Mirror-touch synesthesia is a rare condition which causes individuals to experience the same sensation (such as touch) that another person feels. For example, if someone with this condition were to observe someone touching their cheek, they would feel the same sensation on their own cheek. Synesthesia, in general, is described as a condition in which a stimulus causes an individual to experience an additional sensation. Synesthesia is usually a developmental condition; however, recent research has shown that mirror touch synesthesia can be acquired after sensory loss following amputation.

Three conditions must be met in order to confirm the presence of mirror touch synesthesia. The first condition is that the synaesthetic response, which is defined as the sensation synesthetes feel after observing someone else being touched, should feel like conscious experiences. The second condition is that synesthetic responses are induced by a stimulus that normally does not induce that response. The third condition is that the synesthetic experiences must occur automatically, without conscious thought. In order to examine the prevalence of this condition, a study was conducted at the University College London and University of Sussex. 567 undergraduate participants were recruited and given a questionnaire. From the questionnaire, it was determined that approximately 2.5% of the population experienced mirror-touch synesthesia symptoms. Further studies have shown the prevalence to be 1.6%, meaning that this condition is one of the more common types of synesthesia, along with grapheme-color synesthesia (1.4%) and day-color synesthesia (2.8%). At the moment it is believed that there are two subtypes of the condition. The first type causes a person to feel sensations on the part of their body that mirrors the observed touch. The second type causes a person to feel sensations on the same side of their body as the observed touch.

Studies have attempted to more explicitly define the of synesthetic responses. In most studies, participants are asked to observe someone else being touched and report what kind of synesthetic response they experience. In one particular instance, video clips were used to show different types of observed touch. The of the synesthetic touch is not affected by the location of the observed touch (arm, leg, hand, etc.); however, it is sometimes affected by the spatial orientation of the observed touch. When crossed hands are touched, the hands become uncrossed in the perception of synesthetes. However when the observed hand is upside down, the observed touch does not get rotated. Intensity is also not affected if the observed act consists of someone touching themselves, versus someone touching them. Additionally, the type of object doing the touching has a significant effect on the intensity of the response. If a finger or knife tip is used, a much higher intensity is experienced than if a feather is used. Finally, watching a dummy being touched decreases the intensity of the observed touch significantly. For this reason, it is suspected that in order to experience a synesthetic touch, synesthetes must observe somebody who is capable of feeling sensations.

Mirror touch responses are not limited to feeling touch. Mirror touch synesthetes have a higher ability to feel empathy than non-synesthetes, and can therefore feel the same emotions that someone else may be observed to feel. Additionally, some individuals experience pain when observing someone else in pain, and this is a condition usually developed from birth. Approximately 30% of the normal population experience some form of this condition and around 16% of amputees report synesthetic pain after an amputation. This condition can either be acquired or developed. In the congenital condition, synesthetes experience pain in the same location as the observed pain; however, in the acquired condition, high intensity pain is felt at the same location as the trauma.

In one of the most common forms of synesthesia, individual letters of the alphabet and numbers (collectively referred to as graphemes) are "shaded" or "tinged" with a color. While different individuals usually do not report the same colors for all letters and numbers, studies with large numbers of synesthetes find some commonalities across letters (e.g. A is likely to be red).

Allochiria (from the Greek meaning "other hand") is a neurological disorder in which the patient responds to stimuli presented to one side of their body as if the stimuli had been presented at the opposite side. It is associated with spatial s, usually symmetrical, of stimuli from one side of the body (or of the space) to the opposite one. Thus a touch to the left side of the body will be reported as a touch to the right side, which is also known as somatosensory allochiria. If the auditory or visual senses are affected, sounds (a person's voice for instance) will be reported as being heard on the opposite side to that on which they occur and objects presented visually will be reported as having been presented on the opposite side. Often patients may express allochiria in their drawing while copying an image. Allochiria often co-occurs with unilateral neglect and, like hemispatial neglect, the disorder arises commonly from damage to the right parietal lobe.

