Intracochlear recordings in cochlear implant recipients provide access to the electrically evoked compound action potential (ECAP). ECAP thresholds are potential predictors of speech processor map’s threshold and comfortable loudness levels. The auditory nerve’s refractory properties can influence these levels due to high-rate stimulation with interpulse intervals within the relative refractory period. Recovery functions were investigated at 84 stimulation sites in 14 Nucleus CI24 recipients using neural response telemetry and a modified forward masking technique. This technique introduces a reference masker-probe interval (MPI). In our study, an appropriate value between 300 and 375 µs was determined for this reference MPI, and the use of a reference MPI of 300 µs is suggested for recovery and amplitude growth functions. A median absolute refractory period of about 390 µs and a median time constant of about 425 µs were obtained by fitting an exponential model to the data. Hence, the auditory nerve is usually in relative refractory state when standard neural response telemetry forward masking is selected because of its default MPI of 500 µs. This can bias the measurement of ECAP thresholds. Additionally, the shape of standard forward masking recovery functions was explained by the influence of latency shift of the neural response.

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Research in neural response telemetry (NRT) with the Cochlear 'Nucleus' CI24M Implant began a few years ago, using the first NRT software version (2.04). It has been demonstrated that NRT offers possibilities for fitting the implant speech processor. NRT sessions, however, remain lengthy for the patients, especially for children, and NRT research is seeking to reduce session time. The new version of the NRT software (3.0) allows implant stimulation up to 400 Hz, whereas the previous version stimulated only up to 80 Hz. The time gained is very significant. The goal of the present study was to observe and assess the modifications in NRT produced by these higher stimulus frequencies. Growth functions were measured in six adult patients for electrodes 5-10-15-20 at frequencies of 20, 80, 150, 250 and 365 Hz to determine NRT thresholds (NRT-T). Subjective detection thresholds (T) were also measured for the same electrodes and frequencies. The results showed that the number of valid responses, like the quality and amplitude of the NRT response, quickly decreased at these higher rates. Consequently, the number of measurement points used for the growth function decreased with frequency. Moreover, when frequency increased, the T value decreased while NRT-T increased: the gap between the two thresholds increased with frequency in a linear manner common to all patients. The growth function slopes did not change significantly with frequency.

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