The recognition of human voice in deaf and hearing infants

Purpose: Deafness in infancy has longlasting consequences on brain organization. To achieve the best developmental outcomes in case of profound deafness, cochlear implantation (CI) needs to take place in a critical period during the first year of life, before the cross-modal reorganization of the brain due to auditory deprivation stabilises, preventing the typical development of the auditory cortices. Despite its importance for implantation outcomes, the nature of this critical period and the underlying neural reorganization have not been fully explored.Materials and methods: To fill this knowledge gap, we investigated the cortical responses of 12-month-old profoundly deaf infants and their age-matched controls to sounds produced by a human voice as compared to non-human sounds using functional near-infrared spectroscopy (fNIRS). The deaf infants were tested before undergoing CI surgery and wore hearing aids, allowing them to perceive low frequencies. Human voice stimuli have been shown to trigger brain responses early in development, possibly due to its evolutionary relevance for survival.Results: We found increased brain responses to the human voice in the deaf infants in the bilateral fronto-temporal areas, and their responses correlated with their residual hearing thresholds. These results suggest that even the limited sound stimulation that these deaf infants receive due to their residual hearing allowed the temporal cortices to develop sensitivity to the human voice prior to implantation. The hearing control group showed an inverted hemodynamic response to both voice and non-voice stimuli in the left parietal and right temporal areas, suggesting that by 12 months of age, they habituate rapidly to these very familiar stimuli. While both groups showed an inverted response to the non-voice stimuli, they differed in their responses to voice stimuli.Conclusions: We hypothesize that the responses found in deaf infants to human voice can be consider a good cochlear implant prognosis.