Adaptation of the electrically evoked compound action potential (ECAP) recorded from nucleus CI24 cochlear implant...

This study had three main goals. The first goal was to assess the extent to which neural adaptation varied across cochlear implant users. The second goal was to determine whether adaptation at the level of the auditory nerve was correlated with word recognition ability. The third goal was to determine whether peripheral neural adaptation had an impact on the relationship between the electrically evoked compound action potential (ECAP) thresholds and MAP levels.

Neural response telemetry software was used to record the ECAP in 21 Nucleus cochlear implant users. A series of 110 ECAP recordings were made over a 5-min period at three different stimulation rates: 15, 80, and 300 Hz. The stimulation levels used to record this series of responses were held constant at or near the level the subject identified as his or her maximum comfort level (C-level) for the 300-Hz stimulation rate. Consistent decreases in ECAP amplitude as measured from the beginning to the end of the 5-min stimulation interval were interpreted as evidence of neural adaptation. Regression analysis procedures were then used to assess the relationship between neural adaptation and word recognition.

Significant levels of adaptation were observed for all 21 subjects at stimulation rates of 80 and 300 Hz. Little or no adaptation was observed over the 5-min recording period when the 15-Hz rate was used. The amount of adaptation was greatest at the 300-Hz rate and varied substantially across cochlear implant users. No relationship between the amount of adaptation and word recognition was found. Neither was the degree of adaptation shown to influence the relationship between ECAP thresholds recorded at low rates and the levels used to program the speech processor.

Cochlear implant users experienced varying degrees of long-term adaptation in response to continuous electrical stimulation. The effects of adaptation on the ECAP were apparent even at stimulation rates as low as 80 Hz. Although variations in the amount of adaptation are likely to reflect cross-subject differences in the status of the auditory nerve, no predictable relationship was found between these physiologic measures of peripheral neural function and either word recognition or the relationship between ECAP thresholds and MAP levels.