Synesthesia also has a number of practical applications, one of which is the use of 'intentional synesthesia' in technology.

Synesthesia is found in at least 4.4% of the population, as a high estimate, which is equivalent to 1 in 23 people. This study had also concluded that one common form of synesthesia—grapheme-color synesthesia (colored letters and numbers) – is found in more than one percent of the population, and this latter prevalence of graphemes-color synesthesia has now been independently verified in a yet larger sample. Earlier estimates of the prevalence of synesthesia were based on "best-guess" estimations only ("e.g." 1 in 250,000) or had limitations in their methodologies because they required synesthetes to refer themselves for study ("e.g." 1 in 2000) and for this reason the authors of those studies had been moderate in their claims. Also, some individuals will not self-classify as synesthetes because they do not realize that their perceptions are different from those of everyone else.

The most common forms of synesthesia are those that trigger colors, and the most prevalent of all is day-color. Also relatively common is grapheme-color synesthesia. We can think of "prevalence" both in terms of how common is synesthesia (or different forms of synesthesia) within the population, or how common are different forms of synesthesia within synesthetes. So within synesthetes, forms of synesthesia that trigger color also appear to be the most common forms of synesthesia with a prevalence rate of 86% within synesthetes. In another study, music-color is also prevalent at 18–41%. Some of the rarest are reported to be auditory-tactile, mirror-touch, and lexical-gustatory.

There is research to suggest that the likelihood of having synesthesia is greater in people with autism.

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.)

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.

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).

Riddoch syndrome (also known as the "Riddoch phenomenon") is an ocular affectation often caused by lesions in the occipital lobe which limit the sufferer's ability to distinguish objects. Only moving objects in a blind field are visible, static ones being invisible to the patient. The moving objects are not perceived to have color or detail. The subject may only have awareness of the movement without visual perception of it (gnosanopsia), or the general shape of a moving object may be perceivable as a shadow like outline.

At least one patient was able to use a rocking chair—putting non-moving surroundings in relative motion to her head—to improve her motion perception. She eventually was able to do the same with just voluntary movement of her head.

Mme L. reports that “1, 2, 3 are children without fixed personalities; they play together. 4 is a good peaceful woman, absorbed by down-to-earth occupations and who takes pleasure in them. 5 is a young man, ordinary and common in his tastes and appearance, but extravagant and self-centered. 6 is a young man of 16 or 17, very well brought up, polite, gentle, agreeable in appearance, and with upstanding tastes; average intelligence; orphan. 7 is a bad sort, although brought up well; spiritual, extravagant, gay, likeable; capable of very good actions on occasion; very generous. 8 is a very dignified lady, who acts appropriately, and who is linked with 7 and has much influence on him. She is the wife of 9. 9 is the husband of 8. He is self-centred, maniacal, selfish, thinks only about himself, is grumpy, endlessly reproaching his wife for one thing or another; telling her, for example, that he would have been better to have married a 9, since between them they would have made 18 – as opposed to only 17 with her… 10, and the other remaining numerals, have no personifications”.

Cakins (1893) describes a case for whom “T’s are generally crabbed, ungenerous creatures. U is a soulless sort of thing. 4 is honest, but… 3 I cannot trust… 9 is dark, a gentleman, tall and graceful, but politic under his suavity” .

For synesthete MT “I [is] a bit of a worrier at times, although easy-going; J [is] male; appearing jocular, but with strength of character; K [is] female; quiet, responsible…” .

More recently AP has reported that February is “an introverted female”, while F is a “[male] dodgy geezer”. Similarly, May is reported to be “soft-spoken” and “girly” while M is an “old lady [who] natter[s] a lot”, and while August is “a boy among girls”, A is a female “mother type” (; ).

The prevalence is not known; it is not known whether males or females, or older or younger people, are more likely to have misophonia.

There are no evidence-based treatments for the condition; health care providers generally try to help people cope with it by recognizing what the person is experiencing, and by working on coping strategies with the person. Some small studies have been published on the use of sound therapy similar to tinnitus retraining therapy and on cognitive behavioral therapy and particularly exposure therapy, to help people become less aware of the trigger sound. None of these approaches have been sufficiently studied to determine their effectiveness.

Many applications for iPhone and iPad have been developed to help colorblind people to view the colors in a better way. Many applications launch a sort of simulation of colorblind vision to make normal-view people understand how the color-blinds see the world. Others allow a correction of the image grabbed from the camera with a special "daltonizer" algorithm.

