gms | German Medical Science

The electrically-evoked compound action potential is easy to record in implant patients using intracochlear electrodes and reversed telemetry (NRT in Nucleus implants, or NRI in Clarion implants). However, the usefulness of these measurements for automatic or objective fitting is diminished by the poor correlation between NRT thresholds and the behavioral thresholds and comfortably loud levels needed for fitting. The fact that this correlation gets worse with increasing rate of stimulation for the behavioral measures indicates that the basis of this problem is the variation between people in the way that rate of stimulation affects loudness. In this paper, a model of how rate of stimulation affects loudness will be presented. The model incorporates the effects of temporal integration, neural refractoriness, and loudness growth with current level. The data from four published reports relating the position of the NRT threshold within the behavioral dynamic range for different rates of stimulation supports the use of the model to predict the average difference between NRT and behavioral measures. The variation of individuals away from the predicted average offset between objective and behavioral measures was shown by the model to depend on three subject-dependent factors: the degree of refractory effect as rate increases, the shape of the loudness growth function, and the personal preference for the loudness at which the subject sets their threshold and comfortable levels. Additional objective information that can describe or predict these individual factors will improve the usefulness of NRT measurements for automatic fitting.