Académie royale de Médecine de Belgique

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Résumé de Gordon Terence Plant

VISUAL ILLUSIONS AND HALLUCINATIONS IN HEALTHY AND DISORDERED EYES AND BRAINS

par Gordon Terence PLANT (Moorfields Eye Hospital, London), invité.

Many patients present to ophthalmology or neurology clinics complaining of what are known as "positive" visual symptoms. Rather than loss of vision they experience visual perceptions which are generated in their own eye, optic nerve or brain, not in the outside world. Other patients may experience visual illusions where a real image is distorted in some way. I will begin by describing situations in which we all experience hallucinations, such as in dreams or so-called visual illusions where our eyes deceive us. I will argue that the former are an integral part of brain function and the latter are not in reality "Illusions" rather the brain is presenting us with the most likely explanation for the image falling on the retina, albeit not the correct one. Entoptic phenomena are visual images that are not generated by light falling on the retina but an internal process: some, such as pressing on the eye, can be experienced by all of us. Others, such as flashes of light in retinal or optic nerve disease are pathological and often a great aid in diagnosis. When we consider disorders of the visual cortex of the brain we acknowledge that visual aura is the commonest manifestation of migraine, even so it is often misdiagnosed as epilepsy or a transient ischaemic attack. Patients' descriptions are rich and varied and, I will argue, give us clues as to the organisation of the brain. Lastly I will discuss the illusions of "filling-in" and "completion" where the brain will render complete an image that is incomplete for some reason. Once again there are examples in normal and abnormal vision which challenge our ideas of visual function.