Allochiria is often confused with alloesthesia, also known as false allochiria. True allochiria is a symptom of dyschiria and unilateral neglect. Dyschiria is a disorder in the localization of sensation due to various degrees of dissociation and cause impairment in one side causing the inability to tell which side of the body was touched.

There are two overall forms of synesthesia: projective synesthesia and associative synesthesia. People who project will see actual colors, forms, or shapes when stimulated, as is commonly accepted as synesthesia; associators will feel a very strong and involuntary connection between the stimulus and the sense that it triggers. For example, in the common form chromesthesia (sound to color) a projector may hear a trumpet and see an orange triangle in space while an associator might hear a trumpet and think very strongly that it sounds "orange".

Synesthesia can occur between nearly any two senses or perceptual modes, and at least one synesthete, Solomon Shereshevsky, experienced synesthesia that linked all five senses. Types of synesthesia are indicated by using the notation x → y, where x is the "inducer" or trigger experience, and y is the "concurrent" or additional experience. For example, perceiving letters and numbers (collectively called graphemes) as colored would be indicated as grapheme → color synesthesia. Similarly, when synesthetes see colors and movement as a result of hearing musical tones, it would be indicated as tone → (color, movement) synesthesia.

While nearly every logically possible combination of experiences can occur, several types are more common than others.

Supernumerary phantom limb is a condition where the affected individual believes and receives sensory information from limbs of the body that do not actually exist, and never have existed, in contradistinction to phantom limbs, which appear after an individual has had a limb removed from the body and still receives input from it.

An fMRI study of a subject with a supernumerary phantom left arm was done by Khateb "et al." at the Laboratory of Experimental Neuropsychology at the University of Geneva. When the subject was told to touch her right cheek with the phantom limb, there was increased activity in the motor cortex of her brain in the area roughly corresponding to the left arm. When she announced that she had touched the phantom limb to her cheek, activity was monitored in the area of the somatosensory cortex that corresponded to the right cheek. At times during the experiment, the subject was asked to move the phantom limb to a location that was obstructed or otherwise unfeasible. In these instances, there was similar activation of the motor cortex but no such activity in the somatosensory cortex.

Allochiria has been observed mainly in the context of neglect which is usually due to a lesion that affects the right parietal lobe. In patients with allochiria, their sensibility is retained completely but the patient is not clear as to which side of the body has been touched. Their power of localization is retained but error exists to the side touched and they often refer the irritation to the corresponding part of the limb. In the patients' mind there is doubt or error as to which side of the body is touched.

There are multiple definitions of allochiria. According to Musser, allochiria is the reference of a sensory stimulus to the corresponding location on the opposite location on the opposite side of the body. Judson Bury says that a patient may refer to an impression on one side to a corresponding place on the opposite side of the body. Thus, if a patient is pricked on one limb, he may say that he feels it on the other. Overall, even though different author's definition differs on points such as the type of stimulus, and the symmetry between the site of the stimulus and the seat of its localization, they all agree that an essential feature of allochiria is the deflection of a sensation to the wrong side of the body, which is true allochiria. In none of these definitions is any stress laid on the state of the patient's knowledge of a right or left side and the symptoms are seen as an error in localization.

Obsersteiner laid stress that there is in allochiria no defect in vertical localization but merely confusion in the patient's mind between the opposite sides of the body and come to look upon the symptom as simply any form of bad mistake in localization.

There is in the patient's mind doubt or error as to the side touched while sensibility including the power of localization is otherwise retained. Allochiria has been described as occurring in nerve lesions, Hemiplegia, disseminated sclerosis Multiple sclerosis, tabes dorsalis, unilateral injury to the spinal cord, Ménière's disease, hysteria, symmetrical gangrene, and in connection with touch, pain, the "muscle sense," the temperature sense, sight, smell, taste, hearing, and the electrical reactions.

Allochiria can occur in relation to any or every segment of the body. In some cases allochiria may be , and in others it may be restricted to certain regions of the body, or even only to one part of the body. Allochiria is marked to have connections with a variety of senses and sometimes only certain kinds of stimuli can arouse the appropriate feeling of one sidedness.

