Global effect of deafness on the ELAR latency

ELAR latency was compared with duration of deafness and mean audiometric threshold before implantation to assess the influence of deafness as a whole (i.e., independently of the stimulation electrode site, which has been studied with the M levels) on the conduction time required by the auditory input to reach the auditory cortex. The fact that pure tone audiometric thresholds were not significantly linked to duration of deafness enabled us to study two different aspects of the impact of deafness on the auditory pathway’s integrity (degree of auditory loss versus duration). The latencies of N1 and P2, and interval N1-P2 were associated with duration of deafness before implantation. The degree of auditory loss reflected by the audiogram before implantation was related to the latency of N1 and almost to the latency of P2 (p = 0.056), while it had no effect on interval N1-P2, which implies an effect up to N1 but less strongly beyond. Moreover, the regression analysis showed that the poorer the audiogram before implantation and the longer the duration of deafness, the longer the latency of N1 and P2. It is possible though that the linear relationship between the audiogram and the P2 latency only reflects the relationship between the audiogram and the N1 latency as the regression analysis showed that the interval N1-P2 does not vary linearly according to the audiogram mean. The non significant influence of the audiogram on the interval N1-P2 should be interpreted carefully though because P2 is often more difficult to localize than N1, which results in more variability as shown by the standard errors on figures 3, 5, 6, 7, and 8 and could explain that statistical tests were more significant for N1 than for P2. It is therefore possible that the effect of the audiogram on the interval N1-P2 is “present” even if not significant and that P2 latency does not only reflect N1 latency.

These effects of deafness on central parts of the auditory pathway could be explained by the extent of cross-modal recruitment of the auditory cortex, which increases as the duration of deafness increases (Lee et al., 2001). In addition, the remaining neural fibers dedicated to auditory processing may also be degenerated especially when the duration of deafness is longer (Webster & Webster, 1979; Chouard et al., 1983). Further, EABR waves eIII and eV latencies were found to be related to such pathological parameters (Guiraud et al., in press) and the effects found at higher levels of the auditory pathway could then reflect the fact that latencies are delayed up to the inferior colliculus (i.e., wave eV’s generator) in subjects with longer duration of and/or more severe deafness. The effects of deafness on ELAR latency could then result from less myelinated peripheral processes (Zhou et al., 1995) or less numerous spiral ganglion cells (Smith & Simmons, 1983; Hall, 1990).

In conclusion, this study describes latencies of ELAR waves N1 and P2, and interpeak interval N1-P2 characteristics across stimulation sites for 14 cochlear implant users. The influences of anatomical and pathological parameters on latencies were studied using the same protocol as in a previous study concerning EABR (Guiraud et al., in press). As in the EABR study, interesting correlations were found with the audiogram before cochlear implantation and the duration of deafness showing that ELAR latencies reflect well the integrity of the auditory pathway. However, contrary to EABR latency, they were found not to depend on stimulation parameters such as electrode site or even possibly stimulation intensity since no relationship was found with the M levels. It seems therefore that stimulation parameters affect more the peripheral than the central parts of the auditory pathway. However, comparison between both studies should be interpreted carefully. It could indeed be possible that electrical stimulation affect the brainstem and the auditory cortex differently. N1 and P2 are also broader than the EABR waves IIIe and Ve and their latency measures are generally more variable, which could result in non-significant effects despite the existence of an influence of stimulation parameters. All the same, this study raises the question whether the ELAR would be more suitable than EABR to provide an objective tool to assess the auditory pathway’s integrity. The two studies on EABR and ELAR would therefore need further investigation, which could consist of comparing the influence of various sites and intensities on the relationship between auditory performance (i.e., benefit from the implant) and EEPs recorded from the peripheral versus the central parts of the auditory pathway.