Evolution of non-speech sound memory in postlingual deafness: implications for cochlear implant rehabilitation

Neurofunctional patterns assessed before or after cochlear implantation (CI) are informative markers of implantation outcome. Because phonological memory reorganization in post-lingual deafness is predictive of the outcome, we investigated, using a cross-sectional approach, whether memory of non-speech sounds (NSS) produced by animals or objects (i.e. non-human sounds) is also reorganized, and how this relates to speech perception after CI. We used an fMRI auditory imagery task in which sounds were evoked by pictures of noisy items for post-lingual deaf candidates for CI and for normal-hearing subjects. When deaf subjects imagined sounds, the left inferior frontal gyrus, the right posterior temporal gyrus and the right amygdala were less activated compared to controls. Activity levels in these regions decreased with duration of auditory deprivation, indicating declining NSS representations. Whole brain correlations with duration of auditory deprivation and with speech scores after CI showed an activity decline in dorsal, fronto-parietal, cortical regions, and an activity increase in ventral cortical regions, the right anterior temporal pole and the hippocampal gyrus. Both dorsal and ventral reorganizations predicted poor speech perception outcome after CI. These results suggest that post-CI speech perception relies, at least partially, on the integrity of a neural system used for processing NSS that is based on audio-visual and articulatory mapping processes. When this neural system is reorganized, post-lingual deaf subjects resort to inefficient semantic- and memory-based strategies. These results complement those of other studies on speech processing, suggesting that both speech and NSS representations need to be maintained during deafness to ensure the success of CI.