Abstract

Deafness is a common sensory loss condition that affects a relatively large portion of the population around the world. Profound sensorineural deafness is the condition that we will consider in this work given its compatibility with cochlear implantation. Cochlear implant (CI) is a neuroprosthesis that allows the acoustic signal to bypass the impaired inner hear system, by sending an electrical input directly to the acoustic nerve. Although it represents an important success among neuroprosthesis, it still represents a matter of interest in the field of cognitive neuroscience. Specifically, it is still unclear how it impacts language processing in prelingually and postlingually deaf CI users where the presence and the nature of a critical period for language acquisition has been long debated (Chapter 1). To this aim, we selected well-established ERP components that have been linked to language processing like the N400 and the P600: the first usually is elicited by semantic incongruities and the second is elicited in response to syntactic manipulations (Chapter 2). We adopted a widely employed EEG paradigm where participants had to read a large sample of sentences one word at a time and we also tested subjects with a set of behavioral tasks. Sentences were semantically incongruent or contained a subject-verb number-agreement violation; an equal number of correct sentences were used as a control condition. We tested CI recipients with early deafness onset (< 3 years) or postverbal deafness onset (>3 years) as well as a large population of normal hearing participants that ranged from 12 up to 70 years old. The first study of this thesis will compare ERP components of language processing between preverbal CI users and a group of age-matched hearing controls. Results revealed comparable P600 and a larger N400 in CI users as well as interesting correlations between components and age at implantation that support the existent general consensus in favor of early implantation (Chapter 5). The second study employed the same RSVP paradigm and compared postlingually deaf CI participants with a sample of age peers. This study revealed that, despite the implanted group seems to have similar overall results to NH controls, CI can still impact language processing as revealed by the correlation between the N400 effect and the use of the CI (Chapter 6). Lastly, we analyzed the sample of control participants to better understand how aging can impact ERP components of language processing. We observed that both the N400 and the P600 (but also other components) are modulated by aging (Chapter 4). Together these results proved that the study of language and its neural signatures (ERPs) is of primary interest, not only for clinical purposes but also for the advancement of psycholinguistic knowledge.