Synesthesia’s links with other mental disorders

Figure 1. Daniel Tammet, an autistic with synesthesia
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There is evidence showing links between synesthesia and other mental disorders, such as autism spectrum conditions (ASC). Synesthesia occurs when a stimulation of a specific sensory modality automatically affects the perception of another unstimulated modality. These altered sensory processing are also common with ASC individuals. Compared to other mental disabilities, ASC individuals demonstrate a higher sensory responsiveness [3]. Neuroimaging studies show that an autistic brain develops and functions differently than a typically developing brain, in a similar fashion to a synaesthete. For example, auditory stimuli can trigger a response in visual brain regions in autistic individuals [4]. While synesthesia typically occurs at a rate of 7.2% in normal individuals, with autistic individuals it occurs at a much higher rate of 18.9%[1]. The co-occurrence of ASC and synesthesia can be explained further through genetics as it is likely that synesthesia has a genetic component. 40-50% of synaesthetes have a first-degree relative who is also a synesthete [2]. Specific gene loci have been identified with synesthesia, such as chromosome 2, and this is an area previously linked to autism [5].

1.0 Introduction

Background information

Figure 2.The percentage of individuals with synesthesia.
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On left is autistics, on right is control. Image source: Baren-Cohen et al., 2012

A well-known case study by Baron-Cohen[1] follows a man who has a co-occurrence of synesthesia and autism spectrum disorder (ASC). While synesthesia typically occurs at a rate of 7.2% in normal individuals, with autistic individuals it occurs at a much higher rate of 18.9% [1]. Synesthetes are often characterized as having altered sensory processing, especially in the visual and auditory senses [6]. At the same time, recent research provides evidence that auditory inputs can trigger a response in the visual brain area of autistics [7]. It is suggested that ASC individuals have a developmental bias towards short-range connections [8], more than any other developmental disabilities. These short-range connections lead to a hyper connectivity in local networks - a neural condition also found in synesthetes.

What's happening in the brain?

Figure 3. Autistic brain (right) versus control(left)
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Autistic brain has more short-range connectivity, whereas control has long-range connectivity. Image source: Just et al., 2004

Because of the wide range of synesthetic couplings, the neuronal mechanisms behind synesthesia have yet to be defined. In sequence-space synesthesia, the parietal cortex plays an important role [9]. In Grapheme-colour synesthesia, it is shown that the visual and parietal areas are involved [10]. However, neuroimaging studies show that there are specific regions that apply to all forms of synesthesia. It is suggested that since the parietal cortex receives sensory information, it plays an important role in multisensory processes [11]. From FRMI studies, the parietal cortex was activated during synesthetic experiences [11].

In ASC individuals, local hyper-connectivity can develop in the presence of a deficit of long-range connectivity [19]. In figure 1, we see that normal individuals (image on left) can form long range connection between simultaneous inputs (arrows). However, in autistic individuals (image on right), there is a deficit of this long range connectivity. Instead, what is seen is an increase in localized connectivity. It is suggested that this gives rise to enhanced perception, such as the auditory and visual field, as they are next to each other [4].Therefore, it is suggested that hyperactivity in these local short-range association areas in autistics can lead to synesthesia-like symptoms.

2.0 Neural mechanisms

Enhanced Perceptual Functioning and cross-activation model

The EPF (Enhanced Perception Functioning) model was first proposed by Mottron et al., [12] to account for enhanced perception in autistics. According to the EPF model, increased short-range connectivity between local regions leads to both an increase in low-level (i.e., discrimination) and also mid-level (i.e., pattern detection) processing of those regions. This suggests that discrimination detection (i.e., pitch) and also the ability to recognize visual patterns (i.e., hyperlexia) is enhanced in autistics. For example, in a study done by Plaisted et al(1998)[13], autistics demonstrated a superior performance in discrimination tasks. They were able to discriminate between seven randomly placed circles that differed in their position on the screen. It is suggested that autistics were able to find a pattern (relational distance) between each circle, and therefore discriminate which circles were different or similar [13]. The enhanced low-level perception enables autistics to find irregularities in redundant stimuli. In autistics, this enhanced perception allows them to detect irregularities in stimuli to a higher degree, which could explain a predisposition for autistic individuals to develop synesthetic-like behaviours.

