## 1 Introduction
Several recent studies have assessed cross-modal reorganization in CI users before and after implantation, using fNIRS measures of cortical activation (e.g., extracting beta weights of the generalized-linear-model (GLM) fit to the canonical hemodynamic response function (HRF) to quantify the amplitude of cortical activation [35; 39]), and functional connectivity (e.g., assessing the temporal correlations between time courses of hemodynamic changes of two brain regions [40; 41]). Table S1 (Supplementary Materials) details studies that assessed neural activation to auditory speech via CI hearing, including NIH participants who listened to vocoded speech and CI participants. For NH participants (Table S1a), a general trend of increased activation in the temporal regions with improved intelligibility has been reported in several studies [42; 43; 44]. However, other studies did not find significant differences in temporal activation between natural and vocoded stimuli [45] or reported a difference between the right and left temporal areas, with larger activation in the left temporal regions for vocoded stimuli and in the right temporal regions for natural stimuli [30]. A significant effect of speech intelligibility on frontal region activation was also reported in a small number of studies, with larger activation for degraded stimuli compared to natural stimuli (e.g., [45; 46]), and activation peaking at intermediate levels of intelligibility [43; 47]. For CI participants (Table S1b), on the other hand, variable findings were reported, ranging from larger [32], to similar [38; 48; 49], and to smaller brain activation levels [50] to auditory speech in temporal brain areas compared to NH controls. Furthermore, unlike the results for NH participants, one study that examined the activation to auditory speech in the frontal regions for CI users reported a similar pattern of activation for CI and NH controls [49]. The inconsistency in CI reports may stem from the fact that CI users are a heterogeneous group with variable speech perception abilities. This explanation is supported by two recent studies that showed cortical activation in temporal regions to correlate with the level of speech comprehension of CI users [31; 32]. The lack of coherence across studies may also be related to the fact that most CI studies have tested post-lingually deafened adults, that is, individuals with acquired hearing loss who had normal acoustic hearing during their cognitive and language development years [31; 32; 48; 49; 50]. Post-lingual CI users learned to process speech based on acoustic information, and thus, the pattern of cortical activation to the degraded speech transmitted by the CI device may resemble that of NH individuals who listen to degraded speech via acoustic simulations. Conversely, prelingually deafened CI users experienced impaired acoustic hearing, or electric hearing during their perceptual and spoken language development and their cognitive development (depending on age at implantation), and therefore might be expected to exhibit a different pattern of cortical activation to speech. To our knowledge, only one study has assessed the cortical activation to auditory speech in prelingual CI users using fNIRS [38]. In that study, the activation to visual and auditory speech in temporal brain regions was compared between prelingual CI children and NH children who listened to natural speech (Table S1b). The results showed greater cortical responses to visual speech in CI children, with no significant difference in the pattern of activity to auditory speech between the CI and NH groups. No control group of NH participants who listened to vocoded speech was included in the study. To the best of our knowledge, the only fNIRS study that included a comparison group of NH participants who listened to vocoded (and non-vocoded) speech assessed cortical activation in postlingual CI users [31] (Table S1c).