Tactile hallucination is the false perception of tactile sensory input that creates a hallucinatory sensation of physical contact with an imaginary object. It is caused by the faulty integration of the tactile sensory neural signals generated in the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

and the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direction ...

and sent to the primary somatosensory cortex (SI) and

secondary somatosensory cortex The human secondary somatosensory cortex (S2, SII) is a region of cortex in the parietal operculum on the ceiling of the lateral sulcus. Region S2 was first described by Adrian in 1940, who found that feeling in cats' feet was not only represe ...

(SII). Tactile hallucinations are recurrent symptoms of neurological diseases such as

schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wi ...

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...

, Ekbom's syndrome and

delerium tremens Delirium tremens (DTs) is a rapid onset of confusion usually caused by withdrawal from alcohol. When it occurs, it is often three days into the withdrawal symptoms and lasts for two to three days. Physical effects may include shaking, shiver ...

. Patients who experience phantom limb pains also experience a type of tactile hallucination. Tactile hallucinations are also caused by drugs such as

cocaine Cocaine (from , from , ultimately from Quechua: ''kúka'') is a central nervous system (CNS) stimulant mainly used recreationally for its euphoric effects. It is primarily obtained from the leaves of two Coca species native to South Am ...

and

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

History and background

During ancient Greek times, touch was considered to be an unrefined perceptual system because it differed from the other senses on the basis of the distance and timing of

perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system, ...

of the stimulus. Unlike

vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain und ...

and

audition An audition is a sample performance by an actor, singer, musician, dancer or other performer. It typically involves the performer displaying their talent through a previously memorized and rehearsed solo piece or by performing a work or piece ...

, one perceives touch simultaneously with the medium and the stimulus is always

proximal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position ...

and not distal. By the 17th century, the British empiricist John Locke attributed the word "feeling" with two types of

sensation Sensation (psychology) refers to the processing of the senses by the sensory system. Sensation or sensations may also refer to: In arts and entertainment In literature *Sensation (fiction), a fiction writing mode * Sensation novel, a Britis ...

Weber Weber (, or ; German: ) is a surname of German origin, derived from the noun meaning " weaver". In some cases, following migration to English-speaking countries, it has been anglicised to the English surname 'Webber' or even 'Weaver'. Notable pe ...

distinctly identified these two types of sensation as the sense of touch and common bodily sensibility. This distinction further helped 19th century psychiatrists to distinguish between tactile hallucinations and cenesthopathy. During the 19th century, tactile hallucinations were classified as symptoms associated with

insanity Insanity, madness, lunacy, and craziness are behaviors performed by certain abnormal mental or behavioral patterns. Insanity can be manifest as violations of societal norms, including a person or persons becoming a danger to themselves or to ...

, organic and toxic syndromes and delusional parasitosis yet there was no identification on how such hallucinations were caused. Neuroscientists such as Dr. Oliver Sacks and Dr. V.S. Ramachandran have analyzed and attributed tactile hallucinations as a dysfunctional perception of the brain as opposed to just a

symptom Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showi ...

related to

. They have contributed significantly to propose tactile hallucinations as the false perception of tactile sensory input creating a sensation of touch with an imaginary object.

In schizophrenia

Hallucinations A hallucination is a perception in the absence of an external stimulus that has the qualities of a real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. Hallucination is a combinati ...

of pain and touch are very rare in schizophrenic disorders but 20% of patients with schizophrenia experience some sort of tactile hallucinations along with

visual The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight ...

and auditory hallucinations. The most common tactile hallucination in patients with schizophrenia is a sensation in which a patch of their

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...

is stretched elastically across their head. They vary in intensity, range and speed at which they feel this stretching painful sensation. They are usually triggered by

emotional Emotions are mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definition. ...

cues such as guilt,

anger Anger, also known as wrath or rage, is an intense emotional state involving a strong uncomfortable and non-cooperative response to a perceived provocation, hurt or threat. A person experiencing anger will often experience physical effects, suc ...

fear Fear is an intensely unpleasant emotion in response to perceiving or recognizing a danger or threat. Fear causes physiological changes that may produce behavioral reactions such as mounting an aggressive response or fleeing the threat. Fear ...

and depression. Other types of tactile hallucinations takes on a sexual form in patients with schizophrenia. Patients with schizophrenia may sometimes experience the feeling of being kissed or the feeling of someone lying by their side in response to their

emotion Emotions are mental states brought on by neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or displeasure. There is currently no scientific consensus on a definition. ...

of being lonely. Additionally, they occasionally hallucinate the feeling of small animals such as

snakes Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more j ...

crawling over their body. Such vivid tactile

of an object that is not present results from the unsuccessful attempt of the

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

trying to perceive objects that are novel and that represent unreal situations usually triggered by guilt and fear. Patients with schizophrenia also have a hard time portraying

emotions Emotions are mental states brought on by neurophysiology, neurophysiological changes, variously associated with thoughts, feelings, behavioral responses, and a degree of pleasure or suffering, displeasure. There is currently no scientific ...

as they divert most of their

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...

to control the

pain Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...

from their tactile hallucinations.

Olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, i ...

and tactile hallucinations correlate with one another in terms of prevalence in patients with

. One particular study was conducted by Langdon et al., in which the prevalence of olfactory hallucinations and tactile hallucinations was analyzed in two distinct clinical samples of patients with schizophrenia. One of the samples contained patients with schizophrenia with tactile hallucinations as reported by the

World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level o ...

while the other sample contained cases with negative and positive types of tactile hallucinations in patients with schizophrenia. It was concluded that about 13% to 17% of patients with schizophrenia experience olfactory and tactile hallucinations. The study reported that socio-cultural factors influenced self-reporting of tactile hallucinations. Since hallucinations in general were feared as a symptoms of

, patients were reluctant to seek help for such

symptoms Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showi ...

. Moreover, the study concluded that tactile hallucinations were usually accompanied by several other

hallucinations A hallucination is a perception in the absence of an external stimulus that has the qualities of a real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. Hallucination is a combinati ...

associated with different modalities such as

taste The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste recepto ...

and sight. However, the study failed to recognize the

pathophysiology Pathophysiology ( physiopathology) – a convergence of pathology with physiology – is the study of the disordered physiological processes that cause, result from, or are otherwise associated with a disease or injury. Pathology is t ...

of tactile hallucinations in individuals with schizophrenia.

In Parkinson's disease

About 7% of individuals with

(PD) also experience mild or severe types of tactile hallucinations. Most of these hallucinations are based on the sensation of a particular kind of animal. Several case studies were conducted by Fénelon and his colleagues on patients with PD that had tactile hallucinations. One of his patients described that he sensed "spiders and cockroaches chewing on his lower limb" which was rather painful. Several other patients felt that there was a

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

infestation of their

which caused

lesions A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals. Types There is no designated classif ...

on their skins due to the obsessive need of

itching Itch (also known as pruritus) is a sensation that causes the desire or reflex to scratch. Itch has resisted many attempts to be classified as any one type of sensory experience. Itch has many similarities to pain, and while both are unpleasant ...

. Fénelon also analyzed the particular types of tactile hallucinations experienced, the timing of such experience and certain

drugs A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via inhalat ...

that could eliminate such experience. It was concluded that patients with both PD and tactile hallucinations not only experienced sensations elicited by

insects Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs ...

under their

but also by vivid tactile sensations of people. These hallucinations were aggravated during evening times due to altered arousal states and were alleviated by

dopaminergic Dopaminergic means "related to dopamine" (literally, "working on dopamine"), dopamine being a common neurotransmitter. Dopaminergic substances or actions increase dopamine-related activity in the brain. Dopaminergic brain pathways facilitate do ...

treatment such as the intake of

clozapine Clozapine is a psychiatric medication and is the first atypical antipsychotic (also called second-generation antipsychotic). It is primarily used to treat people with schizophrenia and schizoaffective disorders who have had an inadequate resp ...

. The study also explains that the

of tactile hallucinations is uncertain, however, such hallucinations can be attributed to narcoleptic

rapid eye movement sleep Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream vi ...

disorders due to its concordance with visual hallucinations. Moreover, it emphasizes that individuals who have had PD for a longer period of time have a more severe form of tactile hallucinations than with individuals who have succumbed to this disease for just a short period of time. Clinical drugs used as an antiparkinsonian agent such as Trihexyphenidyl are known to create tactile hallucinations in patients with PD.

Restless legs syndrome

Restless legs syndrome (RLS) causes unpleasant or uncomfortable sensations in the legs and an irresistible urge to move them. Tactile hallucinations in RLS include feelings of itching, pulling, crawling or creeping mainly in the legs, with the accompanying overwhelming urge to move them. These

are more prominent in the late afternoon and at night, often causing

insomnia Insomnia, also known as sleeplessness, is a sleep disorder in which people have trouble sleeping. They may have difficulty falling asleep, or staying asleep as long as desired. Insomnia is typically followed by daytime sleepiness, low energy ...

. The causes of RLS are generally unknown, though there are three major hypotheses: iron deficiency, dopamine insufficiency and genetic inheritance. RLS can also occur due to nerve damage, or

neuropathy Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or or ...

. Treatments for RLS typically focus on symptom relief through supplementing iron, blocking nerve receptors through the use of alpha-2 delta drugs such as

gabapentin Gabapentin, sold under the brand name Neurontin among others, is an anticonvulsant medication primarily used to treat partial seizures and neuropathic pain. It is a first-line medication for the treatment of neuropathic pain caused by diab ...

, or through the use of

opioid Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, replacement therapy for opioi ...

s or

benzodiazepine Benzodiazepines (BZD, BDZ, BZs), sometimes called "benzos", are a class of depressant drugs whose core chemical structure is the fusion of a benzene ring and a diazepine ring. They are prescribed to treat conditions such as anxiety disorders, ...

