**4. Discussion**

The first aim of this study was to establish the lexical access abilities of adult subjects with bilateral severe to profound hearing loss. Study 1 showed superior performance on phonemic fluency compared to semantic fluency in the subjects included. This result is in contrast with the Santos study which showed superior performance on semantic fluency in adults with hearing impairment [24]. However, our study and Santos' study were not conducted in the same language (French vs. Brazilian Portuguese). The time allotted to measure the fluencies was also different (2 min vs. 1 min). It has been shown that semantic fluency is an indicator of a deficit in executive function [25]. Similarly, it is known that hearing impairment affects executive functions [26]. The results of this study could, therefore, demonstrate an executive function deficit more pronounced than the phonological deficit in adults with severe to profound hearing loss. Furthermore, the maintenance of this dorsal phonemic pathway in subjects with post-lingual hearing loss predicts a favourable outcome with a cochlear implant [21].

A positive correlation was observed between the phonemic and semantic Z-scores. Therefore, there is a link between success in phonemic and semantic fluencies, even though phonemic and semantic fluencies involve separate distinct systems. Semantic fluency involves the inferior longitudinal fasciculus, the unciform fasciculus, the temporal part of the inferior fronto-occipital fasciculus and the superior temporal gyrus. Phonemic fluency involves the ascending frontal tract, the frontal part of the inferior fronto-occipital bundle and the superior frontal gyrus [27]. The results of our study suggest a close link between these two pathways and that they can work together.

People with congenital hearing loss have better semantic lexical access than people with acquired hearing loss. This can be explained by the fact that people with congenital hearing loss have insufficient phonological decomposition dating to the prelingual period. Therefore, they would preferentially use the ventral (occipito-temporal) semantic pathway [20].

Secondly, we assessed the impact of cochlear implantation on phonemic and semantic lexical access. Study 2 showed a positive impact of the cochlear implant on phonemic fluency as early as 3 months post implant. Thus, cochlear implantation allowed for better phonological representation and better access to the dorsal pathway. This benefit of the cochlear implant appears relatively early after surgery. These results are important because most studies have highlighted cognitive changes at six months or one year but not so early. Indeed, since ten years the clinical research on the cognitive improvement by cochlear implantation has become increasingly recognized. Especially, studies have showed improvements in different cognitive functions as processing speed, cognitive flexibility and working memory [28–30]. These studies have conducted prospective longitudinal studies as early as 6 months after cochlear implantation but never as early as 3 months. So, our results suggest that verbal fluency improvement start since 3 months after implantation.

Study 2 showed no significant difference in pre- and post-cochlear implantation fluency between groups based on the origin of the hearing loss. The contribution of the cochlear implant on phonemic and semantic fluency is as effective for an acquired hearing loss as for a congenital one at 3 months post-implant. No correlation was found between differences in post- and pre-implant fluency and implant auditory gain. At 3 months post-implantation, the benefit of the cochlear implant is not specific to the phonemic and semantic pathways. The distribution between the pathways is not determined.

Finally, the improvement in participants verbal fluency, even elderly, and this very quickly, adds proof of the interest of cochlear implantation for cognitive stimulation and the prevention of cognitive decline as suggested by other studies [31].

It would be interesting to continue this study with a longer post cochlear implantation observation time to see if a correlation could be obtained between implant gain and the difference in phonemic and semantic fluency. It might also be interesting to analyse clustering (retrieval of words by phonemic or semantic subcategory) and switching (moving from one subcategory to another) in verbal fluency tasks in people with hearing impairment.
