Although often termed a "neurological condition," synesthesia is not listed in either the DSM-IV or the ICD since it most often does not interfere with normal daily functioning. Indeed, most synesthetes report that their experiences are neutral or even pleasant. Like perfect pitch, synesthesia is simply a difference in perceptual experience.

The simplest approach is test-retest reliability over long periods of time, using stimuli of color names, color chips, or a computer-screen color picker providing 16.7 million choices. Synesthetes consistently score around 90% on reliability of associations, even with years between tests. In contrast, non-synesthetes score just 30–40%, even with only a few weeks between tests and a warning that they would be retested.

Grapheme-color synesthetes, as a group, share significant preferences for the color of each letter (e.g. A tends to be red; O tends to be white or black; S tends to be yellow etc.) Nonetheless, there is a great variety in types of synesthesia, and within each type, individuals report differing triggers for their sensations and differing intensities of experiences. This variety means that defining synesthesia in an individual is difficult, and the majority of synesthetes are completely unaware that their experiences have a name.

Neurologist Richard Cytowic identifies the following diagnostic criteria for synesthesia in his "first" edition book. However, the criteria are different in the second book:

1. Synesthesia is involuntary and automatic.

2. Synesthetic perceptions are spatially extended, meaning they often have a sense of "location." For example, synesthetes speak of "looking at" or "going to" a particular place to attend to the experience.

3. Synesthetic percepts are consistent and generic (i.e. simple rather than pictorial).

4. Synesthesia is highly memorable.

5. Synesthesia is laden with affect.

Cytowic's early cases mainly included individuals whose synesthesia was frankly projected outside the body (e.g. on a "screen" in front of one's face). Later research showed that such stark externalization occurs in a minority of synesthetes. Refining this concept, Cytowic and Eagleman differentiated between "localizers" and "non-localizers" to distinguish those synesthetes whose perceptions have a definite sense of spatial quality from those whose perceptions do not.

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.)

The most common tests perform to diagnose cerebral achromatopsia are the Farnsworth-Munsell 100-hue test, the Ishihara plate test, and the color-naming test. Testing and diagnosis for cerebral achromatopsia is often incomplete and misdiagnosed in doctor’s offices.

Remarkably, almost 50% of tested patients diagnosed with cerebral achromatopsia are able to perform normally on the color-naming test. However, these results are somewhat in question because of the sources from which many of these reports come. Only 29% of cerebral achromatopsia patients successfully pass the Ishihara plate test, which is a more accepted and more standardized test for color blindness.

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.

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).

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.

There are no standard diagnostic criteria. Misophonia is distinguished from hyperacusis, which is not specific to a given sound and does not involve a similar strong reaction, and from phonophobia, which is a fear of a specific sound, but it may occur with either.

It is not clear whether people with misophonia usually have comorbid conditions, nor whether there is a genetic component.

The diagnosis of misophonia is not recognized in the DSM-IV or the ICD 10, and it is not classified as a hearing, neurological, or psychiatric disorder. It may be a form of sound–emotion synesthesia, and has parallels with some anxiety disorders. As of 2015 it was not clear if misophonia should be classified as a symptom or as a condition.

The most apparent distinguishing characteristic between congenital achromatopsia and cerebral achromatopsia is the sudden onset of color vision loss following a severe head injury or damage to the occipital lobe following a stroke or similar ischemic event. Non-invasive imaging techniques can be the most helpful in determining whether the area of damage following a traumatic event is an correlated with color-vision processing. Simple diagnostic tools can also be used to determine whether a patient is a likely candidate for further testing, as advancing imaging procedures can often prove expensive and unnecessary.

Co-morbid factors can be valuable indicators of the likelihood of cerebral achromatoptsia. One disorder often seen alongside cerebral achromatopsia is prosopagnosia, the inability to recall or recognize faces. The correlation is still the subject of ongoing research, but the most telltale clue in this association is the close proximity of brain lesions seen in prosopagnosics and cerebral achromatopsiacs without prosopagnosia. Figure 1 illustrates overlap of brain lesions compiled from numerous case reports of both disorders. A common area of damage associated with both disorders can be seen in the right occipital lobe.