The GNOME desktop environment provides colorblind accessibility using the gnome-mag and the libcolorblind software. Using a gnome applet, the user may switch a color filter on and off, choosing from a set of possible color transformations that will displace the colors in order to disambiguate them. The software enables, for instance, a colorblind person to see the numbers in the Ishihara test.

There is generally no treatment to cure achromatopsia. However, dark red or plum colored filters are very helpful in controlling light sensitivity.

Since 2003, there is a cybernetic device called eyeborg that allows people to perceive color through sound waves. Achromatopsic artist Neil Harbisson was the first to use such a device in early 2004, the eyeborg allowed him to start painting in color by memorizing the sound of each color.

Moreover, there is some research on gene therapy for animals with achromatopsia, with positive results on mice and young dogs, but less effectiveness on older dogs. However, no experiments have been made on humans. There are many challenges to conducting gene therapy on humans. See Gene therapy for color blindness for more details about it.

Cerebral achromatopsia is a type of color-blindness caused by damage to the cerebral cortex of the brain, rather than abnormalities in the cells of the eye's retina. It is often confused with congenital achromatopsia but underlying physiological deficits of the disorders are completely distinct.

Optometrists can supply colored spectacle lenses or a single red-tint contact lens to wear on the non-dominant eye, but although this may improve discrimination of some colors, it can make other colors more difficult to distinguish. A 1981 review of various studies to evaluate the effect of the X-chrom contact lens concluded that, while the lens may allow the wearer to achieve a better score on certain color vision tests, it did not correct color vision in the natural environment. A case history using the X-Chrom lens for a rod monochromat is reported and an X-Chrom manual is online.

Lenses that filter certain wavelengths of light can allow people with a cone anomaly, but not dichromacy, to see better separation of colors, especially those with classic "red/green" color blindness. They work by notching out wavelengths that strongly stimulate both red and green cones in a deuter- or protanomalous person, improving the distinction between the two cones' signals. As of 2013, sunglasses that notch out color wavelengths are available commercially.

Patients with cerebral achromatopsia deny having any experience of color when asked and fail standard clinical assessments like the Farnsworth-Munsell 100-hue test (a test of color ordering with no naming requirements). Patients may often not notice their loss of color vision and merely describe the world they see as being "drab". Most describe seeing the world in "shades of gray". This observation notes a key difference between cerebral and congenital achromatopsia, as those born with achromatopsia have never had an experience of color or gray.

Some studies have demonstrated improvements in reading and spelling performance of individual children with surface dyslexia. Many of the interventions that exist are based on the dual route model of reading and utilize a targeted approach based on the individual assessment results.

Case studies conducted by Law and Cupples (2015) recommend first identifying specific oral reading difficulties experienced by the individual with surface dyslexia and based on the reading patterns identified designing a theoretically motivated and targeted treatment program. One of the interventions involved targeting visual-orthographic processing by increasing the efficiency by which surface dyslexics identified nonwords. The second intervention involved training in the identification and decoding of common letter patterns in irregular words.

The affected individual may not realize that they have a visual problem and may complain of becoming "clumsy" or "muddled" when performing familiar tasks such as setting the table or simple DIY.

Anosognosia, a lack of awareness of the deficit, is common and can cause therapeutic resistance. In some agnosias, such as prosopagnosia, awareness of the deficit is often present; however shame and embarrassment regarding the symptoms can be a barrier in admission of a deficiency. Because agnosias result from brain lesions, no direct treatment for them currently exists, and intervention is aimed at utilization of coping strategies by patients and those around them. Sensory compensation can also develop after one modality is impaired in agnostics

General principles of treatment:

- restitution

- repetitive training of impaired ability

- development of compensatory strategies utilizing retained cognitive functions

Partial remediation is more likely in cases with traumatic/vascular lesions, where more focal damage occurs, than in cases where the deficit arises out of anoxic brain damage, which typically results in more diffuse damage and multiple cognitive impairments. However, even with forms of compensation, some afflicted individuals may no longer be able to fulfill the requirements of their occupation or perform common tasks, such as, eating or navigating. Agnostics are likely to become more dependent on others and to experience significant changes to their lifestyle, which can lead to depression or adjustment disorders.

Dichromacy ("di" meaning "two" and "chroma" meaning "color") is the state of having two types of functioning color receptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights. By comparison, trichromats require three pure spectral lights to match all colors that they can perceive, and tetrachromats require four.

