A perception in the absence of external stimulus that has qualities of real perception. 


They are vivid, and are perceived to be located in external objective space. 


They are distinguishable from several phenomena including dreaming, which does not involve wakefulness; pseudohallucination, which does not mimic real perception, and is accurately perceived as unreal; illusion, which involves distorted or misinterpreted real perception; and imagination, which does not mimic real perception and is under voluntary control.



Hallucinations can occur in any sensation-visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive and chronoceptive.



A disturbances a mild form of hallucination and can occur in most of the senses above:seeing movement in peripheral vision, or hearing faint noises or voices. 



In schizophrenia auditory hallucinations are common.



Auditory hallucinations may be associated with benevolent or malicious feelings about themselves.



The visual counterpart is the feeling of being looked or stared at, usually with malicious intent.



Frequently, auditory hallucinations and their visual counterpart are experienced together.



Hypnagogic hallucinations and hypnopompic hallucinations are considered normal: Hypnagogic hallucinations can occur as one is falling asleep and hypnopompic hallucinations occur when one is waking up. 



Hallucinations can also be associated with drug use, sleep deprivation, psychosis, neurological disorders, and delirium tremens.



Hallucinations can  affect different senses, sometimes occurring simultaneously, creating multiple sensory hallucinations for those experiencing them.



A visual hallucination is the perception of an external visual stimulus where none exists.



Visual hallucinations may be simple or complex:



Simple visual hallucinations (SVH) are lights, colors, geometric shapes, and indiscrete objects. 



Complex visual hallucinations (CVH) are clear, lifelike images or scenes such as people, animals, objects, and places.



Auditory hallucinations, also known as paracusia.



Auditory hallucinations are the perception of sound without outside stimulus. 



Auditory hallucinations are the most common type of hallucination.



Auditory hallucinations are divided into two categories: elementary and complex. 



Elementary hallucinations are the perception of sounds such as hissing, whistling, an extended tone, and more. 



Complex hallucinations are those of voices, music, or other sounds that may or may not be clear.



With complex auditory hallucinations, they may be familiar or unfamiliar, friendly or aggressive, among other possibilities. 



Hallucinations of a single individual person of one or more talking voices is highly associated with psychotic disorders such as schizophrenia.



Many people suffering from diagnosable mental illness may sometimes hear voices as well.



Disorders such as lateral lobe epilepsy, Wilson’s disease, various endocrine diseases, numerous metabolic disturbances, multiple sclerosis, systemic lupus erythematosus, porphyria, sarcoidosis, and others diseases can present with paracusia.



Musical hallucinations may be the result  from hearing-loss, lateral temporal lobe epilepsy, arteriovenous malformation, stroke, lesion, abscess, or tumor.



High caffeine consumption associated with increase in likelihood of experiencing auditory hallucinations: as few as five cups of coffee a day, with approximately 500 mg of caffeine, could trigger the phenomenon.



Command hallucinations are hallucinations in the form of commands, and are often associated with schizophrenia.



Phantosmia refers to olfactory hallucinations, smelling an odor that is not actually there.



Parosmia is the inhaling of a real odor but perceiving it as different scent than remembered.



Olfactory hallucinations are distortions to the sense of smell that, in most cases, are not caused by anything serious and usually go away on their own in time.



Olfactory hallucinations can result from conditions: nasal infections, nasal polyps, dental problems, migraines, head injuries, seizures, strokes, or brain tumors, environmental exposures to smoking, exposure to certain types of chemicals (e.g., insecticides or solvents), or radiation treatment for head or neck cancer.



Olfactory hallucinations can be a symptom of certain mental disorders such as depression, bipolar disorder, intoxication or withdrawal from drugs and alcohol, or psychotic disorders.



 Olfactory hallucination perceived odors are usually unpleasant:  smelling burned, foul spoiled, or rotten in nature.



Tactile hallucinations refers to the illusion of tactile sensory input.



It is appreciated as simulating various types of pressure to the skin or other organs. 



One subtype of tactile hallucination, formication, is the sensation of insects crawling underneath the skin and is frequently associated with prolonged cocaine use.



A gustatory hallucination is the perception of taste without a stimulus, and are typically strange or unpleasant, are relatively common among individuals who have certain types of focal epilepsy, especially temporal lobe epilepsy. 



The general somatic sensations of a hallucinatory nature occur when an individual feels that their body is being mutilated.



Hypnagogic hallucinations occur 


just before falling asleep.



