Synesthesia is an extraordinary perceptual phenomenon, in which individuals experience unusual percepts elicited by the activation of an unrelated sensory modality or by a cognitive process.
Emotional reactions are commonly associated. The condition prompted philosophical debates on the nature of perception and impacted the course of art history. It recently generated a considerable interest among neuroscientists, but its clinical significance apparently remains underevaluated. This review focuses on the recent studies regarding variants of color synesthesia, the commonest form of the condition. Synesthesia is commonly classified as developmental and acquired.
Developmental forms predispose to changes in primary sensory processing and cognitive functions, usually with better performances in certain aspects and worse in others, and to heightened creativity. Acquired forms of synesthesia commonly arise from drug ingestion or neurological disorders, including thalamic lesions and sensory deprivation e. Cerebral exploration using structural and functional imaging has demonstrated distinct patterns in cortical activation and brain connectivity for controls and synesthetes.
Decoding the genetics of synaesthesia
Artworks of affected painters are most illustrative of the nature of synesthetic experiences. Results of the recent investigations on synesthesia offered a remarkable insight into the mechanisms of perception, emotion and consciousness, and deserve attention both from neuroscientists and from clinicians. Synesthesia is an extraordinary perceptual phenomenon, in which the world is experienced in unusual ways.
In this condition, a particular stimulation in a given sensory modality e. An illustrative presentation of the condition would be that of a given person in whom hearing the sound of a trumpet consistently elicits the vision of brightly colored triangles dancing in front of his eyes, in association with a sensation of pressure on his arms, letting him feel uncomfortable to sit still. Synesthetic experiences have had over the centuries far-reaching sociocultural implications.
They prompted philosophical debates on the nature of perception, consciousness and even talent and creativity, and significantly impacted the course of art history, notably at the turn of the 20th Century [ 3 — 6 ]. Moreover, favored by the emergence of sophisticated tools for functional brain exploration, they have generated a considerable interest among neuroscientists [ 6 , 7 ]. Clinical significance of synesthesia, however, is still largely underevaluated.
Although some synesthetic phenomena express the presence of a disease, developmental synesthesia as a rule is considered an individual cognitive variant in the normal population [ 8 ].
Unfortunately, the astonishing features of these percepts have too often led the entourage of affected persons, including physicians, to wrongfully consider them as confabulators, drug users, or schizophrenics [ 7 ].
In this regard, the following history reported by Vincent Van Gogh is representative. While in the painter was taking piano lessons, his teacher noticed that he was continually relating the sounds of the piano keys with specific colors; considering then that his pupil was insane, the teacher sent him away [ 9 ].
It is therefore understandable that synesthetes i. For that very reason, scientific studies probably underestimate synesthesia prevalence in the general population. Developmental synesthesia appears to be the most frequent type of this condition, with a 4. It can run in families and demonstrate Mendelian transmission [ 14 ].
Different forms of synesthesia can be observed in the same person or in the same family [ 15 ]. The condition is occasionally associated with autism spectrum disorders, like Asperger syndrome [ 16 ]. The following criteria have been proposed to help establishing a diagnosis of developmental synesthesia: induced percepts should be elicited by a specific stimulus, they should be automatically generated, and typically have percept-like qualities [ 8 , 17 , 18 ]. Usually, pairings of inducers and concurrents are specific i.
Acquired forms of synesthesia have also been reported, essentially associated with neurologic disorders or following psychotropic drug ingestion [ 20 — 23 ].
So far, over 60 types of synesthetic phenomena have been described. The apparently most common form with a The second most prevalent form is time unit e.
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Inducers and concurrents also include smells, tastes, temperatures, personalities and emotions [ 26 ], and can be multiple during a single synesthetic experience. Thus, percepts induced by grapheme—color synesthesia are occasionally accompanied by shape, texture, movement features, and even nonvisual percepts such as smells and tastes, particularly emotions [ 31 , 32 ].
Synesthetic colors generated in grapheme—color synesthesia are determined by systematic rules rather than randomly occurring, and based on the psycholinguistic mechanisms of language processing. Visual segregation test demonstrating improved digit identification performances by grapheme—color synesthetes. Adapted with permission [ 27 ]. Reproduced with permission.
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It is characterized by the generation of specific colored percepts upon conceptual representation of swimming in a particular style i. This phenomenon could be triggered by either presenting a picture of a swimming individual or asking the tested individual to think about a given swimming style. It was speculated that this synesthetic type was caused by overactivity in the mirror neuron system responding to the specific representation [ 37 ].
Painters commonly demonstrate unique skills in the observation of visual phenomena, in which depiction offers an invaluable source of information for neuroscientists investigating visual function in health and disease [ 42 — 46 ]. Among this population, the prevalence of synesthesia was found higher than in general population; in addition, their percepts are frequently represented in their artworks [ 47 ]. Thus, Kandinsky's nonfigurative paintings and theory of synesthesia [ 53 ], prompted by his experience of extraordinary visions of lines and colors elicited by the sound of musical instruments, paved the way to abstract art and thus marked a turning point in history of art [ 47 ].
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A recent analysis of Kandinsky's works using the Implicit Association Test found no implicit association between the original color—form combinations, and authors concluded that these are probably not a universal property of the visual system [ 44 ]. Most informative indications on the character of percepts commonly observed by synesthetic painters, as well as on the compulsive manner they depict their visions, were provided by Carol Steen, a remarkable synesthetic painter [ 54 ].
To depict the brightness of colors perceived, synesthetic painters reportedly often apply with speed, pure, unmixed oil paint, or watercolor straight from the tube.
