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The well-known 18th century painting "The Nightmare" by Henri Fuseli depicts what very likely is a sleep paralysis hallucination (or maybe a night terror). An incubus, or male demon, sits on the paralyzed woman's chest, applying the pressure that is so often felt there during sleep paralysis episodes.
Hallucinations are misconceived perceptions of sensory input in the absence of a real external stimulus. They are a disorder of perception with a strong relevance and comorbidity with numerous medical as well as psychiatric disorders1,2. Hallucinations are a core symptom of schizophrenia, although, they may be present in normal individuals 3. Sensory deprivation in healthy individuals is known to induce hallucinations 4. Recreational and pharmaceutical drugs have also been known to stimulate hallucinations. Methylphenidate, for example, is a central nervous system stimulant that can cause visual hallucinations following overdose 5. Hallucinations are not exclusive to the more common visual and auditory sensory modalities; further subtypes include tactile, olfactory, and gustatory hallucinations.

Schizophrenic patients are often studied to understand the neurobiological basis of hallucinations. Neuroimaging is used to localize different brain areas associated with hallucinations, with a focus on differences in structure, function and connectivity 1. Hallucinations in affected individuals are thought to occur due to the misattribution of self-generated sensory stimulation to an external stimulus. Disruption in brain hodology may contribute to these self-monitoring deficits, in which the distinction between internal and external becomes ambiguous 4. Consistent with this theory, abnormalities in connectivity have been shown to persist in hallucinating patients between each episode. Current findings also implicate that improper functioning in the prefrontal premotor, cingulate, subcortical and cerebellar areas may contribute to the hallucinatory experience1. Further findings show that hallucinations follow large volume reductions in the dorsolateral prefrontal cortex suggesting a mechanism for involuntary sensory misperceptions 1,4.

Hallucinations may vary in the emotional experiences that they evoke, either eliciting neutral, positive, or in some cases distressing and even disabling experiences 3.

Pharmacological treatments such as benzodiazepines, neuroleptic medications, β-blockers, antidepressants and anticonvulsants have been proven effective in the treatment of multiple sensory hallucinations 2,6,7. Effective non-pharmacological treatments include transcutaneous electrical nerve stimulation, mirror therapy, surgical intervention and psychotherapeutic interventions (such as CBT) 2,6,7.

Content
  1. Visual Hallucinations (Shelda Lloyd)
    • Classifications
    • Underlying mechanisms
    • Charles Bonnet Syndrome
    • Peduncular Hallucinosis
    • Anton’s Syndrome
    • Treatment Methods
  2. Auditory Hallucinations (Rajesan Rajendran)
    • Verbal Auditory Hallucination
      • Neurobiological correlates
      • rTMS treatment
    • Nonverbal Auditory Hallucinations
      • Musical Hallucinations
        • Pathophysiology
        • Treatment
      • Subjective Tinnitus
        • Treatment
  3. Tactile Hallucinations(Ramandeep Bolla)
    • Phantom Limb Epidemiology
    • Phantom Limb Pain
    • Neurological Basis
      • Peripheral Mechanisms
      • Central Mechanisms
      • Spinal Mechanisms
    • Treatments
      • Pharmacological Treatment
      • Non-pharmacological Treatment
  4. Olfactory and Gustatory Hallucinations (Sallini Kalachandran)
    • Causes & Comorbid Disorders
    • Proposed Neurobiological Mechanism
    • Surgical Resolution and Other Treatments
    • Clinical Significance
  5. Drug Induced Hallucinations (Mariette Soosaipillai)
    • Overview of Drug-Induced Hallucinations
    • Hallucinogens: Receptors, Neurotransmitters and the Brain
    • Recreational Drugs
    • Pharmaceutical Drugs
    • Short-term and Long-term Effects
    • Treatments



References
  1. Allen, P., Laroib, F., McGuirea, P. K. & Aleman, A. Hallucinating brain: A review of structural and functional Neuroimaging studies of hallucinations. Neurosci. & Biobehav. Reviews 32, 175-191 (2008)
  2. Teeple, R. C., Caplan, J. P. & Stern, T. A. Visual hallucinations: differential diagnosis and treatment. J. Clin. Psychiatry 11 (1), 26-32 (2009)
  3. Collerton, D., Perry, E. & McKeith, I. Why people see things that are not there: A novel perception and attention deficit model for recurrent complex visual hallucinations. Behav. And Brain Sci. 28, 737-794 (2005)
  4. Boksa, P. On the neurobiology of hallucinations. J. Psychiatry Neurosci. 34(4), 260-262 (2009)
  5. Ayelet, H. & Avinoam, S. Methylphenidate induction of complex visual hallucinations. J. of Child Neurology 24, 1005-1007 (2009)
  6. Ffytche, D. H. The hodology of hallucinations. Cortex 44, 1067-1083 (2008)
  7. Subedi, B. & Grossberg, G. T. Phantom limb pain: mechanisms and treatment approaches. Pain Research and Treatment 1, 1-8 (2011)