Autoscopy is the experience in which an individual perceives the surrounding environment from a different perspective, from a position outside of his or her own body. Autoscopy comes from the ancient Greek ("self") and ("watcher").

Autoscopy has been of interest to humankind from time immemorial and is abundant in the folklore, mythology, and spiritual narratives of most ancient and modern societies. Cases of autoscopy are commonly encountered in modern psychiatric practice. According to neurological research, autoscopic experiences are hallucinations.

Experiences - are characterized by the presence of the following three factors:

- disembodiment, an apparent location of the self outside one's body;

- impression of seeing the world from an elevated and distanced visuo-spatial perspective or extracorporeal, but egocentric visuo-spatial perspective;

- impression of seeing one's own body from this perspective (autoscopy).

Laboratory of Cognitive Neuroscience, École Polytechnique Fédérale de Lausanne, Lausanne, and Department of Neurology, University Hospital, Geneva, Switzerland, have reviewed some of the classical precipitating factors of autoscopy. These are sleep, drug abuse, and general anesthesia as well as neurobiology. They have compared them with recent findings on neurological and neurocognitive mechanisms of the autoscopy. The reviewed data suggest that autoscopic experiences are due to functional disintegration of lower-level multisensory processing and abnormal higher-level self-processing at the temporoparietal junction.

Grapheme-color synaesthesia or colored grapheme synesthesia is a form of synesthesia in which an individual's perception of numerals and letters is associated with the experience of colors. Like all forms of synesthesia, Grapheme-color synesthesia is involuntary, consistent, and memorable. Grapheme-color synesthesia is one of the most common forms of synesthesia, and because of the extensive knowledge of the visual system, one of the most studied.

While it is extremely unlikely that any two synesthetes will report the same colors for all letters and numbers, studies of large numbers of synesthetes find that there are some commonalities across letters (e.g., "A" is likely to be red). Early studies argued that grapheme-color synesthesia was not due to associative learning, such as from playing with colored refrigerator magnets. However, one recent study has documented a case of synesthesia in which synesthetic associations could be traced back to colored refrigerator magnets. Despite the existence of this individual case, the majority of synesthetic associations do not seem to be driven by learning of this sort. Rather, it seems that more frequent letters are paired with more frequent colors, and some meaning-based rules, such as ‘b’ being blue, drive most synesthetic associations.

There has been a lot more research as to why and how synesthesia occurs with more recent technology and as synesthesia has become more well known. It has been found that grapheme-color synesthetes have more grey matter in their brain. There is evidence of an increased grey matter volume in the left caudal intra- parietal sulcus (IPS). There was also found to be an increased grey matter volume in the right fusiform gyrus. These results are consistent with another study on the brain functioning of grapheme-color synesthetes. Grapheme-color synesthetes tend to have an increased thickness, volume, and surface area of the fusiform gyrus. Furthermore, the area of the brain where word, letter, and color processing are located, V4a, is where the most significant difference in make-up was found. Though not certain, these differences are thought to be part of the reasoning for the presence of grapheme-color synesthesia.

Individuals with grapheme-color synesthesia rarely claim that their sensations are problematic or unwanted. In some cases, individuals report useful effects, such as aid in memory or spelling of difficult words.

These experiences have led to the development of technologies intended to improve the retention and memory of graphemes by individuals without synesthesia. Computers, for instance, could use "artificial synesthesia" to color words and numbers to improve usability. A somewhat related example of "computer-aided synesthesia" is using letter coloring in a web browser to prevent IDN homograph attacks. (Someone with synesthesia can sometimes distinguish between barely different looking characters in a similar way.)

Anaphia, also known as tactile anesthesia, is a medical symptom in which there is a total or partial absence of the sense of touch.

Anaphia is a common symptom of spinal cord injury and neuropathy.