Faithfully representing their perceptions may require breaking some long-standing rules, a feature that — as underlined by Carol Steen — characterizes modern art. The artist also specified that her visions were never representational nor figurative.
Emotional reactions play a prominent role in synesthetic processes. They are commonly experienced in such conditions [ 32 , 55 ], acting as either inducer, concurrent, or modulator [ 32 , 56 ]. Some synesthetes indicate that all disagreeable events generate same color, specific for the given individual.
Saturation of evoked colors is susceptible to be altered by mood [ 32 ]. These perceptual phenomena mainly consist of colored shapes, less commonly of flavors, smells, sounds, or temperatures, and are associated with a higher degree of trance and loss of environmental boundaries [ 63 ]. Over decades, Chagall repeatedly depicted using intense green or blue colors, the faces of central characters in his paintings [ 58 ]. This most probably reflected a variant of personality—color synesthesia.
For comments, see the legend of Fig. Cerebral structures processing emotion are altered in developmental synesthestes. Acquired cerebral disorders are also susceptible to cause emotional synesthetic percepts. Occurrence, expression, and the underlying mechanisms of affect-related forms of synesthesia have recently been reconsidered [ 56 ].
Constitutional synesthesia predisposes to better performances in certain aspects and worse in others.
Speech perception deficit could be a consequence of the impaired motion perception, namely the biological movement of lips or of a much wider deficit in multisensory integration [ 71 ]. Improved perception can occur within both inducing stimulus and concurrent domains [ 68 ]. Memory was also found enhanced when using synesthetic percepts [ 25 , 73 ]. Improved performances depend partly on preconscious mechanisms, operating early in sensory processing [ 74 ].
Thalamic insult may induce large-scale reorganization of the brain, modify the balance between excitatory and inhibitory connections in high-order visual areas, and favor the development of synesthesia [ 80 ]. Sensory deprivation favors the occurrence of synesthetic phenomena.
With blind people, nonvisual stimuli tend to elicit various percepts in the suppressed sensory modality, including colored photisms [ 84 , 85 ] presumably by cross-modal activation of the deafferented cortex [ 86 ].
Sound-induced photisms in visually affected people are a well recognized phenomenon [ 87 ]. Six late-blind individuals were recently reported experiencing colored phenomena when hearing or thinking about letters, numbers, and time-related terms [ 88 , 89 ].
In one of these individuals, touching Braille characters induced colored photisms. A patient of ours, blinded by bilateral arteritic anterior ischemic optic neuropathy, reported perceiving colored photisms when brushing his teeth or hearing a hand clap personal observation.
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Brain lesions disrupting canonical networks and sensory input to associative areas are also susceptible to induce synesthetic-like hallucinatory syndromes. A right monophtalm patient with right parosmia reported intricate visual and olfactory hallucinations following a right occipitotemporal stroke [ 91 ].
The patient described seeing people with strong odors. The presumed mechanism of these hallucinations was the desinhibition of the connections from the visual association areas to perirhinal and parahippocampal gyri [ 92 ]. Sensory substitution devices SSDs have been developed to provide blind individuals with information on their visual surrounding.
They convey visual information through another sensory modality, like audition [ 93 ]. Visual-to-auditory SSDs proceed by online translation of camera-captured views into sounds, which represent the visual features of the scene [ 93 , 94 ].
Whether — and to what extent — SSD users also perceive the auditory stimulus as a sound is debated [ 97 , 98 ]. Sensory substitution, however, differs in some respect from the naturally occurring synesthesia.
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Indeed, intended to reliably figure the visual surrounding, percepts elicited by SSDs are elaborated, whereas regular synesthetic phenomena exhibit essentially idiosyncratic features [ 8 ]. Further, in contrast to SSD-provoked synesthetic experiences, in developmental synesthesia, inducers do not conform to sensorimotor contingencies of the concurrent modality [ 98 ]. Assumptions have been made on the mechanisms underlying synesthesia, including hyperconnectivity between cortical areas [ 99 ], reduced level of feed-back from inhibitory cerebral structures [ 2 ], learned association in early life [ ], and a normal perceptual mechanism incompletely suppressed in synesthetes [ 17 ].
Neurocognitive models have been elaborated [ — ]. In recent years, brain-imaging studies brought further evidence that synesthetes connect more inside and between sensory regions and less with remote areas, especially the frontal cortex.
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Indeed, these individuals exhibit increased intranetwork connectivity in medial visual, auditory and intraparietal networks, and internetworks connectivity between the medial and lateral visual networks, the right frontoparietal network and between the lateral visual and auditory networks.
In contrast, nonsynesthetes have more intranetwork connections within frontoparietal network [ ]. Cerebral activation revealing distinct activity patterns for controls and synesthetes during grapheme and pseudo-grapheme presentation. Synesthetes demonstrate the most significant activity in the bilateral posterior inferior temporal gyri.
Involvement of the bottom-up and top-down mechanisms has further been considered [ , — ]. The bottom-up model stipulates that the concurrent representation is prompted by the inducer representation via over represented and overactive horizontal connections, whereas the top-down model proposes that the inducer stimulates the concurrent percept via an input from a convergent, higher order integrator [ 2 ]. Using dynamic causal modeling, Van Leeuwen et al.
Reduction of long-range couplings in the theta frequency band could facilitate the top-down feedback. Cytoarchitectonic maturation of the primary sensory areas and the development of their specific connections are highly dependent on the thalamic input [ ]. Enucleation in prenatal macaque drastically alters the equivalents of V1 and V2 visual cortices, and induces rich noncanonical connections with somatosensory, auditory, and frontal areas [ ], resembling transient fetal connections [ ].
Thus, the visual cortex ends up treating other types of information.