Neural and Behavioral Adaptation to Listening with a Unilateral Cochlear Implant (CI)

The human auditory system must encode sounds and separate relevant input from noise. Hearing loss hurts both processes, but profound hearing loss and deafness can be treated with a Cochlear Implant (CI). Here, we probe the human brain’s adaptation to electrical hearing with a CI in unilaterally implanted CI users with normal or aided contralateral hearing, in the first and seventh month after CI activation. In addition to clinical measures of speech comprehension, electroencephalography (EEG) recordings during passive listening to amplitude-modulated (AM) sounds and an active spatial listening task with competing talkers were analyzed. We demonstrate that, first, adaptation to hearing with a CI is accompanied by decreasing cortical overrepresentation and increasing auditory temporal coding fidelity: After six month of CI use, auditory phase-locking during passive listening relatively decreased for 4-Hz AM sounds but increased for 40-Hz AM sounds. Second, strongest benefits of listening with the CI (i.e., CI on vs. off) in the spatial-listening task occurred when the CI ear was primarily exposed to target sound. Here, the power of lateralized alpha oscillations (∼10 Hz) served as a robust signature of spatial attention, independent of time (1 ^st vs 7 ^th month) and listening modality (with vs. without CI). Stronger lateralized alpha modulation in the hemisphere contralateral to the non-implanted ear suggests asymmetrical allocation of auditory spatial attention, operating preferentially on the input to the non-implanted ear. These findings highlight CI-related plasticity in auditory perception and a persistent attentional bias toward the non-implanted ear, emphasizing the need to consider bilateral auditory input in CI rehabilitation.