Although not all phantom limbs are painful, people will sometimes feel as if they are gesturing, feel itches, twitch, or even try to pick things up. The missing limb often feels shorter and may feel as if it is in a distorted and painful position. Occasionally, the pain can be made worse by stress, anxiety, and weather changes. Phantom limb pain is usually intermittent. The frequency and intensity of attacks usually declines with time.

Some people who have undergone gender reassignment surgery have reported the sensation of phantom genitals. The reports were less common among post-operative transgender women, but did occur in transgender men. Similarly, subjects who had undergone mastectomy reported experiencing phantom breasts; these reports were substantially less common among post-operative transgender men, but did occur in transgender women.

As of 2016 the literature on misophonia was limited. Some small studies show that people with misophonia generally have strong negative feelings, thoughts, and physical reactions to specific sounds, which the literature calls "trigger sounds". These sounds are apparently usually soft, but can be loud. One study found that around 80% of the sounds were related to the mouth (eating, slurping, chewing or popping gum, etc.), and around 60% were repetitive. A visual trigger may develop related to the trigger sound. It also appears that a misophonic reaction can occur in the absence of an actual sound.

Reactions to the triggers can include aggression toward the origin of the sound, leaving, or remaining in its presence but suffering, trying to block it, or trying to mimic the sound.

The first misophonic reaction may occur when a person is young and can originate from someone in a close relationship, or a pet.

People with misophonia are aware they experience it and some consider it abnormal; the disruption it causes in their lives ranges from mild to severe. Avoidance and other behaviors can make it harder for people with this condition to achieve their goals and enjoy interpersonal interactions.

A phantom limb is the sensation that an amputated or missing limb is still attached. Approximately 60 to 80% of individuals with an amputation experience phantom sensations in their amputated limb, and the majority of the sensations are painful. Phantom sensations may also occur after the removal of body parts other than the limbs, e.g. after amputation of the breast, extraction of a tooth (phantom tooth pain) or removal of an eye (phantom eye syndrome).

Somatoparaphrenia is a type of monothematic delusion where one denies ownership of a limb or an entire side of one's body. Even if provided with undeniable proof that the limb belongs to and is attached to their own body, the patient produces elaborate confabulations about whose limb it really is, or how the limb ended up on their body. In some cases, delusions become so elaborate that a limb may be treated and cared for as if it were a separate being.

Somatoparaphrenia differs from a similar disorder, asomatognosia, which is characterized as loss of recognition of half of the body or a limb, possibly due to paralysis or unilateral neglect. For example, asomatognosic patients may mistake their arm for the doctor's. However, they can be shown their limb and this error is temporarily corrected.

Somatoparaphrenia has been reported to occur predominately in the left arm of one's body, and it is often accompanied by left-sided paralysis and anosognosia (denial or lack of awareness) of the paralysis. The link between somatoparaphrenia and paralysis has been documented in many clinical cases and while the question arises as to whether paralysis is necessary for somatoparaphrenia to occur, anosognosia is not, as documented by cases with somatoparaphrenia and paralysis with no anosognosia.

Hypoesthesia (or hypesthesia) refer to a reduced sense of touch or sensation, or a partial loss of sensitivity to sensory stimuli. In everyday speech this is sometimes referred to as "numbness".

Hypoesthesia is one of the negative sensory symptoms associated with cutaneous sensory disorder (CSD). In this condition, patients have abnormal disagreeable skin sensations that can be increased (stinging, itching or burning) or decreased (numbness or hypoesthesia). There are no other apparent medical diagnoses to explain these symptoms.

Cutaneous hyperesthesia has been associated with diagnosis of appendicitis in children but this symptom was not supported by the evidence.

Hypoesthesia originating in (and extending centrally from) the feet, fingers, navel, and/or lips is one of the common symptoms of beriberi, which is a set of symptoms caused by thiamine deficiency.

Hypoesthesia is also one of the more common manifestations of decompression sickness (DCS), along with joint pain, rash and generalized fatigue.

Misophonia, literally "hatred of sound", was proposed in 2000 as a condition in which negative emotions, thoughts, and physical reactions are triggered by specific sounds.

