## 4 Discussion
Furthermore, animal studies suggest that when implantation occurs at a later age, or after the sensitive period of synaptic pruning in the auditory pathway, it may restore some of the tonotopic organization of the primary auditory cortex without functionally improving synaptic efficiency (e.g., [77]). Thus, late-implanted individuals may have impaired spectral processing of the auditory signal [54], which may result in poor speech perception, particularly in noisy conditions. Indeed, in our current study, five of the seven late-implanted CI users had poor SIN perception. They also had significantly smaller activation in response to SIQ compared to NH participants, and inversed response to SIN in fNIRS channels reflecting mainly the middle temporal gyrus (MTG). Given that this temporal region is thought to be recruited to perform auditory analyses and early speech decoding processes [47; 78], the finding of a smaller response to SIQ provides new evidence that suggests that speech understanding in late-implanted individuals with prelingual deafness is observed by less efficient speech processing mechanisms. The inversed HbO response shown in the right and left MTG areas to SIN for the poor CI participants may have several explanations. One explanation, derived from fMRI studies, may postulate that the negative response indicated a blood \"stealing\" effect, that is, redirection of blood from a passive cortical region to a region with high neuronal activity [79]. However, a careful inspection of the poor CI data, does not reveal elevated beta-values in close-by channels. A different explanation may suggest that the negative response represented a suppression of cortical activation in the MTG regions for the poor CI participants. Such deactivation was previously suggested in fMRI studies to represent functional inhibition that occurs in cortical areas that are unnecessary for task performance, to maintain efficient processing [80]. Recent findings that combined fNIRS and EEG data argue, however, that negative fNIRS HbO responses to auditory signals may not be driven by a decrease in neural activity in the auditory cortex [81]. This disagreement may be resolved in future studies that will specifically examine the phenomenon of inversed HbO response to auditory signals. It should be noted that age-at-implantation cannot solely account for the differences shown in the MTG pattern of activity between the good and poor CI performers, as two late-implanted CI users were included in the good CI group and three poor CI performers were implanted early. Other contributing factors, excluding auditory history, should therefore be considered, including factors like the linguistic background of the CI participants. Neural activity in the MTG has been suggested to underlie the recognition of sounds as words and the comprehension of the syntactic properties of words by combining phonetic and semantic cues [82; 83; 84]. Hence, deficits in phonemic and/or syntactic knowledge may yield reduced MTG activation to speech for poor CI performers, limiting their ability to segregate the speech stream into syllables and words, and thus, causing poor SIN understanding. This reasoning aligns with studies that show poor linguistic skills in some prelingual CI users when compared to NH subjects [3; 9; 85; 86; 87; 88]. Unfortunately, we did not test linguistic abilities in this study. Future fNIRS studies with CI patients may include comprehensive linguistic assessments to test this hypothesis. The present findings, reported for the first time for prelingual CI users, are in accordance with previous fNIRS reports on a significant relationship between patterns of activation to auditory and visual speech and speech understanding in CI groups that included post-lingually deafened, or a mix of prelingually and post-lingually deafened participants [31; 32; 35; 39; 40]. They are also consistent with the PET data that showed that the degree of cortical response to speech correlated with speech perception in post-lingual CI users [25; 89].