Abstract
This article explores the coding of frequency information in the auditory system from the viewpoint of what has been learnt from cochlear implants. Cochlear implants may provide a window on central auditory nervous system function by creating the possibility to separate place and temporal information. An existing model of frequency discrimination in the acoustically stimulated auditory system is extended to include electrical stimulation. To be able to predict frequency difference limens for acoustic stimulation, an important assumption is that one spike per stimulus cycle is available, which may be provided by the existence of a volley principle. It is shown that to predict frequency difference limens for electrical stimulation of the auditory system, it must also be assumed that electrical stimulation causes desynchronization at a central auditory nervous system integration centre. With these assumptions, the model predicts the degradation in frequency discrimination that occurs for electrical stimulation. Finally, it is shown that cochlear implants have not yet proven conclusively that either rate-place coding or temporal coding is predominant in the auditory system.
Highlights
A long-standing question about frequency analysis in the auditory system is how frequency information is represented: is frequency coded as a temporal code or as a place code (Moller, 1999) or as both? Pure tones are represented as both rate-place information and temporal information in the discharge patterns of auditory nerve fibres and the central auditory nervous system, but the extent to which the auditory system uses either representation is unknown
Phase-lock coding is a temporal mechanism, wherein the auditory system presumably uses the synchronization of neural discharges to individual cycles of periodic stimuli as a cue to determine the frequency of a pure tone
Psychoacoustic data from cochlear implants seem to refute the idea that temporal coding mechanisms are utilized by the central auditory system to extract frequency information from the neural spike train, as the frequency difference limens are much poorer than for normal-hearing listeners, even though there is much more synchronization to the stimulus waveform in electrical stimulation
Summary
A long-standing question about frequency analysis in the auditory system is how frequency information is represented: is frequency coded as a temporal code or as a place code (Moller, 1999) or as both? Pure tones are represented as both rate-place information (rate-place coding) and temporal information (phase-lock coding) in the discharge patterns of auditory nerve fibres and the central auditory nervous system, but the extent to which the auditory system uses either representation is unknown. Based on Moller's (1999) strong arguments in favour of a phase-lock code for frequency, and the success of the simple inter-spike interval based model of Goldstein and Srulovicz (1977) in predicting the shape and magnitude of frequency discrimination thresholds, a new model of frequency discrimination was presented in Hanekom and Kriiger (2001). This model used a simple description of the statistics of phase-locking, which enabled the authors to construct an optimal estimation mechanism by which frequency information can be extracted from one or more neural spike trains. The emphasis is on the interpretation of the frequency discrimination performance of an optimal estimator, presumably located somewhere in the central auditory nervous system, given the statistics of acoustically and electrically evoked spike trains
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