Short- and long-latency components of the eCAP reveal different refractory properties

The refractory recovery function (RRF) measures the electrically evoked compound action potential (eCAP) in response to a second pulse (probe) after masking by a first pulse (masker). This RRF is usually used to assess the refractory properties of the electrically stimulated auditory nerve (AN) by recording the eCAP amplitude as a function of the masker probe interval. Instead of assessing eCAP amplitudes only, recorded waveforms can also be described as a combination of a short-latency component (S-eCAP) and a long-latency component (L-eCAP). It has been suggested that these two components originate from two different AN fiber populations with differing refractory properties. The main objective of this study was to explore whether the refractory characteristics revealed by S-eCAP, L-eCAP, and the raw eCAP (R-eCAP) differ from each other. For clinical relevance, we compared these refractory properties between children and adults and examined whether they are related to cochlear implant (CI) outcomes.

In this retrospective study, the raw RRF (R-RRF) was obtained from 121 Hi-Focus Mid-Scala or 1 J cochlear implant (Advanced Bionics, Valencia, CA) recipients. Each R-eCAP of the R-RRF was split into an S-eCAP and an L-eCAP using deconvolution to produce two new RRFs: S-RRF and L-RRF. The refractory properties were characterized by fitting an exponential decay function with three parameters: the absolute refractory period (T); the saturation level (A); and the speed of recovery from nerve refractoriness (τ), i.e., a measure of the relative refractory period. We compared the parameters of the R-RRF (R_T, R_A, Rτ) with those obtained from the S-RRF (S_T, S_A, Sτ) and L-RRF (L_T, L_A, Lτ) and investigated whether these parameters differed between children and adults. In addition, we examined the associations between these parameters and speech perception in adults with CI. Linear mixed modeling was used for the analyses.

We found that T_R was significantly longer than S_T and L_T, and S_T was significantly longer than L_T. R_A was significantly larger than S_A and L_A, and S_A was significantly larger than L_A. Also, Sτ was significantly longer in comparison to Rτ and Lτ, but no significant difference was found between Rτ and Lτ. Children presented a significantly larger S_A and L_A and a shorter R_T in comparison to adults. Shorter Sτ was significantly associated with better speech perception in adult CI recipients, but other parameters were not.

We demonstrated that the two components of the eCAP have different refractory properties and that these also differ from those of the R-eCAP. In comparison with the R-eCAP, the refractory properties derived from the S-eCAP and L-eCAP can reveal additional clinical implications in terms of the refractory difference between children and adults as well as speech performance after implantation. Thus, it is worthwhile considering the two components of the eCAP in the future when assessing the clinical value of the auditory refractory properties.