Longitudinal speech understanding outcomes in cochlear implant recipients are associated with neural factors identified using psychophysics and functional brain imaging

Objective: There remain large variations in cochlear implant outcomes which are poorly explained by patient history factors. This study aims to better understand how different neural limitations along the auditory processing pathway are associated with speech understanding outcomes.Design: 43 cochlear implant recipients (44 ears) were recruited to participate in a longitudinal study to evaluate limitations at various locations along the auditory processing pathway. At the periphery, intra-cochlear neural health was estimated using focused psychophysical thresholds. Higher-order across-electrode intensity discrimination ability was assessed using psychophysical “spectral tilt”. At the cortical level, cross-modal activation of the auditory cortex was assessed using functional near infra-red spectroscopy. Speech understanding outcomes were evaluated in competing multi-talker babble noise at one-year post device switch-on.Results: We found that greater abilities to discriminate smaller across-electrode stimulation intensities (R2 = 0.138, p = 0.047) and smaller cross-modal activations of the auditory cortex (R2 = 0.216, p = 0.01) were significantly associated with better speech understanding outcomes in cochlear implant recipients. Furthermore, both measures contributed significantly to better predict speech understanding outcomes in a multiple regression model (Adj R2 = 0.312, p = 0.009).Conclusion: The results suggest that recipient-specific neural factors contribute significantly towards individual differences in speech understanding outcomes. Different recipients can be affected by different limitations, which can be identified using these tests, and therefore lead to potential patient-specific methods to improve their speech understanding.