Hypnagogic hallucinations affect a high proportion of the population: up to 37%  experience  them twice a week.



Hypnagogic hallucinations can last from seconds to minutes.



Hypnagogic hallucinations occur while the person  remains aware of the true nature of the images. 



Hypnagogic hallucinations may be associated with narcolepsy. 



Hypnagogic hallucinations are rarely associated with brainstem abnormalities.



Peduncular hallucinosis refers to hallucinations from the neural tract running to and from the pons on the brain stem. 



Peduncular hallucinations usually occur in the evenings.



With peduncular hallucinations the individual is fully conscious and then can interact with the hallucinatory characters for extended periods of time. 



One form  of visual hallucination is delirium tremens. 



DTs associated with agitation and confusion, especially in the later stages of this disease. 



Insight is reduced with DTs.



Sleep is disturbed with DTs.



Parkinson’s disease and  Lewy body dementia have similar hallucinatory symptoms. 



Migraine aura and scintillating scotoma type of hallucination is usually experienced during the recovery from a comatose state. 



((Charles Bonnet syndrome)) refers to visual hallucinations experienced by a sight impaired person. 



Visual hallucinations can be due to focal seizures..



Visual hallucinations due to focal seizures differ depending on the region of the brain where the seizure occurs. 



Visual hallucinations during occipital lobe seizures are brightly colored,  and may move their geometric shapes across the visual field.



Visual hallucinations during occipital lobe seizures may multiply, or form concentric rings and persist from a few seconds to a few minutes. 



Visual hallucinations are usually unilateral and localized to one part of the visual field on the contralateral side of the seizure focus.



Unilateral hallucinatory visions move horizontally across the visual field from the contralateral side and move toward the ipsilateral side.



Temporal lobe seizures, can produce complex visual hallucinations of people, scenes, animals, and more as well as distortions of visual perception. 



Temporal lobe seizures visual perception 


distortions may include size distortion, distorted perception of movement, a sense that surfaces such as ceilings and even entire horizons are moving farther away.



Drug induced hallucinations (hallucinogens). dissociatives, and deliriants, including many anticholinergic drugs and certain stimulants cause visual and auditory hallucinations. 



Psychedelics agents such as lysergic acid diethylamide (LSD) and psilocybin can cause hallucinations.



Hallucinations, particularly auditory, are known side effects of opioids.



Synthetic opioids: pentazocine, levorphanol, fentanyl, pethidine, methadone are more associated with this side effect than natural opioids like morphine and codeine and semi-synthetics like hydromorphone, which appears to be a stronger correlation with the relative analgesic strength. 



They can be caused by sensory deprivation when it occurs for prolonged periods of time.



Deprivation induced hallucination: visual deprivation-blindfolded, darkness, or auditory.



Anomalous benign hallucinations experiences, such as so-called benign hallucinations may occur in a person with good mental and physical health, 


even in the apparent absence of a transient trigger factor.



Studies suggest approximately 10% of the population has experienced at least one hallucinatory episode in the course of their life. 



Hallucinations are associated with structural and functional abnormalities in the sensory cortices. 



Reduced grey matter in regions of the superior temporal gyrus/middle temporal gyrus, including Broca’s area, is associated with auditory hallucinations.



Acute hallucinations are associated with increased activity in the same regions along with the hippocampus, parahippocampus, and the right hemispheric homologue of Broca’s area in the inferior frontal gyrus.



Grey and white matter abnormalities in visual regions are associated with visual hallucinations.



 It is held that dysfunction in sensory regions underly hallucinations.



Thalamoocortical circuits, composed of projections between thalamic and cortical neurons and adjacent interneurons, underlie electrophysical characteristics that are the basis for  sensory processing. 




Thalamoocortical neuronal circuit input enables modulation of sensory neurons, and dysfunction in sensory afferents may result in pre-existing expectations and modulation of  sensory experience, potentially resulting in the generation of hallucinations. 



Hallucinations are associated with less accurate sensory processing.



Hallucinations are associated with the absence of reduction in P50 amplitude in response to the presentation of a second stimuli.



Dysfunctional dopamine signaling may lead to abnormal sensory processing.



Treatments for hallucinations caused by mental disease, antipsychotic and atypical antipsychotic medication may utilized.



Abstaining from hallucinogenic drugs, stimulant drugs, managing stress levels, living healthily, and getting adequate sleep can help reduce the prevalence of hallucinations. 




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