Synesthetic experiences impact the cognitive functions to a larger extent than believed in the past [65,66▪▪,67,68]. Constitutional synesthesia predisposes to better performances in certain aspects and worse in others. Although having better color perception compared with nonsynesthetes [69,70], synesthetes present impaired motion [66▪▪,67] and speech perception . 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 . In grapheme–color and tone–color synesthetes, increased gray matter volume in the left posterior fusiform gyrus and decreased gray matter volume of the anterior part of the same gyrus and in the left MT/V5 support these hypotheses . Improved perception can occur within both inducing stimulus and concurrent domains . Memory was also found enhanced when using synesthetic percepts [25,73].
Improved performances depend partly on preconscious mechanisms, operating early in sensory processing . Thus, a recent investigation using pictures containing hidden letters found that grapheme–color projectors recognized the letters faster than nonsynesthetes; interestingly, tested individuals noted that concurrent colors were generated before conscious letter recognition [75▪]. Grapheme–color synesthesia even allows computing via synesthetically perceived colors  and as expected, emotional experience modulates synesthetes’ performances .
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 . Sound-induced photisms in visually affected people are a well recognized phenomenon . 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). We also recently observed an unusual case of a late-blind individual suffering from retinitis pigmentosa who volunteered consistently ‘seeing’ his limbs when moving them, a phenomenon presumably related to cross-modal activation of his visual cortex by proprioceptive inputs [90▪].
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 . 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 .
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 . Visual-to-auditory SSDs proceed by online translation of camera-captured views into sounds, which represent the visual features of the scene [93,94]. Users of such devices commonly claim to ‘see’ the objects figured by sounds, and therefore sensory substitution has been considered a kind of synthetic synesthesia . Interestingly, functional magnetic resonance imaging (fMRI) investigations using a visual-to-auditory SSD, both in blindfolded healthy individuals  and in congenitally blind individuals [96▪], showed activation of visual areas. 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. Indeed, intended to reliably figure the visual surrounding, percepts elicited by SSDs are elaborated, whereas regular synesthetic phenomena exhibit essentially idiosyncratic features . Further, in contrast to SSD-provoked synesthetic experiences, in developmental synesthesia, inducers do not conform to sensorimotor contingencies of the concurrent modality .
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. 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 . When presented with inducers, synesthetes exhibit a clustering pattern of activated brain areas uniting more visual regions, whereas nonsynesthetes activate particularly frontal and parietal regions [107▪▪] (Fig. 6).
Involvement of the bottom-up and top-down mechanisms has further been considered [105,108–111]. 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 .
It was suggested that congenital alterations in thalamic circuitry might be responsible for atypical cortical morphology and connections, found with different synesthetic phenotypes [64▪▪,115]. 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. Likewise, congenitally blind humans exhibit occipital cortex activation following auditory or somatosensory stimulation [96▪]. It is therefore conceivable that in developmental synesthesia, congenitally anomalous sensory input leads to abnormal synaptic pruning and differences in brain connectivity. In grapheme–color synesthetes, low white matter densities in pulvinar, medial and lateral ventral posterior nuclei, and low fractional anisotropy in medial dorsal and ventral anterior nuclei suggest a constitutional disconnection and hypoconnection between thalamus and cerebral cortex [64▪▪]. The concerned white matter tracts project to the left prefrontal cortex and bilateral temporal and posterior parietal cortex, regions that in synesthetes are distinct both in structure and function. Secondary synesthesia after thalamic stroke also support the involvement of thalamic output in synesthetic phenomena [24▪,80–83].
Over the last few years, substantial advances have been made in the understanding of synesthesia, and hence more globally in the comprehension of perception and consciousness. Fortunately, awareness of this condition in the societal environment also significantly improved, finally allowing synesthetes to feel relieved by the so badly needed recognition of their particular situation. In a near future, in addition to the expected deepening of the explorations undertaken, elaborating a more comprehensive definition of synesthesia would be welcomed. Currently used criteria are rather restrictive for a condition that is quite polymorphic in nature. This process, however, is customary in the history of medicine, which consists of initially establishing a restricted definition to encapsulate the core of the condition and then broadening it, taking into account the numerous subtle presentations encountered.
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