distinctive mode of perception in their childhood. some have learned how to apply their ability in daily life and work. synesthetes have used their abilities in memorization of names and telephone numbers, mental arithmetic, and more complex creative activities like producing visual art, music, and theater. despite the commonalities which permit the definition of the broad phenomenon of synesthesia, individual experiences vary in numerous ways. this variability was first noticed early in synesthesia research. some synesthetes report that vowels are more strongly colored, while for others consonants are more strongly colored. self-reports, interviews, and autobiographical notes by synesthetes demonstrate a great degree of variety in types of synesthesia, the intensity of synesthetic perceptions, awareness of the perceptual discrepancies between synesthetes and non-synesthetes, and the ways synesthesia is used in work, creative processes, and daily life. synesthetes are very likely to participate in creative activities. it has been suggested that individual development of perceptual and cognitive skills, in addition to one's cultural environment, produces the variety in awareness and practical use of synesthetic phenomena. synesthesia may also give a memory advantage. in one study, conducted by julia simner of the university of edinburgh, it was found that spatial sequence synesthetes have a built-in and automatic mnemonic reference. whereas a non-synesthete will need to create a mnemonic device to remember a sequence (like dates in a diary), a synesthete can simply reference their spatial visualizations. mechanism regions thought to be cross-activated in grapheme–color synesthesia (green=grapheme recognition area, red=v4 color area) as of 2015, the neurological correlates of synesthesia had not been established. dedicated regions of the brain are specialized for given functions. increased cross-talk between regions specialized for different functions may account for the many types of synesthesia. for example, the additive experience of seeing color when looking at graphemes might be due to cross-activation of the grapheme-recognition area and the color area called v4 (see figure). this is supported by the fact that grapheme–color synesthetes can identify the color of a grapheme in their peripheral vision even when they cannot consciously identify the shape of the grapheme. an alternative possibility is disinhibited feedback, or a reduction in the amount of inhibition along normally existing feedback pathways. normally, excitation and inhibition are balanced. however, if normal feedback were not inhibited as usual, then signals feeding back from late stages of multi-sensory processing might influence earlier stages such that tones could activate vision. cytowic and eagleman find support for the disinhibition idea in the so-called acquired forms of synesthesia that occur in non-synesthetes under certain conditions: temporal lobe epilepsy, head trauma, stroke, and brain tumors. they also note that it can likewise occur during stages of meditation, deep concentration, sensory deprivation, or with use of psychedelics such as lsd or mescaline, and even, in some cases, marijuana. however, synesthetes report that common stimulants, like caffeine and cigarettes do not affect the strength of their synesthesia, nor does alcohol.: 137–40 a very different theoretical approach to synesthesia is that based on ideasthesia. according to this account, synesthesia is a phenomenon mediated by the extraction of the meaning of the inducing stimulus. thus, synesthesia may be fundamentally a semantic phenomenon. therefore, to understand neural mechanisms of synesthesia the mechanisms of semantics and the extraction of meaning need to be understood better. this is a non-trivial issue because it is not only a question of a location in the brain at which meaning is "processed" but pertains also to the question of understanding – epitomized in e.g., the chinese room problem. thus, the question of the neural basis of synesthesia is deeply entrenched into the general mind–body problem and the problem of the explanatory gap. genetics main article: genetics of synesthesia due to the prevalence of synesthesia among the first-degree relatives of people affected, there may be a genetic basis, as indicated by the monozygotic twins studies showing an epigenetic component. synesthesia might also be an oligogenic condition, with locus heterogeneity, multiple forms of inheritance, and continuous variation in gene expression. while the exact genetic loci for this trait haven't been identified, research indicates that the genetic constructs underlying synesthesia are most likely more complex than the simple x-linked mode of inheritance that early researchers believed it to be. further, it remains uncertain as to whether synesthesia perseveres in the genetic pool because it provides a selective advantage, or because it has become a byproduct of some other useful selected trait. women have a higher chance of developing synesthesia, as demonstrated in population studies conducted in the city of cambridge, england where females were 6 times more likely to have it. as technological equipment continues to advance, the search for clearer answers regarding the genetics behind synesthesia will become more promising. although often termed a "neurological condition," synesthesia is not listed in either the dsm-iv or the icd since it usually 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. reaction times for answers that are congruent with a synesthete's automatic colors are shorter than those whose answers are incongruent. 