## 4 Discussion  
The findings of the present study demonstrate different patterns of cortical activation in temporal regions covering mostly the MTG in response to speech in NH listeners who listen to natural stimuli and prelingually deafened CI users, depending on the SRTn of the latter. Specifically, while NH listeners showed large cortical activation, (i.e., high positive HbO beta values in the temporal areas), to speech in quiet and in noise, prelingual CI users with poor SRTn showed significantly smaller cortical activation to speech, with positive beta values for speech in quiet, and negative beta values, representing reduced HbO supply, for speech amid noise. On the other hand, CI users with good SRTn showed somewhat smaller, but not significantly different, beta values when compared with the NH group. Interestingly, the NH participants who listened to vocoded stimuli showed an activation pattern that was not significantly different from that of the NH participants who listened to natural stimuli or the CI groups. Nevertheless, beta values in response to SIN in the right and left MTG were correlated with SRTn across all participants, suggesting that larger brain activation in these temporal areas relates to better speech understanding in noise, irrespective of the auditory background. Our novel main finding illustrates the relationship between the pattern of cortical activation and SIN understanding for CI users, with CI participants who have poor SIN understanding showing smaller response to SIQ and inversed response to SIN in temporal regions, compared with NH controls. These findings may support the notion that visual repurposing of the auditory regions have occurred for some of the CI users during the period of auditory deprivation, that is, before implantation (for a review see: [71]). As a result, their processing of auditory stimuli in these cortical regions may have been reduced (e.g., [39]). To date, the only study that assessed cortical activation to speech using fNIRS in a group of prelingually deafened CI users (children) reported no significant difference in activation to auditory speech in temporal regions between CI and NH participants [38]. This discrepancy may be explained by the fact that in [PERSON] et al. 's study, only three of the 19 CI participants had poor speech perception ability; thus, no statistical analysis separating the good and poor CI patients could have been conducted. On the other hand, in our current study, most CI participants (8/14) had poor SIN perception. Another reason for the differences between the two studies may be related to the different fNIRS paradigm that was used to assess cortical activation to speech in [PERSON] et al. 's study, and/or the different approach used for ROIs identification. Figure 6: ROI-mean beta values for the SIN condition as a function of speech reception thresholds in noise (SRTn) across participants (n = 41; 15 NH, 15 NHV, 11 CI). Left panel for Left SIN and right panel for Right SIN. NH = normal-hearing participants who listened to natural stimuli, NHV = normal-hearing participants who listened to vocoded stimuli, CI = cochlear implant users. Individual examination of the poor CI performers in the current study shows that more than half (5/8) of these participants were implanted late, i.e., they were rehabilitated after the \"critical\" or \"sensitive\" early life period wherein neuronal properties are particularly susceptible to shaping by experience [72; 73]. Electrophysiology studies in children with CI show significant differences in evoked middle latency response (eMLR) (e.g., [74]) and in cortical auditory-evoked potentials (CAEP) (e.g., [75]) between children implanted before 3.5-5 years of age and children implanted later, supporting the notion that the most sensitive period for auditory deprivation is up to approximately 4 years of age [76].