Misophonia is not classified as an auditory, neurological, or psychiatric condition, there are no standard diagnostic criteria, it is not recognized in the DSM-IV or the ICD-10, and there is little research on how common it is or the treatment. Proponents suggest misophonia can adversely affect ability to achieve life goals and to enjoy social situations. Treatment consists of developing coping strategies through cognitive behavioral therapy and exposure therapy.

Ordinal-linguistic personification (OLP, or personification for short) is a form of synesthesia in which ordered sequences, such as ordinal numbers, days, months and letters are associated with personalities and/or genders (). Although this form of synesthesia was documented as early as the 1890s (; ) researchers have, until recently, paid little attention to this form (see History of synesthesia research).

Phantom pain involves the sensation of pain in a part of the body that has been removed.

The hallmark sign of Alice in Wonderland syndrome (AIWS) is a migraine, and AIWS may in part be caused by the migraine. AIWS affects the sense of vision, sensation, touch, and hearing, as well as one's own body image.

A prominent and often disturbing symptom are experiences of altered body image. The person may find that they are confused as to the size and shape of parts of (or all of) their body. They may feel as though their body is expanding or getting smaller. Alice in Wonderland syndrome also involves perceptual distortions of the size or shape of objects. Other possible causes and signs of the syndrome include migraines, use of hallucinogenic drugs, and infectious mononucleosis.

Patients with certain neurological diseases have experienced similar visual hallucinations. These hallucinations are called "Lilliputian," which means that objects appear either smaller or larger than they actually are.

Patients may experience either micropsia or macropsia. Micropsia is an abnormal visual condition, usually occurring in the context of visual hallucination, in which affected persons see objects as being smaller than those objects actually are. Macropsia is a condition where the individual sees everything larger than it actually is.

A relationship between the syndrome and mononucleosis has been suggested.

One 17-year-old male, Michael Huang, described his odd symptoms. He said, "quite suddenly objects appear small and distant (teliopsia) or large and close (peliopsia). I feel as I am getting shorter and smaller 'shrinking' and also the size of persons are not longer than my index finger (a lilliputian proportion). Sometimes I see the blind in the window or the television getting up and down, or my leg or arm is swinging. I may hear the voices of people quite loud and close or faint and far. Occasionally, I experience attacks of migrainous headache associated with eye redness, flashes of lights and a feeling of giddiness. I am always conscious to the intangible changes in myself and my environment."

The eyes themselves are normal, but the person will often 'see' objects as the incorrect size, shape or perspective angle. Therefore, people, cars, buildings, houses, animals, trees, environments, etc., look smaller or larger than they should be, or that distances look incorrect; for example, a corridor may appear to be very long, or the ground may appear too close.

The person affected by Alice in Wonderland Syndrome may also lose the sense of time, a problem similar to the lack of spatial perspective. In other words, time seems to pass very slowly, akin to an LSD experience. The lack of time, and space, perspective leads to a distorted sense of velocity. For example, one could be inching along ever so slowly in reality, yet it would seem as if one were sprinting uncontrollably along a moving walkway, leading to severe, overwhelming disorientation. This can then cause the person to feel as if movement, even within his or her own home, is futile.

In addition, some people may, in conjunction with a high fever, experience more intense and overt hallucinations, seeing things that are not there and misinterpreting events and situations.

Other minor or less common symptoms may include loss of limb control and general dis-coordination, memory loss, lingering touch and sound sensations, and emotional experiences.

There are various types of sensations that may be felt:

- Sensations related to the phantom limb's posture, length and volume e.g. feeling that the phantom limb is behaving just like a normal limb like sitting with the knee bent or feeling that the phantom limb is as heavy as the other limb. Sometimes, an amputee will experience a sensation called telescoping. This is the feeling that the phantom limb is gradually shortening over time.

- Sensations of movement (e.g. feeling that the phantom foot is moving).

- Sensations of touch, temperature, pressure and itchiness. Many amputees report of feeling heat, tingling, itchiness, and pain.

