4.1.4 Disorders of perception

Sensation, perception and the main categories

Sensation turns physical input into neural signals, and perception interprets those signals. Attention, expectation and reduced sensory input can all change what is perceived.

Illusions and hallucinations together are called perceptual deceptions.

Sensory distortions

• Hypoacusis explains why a delirious patient seems not to respond to ordinary speech, so staff should speak clearly and loudly.

• Micropsia is one half of dysmegalopsia and macropsia the other; dysmegalopsia proper is asymmetric, unlike their uniform change.

• Paraprosopia is a distortion of a face that is still recognised, the facial form of metamorphopsia.

• Autoparaprosopia is the same change seen in one's own reflection.

• Palinopsia is the return or persistence of the visual image of a real object after it has gone, often within an area of visual field loss.

• Teichopsia, an expanding zigzag fortification pattern, is the classic visual aura of migraine with aura, evolving over minutes with scotoma and size distortion.

• Brief, stereotyped episodes of distortion should raise the possibility of seizures.

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Familiarity, memory and imagery

• Eidetic imagery is exceptionally vivid, detailed visual recall of a scene after brief exposure, summoned and dismissed at will, with awareness that it is not real.

• Eidetic imagery is a memory phenomenon, not a disorder of perception, and it is commoner in children.

• Visual imagery vividness falls across the lifespan, and the decline is steeper in males.

• Aphantasia is essentially absent in adolescence and grows more prevalent with age.

Illusions

• An illusion is the misperception of a real external stimulus that corrects on closer inspection.

• Illusions are usually normal experiences and occur most often in vision.

• An illusion differs from a hallucination, which has no stimulus, and from a pseudohallucination or a delusion.

• Poor lighting, sensory impairment, inattention, fear and fatigue promote illusions, so correcting vision, hearing and the environment is part of delirium care.

• Pareidolia is a normal experience, independent of mood, held with insight and coexisting with the accurate percept.

• Different observers may see the same pareidolic pattern, and pareidolia also occurs in neurological illness.

Agnosias and parietal signs

Prosopagnosia is the reverse of paraprosopia: in prosopagnosia the face looks normal but is not known, and in paraprosopia it looks distorted but is still known.

Describing hallucinations

• Describe the modality, complexity, clarity, location and reality testing, and ask about sleep, sensory loss, illness, medication and substances.

• Elementary hallucinations are simple, unformed perceptions such as flashes, sparks, clicks, bangs or tones.

• Complex (formed) hallucinations have organised content such as a word, figure or tune.

• Glancing aside or muttering does not by itself establish that a person is hallucinating.

Hallucinations of sleep, drugs and age

• A hypnagogic hallucination is a brief false perception in the transition into sleep, and it is common in people with no mental disorder.

• A hypnopompic hallucination arises in the transition from sleep to waking.

• Community surveys attribute most hallucinatory experiences to sleep and sleep disturbance (hypnagogic and hypnopompic experiences, insomnia, daytime sleepiness, narcolepsy) rather than to any psychiatric group.

• Daytime sleepiness, cataplexy or sleep paralysis alongside sleep-transition hallucinations should prompt assessment for narcolepsy.

• Flashbacks, the spontaneous, transient return of drug-induced perceptual effects after the drug has gone, are characteristic of hallucinogens such as lysergic acid diethylamide (LSD).

• Visual hallucinations in an older patient point to an organic cause, and clear consciousness neither excludes this nor favours a functional psychosis.

True hallucinations and pseudohallucinations

• Pseudohallucinations are typical of bereavement and are listed among the features of Ganser syndrome.

• Pseudohallucinations are not specific to any disorder: they occur in psychotic, organic, drug-induced, personality and grief states, and occasionally in healthy people.

• The term is used inconsistently, for either an internally located experience or one recognised as unreal, so also record location, perceptual quality, controllability, conviction and distress.

• Hallucinosis describes hallucinations in relatively clear consciousness, though insight is not always kept.

Auditory hallucinations

• Auditory hallucinations range from sounds and music to speech, and they are the most frequent modality in major depression with psychotic features.

• Alcoholic hallucinosis gives mainly auditory hallucinations in clear consciousness and is distinct from delirium, where hallucinations in clouded consciousness are typically visual.

• Voices in schizophrenia vary in frequency, from a few times a week to near constant, so unbroken all-day presence is not typical.

Grammatical person and first-rank status

The grammatical person of a voice is set by the pronoun it uses for the patient, not by whose voice it sounds like.

• First-rank symptoms remain useful but do not uniquely identify schizophrenia.

• A voice using first-person speech should be clarified rather than assumed to be thought echo.

• Risk from command hallucinations depends on what is commanded, the voice's perceived power, past compliance, distress, perceived control and coping, not on the command alone.

Visual and musical hallucinations

• Charles Bonnet syndrome is a release phenomenon of visual loss from eye disease, most often age-related macular degeneration, in cognitively intact people; it is not a mental illness or dementia.

• Charles Bonnet syndrome produces complex visual hallucinations in clear consciousness with insight usually kept, unlike delirium and psychosis.

• Management is to explain the phenomenon, optimise vision and assess any new symptoms, rather than assume dementia or start an antipsychotic.

• Musical hallucinations arise from damage to, or deprivation of, the auditory system (ear, brainstem, temporal cortex), mainly with acquired hearing loss, as the auditory parallel of Charles Bonnet syndrome.

• Musical hallucinations also occur in some psychiatric disorders, but a lesion outside the auditory pathways is not a recognised cause.

• Lilliputian hallucinations are visual hallucinations of miniature people or animals, pointing to an organic cause: classically delirium tremens, also Charles Bonnet syndrome and Lewy body dementia.

• Brobdingnagian hallucinations are hallucinations of giant people or objects, named after the land of giants in Gulliver's Travels.

A Lilliputian hallucination is told from micropsia by the stimulus: in micropsia a real object is seen smaller.

Hallucinations of smell, taste and the body

• Somatic passivity adds to a somatic hallucination a delusional belief in an outside agency, but bodily hallucinations do not automatically mean passivity.

• Kinaesthetic hallucinations include vestibular variants and occur in vestibular disease and other intoxications.

• Acute cannabis intoxication can cause kinaesthetic hallucinations as a brief episode after a first dose.

Hallucinations defined by location and trigger

• Functional is told from reflex hallucination by modality: a functional hallucination stays in the trigger's sense, while a reflex hallucination crosses to another.

• Synaesthesia may be a lifelong, stable pairing, such as letters evoking colours, or may follow hallucinogens such as LSD.

Autoscopic phenomena

• Autoscopic hallucination (phantom mirror image) is seeing an image of one's own body or double in outer space and recognising it as oneself, while the self stays in one's own body.

• Heautoscopy is seeing a double of oneself with uncertainty about which body holds the self, the viewpoint alternating between the two.

• An out-of-body experience locates the self outside the physical body.

• The doppelganger phenomenon, awareness of one's own double close by without any percept, is a disturbance of ideation rather than perception.

• Autoscopy arises from temporoparietal and parieto-occipital dysfunction, reported in epilepsy, migraine, focal posterior lesions and toxic-infective states, but not in frontal lobe dementia.

• Among psychiatric disorders, autoscopy is most often reported in depression.

• Near-death experiences are subjective reports of out-of-body experience with autoscopy, depersonalisation and a transcendental element, and they do not include observed organic or motor signs.

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Patterns by clinical context