Chromesthesia
Chromesthesia or sound-to-color synesthesia is a type of synesthesia in which sound involuntarily evokes an experience of color, shape, and movement. Individuals with sound-color synesthesia are consciously aware of their synesthetic color associations/perceptions in daily life. Synesthetes that perceive color while listening to music experience the colors in addition to the normal auditory sensations. The synesthetic color experience supplements, but does not obscure real, modality-specific perceptions. As with other forms of synesthesia, individuals with sound-color synesthesia perceive it spontaneously, without effort, and as their normal realm of experience. Chromesthesia can be induced by different auditory experiences, such as music, phonemes, speech, and/or everyday sounds.
Individual variance
The color associations, that is, which color is associated to which sound, tone, pitch, or timbre is highly idiosyncratic, but in most cases, consistent over time. Individuals with synesthesia have unique color pairings. 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. There are forms of pseudo-chromesthesia that may be explained by associations synesthetes have made and forgotten from childhood.As with other types of synesthesia, sound-color synesthesia can be divided into groups based on the way the colors are experienced. Those that 'see' or perceive the color in external space are called projectors, and those that perceive the color in the mind's eye are often called associators, but these terms can be misleading to understanding the nature of the experience. For most synesthetes, the condition is not wholly sensory/perceptual.
For some individuals, chromesthesia is only triggered by speech sounds, while others' chromesthesia can be triggered by any auditory stimuli. In a study investigating variability within categories of synesthesia, 40% of subjects with chromesthesia for spoken words reported that voice pitch, accent, and prosody influenced the synesthetic color, whereas few subjects reported that volume or speed of talking had any influence. Within these subjects, many reported that the speaker's emotional inflection could influence the synesthetic color, but only two reported that their own mood had such influence. Of participants categorized as having synesthesia for music in this study, 75% reported concurrents exclusively when listening to notes being played. When asked whether the experience of the concurrent could be voluntarily controlled, only 33% of participants indicated an ability to smother, ignore, or willfully evoke their concurrents without great effort. Attention to the inducing stimulus was reported as influential in 59% of participants. Other contributing factors included concentration level, fatigue, sleep habits, fever, emotions, and substances, such as caffeine or alcohol.
Sound-color synesthesia is far more common than color-sound synesthesia, although there are reported cases where sounds and colors activate bidirectionally. One individual sees colors when she hears sounds and also hears sounds when she sees colors. This type of synesthesia interferes greatly with daily life. This individual's associations were highly consistent over time, but the associations were not necessarily the same in either direction. Another individual who had absolute pitch, as well as Chromesthesia, claimed that her absolute pitch was less stable than her Chromesthesia.
There may be an effect of semantic mediation in some individuals with sound-color synesthesia. One subject self-triggered notes on a synthesizer and noted the color associations. 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.
History
The terms synesthesia and chromesthesia have developed and evolved considerably throughout history. The first documented synesthete was Georg Tobias Ludwig Sachs in 1812. Although he did not give a specific name to his experience, in a medical dissertation regarding his albinism, he mentioned obscure ideas and described how colored ideas appeared to him. Even earlier than Sachs, however, Johann Gottfried Herder discussed similar ideas in his Treatise on the Origin of Language in 1772. He talked about how people, "through a sudden onset immediately associate with this sound that color".The first concrete term associated with chromesthesia was given by Charles-Auguste-Édouard Cornaz in an eye disease dissertation in 1848. Color blindness was a common condition known as chromatodysopsia and, since Cornaz saw chromesthesia as the opposite, he named it hyperchromatopsia or perception of too many colors.
In 1881, Eugen Bleuler and Karl Bernhard Lehmann were the first to establish six different types of what they called secondary sensations or secondary imaginations. The first, which was the most common, was sound photisms. They described it as "light, color, and form sensations which are elicited through hearing". Their book was reviewed by an Austrian newspaper, where the term colored hearing, still commonly used today to describe chromesthesia, first appeared.
Research on synesthesia in the United States began in 1892. And, since 1895, the term finally expanded from pure sound-to-color experiences to a wide range of phenomena, including grapheme-color synesthesia, mirror-touch synesthesia, and lexical-gustatory synesthesia. The rise of behaviorism between 1920 and 1940 resulted in a considerable decline in interest for synesthesia, as it was seen as "little more than a learned association". The number of scientific papers on the topic rebounded around 1980 and exponentially increased in the 21st century, where substantial progress has been made to study it empirically and understand the mechanisms at work.
Mechanisms
Synesthesia is established in early childhood, when the brain is most plastic. 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". Furthermore, it manifests as the dominant process in distributed systems, or neural networks, which are dynamic, auto-assemble and self-calibrate. That is what is understood so far, but the specific mechanisms by which synesthesia occurs are still unclear and a general consensus has not yet been reached. There are two main hypotheses: Cross-activation theory and Disinhibited feedback model.Cross-activation theory
The cross-activation theory of synesthesia was formulated by V.S. 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. This is most apparent in grapheme-color synesthesia, because the brain regions for color processing and visual word form processing are adjacent. 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.Neonates have increased connectivity between different brain areas, but these hyper-connections are cut back during development. 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. Another hypothesis is that unusual branching of neurons causes more numerous synaptic connections and cross-activation. 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.
Cross-activation may occur at the fusiform gyrus in projector synesthetes and at the angular gyrus in associator synesthetes.
One problem with the cross-activation theory is that synesthesia should be present from birth, but is only evident from mid-childhood.
Disinhibited feedback model
The disinhibited feedback model is an alternative to the cross-activation theory. 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. Disinhibited feedback could account for the fact that chromesthesia can be acquired by damage to the retino-cortical pathway or transiently induced through chemical agents, sensory deprivation, meditation, etc.In all brains, there are anatomical cross-connections where inhibition and excitation are counterbalanced. However, excitation prevails in synesthetes and this disinhibits other structures "to elicit sensory sensations in a second sensory area". One theory that explains how this occurs is neurotransmitter-mediated inhibition. Local inhibitory networks are supposed to confine cortical firing to a specific region, but it leads to a spread of cortical firing, when these networks are blocked by bicuculline.
Forward feeding connections in the brain that receive converging signals from multiple pathways are reciprocated by feedback connections. In most people, feedback connections are sufficiently inhibited to avoid synesthetic induction of a concurrent perception. In synesthetes, it is suggested that feedforward signaling in the inducer pathway could activate neurons, to which both inducer and concurrent pathways converge, and that feedback signaling is capable of propagating down the concurrent pathway to activate the concurrent representation. In this mechanism, feedforward activity from the inducer leads to feedback activation of the concurrent representation.