Clinical features of CRPS have been found to be inflammation resulting from the release of certain pro-inflammatory chemical signals from the nerves, sensitized nerve receptors that send pain signals to the brain, dysfunction of the local blood vessels' ability to constrict and dilate appropriately, and maladaptive neuroplasticity.

The signs and symptoms of CRPS usually initially manifest near the site of a (typically minor) injury. The most common symptoms are pain sensations, including burning, stabbing, grinding, and throbbing. Moving or touching the limb is often intolerable. The patient may also experience muscle spasms; local swelling; extreme sensitivity to things such as wind and water, touch and vibrations; abnormally increased sweating; changes in skin temperature (usually hot but sometimes cold) and color (bright red or a reddish violet); softening and thinning of bones; joint tenderness or stiffness; changes in nail and hair growth and/or restricted or painful movement. Drop attacks (falls), almost fainting, and fainting spells are infrequently reported, as are visual problems. The symptoms of CRPS vary in severity and duration. Since CRPS is a systemic problem, potentially any organ can be affected.

The pain of CRPS is continuous although varies in severity. It is widely recognized that it can be heightened by emotional or physical stress.

Previously it was considered that CRPS had three stages; it is now believed that people affected by CRPS do not progress through these stages sequentially. These stages may not be time-constrained and could possibly be event-related, such as ground-level falls or re-injuries of previously damaged areas. Thus, rather than a progression of CRPS from bad to worse, it is now thought, instead, that such individuals are likely to have one of the three following types of disease progression:

1. "Stage" one is characterized by severe, burning pain at the site of the injury, muscle spasms, joint stiffness, restricted mobility, rapid hair and nail growth, and vasospasm. The vasospasm is that which causes the changes in the color and temperature of the skin. Some may experience hyperhydrosis (increased sweating). In mild cases this stage lasts a few weeks, in which it can subside spontaneously or respond rapidly to treatment (physical therapy, pain specialist).

2. "Stage" two is characterized by more intense pain. Swelling spreads, hair growth diminishes, nails become cracked, brittle, grooved and spotty, osteoporosis becomes severe and diffuse, joints thicken, and muscles atrophy.

3. "Stage" three is characterized by irreversible changes in the skin and bones, while the pain becomes unyielding and may involve the entire limb. There is marked muscle atrophy, severely limited mobility of the affected area, and flexor tendon contractions (contractions of the muscles and tendons that flex the joints). Occasionally the limb is displaced from its normal position, and marked bone softening and thinning is more dispersed.

Alice in Wonderland syndrome is a disorienting neuropsychological condition that affects perception. People experience size distortion such as micropsia, macropsia, pelopsia, or teleopsia. Size distortion may occur of other sensory modalities.

It is often associated with migraines, brain tumors, and the use of psychoactive drugs. It can also be the initial symptom of the Epstein–Barr virus (see mononucleosis). AiWS can be caused by abnormal amounts of electrical activity causing abnormal blood flow in the parts of the brain that process visual perception and texture.

Anecdotal reports suggest that the symptoms are common in childhood, with many people growing out of them in their teens. It appears that AiWS is also a common experience at sleep onset, and has been known to commonly arise due to a lack of sleep.

Dysesthesia can generally be described as a class of neurological disorders. It can be further classified depending on where it manifests in the body, and by the type of sensation that it provokes.

Cutaneous dysesthesia is characterized by discomfort or pain from touch to the skin by normal stimuli, including clothing. The unpleasantness can range from a mild tingling to blunt, incapacitating pain.

Scalp dysesthesia is characterized by pain or burning sensations on or under the surface of the cranial skin. Scalp dysesthesia may also present as excessive itching of the scalp.

Occlusal dysesthesia, or "phantom bite", is characterized by the feeling that the bite is "out of place" (occlusal dystopia) despite any apparent damage or instability to dental or oromaxillofacial structures or tissue. Phantom bite often presents in patients that have undergone otherwise routine dental procedures. Short of compassionate counseling, evidence for effective treatment regimes is lacking.