Les hallucinations visuelles (HV) sont un symptôme psychotique commun retrouvé de manière transdiagnostique dans les troubles de la vision, les troubles neurologiques et les troubles psychiatriques. Les différents modèles cognitifs des HV s’accordent sur la présence d’une perturbation du traitement de l’information visuelle bottom-up et des processus tardifs top-down pour expliquer la survenue des HV. Dans un première partie de notre travail, nous avons étudié les caractéristiques communes entre les pathologies caractérisées par la présence d’HV et avons constaté que toutes présentent des anomalies structurales ou fonctionnelles de la rétine mais que peu d’études ont exploré le lien entre anomalies fonctionnelles de la rétine et HV. Nous avons ensuite étudié le fonctionnement de la rétine chez des patients souffrant de schizophrénie présentant ou non des HV. Nos résultats ont montré la présence d’anomalies fonctionnelles touchant le système des photorécepteurs et les cellules ganglionnaires de la rétine chez les patients souffrant de schizophrénie et des anomalies fonctionnelles du système des bâtonnets spécifiquement chez les patients présentant des HV. Les anomalies du traitement sensoriel sont une condition nécessaire mais insuffisante pour expliquer la survenue des HV. Ainsi nous avons exploré l’influence du traitement des émotions dans les HV, par une tâche d’amorçage émotionnel sur le traitement de stimuli ambigus chez des sujets sains prônes aux hallucinations et des patients souffrant de schizophrénie. Les premiers résultats intermédiaires présentés montrent en partie que la manipulation d'une amorce émotionnelle pourrait influencer la perception de stimuli ambigus chez les sujets sains prônes aux hallucinations. Ces arguments sont à confirmer par un nouveau protocole de recherche à adapter. Nos résultats nous permettent de discuter l’adaptation d’un modèle des HV dans la schizophrénie. / Visual hallucinations (VH) are a common psychotic symptom present in eye disease, neurological disorders and psychiatric disorders. Cognitive models of VH postulate the disruption of both bottom-up visual information processing and late top-down processes to explain the emergence of VH. In the first part of our work, we have reviewed the common characteristics between pathologies characterized by VH. We have found that structural or functional abnormalities of the retina were systematically present, but few studies have explored the link between these abnormalities and VH. Hence, we have explored the retinal functions in schizophrenia patients with or without VH. We have found photoreceptors and retinal ganglion cells abnormalities in patients with schizophrenia and abnormalities of the rod system specifically in patients with VH. Abnormalities of sensory processing are a necessary but insufficient condition to explain the occurrence of VH. Thus, we have explored the influence of emotion processing in healthy hallucinations prone subjects and schizophrenia patients in an emotional priming task requiring the treatment of ambiguous stimuli. We expose intermediate results which show that the manipulation of an emotional primer can influence the perception of ambiguous stimuli in healthy hallucinations prone subjects. These results need replications in a new research protocol. Our results allow us to discuss the adaptation of a model of VH in schizophrenia.

Charles Bonnet syndrome (CBS) is defined as complex persistent visual hallucinations in the absence of mental disorder. It is associated with advanced age and poor vision. It is common, with prevalence estimates of up to 63% among older people with significant visual impairment. CBS would not be diagnosed in the presence of dementia, but its relationship to milder cognitive impairment is unclear. The few studies that have examined this are underpowered and provide contradictory results. There are 16 case reports of dementia emerging in people with a diagnosis of CBS. These cases raise the possibility of an association between impaired insight at diagnosis of CBS and the subsequent development of dementia. This thesis reports the findings of a prospective cohort study which describes changes in cognitive functioning over one year in patients with CBS and age-matched controls. Participants were recruited from low vision and glaucoma assessment clinics. A clinical assessment was carried out by an old age psychiatrist, and participants had a detailed assessment of visual functioning. This thesis also describes the findings of the first study to use voxel-based morphometry (VBM) to investigate changes in volume of grey and white matter in CBS. Participants were recruited from the same clinics as the cohort study, and underwent MRI scanning on a 1.5T scanner, to a protocol designed to produce 1mm3 voxels. Twelve participants with CBS and ten controls were followed up. Two people in the CBS group developed dementia, while none did in the control group. The CBS group showed a mean change in the score on the Addenbrooke’s cognitive examination (ACE-R) of -3.7 points, compared to a change of +1.4 in the control group. This difference was not statistically significant. The CBS participants performed worse on the verbal fluency item of the ACE-R, a difference which was statistically significant. The VBM analysis was conducted on 11 CBS participants and 11 controls. The CBS group showed an increase in grey matter volume in the right cerebellar hemisphere. This difference retained significance after family-wise error correction, non-stationary correction, and ANCOVA to control for the effects of possible confounders. As far as the author is aware, these are the most methodologically robust studies to date to have investigated cognition and morphological brain changes in CBS. The findings of the cohort study were inconclusive. However, the two cases of dementia in CBS patients add weight to the suspicion that this is a clinically important outcome in the condition, and the finding of abnormalities in frontal lobe testing in participants with CBS fits with a theoretical model of visual hallucination generation. Moreover, this type of research appears to be acceptable to a frail and visually disabled population, and studies powered to investigate this issue more fully would be feasible. The VBM findings report the presence of underlying structural brain abnormalities in CBS, in a region not usually associated with visual hallucinations. Possible links with Lewy body dementia, and implications for theories of visual hallucinations, are discussed.