Peripheral neural synchrony in post-lingually deafened adult cochlear implant users

OBJECTIVE: We recently developed a noninvasive method for quantifying neural synchrony in the cochlear nerve (i.e., peripheral neural synchrony) in cochlear implant (CI) users, which allows for evaluating this important physiological phenomenon in human CI users for the first time in the literature. This paper reports this new method in detail. In addition, this study assessed how peripheral neural synchrony was correlated with temporal resolution acuity and speech perception outcomes measured in quiet and in noise in post-lingually deafened adult CI users. It tested the hypotheses that 1) the degree of peripheral neural synchrony varied among CI users, and 2) peripheral neural synchrony was an important factor for temporal resolution acuity and speech perception outcomes in noise in post-lingually deafened adult CI users. DESIGN: Study participants included 18 post-lingually deafened adult CI users with a Cochlear(™) Nucleus(®) device. Two study participants were implanted bilaterally, and each ear was tested separately. For each of the 20 ears tested in this study, peripheral neural synchrony, quantified using an index named the phase locking value (PLV), was measured at four electrode locations across the electrode array (default electrodes: 3, 9, 15 and 21) based on 400 sweeps of the electrically evoked compound action potential (eCAP). The PLV is a measure of trial-by-trial phase coherence among eCAP sweeps/trials and quantifies the degree of peripheral neural synchrony. Temporal resolution acuity was evaluated by measuring the within-channel gap detection threshold (GDT) using a three-alternative, forced-choice procedure, targeting 79.4% correct on the psychometric function in a subgroup of 9 participants (10 ears). For each ear tested in these participants, GDTs were measured at two electrode locations with a large difference in PLVs. For 15 ears tested in 14 participants, speech perception performance was evaluated using Consonant-Nucleus-Consonant (CNC) word lists presented in quiet and in noise at signal-to-noise ratios (SNRs) of +10 and +5 dB. A Linear Mixed-effects Model (LMM) was used to evaluate the effect of electrode location on the PLV after controlling for the stimulation level effect. A partial correlation test was used to evaluate the association between the PLV and GDT while controlling for the effect of stimulation level on GDT. GDTs measured for electrode pairs with different PLVs were compared using a paired-samples t-test. The association between PLVs and CNC word scores measured in different conditions, as well as the association between the PLV and the degree of noise effect on CNC word scores were evaluated using Pearson product-moment correlation tests with Bonferroni correction for multiple comparisons. RESULTS: PLVs varied substantially across study participants and electrode locations. There was a significant correlation between the PLV and GDT, where higher PLVs were associated with lower GDTs. PLVs were not significantly correlated with CNC word scores measured in any listening condition or the effect of competing background noise presented at a SNR of +10 dB on CNC word scores. In contrast, there was a moderate, negative correlation between the PLV and the degree of noise effect on CNC word scores for a competing background noise presented at a SNR of +5 dB. CONCLUSIONS: This newly developed method can be used to assess peripheral neural synchrony in living CI users, a physiological phenomenon that potentially plays a vital role in determining auditory perception outcomes in electrical hearing. Poorer peripheral neural synchrony is associated with lower temporal resolution acuity and larger detrimental effect of competing background noise on speech perception performance in post-lingually deafened adult CI users.