The Ishihara color test, which consists of a series of pictures of colored spots, is the test most often used to diagnose red–green color deficiencies. A figure (usually one or more Arabic digits) is embedded in the picture as a number of spots in a slightly different color, and can be seen with normal color vision, but not with a particular color defect. The full set of tests has a variety of figure/background color combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope, described above, is also used in diagnosing anomalous trichromacy.

Because the Ishihara color test contains only numerals, it may not be useful in diagnosing young children, who have not yet learned to use numerals. In the interest of identifying these problems early on in life, alternative color vision tests were developed using only symbols (square, circle, car).

Besides the Ishihara color test, the US Navy and US Army also allow testing with the Farnsworth Lantern Test. This test allows 30% of color deficient individuals, whose deficiency is not too severe, to pass.

Another test used by clinicians to measure chromatic discrimination is the Farnsworth-Munsell 100 hue test. The patient is asked to arrange a set of colored caps or chips to form a gradual transition of color between two anchor caps.

The HRR color test (developed by Hardy, Rand, and Rittler) is a red–green color test that, unlike the Ishihara, also has plates for the detection of the tritan defects.

Most clinical tests are designed to be fast, simple, and effective at identifying broad categories of color blindness. In academic studies of color blindness, on the other hand, there is more interest in developing flexible tests to collect thorough datasets, identify copunctal points, and measure just noticeable differences.

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.

Sensory modality testing allows practitioners to assess for generalized versus specific deficits, distinguishing visual agnosias from optic aphasia, which is a more generalized deficit in semantic knowledge for objects that spans multiple sensory modalities, indicating an impairment in the semantic representations themselves.

Specialists, like ophthalmologists or audiologists, can test for perceptual abilities. Detailed testing is conducted, using specially formulated assessment materials, and referrals to neurological specialists is recommended to support a diagnosis via brain imaging or recording techniques. The separate stages of information processing in the object recognition model are often used to localize the processing level of the deficit.

Testing usually consists of object identification and perception tasks including:

- object-naming tasks

- object categorization or figure matching

- drawing or copying real objects or images or illustrations

- unusual views tests

- overlapping line drawings

- partially degraded or fragmented image identification

- face or feature analysis

- fine line judgment

- figure contour tracking

- visual object description

- object-function miming

- tactile ability tests (naming by touch)

- auditory presentation identification

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 verbal fluency test is a widely and commonly used test to assess for frontal lobe dysfunction in patients.

- Procedure:

Participants are asked to generate words beginning with letters that had previously been introduced to them (e.g.: generate a word beginning with 'A' or 'R'). They are given three 1-min trials (one trial per letter). The goal is to say as many different words possible that begin with the given letter.

- Results:

The Verbal fluency test can assess for damage in the prefrontal lobes, which has been associated with patients suffering from source amnesia. Patients with frontal lobe disorder have trouble putting verbal items into a proper sequential order, monitor personal behaviors as well as a deficient judgment in recency. All of these behaviors are required for the proper recall of the source of a memory.

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.

The Wisconsin Card Sorting Test is widely used in clinical settings to test for cognitive impairments, such as frontal lobe disorder which has been associated with source amnesia.

- Procedure:

The visuo-spatial component of this test is devised of two sets of 12 identical cards. The figures on the cards differ with respect to color, quantity, and shape. The participants are then given a pile of additional cards and are asked to match each one to one of the previous cards.

- Results:

Patients suffering from frontal lobe dysfunction and ultimately source amnesia, will have much greater difficulty finishing this task successfully through method of strategy.

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.

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 three determining elements of a dichromatic opponent-colour space are the missing colour, the null-luminance plane, and the null-chrominance plane. The description of the phenomena itself does not indicate the colour that is impaired to the dichromat, however, it does provides enough information to identify the fundamental colour space, the colours that are seen by the dichromat. This is based on testing both the null-chrominance plane and null-luminance plane which intersect on the missing colour. The cones excited to a corresponding colour in the colour space are visible to the dichromat and those that are not excited are the missing colours.

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.

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.

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.