Plasticity following auditory deafferentation and reafferentation

The present thesis investigates the effects of auditory deafferentation and reafferentation with a unimodal and multisensory perspective. Aim of the thesis is the understanding of issues concerning functional plasticity resulting from long-term auditory deprivation, and the effects of reafferentation through a cochlear implant (CI) on audition, vision, and their interaction. The thesis is divided into three parts: Part I explores the effects of auditory deafferentation on the visual modality to understand whether a long-term sensory deprivation leads one of the remaining senses to reorganise in a cross-modal fashion. In particular, Chapter 1 reviews animal and human findings on cross-modal plasticity after sensory deafferentation and introduces the particular case of deafness, focusing on the sensory modality that seems to reorganise the most after profound deafness: vision. In Chapter 2 I present the study we conducted to explore an underinvestigated issue of cross-modal reorganisation after long-term auditory deprivation. We investigated visual temporal processing in a group of profoundly deaf individuals by testing their ability to make temporal order judgments. Our results show comparable accuracy in processing visual temporal sequences in deaf individuals and hearing controls, but an enhanced reactivity in the deaf population particularly when responding to stimuli appearing towards the periphery of the visual field. Our findings suggest that long-term auditory deprivation does not alter temporal processing abilities, and that the reactivity observed in the deaf group may instead constitute a central aspect of the functional changes occurring after auditory deafferentation. Part II of the thesis addresses the effects of auditory reafferentation through a cochlear implant on the adult auditory system. Chapter 3 reviews findings that document plasticity in the adult brain and the role of experience in determining the extent for plasticity to occur. In addition, a review on auditory spatial hearing introduces the two studies we conducted to investigate the recovery of sound localisation abilities after bilateral and unilateral cochlear implantation (chapter 4 and 5, respectively). Results from the first study show that partial recovery of spatial hearing after bilateral implantation occur with different time course as a function of the recipient’s experience with auditory cues. Results from the second study show that some sound localisation abilities can emerge even in prelingually deafened adults fitted with a single implant, at least in a laboratory setting. Importantly, this ability appears to be constraint by the years of experience with the CI, and again as a function of previous auditory experience of the CI recipient. Part III addresses the question of the effects of auditory reafferentation on the visual system and its interaction with audition. Chapter 6 reviews the issue of cross-modal plasticity after auditory reafferentation. In particular, we investigated whether visual abilities are modified after cochlear implantation in a group of prelingual and postlingual deaf recipients (Chapter 7). In this study we found that prelingual deaf recipients, compared to postlingual deaf, had an advantage in detecting the onset of rapidly presented visual stimuli in the periphery of the visual field. In a further experiment (Chapter 8) we investigated whether auditory and visual information are integrated after cochlear implantation in prelingual and postlingual deaf recipients and found that their abilities are comparable to hearing controls. Finally, Chapter 9 summarises all the presented results and draws the major conclusions.