may exclude themselves on the basis of not fitting the prescribed definitional criteria. this is also significant to the extent that synesthetic individuals have a limited ability to differentiate their experience from that of nonsynesthetic individuals. a possible resolution of these issues would involve defining synesthesia on a neurological basis. such a unifying neurobiological cause has yet to be found, but if it exists, it would deepen understanding of the phenomenon in ways that the behavioral definition has failed to do. drug-induced chromesthesia gautier, under the influence of hashish, depicted his chromesthetic perceptions over the piano as lines of color arising from the music. chromesthesia can be transiently induced with chemical agents through the manipulation of neurotransmitters. these substances can also modulate existing synesthesia. psychoactive drugs including lsd, mescaline, psilocybin, and ayahuasca are non-selective serotonin agonists that elicit spontaneous synesthesia, specially sound-to-color. the first to report drug-induced chromesthesia was théophile gautier in 1845. under the influence of hashish, he described: "my hearing was developed extraordinarily; i heard the noise of colors. green, red, blue, yellow sounds reached me in perfectly distinguishable waves". gautier made a sketch of gustave moreau playing the piano, where he depicted his chromesthetic experiences as lines of color above the instrument. recent scientific studies, with enhanced methodologies, suggest that drug-induced synesthesia is substantially different from congenital synesthesia. psychoactive substances "affect ongoing streams of transmission rather than causing stimulus-induced activation". the most common type of synesthesia elicited with chemical agents is chromesthesia. still, frequent inducers include auditory and visual stimuli, especially music - which could explain the prevalence of sound-to-color synesthesia over other types of synesthesia. heinrich klüver's form constants: tunnels, spirals, honeycombs gratings, and cobwebs heinrich klüver categorized recurring geometric shapes under the influence of peyote in the 1920s. he called these the form constants: tunnels, spirals, honeycombs gratings, and cobwebs. these also apply to both drug-induced and natural hallucinations, which appear in near-death experiences, sensory deprivation, waking up or falling asleep, and during migraines. according to klüver, all hallucinations consist of shapes in one of these categories and 'atypical' hallucinations are simply variations. the form constants are common in chromesthetic experiences. psychedelics greatly enhance suggestibility, so it is fairly common to mistake hallucinations with chromesthesia; especially considering that all measures of color perception including brightness, saturation, luminance, contrast, and hue are affected due to chemical agents. drug-induced chromesthesia, as opposed to congenital chromesthesia, is not consistent or automatic. furthermore, bottom-up processing is responsible for experiences under drug influence, so external stimuli and context are not as critical. several studies, both direct (intentionally trying to induce synesthesia) and indirect (participants respond to a set of questions, including one about synesthetic experiences), suggest that the induction of synesthesia with chemical agents is possible. nevertheless, most studies "suffer from a large number of limitations including a lack of placebo control, double-blinds, and randomized allocation". music and chromesthesia individuals with chromesthesia are more likely to play musical instruments and be artistically inclined. furthermore, "both the hobbies and occupations of synesthetes are skewed toward the creative industries." people with synesthetic propensities are more metaphorical since the same genes cause them to relate concepts and ideas and, thus, be more creative. this could explain the higher incidence of musicians who have synesthesia. however, musical experience does not assist the ability to consistently match colors to tones. against natural expectation, studies have found that possession of absolute pitch increased local variance in matching ability. one possible explanation for this is that because absolute pitch is subject to chance error, occasionally incorrectly inferred note names could compete with the pitch-induced color on particular trials. another possibility is that people with absolute pitch can label tighter pitch recognition categories than normal, introducing a greater number of category boundaries to cross between distinct tones. composers with chromesthesia franz liszt was a composer who was known for asking performers to play with color. he was noted telling his orchestra to play the music in a "bluer fashion," since that is what the tone required. synesthesia was not a common term in liszt's time; people thought he was playing a trick on them when he referred to a color instead of a musical term. leonard bernstein openly discussed his chromesthesia, which he described as a "timbre to color." although he does not reference specific songs as being a certain color, he does explain the way it should sound to the artist performing. there are recordings of him stopping orchestras and singers when they are changing the "timbre." if someone changes the "timbre" or tone in a piece, it does not necessarily change the sound to the listener, but the composer with chromesthesia will automatically know. amy beach was another composer who had synesthesia. according to her perspective, each key signature was associated with a particular color. if an artist changed the key to suit their voice, then she would become upset because it would change the intended sound, portrayal, and emotion of the piece. tori amos described to see music as colors. olivier messiaen was influenced by the color of musical keys for his compositions. alexander scriabin. it is debatable whether scriabin had chromesthesia or if his analogies were purely associative alexander scriabin the theosophist "meanings of colors" of thought-forms and human aura associated with feelings and emotions alexander scriabin was a russian composer and pianist. he is famously regarded as a synesthete, but there is a lot of controversy surrounding whether he had chromesthesia or not. scriabin was a major proponent of theosophy, which had a system associating colors to feelings and emotions. this influenced the musician, who distinguished "spiritual" tonalities (like f-sharp major) from "earthly, material" ones (c major, f major). furthermore, alexander scriabin developed a "keyboard with lights" or clavier à lumières, which directly matched musical notes with colors. "scriabin believed integration of colored light within a symphonic work would act as a 'powerful psychological resonator for the listener'". that is why he created the clavier à lumières for his color-symphony prometheus: the poem of fire.this consisted of a color organ, which projected colors on a screen. the musicologist sabaneyev first published a table of scriabin's sound-to-color mapping in 1911: scriabin's sound-to-color associations note color c red g orange-pink d yellow a green e whitish-blue b similar to e f♯ blue, bright d♭ violet a♭ purplish-violet e♭ steel color with metallic sheen b♭ similar to e flat f red, dark scriabin was friends with composer nikolai rimsky-korsakov, who was a synesthete, and their sound-to-color associations were not the same. specifically, rimsky-korsakov made a distinction between major and minor scales and his associations had a "more neutral, spontaneous character". still, different individuals respond to some sounds and not others, and sound-to-color associations vary greatly between them. scriabin's sound-to-color associations arranged into a circle of fifths, demonstrating its spectral quality when the notes are ordered by the circle of fifths, the colors are in order of a spectrum, which casts doubt on whether scriabin experienced chromesthesia: scriabin's sound-to-color circle of fifths note color c red g orange d yellow a green e sky blue b blue f♯/ g♭ bright blue or violet d♭ violet or purple a♭ violet or lilac e♭ flesh or steel b♭ rose f deep red whether scriabin had chromesthesia or not, his work was greatly influenced by the particularities of this phenomenon. he created a system that associated colors to tones and aimed to create holistic sensory experiences with his compositions. not only did he experiment with colors, but also with "the generation of scents and sensation of touch and taste".