Recovery Function of the Late Auditory Evoked Potential in Cochlear Implant Users and Normal-Hearing Listeners

Abstract

It has been theorized that neural recovery is related to temporal coding of speech sounds. The recovery function of cortically generated auditory evoked potentials has not been investigated in cochlear implant (CI) users. This study characterized the recovery function of the late auditory evoked potential (LAEP) using a masker-probe paradigm in postlingually deafened adult CI users and young normal-hearing (NH) listeners. A case-control study of the late auditory evoked potentials using electrophysiological technique was performed. The LAEP was evoked by 1 kHz tone bursts presented in pairs, with the first stimuli as the maskers and the second stimuli as the probes. The masker-probe intervals (MPIs) were varied at 0.7, 1, 2, 4, and 8 sec, with an interpair interval of 12 sec. Nine CI users and nine NH listeners participated in this study. The normalized amplitude from the probe response relative to the masker response was plotted as a function of the MPI to form a recovery function. The latency shift for the probe response relative to the masker response was calculated. The recovery function was approximately linear in log scale of the MPI in NH listeners, while it showed somewhat different recovery patterns with a large intersubject variability in CI users. Specifically, although the probe response was approximately 60 percent of the masker response for the MPI of 0.7 sec in both groups, the recovery function of CI users displayed a nonlinear pattern, with a steeper slope than that of NH listeners. The probe response completely recovered at the MPI of 4 sec in NH listeners and at the MPI of 2 sec in CI users. N1 and P2 latencies from probe responses were shorter than those from masker responses in NH listeners, while no latency difference was found between probe responses and masker responses in CI users. Our interpretation of these findings is that the faster recovery of the LAEP in CI users is related to abnormal adaptation mechanisms and a less prominent role of the components with longer latencies in the LAEP of CI users. Other mechanisms such as the compromised inhibitory regulation in the auditory system and the aging effect in CI users might also play a role. More research needs to be done to determine whether the slope of the LAEP recovery function is correlated with speech-perception performance.