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 the 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. many tests exist for synesthesia. each common type has a specific test. when testing for grapheme–color synesthesia, a visual test is given. the person is shown a picture that includes black letters and numbers. a synesthete will associate the letters and numbers with a specific color. an auditory test is another way to test for synesthesia. a sound is turned on and one will either identify it with a taste or envision shapes. the audio test correlates with chromesthesia (sounds with colors). since people question whether or not synesthesia is tied to memory, the "retest" is given. one is given a set of objects and is asked to assign colors, tastes, personalities, or more. after a period of time, the same objects are presented and the person is asked again to do the same task. the synesthete can assign the same characteristics because that person has permanent neural associations in the brain, rather than memories of a certain object. the automaticity of synesthetic experience. a synesthete might perceive the left panel like the panel on the right. 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: synesthesia is involuntary and automatic. 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. synesthetic percepts are consistent and generic (i.e., simple rather than pictorial). synesthesia is highly memorable. 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. prevalence estimates of prevalence of synesthesia have ranged widely, from 1 in 4 to 1 in 25,000–100,000. however, most studies have relied on synesthetes reporting themselves, introducing self-referral bias. in what is cited as the most accurate prevalence study so far, self-referral bias was avoided by studying 500 people recruited from the communities of edinburgh and glasgow universities; it showed a prevalence of 4.4%, with 9 different variations of synesthesia. this study 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 since been independently verified in a sample of nearly 3,000 people in the university of edinburgh. 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 spectrum condition. history the interest in colored hearing dates back to greek antiquity when philosophers asked if the color (chroia, what we now call timbre) of music was a quantifiable quality. isaac newton proposed that musical tones and color tones shared common frequencies, as did goethe in his book theory of colours. there is a long history of building color organs such as the clavier à lumières on which to perform colored music in concert halls. the first medical description of "colored hearing" is in an 1812 thesis by the german physician georg tobias ludwig sachs. the "father of psychophysics," gustav fechner, reported the first empirical survey of colored letter photisms among 73 synesthetes in 1876, followed in the 1880s by francis galton. carl jung refers to "color hearing" in his symbols of transformation in 1912. in the early 1920s, the bauhaus teacher and musician gertrud grunow researched the relationships between sound, color, and movement and developed a 'twelve-tone circle of colour' which was analogous with the twelve-tone music of the austrian composer arnold schönberg (1874-1951). she was a participant in at least one of the congresses for colour-sound research (german:kongreß für farbe-ton-forschung) held in hamburg in the late 1920s and early 1930s. research into synesthesia proceeded briskly in several countries, but due to the difficulties in measuring subjective experiences and the rise of behaviorism, which made the study of any subjective experience taboo, synesthesia faded into scientific oblivion between 1930 and 1980. as the 1980s cognitive revolution made inquiry into internal subjective states respectable again, scientists returned to synesthesia. led in the united states by larry marks and richard cytowic, and later in england by simon baron-cohen and jeffrey gray, researchers explored the reality, consistency, and frequency of synesthetic experiences. in the late 1990s, the focus settled on grapheme → color synesthesia, one of the most common and easily studied types. psychologists and neuroscientists study synesthesia not only for its inherent appeal but also for the insights it may give into cognitive and perceptual processes that occur in synesthetes and non-synesthetes alike. synesthesia is now the topic of scientific books and papers, ph.d. theses, documentary films, and even novels. since the rise of the internet in the 1990s, synesthetes began contacting one another and creating websites devoted to the condition. these rapidly grew into international organizations such as the american synesthesia association, the uk synaesthesia association, the belgian synesthesia association, the canadian synesthesia association, the german synesthesia association, and the netherlands synesthesia web community. society and culture notable cases solomon shereshevsky, a newspaper reporter turned mnemonist, was discovered by russian neuropsychologist alexander luria to have a rare fivefold form of synesthesia, of which he is the only known case. words and text were not only associated with highly vivid visuospatial imagery but also sound, taste, color, and sensation. shereshevsky could recount endless details of many things without form,