Dichromacy in humans is a color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, predominantly affecting males. Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions.

Chromophobia (also known as chromatophobia or chrematophobia) is a persistent, irrational fear of, or aversion to, colors and is usually a conditioned response. While actual clinical phobias to color are rare, colors can elicit hormonal responses and psychological reactions.

Chromophobia may also refer to an aversion of use of color in products or design. Within cellular biology, "chromophobic" cells are a classification of cells that do not attract hematoxylin, and is related to chromatolysis.

Achromatopsia (ACHM), also known as total color blindness, is a medical syndrome that exhibits symptoms relating to at least five conditions. The term may refer to acquired conditions such as cerebral achromatopsia, also known as color agnosia, but it typically refers to an autosomal recessive congenital color vision condition, the inability to perceive color and to achieve satisfactory visual acuity at high light levels (typically exterior daylight). The syndrome is also present in an incomplete form which is more properly defined as dyschromatopsia. It is estimated to affect 1 in 40,000 live births worldwide.

There is some discussion as to whether achromats can see color or not. As illustrated in "The Island of the Colorblind" by Oliver Sacks, some achromats cannot see color, only black, white, and shades of grey. With five different genes currently known to cause similar symptoms, it may be that some do see marginal levels of color differentiation due to different gene characteristics. With such small sample sizes and low response rates, it is difficult to accurately diagnose the 'typical achromatic conditions'. If the light level during testing is optimized for them, they may achieve corrected visual acuity of 20/100 to 20/150 at lower light levels, regardless of the absence of color. One common trait is hemeralopia or blindness in full sun. In patients with achromatopsia, the cone system and fibres carrying color information remain intact. This indicates that the mechanism used to construct colors is defective.

"Seeing pink elephants" is a euphemism for drunken hallucination caused by alcoholic hallucinosis or delirium tremens. The term dates back to at least the early 20th century, emerging from earlier idioms about snakes and other creatures. An alcoholic character in Jack London's 1913 novel "John Barleycorn" is said to hallucinate "blue mice and pink elephants".

There are various kinds of color blindness:

- Protanopia is a severe form of red-green color blindness, in which there is impairment in perception of very long wavelengths, such as reds. To these individuals, reds are perceived as beige or grey and greens tend to look beige or grey like reds. It is also the most common type of dichromacy today. This problem occurs because patients do not have the red cone cells in the retina. Protanomaly is a less severe version.

- Deuteranopia consists of an impairment in perceiving medium wavelengths, such as greens. Deuteranomaly is a less severe form of deuteranopia. Those with deuteranomaly cannot see reds and greens like those without this condition; however, they can still distinguish them in most cases. It is very similar to protanopia. In this form, patients do not have green cone cells in the retina, which makes it hard to see the green color.

- A rarer form of color blindness is tritanopia, where there exists an inability to perceive short wavelengths, such as blues. Sufferers have trouble distinguishing between yellow and blue. They tend to confuse greens and blues, and yellow can appear pink. This is the rarest of all dichromacy, and occurs in around 1 in 100,000 people. Patients do not have the blue cone cells in the retina.

Surface dyslexia is a type of dyslexia, or reading disorder. According to Marshall & Newcombe's (1973) and McCarthy & Warrington's study (1990), patients with this kind of disorder cannot recognize a word as a whole due to the damage of the left parietal or temporal lobe. Individuals with surface dyslexia are unable to recognize a word as a whole word and retrieve its pronunciation from memory. Rather, individuals with surface dyslexia rely on pronunciation rules. Thus, patients with this particular type of reading disorder read non-words fluently, like "yatchet", but struggle with words that defy pronunciation rules (i.e. exception words). For example, a patient with surface dyslexia can correctly read regular words like "mint", but will err when presented a word that disobeys typical pronunciation rules, like "pint". Often, semantic knowledge is preserved in individuals with surface dyslexia.

Associative visual agnosia is a form of visual agnosia. It is an impairment in recognition or assigning meaning to a stimulus that is accurately perceived and not associated with a generalized deficit in intelligence, memory, language or attention. The disorder appears to be very uncommon in a "pure" or uncomplicated form and is usually accompanied by other complex neuropsychological problems due to the nature of the etiology. Afflicted individuals can accurately distinguish the object, as demonstrated by the ability to draw a picture of it or categorize accurately, yet they are unable to identify the object, its features or its functions.