Phantom limbs

Phantom limb pain is a type of tactile hallucination because it creates a

of excruciating

in a

limb Limb may refer to: Science and technology * Limb (anatomy), an appendage of a human or animal *Limb, a large or main branch of a tree *Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb *Limb, in botany, ...

that has been amputated. In 1996, VS Ramachandran conducted a research on several amputees to pinpoint the

neural In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...

reasons behind these illusionary pains. Most of these amputees that had an unbearable phantom limb pain are reported by patients whose limb was paralyzed before amputation. VS Ramachandran proposed the "learned paralysis" hypothesis. The hypothesis suggested that every time the patients tried to move their paralyzed limb, they received sensory feedback (through

and

proprioception Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...

) that the limb did not move. This feedback hardwired itself into the brain circuitry, so that, even when the limb was no longer present, the brain had learned that the phantom limb was paralyzed. As a treatment for phantom limb pains, VS Ramachandran devised a mirror box that would superimpose the mirror image of the normal arm in place of the missing arm and the patient would immediately be relieved of the pain. This suggested that the brain had a plastic nature in the somatosensory system and the

had reorganized its somatosensory region to accommodate for this new change. Patients that experience this phantom limb pain are very important in research studies for their role in determining

brain plasticity A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

. The vivid tactile sensation of the arm that is no longer present suggest the highly complex nature of the brain to reorganize different functions which were once thought to be hardwired to specific regions (localization).

Inducement through drugs

Organic and

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subs ...

syndromes can also induce tactile hallucinations. The use of

for recreational purposes has been reported to induce tactile hallucinations. They usually have sensations of moving itches and crawling

Cocaine Cocaine (from , from , ultimately from Quechua: ''kúka'') is a central nervous system (CNS) stimulant mainly used recreationally for its euphoric effects. It is primarily obtained from the leaves of two Coca species native to South Am ...

and

can induce rapid firing of neuronal cells of the somatosensory region of the

leading to vivid perception of illusionary bugs on the skin. Additionally, as mentioned above, Trihexyphenidyl is an antiparkinsonian drug that creates tactile hallucination. The mechanism through which these drugs induce tactile hallucinations is still unknown.

Cenesthopathy

Cenesthopathy is a rare medical term used to refer to the feeling of being ill and this feeling is not localized to one region of the body. Cenesthopathic hallucinatory experiences are caused by the hyperactive neuronal stimulation of the primary somatosensory cortex due to a disorder or a damage to this area. There are two theories that are established to portray sensation of unified bodily feeling. One of these theories is called associationism, which states that cenesthesia is an amalgamation of propioceptive and interoceptive sensations. Faculty psychology is the other theory which states that there is a particular

region where all of the sensory information converged and the integration of this information gives one cenesthetic sensation. The latter theory became more predominant and it established two types of cenestopathic hallucinations namely "painful" and "paraesthetic". Patients that experience "painful" type of cenesthopathic hallucination felt that their organs were stretched apart and twisted. On the other hand, patients with "paraesthetic" cenesthopathic hallucination experience severe hallucinatory itching.

Pathophysiology

Schematic of cortical areas involved with pain processing and fMRI

Tactile hallucinations are the result of a dysfunctional somatosensory and a dysfunctional

awareness Awareness is the state of being conscious of something. More specifically, it is the ability to directly know and perceive, to feel, or to be cognizant of events. Another definition describes it as a state wherein a subject is aware of some info ...

regions of the brain. Tactile sensory input is produced and conducted through the

and

and it is received at the primary somatosensory cortex. Once it has reached the primary somatosensory cortex, it is distributed across the

and it will not be processed unless it is important and one pays close attention to the information based on a specific context.

Consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...

to these specific tactile sensations is generated only through multiple feedback loops passing through higher cortical areas such as secondary somatosensory area, parietal,

insular cortex The insular cortex (also insula and insular lobe) is a portion of the cerebral cortex folded deep within the lateral sulcus (the fissure separating the temporal lobe from the parietal and frontal lobes) within each hemisphere of the mammalian ...

and premotor areas. The intensity of the tactile stimulus is directly proportional to the area of the primary somatosensory region activated. A feedback mechanism from different cortical areas results in the awareness of touch. Even with complete sensory deprivation, discrete tactile memories can trigger spontaneous firing of impaired neurons. Therefore, individuals with various psychiatric disorders are more prone to tactile hallucinations than normal individuals. : Tactile hallucinations are especially possible due to faulty sensory integration of neuronal signals in the primary and secondary somatosensory system with neuronal signals in the parietal cortex,

and premotor cortex. Moreover, the posterior insula is responsible for mental body schema representation and can produce tactile hallucination if defected. Additionally, the regions of the brain involved in tactile hallucinations are similar to the regions of the brain involved in pain.

References

{{reflist Hallucinations Symptoms and signs of mental disorders Articles containing video clips