Contributions of aging, auditory nerve health, and speed of processing to psychophysical gap detection thresholds in cochlear implant users.

Precise encoding of temporal speech cues is critical for speech understanding and are especially important for cochlear implant (CI) users, who receive limited spectral information. Silent gap detection is a simple index of auditory temporal precision, yet mechanisms and stimulus parameters driving variability in CI gap detection thresholds (GDTs) remain unclear. The overall aim of this study was to describe how AN density, aging, pulse rate, and cognitive speed of processing contribute to GDTs in adult CI users. Twenty post-lingually deafened adult Cochlear™ CI users (37-87 years) participated. We recorded electrically evoked compound action potentials (ECAPs) and psychophysical gap detection using direct stimulation techniques. ECAP interphase gap (IPG) slope effect was used as an estimate of AN density. GDTs were measured with 500 and 3500 pps pulse trains on two electrodes per ear paired with the highest and lowest IPG effect. Cognitive processing speed was assessed with the Connections Test. CNC word recognition was measured in the sound field. GDTs increased (worsened) with increasing age; but decreased (improved) overall when using a higher compared to lower pulse rate but more so for younger compared to older CI users. Estimates of AN density did not predict GDTs. Slower cognitive processing speed was associated with poorer GDTs, when using higher but not lower pulse rates. These results suggest that, at higher pulse rates central/cognitive mechanisms but not AN density status, predict encoding of temporal gaps in adult CI users particularly when using higher pulse rate stimuli.