Investigating speech processing in paediatric and adult CI users using combined fNIRS/EEG measurements
In case of severe hearing loss or congenital deafness, cochlear implants (CIs) represent the method of choice to restore hearing and enable language acquisition. Yet, little is known about how exactly the cortical processing of speech differs between acoustic and electrical hearing. In a first study, we thus tested unilateral adult CI users with preserved normal hearing in the other ear by separately presenting both ears with vowel sequences, while simultaneously recording functional near-infrared spectroscopy (fNIRS) and EEG data. Results showed smaller and delayed auditory cortex activity when the CI ear was stimulated. In a second study, paediatric CI users were tested during and after the first year of CI use and compared to an age-matched normal-hearing control group. In response to vowel sequences as well as to running speech, cortical activity was again markedly smaller in CI-based hearing. Despite trends in this direction, activity levels did not increase significantly with more CI experience. However, the less experienced CI group showed an abnormal shift of activity to the right hemisphere in response to running speech that was not observed in the other two groups. Overall, these data hence showed that, except from an initial adaptation phase, activity patterns were qualitatively similar but attenuated in electric hearing.