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6 The Neuropsychology of Visual Hallucinations in Parkinson’s Disease and the Continuum Hypothesis Ksenija Maravic da Silva Abstract The present work investigated neuropsychological aspects of visual hallucinations (VHs) with a particular focus on hallucinations in Parkinson’s disease (PD). The first aim was to investigate the role of different neuropsychological risk factors in development of VHs in PD. Specifically, the roles of medication, perception, executive functions, sleep, and personality were investigated, using a set of standardized neuropsychological tasks and questionnaires in hallucinating and nonhallucinating PD patients, as well as in an age-matched control group. The findings suggest that VHs in PD are a complex multifactor effect of different risk factors, primarily the dysfunctions of the visual system and the system that regulates rapid eye movement sleep and arousal. The second aim of the paper was to make a novel contribution by investigating whether the same risk factors are implicated in proneness to VHs as they are in VHs in PD. To investigate the role of neuropsychological factors in proneness to hallucinations in the normal population (namely, the role of perception, executive functioning, sleep, and personality), the studies used a set of standardized neuropsychological tasks, questionnaires, and electrophysiological testing in high-prone individuals from the healthy normal population. The findings suggest that the predisposition to VHs in the healthy young population is a multifactor effect of strikingly similar risk factors to those found in PD. However, in addition to the dysfunctions of the visual and arousal system, proneness to hallucinations in the normal population is also predicted by a specific personality profile. 1 Introduction This chapter summarizes the recent work investigating neuropsychological aspects of visual hallucinations (VHs) with a particular focus on hallucinations in Parkinson’s disease (PD). The theoretical framework draws on integrative models (Barnes, Boubert, Harris, Lee, & David, 2003; Diederich, Goetz, & Stebbins, 2005) that suggest that hallucinations stem from concurrent neuropsychological dysfunctions. Specifically, VHs in PD are attributed to the dopaminergic medications, perceptual abnormalities, and executive and sleep dysfunctions; however, their role remains unclear and calls for detailed investigation. 106 K. Maravic da Silva Apart from PD, VHs also occur in a range of disorders, such as in Alzheimer’s disease, Lewy body dementia, narcolepsy-cataplexy syndrome, Charles Bonnet syndrome (VHs of the blind), schizophrenia, hallucinogen-induced states, brain tumors, liver disease, delirium, and epilepsy (Collerton, Perry, & McKeith, 2005; Dauvilliers, Billiard, & Montplaisir, 2003; Fountain, 2001; Manford & Andermann, 1998). Moreover, VHs are frequent in normal individuals, usually in drowsy states of consciousness (Aleman, Böcker, & De Haan, 1999; Girard & Cheyne, 2006; Lopez-Rodrigo, Paino Pineiro, Martinez Suarez, Caro, & Lemos Giraldez, 1997; Ohayon, 2000; Ohayon, Priest, Caulet, & Guilleminault, 1996). Hallucinations are therefore not always indicative of pathology , and several authors have proposed that psychosis-like experiences exist on a continuum, ranging from mild visual disturbances in the healthy general population to the sometimes bizarre full-blown hallucinations in psychiatric illness (Crow, 1998; Slade & Bentall, 1988). This idea has been known as the continuum hypothesis, stating that psychosis-like experiences are distributed (although to varying extent) throughout the general population, and full-blown psychosis represents the most extreme end of the population continuum (Hanssen et al., 2003; L. C. Johns & van Os, 2001; E. Peters, Joseph, Day, & Garety, 2004; E. R. Peters, Joseph, & Garety, 1999; Stefanis et al., 2002; Verdoux & van Os, 2002). Therefore, VHs in PD and proneness to VHs in a normal population might share similar predispositions, which are expressed in a lesser degree in the normal population. 1.1 Aims Stemming from the integrative models, the first aim was to investigate the role of different neuropsychological risk factors in the development of VHs in PD. Specifically, this chapter presents studies investigating the roles of medication, perception, executive functions, sleep, and personality, using a set of standardized neuropsychological tasks and questionnaires in hallucinating and nonhallucinating PD patients, as well as in an age-matched control group. Based on the continuum hypothesis (Crow, 1998; Slade & Bentall, 1988), the second aim was to make a novel contribution by investigating whether the same risk factors are implicated in proneness to VHs as they are in VHs in PD. To investigate the role of neuropsychological factors in proneness to hallucinations in the normal population (namely, the role of abnormalities of perception, executive functioning, sleep, and personality), the studies used a set of standardized neuropsychological tasks, questionnaires , and an electroencephalographic neuroimaging technique (EEG) in high-prone individuals from the healthy normal population. Finally, comparing VHs in a well-recognized neurological disorder (PD) to high hallucination...

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