Chromesthesia
[3] Chromesthesia can be induced by different auditory experiences, such as music, phonemes, speech, and/or everyday sounds.
However, studies to date have reported that synesthetes and non-synesthetes alike associate high pitched sounds with lighter or brighter colors and low pitched sounds with darker colors, indicating that a common mechanism may underlie those associations in normal adult brains.
[8] Of participants categorized as having synesthesia for music in this study, 75% reported concurrents exclusively when listening to notes being played.
[8] Other contributing factors included concentration level, fatigue, sleep habits, fever, emotions, and substances, such as caffeine or alcohol.
When the synthesizer was transposed without her knowledge, she reported identical color associations to the notes that she believed she was hearing, rather than the absolute pitch of the tones.
[11] Even earlier than Sachs, however, Johann Gottfried Herder discussed similar ideas in his Treatise on the Origin of Language in 1772.
[13] Their book was reviewed by an Austrian newspaper, where the term colored hearing, still commonly used today to describe chromesthesia, first appeared.
[14] The number of scientific papers on the topic rebounded around 1980 [1] and exponentially increased in the 21st century, where substantial progress has been made to study it empirically and understand the mechanisms at work.
There is a genetic predisposition for the condition, but the specific type is determined by environment and learning, which explains why "mappings differ across individuals, but are not strictly random".
Ramachandran and E.M. Hubard, based on converging evidence from studies of synesthesia that sensory areas for processing real and synesthetic information tend to be neighboring brain regions.
[17] Individuals with chromesthesia show activation of brain areas involved in visual processing, such as V4, immediately after the auditory perception, indicating an automatic linking of sounds and colors.
[1] The reason for this cross-activation is unclear, but one hypothesis is that the increased connectivity between adjacent brain regions is due to a reduction in the pruning of neuronal networks during childhood.
These hypotheses align with Daphne Maurer's neonatal hypothesis, which states that all newborns are synesthetes, but the condition disappears at around the age of three months.
[16] The disinhibited feedback model rejects the assumption of increased connectivity in synesthetes and proposes that the cross-activation is due to a decrease of inhibition in the networks present in the normal adult brain.
[16] Disinhibited feedback could account for the fact that chromesthesia can be acquired by damage to the retino-cortical pathway [5] or transiently induced through chemical agents, sensory deprivation, meditation, etc.
[16] Functional magnetic resonance imaging studies implicate the left superior temporal sulcus for the integration of auditory and visual information.
[5] The literature contains conflicting definitional criteria for synesthesia, which could bias selection of research subjects and interpretation of results.
While this description of synesthesia is useful in describing the condition, it should not be interpreted literally and used as selection criteria for scientific exploration.
[3] 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.
[1] Psychoactive drugs including LSD, mescaline, psilocybin, and ayahuasca are non-selective serotonin agonists that elicit spontaneous synesthesia, specially sound-to-color.
[21] Gautier made a sketch of Gustave Moreau playing the piano, where he depicted his chromesthetic experiences as lines of color above the instrument.
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.
[1] Psychedelics greatly enhance suggestibility, so it is fairly common to mistake hallucinations with chromesthesia;[20] especially considering that all measures of color perception including brightness, saturation, luminance, contrast, and hue are affected due to chemical agents.
Nevertheless, most studies "suffer from a large number of limitations including a lack of placebo control, double-blinds, and randomized allocation".
Against natural expectation, studies have found that possession of absolute pitch increased local variance in matching ability.
[4] 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.
[24] Tori Amos has described seeing music as structures of light since early childhood, an experience consistent with chromesthesia.
[27] Furthermore, Alexander Scriabin developed a "keyboard with lights" or clavier à lumières, which directly matched musical notes with colors.
[27] "Scriabin believed integration of colored light within a symphonic work would act as a 'powerful psychological resonator for the listener'".
Specifically, Rimsky-Korsakov made a distinction between major and minor scales and his associations had a "more neutral, spontaneous character".
