Reprint from International Cochlear Implant, Speech and Hearing Symposium - Melbourne 1994 CROSS-FIBRE INTERSPIKE INTERVAL PROBABILITY DISTRIBUTION IN ACOUSTIC STIMULATION—A COMPUTER MODELLING STUDY

Electrical stimulation strategies for cochlear implants may be improved by studying temporal frequency coding in single auditory fibres and across fibres in acoustic stimulation [1]. In single nerve fibres, phase locking between action potentials and the acoustic stimulus can only be maintained at frequencies below 600 Hz. At these frequencies, the time interval between successive action potentials, called Interspike Interval (ISI), is the same as the period of the stimulus, and it can therefore be used to code frequency within single fibres. At higher frequencies, the phase locking of individual nerve fibres diminishes but it may still be possible to retain phase locking properties by combining the action potentials in an ensemble of nerve fibres. In an ensemble of fibres innervating different regions in the cochlea, the ISI in each nerve is affected by factors such as the spectral shape of the stimulus, the characteristic frequency, and the maximum firing rate of the nerve. The ISI between the fibres, however, is further affected by the propagation or phase delay of the travelling wave. It is therefore uncertain how these factors would affect frequency coding across fibres. It is possible that the propagation delay between the fibres 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 fibres in an ensemble may result in a higher degree of synchrony such that the predominant intervals in an ensemble is preserved over a wider frequency range than in a single fibre. 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 travelling wave? These are important questions that need to be studied and answered.