Across-Frequency Temporal Processing and Speech Perception in Cochlear Implant Recipients and Normal Hearing Listeners

ABSTRACT Objective: Speech recognition performance among cochlear implant (CI) recipients is highly variable and is influenced by their ability to perceive rapid changes within the acoustic signal (i.e., temporal resolution). A behavioral gap detection test is commonly used to assess temporal processing however it requires active participation, and therefore may be infeasible for young children and individuals with disabilities. Alternatively, cortical auditory evoked potentials (CAEPs) can be elicited by a silent gap embedded in a longer duration stimulus and have been used as an objective measure of temporal resolution. Only a few studies have examined within-frequency gap detection (identical pre- and post-gap frequency), most of which were conducted with normal hearing (NH) individuals and did not include speech perception. The purpose of the study is to evaluate behavioral and electrophysiological measures of within-frequency temporal processing and speech perception in NH and CI recipients. Design: Eleven post-lingually deafened adult CI recipients (n = 15 ears; mean age = 50.4 yrs.) and eleven age- and gender-matched NH individuals (n = 15 ears; mean age = 49.0 yrs.) were recruited. Speech perception was assessed with the CNC word test, AzBio sentence test, and BKB Speech-in-Noise test. Within-frequency (2 kHz pre- and post-gap tone) behavioral gap detection thresholds (GDT) were measured using an adaptive, two-alternative, forced-choice paradigm. Within-frequency CAEPs were measured using four gap duration conditions based on the individual9s behavioral GDT including a supra-threshold (GDTx3), threshold (GDT), sub-threshold (GDT/3), and reference (no gap) condition. Mixed effect models examined group differences in speech perception, behavioral GDTs, and CAEP amplitude and latency. Correlation analyses examined the relationship between the CAEP response, behavioral measures of speech perception and temporal processing, and demographic factors. Results: CI recipients had significantly poorer speech perception scores with no significant differences in behavioral within-frequency GDTs compared to NH participants. CI recipients had poorer CAEP waveform morphology, smaller N1, larger P2 amplitude, and increased P1 latency compared to NH participants. Additionally, older participants displayed smaller N1-P2 amplitude compared to younger participants. Bivariate group correlation analysis showed that individuals with poorer within-frequency GDTs displayed significantly poorer performance on the AzBio sentences in noise and BKB Speech-in-Noise test. Multivariate canonical correlation analysis showed a significant relationship between the within-frequency CAEP amplitude and latency and behavioral measures of speech perception and temporal processing. Conclusions: CI recipients had poorer speech understanding in quiet and noise yet similar behavioral GDTs compared to NH participants. NH participants showed the anticipated trend of increased N1-P2 amplitude as CAEP gap duration increased. However, CAEP amplitude and latency remained relatively stable across gap duration conditions for CI recipients. Instead, significant group and age effects for CAEP peak amplitude and latency were found that can likely be attributed to differences in cortical neuron density, adaptation, and recovery between the groups. Lastly correlation analysis indicates that individuals with poorer temporal processing are likely to have adequate speech perception in quiet but worse speech understanding in noise.