominant intervals in an ensemble are preserved over a wider frequency range than in a single fiber. Are these accurate postulations of the physical system? In a future electrical stimulation strategy that incorporates temporal frequency coding, is it necessary to mimic the spatial-temporal delay in the firing patterns caused by the traveling wave? These are important questions that need to be studied and answered. (From Introduction)