Hallucinations

Hallucinations can be defined as subjectively perceived sensory input despite the absence of external stimuli. For visual hallucinations, this presents as seeing something that is physically “not there.” Visual hallucinations do not disappear with eye closure. Hallucinations can also be caused by occipitoparietotemporal lesions, transient ischemic attacks, and migraines. Monocular hallucinations are always due to abnormalities of the optic nerve or the photoreceptors. Images that are seen can be as simple as dots, spots, streaks or lines, or they can be as complex as faces, animals, cars, or landscapes. Thus, visual hallucinations are typically separated into two broad categories: unformed (also known as simple or elementary) and formed (also known as complex). Unformed visual hallucinations include phosphenes, chromatopsias (xanthopsia, brunescence, erythropsia, cyanopsia, chloropsia, ianthinopsia) and teichopsia or fortification spectrum. Formed visual hallucinations typically involve processes such as metamorphopsia, micropsia, macropsia, teleopsia, pelopsia, microsomatognosia, macrosomatognosia, mosaic vision, and Alice in Wonderland syndrome. Etiologies of these abnormalities include, but are not limited to, refractive problems, retinal deformation, maculopathy, cerebral lesions and migraine.