Objective Measures of Listening Effort in CI Users: Brain Oscillations and Neural Entrainment to Naturalistic Stimuli

Background: Cochlear implant (CI) users often struggle while listening in adverse listening environments and often report feeling exhausted. In order to follow conversations, attention and effort must be exerted and may be related to listening effort (LE). The physiological mechanisms of LE are not well understood.

Methods: Adult CI users were tested while watching the television series “The Office” while recording from 64 channels of EEG. The sound was delivered in a circular 8 speaker array. The front facing speaker played the sound of the movie while the other 7 speakers played multitalker babble. Three SNRs were used +5, +10, and +15 dB. The audio output of the movie was simultaneously digitized as a channel on the EEG. Subjective reports of task difficulty were assessed using the NASA Task Load Index for each of the SNRs.

Results: Two types of analyses were performed: (1) spontaneous EEG, not time-locked to the movie audio, and (2) cortical coherence to the audio envelope. Increased power in the alpha rhythm frequency (8-12 Hz) was observed with increased LE in frontal regions. Significant coherence was observed between the movie audio and brain rhythms in the 4-6 Hz range. This increased coherence was observed bilaterally in the temporal cortex. Significant correlations between the degree of coherence and LE were observed in left frontal regions.

Conclusions: This study show that audio-brain coherence can be measured in CI users. The degree of coherence and the alpha rhythm appear to be potential biomarkers of LE in the CI population.