Similar to the EPF model (enhanced perception due to localized connections in autistics), an increase in short-range connectivity can also be seen in synesthetes. A “cross-activation” theory is proposed by Ramachandran and Hubbard [16]. Cross-activation suggests that a synesthete’s brain has increased associations between adjacent sensory regions [16]. By studying synesthetic grapheme-colour individuals, Ramachandran and Hubbard[16] were able to demonstrate a cross-wiring activation between two adjacent sensory brain regions: the “colour-centre” (area V4 or V8) and the grapheme area of the fusiform gyrus (see figure 4) [16]. When the grapheme area was activated, the increased brain activity in the “colour centre” of synesthetes was unique to only the “colour-centre” and not to the other brain regions, suggesting that synesthetes have increased short-range connectivity between those two local areas [16].

Figure 4. Grapheme-colour synaesthesia
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Grapheme-colour synaesthesia results from cross-activation of grapheme (in green) and colour (in red) areas, which are located adjacent to each other in the fusiform gyrus. Image adapted from Hubbard, E.M., (2007). Image source:

The research done by Ralabelhandran and Hubbard[16] and Plaisted et al[13]., demonstrate that autistics and synesthetes share a common neural mechanism: an increase in short-range connectivity between adjacent sensory regions of the brain, which may explain the co-occurrence of both disorders.

Veridical mapping

Veridical mapping is the ability to detect perceptual (i.e., colour) or structural (i.e, shape) regularities between two or more coding units (i.e, letters or numbers). For example, in Synesthesia, veridical mapping of an intrinsic perception (such as a colour) is paired with a representation (such as the pattern of a letter). Veridical mapping is an extension of the EPF model, and was initially proposed by Mottron et al., 2009[12] as an explanation to the high co-occurrence of savant syndrome in autism.

Figure 5. Example of colour-number synesthesia
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In synesthetic individuals, veridical mapping gives them enhanced processing capacities, as they are able to integrate a perception into a conscious representation. For example, in a case study done by Pring and Hermelion (2002)[14], a synesthete for number-letter associations, the individual is able to quickly learn new number-letter associations. Through veridical mapping, the individual is able to detect and manipulate an input and store it for future purposes[14].

In a case study of an autistic individual (FC) who had synesthesia, numbers were associated with a personality (i.e., nice/not nice) [20]. FC’s way of computing mental additions although atypical, resulted in accurate calculations . It is suggested that FC’s veridical mapping consists of finding patterns in numbers and then coding these patterns in order to carry out the calculation [20]. This suggests that veridical mapping plays a role in the predisposition of autistics to develop the co-occurrence of synesthesia.

A similar result was found for Tammet, an Asperger’s with synesthesia[15]. Tammet’s synesthesia allowed him to associate colours and different textures with numbers. This enabled him to map out number patterns and be able to memorize and calculate complex mathematical operations. Through veridical mapping, it is suggested that autistics are able to detect regularities in two or more similar inputs [15]. In Tammet’s case, a pattern was detected in numbers as veridical mapping enabled him to associate perceptual input (colour) with non-perceptual input (numbers)[15]. As a result, it is suggested that veridical mapping can lead to a higher predisposition of synesthesia, as there is an association between perceptual (i.e, colour) and non-perceptual (i.e., a number pattern) inputs[15]

3.0 Genetic basis

Numerous studies demonstrate that synesthesia has a familial trend. About half of synesthetes report also having a synesthete first-degree relative [1]. In a study done by Asher et al.,[5] they found that there were four different loci that played an important role in brain development, such as neuronal migration and pruning. This aligns with the cross-activation theory, where synesthesia leads to the activation of two normally segregated brain regions. These results suggest that the development of synesthesia has a genetic disposition, but because of the wide range of synesthetic experiences, the synesthetic coupling is influenced by other factors, such as the environment [2].

Recently, a whole-genome wide study report that the same gene loci that are implicated for synesthesia are the same ones that were previously linked to autism [5]. A whole-genome wide scan of auditory-visual synesthesia was linked to the chromosomes 2q24, 5q33, 6p12 and 12p12. The chromosome 2q24 was previously linked to autism [18][17], and had the highest LOD score, suggesting that it is genetically linked [5]. In addition, the genes TBR1 and SCN3A have both been implicated in autism and synesthesia.

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