Synaesthesia - Jamie

A Varying of Perceptions
Jamie Maguire

There is a complex and varied phenomenon experienced by some people that science is only beginning to unravel. Synaesthesia is a condition in which one sensory stimulus involuntarily triggers another, such as feeling physical shapes when eating food or seeing colors when hearing words (Hubbard & Ramachandran, 2005). In order to differentiate synaesthesia from similar drug or accident induced sensations, the phenomenon must start by the age of four, be vivid, involuntary, and separate from the imagination, as well as not be the product of a hallucination or drug (Baron-Cohen, Bor, Billington, Asher, et al, 2007). Synaesthetic experiences fluctuate from person to person, but remain constant over time for each individual. It is uncertain how common of an anomaly synaesthesia actually is. Approximations of the regularity of synaesthetes range from 1 in 20 people (Hubbard & Ramachandran, 2005) to 1 in 25,000 (Leventhall, Pelmear, & Benton, 2003), the most recent estimates tend to favor greater regularity (Steven, Blakemore, 2003).
The term “synaethesia” applies to a wide range of cognitive experiences. The most common of which is grapheme-color, in which written or imagined letters, numbers, or words evoke a specific color (Rich, Bradshaw, Mattingley, 2005). For example, the letter ‘a’ might appear to be a pale green. Other forms of synaesthesia include auditory-visual, where particular sounds cause a person to ‘see’ colors; lexical-gustatory, meaning that the hearing or reading of certain words induces a specific taste (Rich, Bradshaw, Mattingley, 2005). Still more modes of expression are touch by vision, shapes by tastes, numbers or letters having personality traits, and seeing units of time (such as days or months) in a spatial arrangement (Barnett, Finucane, Asher, Bargary, et al, 2007). Synaethesiates also vary in whether they perceive the experience as in their mind or in external space. Those that identify their experience as in their ‘mind’s eye’ are termed associators, and synaesthesiates that describe the occurrence outside of their mind in space are known as projectors. The projected phenomenon is typically considered stronger where as current data indicates that association is more common (Dixon, Smilek, & Merikle, 2004).
Francis Galton is typically credited with being the first to formally describe synaesthesia in 1881 (Asher, Lamb, Brocklebank, Cazier, et al, 2009). He recognized that the phenomenon was experienced differently by each individual. Galton also noted that the condition is strongly hereditary (Galton, 1881), as well as being more common in females than males (Galton, 1883). Attempting to explain the variability in experiences, he hypothesized that specific color associations came from early childhood when learning letters and numbers. Despite Galton’s detailed initial observations, it would be about another 100 years before more in-depth studies began (Hubbard & Ramachandran, 2005).
Since Galton’s time, more hypotheses about the physical cause of the anomaly have been formed. Two competing neurological models currently exist. The first, commonly called the pruning model, states that synesthesiates have increased neural connectivity. Alternatively, the disinhibited feedback model claims that there is augmented communication between brain regions caused by malfunctioning inhibitory processes (Hubbard, 2007).
As scientific studies of synaesthesia became more common, Galton’s observation that the trait appeared to be more prominent in females seemed to be correct (Baron-Cohen, Burt, Harrison, Goldstein, & Bolton, 1996). This in turn led to the deduction that synaesthesia might an X-linked gene (Bailey & Johnson, 1997). In fact, initial studies indicated such a gender bias that it was proposed that synaesthesia was not only dominantly X-linked, but also associated with lethality in males (Robertson & Sagiv, 2005). The strong familial passing of the trait seemed to indicate that the condition was a dominant characteristic, following simple Mendelian inheritance (Asher, Lamb, Brocklebank, Cazier, et al, 2009).
Further probing of synaesthsia has undermined most of these previous observations. Larger sample sizes and studies not based on self-referral have revealed that synaesthetic women do not out-number synaesthetic men by a statistically relevent amount (Simner, Mulvenna, Sagiv, Tskanikos, et al, 2006). Additionally, since synaethesiates have been found to be just as likely to give birth to sons as daughters, there is no support that synaesthesia causes male lethality (Ward & Simner, 2005). A study published in 2009 confirmed the first known cases of male-to-male transmission of synaesthesia (Asher, Lamb, Brocklebank, Cazier, et al, 2009). To further complicate the inheritance and expression of synaesthesia, there are rare cases in which a person born with synaesthesia who became blind early in life experienced the phenomenon as colored images when reading Braille, which may point to some environmental influences on how synaesthesia is expressed (Steven & Blakemore, 2003). There is even one documented case of monozygotic twins in which only one experiences synaesthesia (Smiek, Moffatt, Pasternak, White, et al, 2001). Clearly, the genetics involved is not as straight forward as was first believed.
Barnett et al carried out a study of 92 synaesthesiates in 53 families that explored variances of synaesthetic experiences among first-degree relatives. This study found that 27% of the families with multiple synaesthesiates had members with different types of synaesthesia, that is to say, one family member may have linguistic-color synaesthesia while another may experience word-taste. Even in those families in which only one form of synaesthesia was present, all of which were grapheme-color, the individual experience was still highly varied. Within families that had only grapheme-color synaesthesia, it was possible for various inducers, associators, and projectors to co-transpire; and family members had no increased likelihood of having the same color associations than unrelated synaesthesiates. These results seem to infer that there is a fundamental genetic mechanism for all types of synaethesia. The authors of the study suggest that what is inherited is a predisposition for synaesthesia, which may or may not be phenotypically expressed, and that the means by which the condition is expressed is dependent on discrepancies in development and early experience (2008). Such a mechanism encompasses all of the known irregularities in the passing on and expression of synaesthesia.
At this point in time, only one attempt has actually been made to map out the genes involved in synaesthesia. This investigation, led by Julian Asher, looked specifically at auditory-visual synaesthesia and found probable links on chromosome 2q24.1, 5q33.1, 6p12.3, and 12p12.1. These results indicate that synaesthesia has multiple modes of inheritance on different causative locations on human DNA. Such findings support the variety of forms of synaesthesia expression and are also consistent with other neurodevelopmental disorders that are believed to be strongly affected by both gene-gene and gene-environment interactions (Asher, Lamb, Brocklebank, Cazier, et al, 2009).
The positions of these loci are also interesting in their relationship to other neurological conditions that affect similar functions or have even been linked with synaesthesia. Chromosome 2q24.1 has been linked to autism, a disorder to which synaesthesia is sometimes reported as a symptom. On chromosome 5, the region coupled with synaesthesia includes the gene DPYSL3 which is involved in the growth of axons and differentiation of neurons. The affected area on chromosome 6 has been is believed to play a role in linguistic processing and decoding and is also associated with dyslexia. Chromosome 12p12.1 contains an N-methyl-D-aspartate (NMDA) receptor gene. NMDA receptors have been found to affect memory, with over expression enhancing learning and memory in mice and are evidenced to play a role in savantism and autism, as well as synaesthesia (Asher, Lamb, Brocklebank, Cazier, et al, 2009). The hypothesized link between synaesthesia, savantism, and autism led one synaesthetic savant to be tested for autism. From this test it was determined that he had a functioning form of autism known as Asperger Syndrome, lending credence to interrelatedness between the three conditions (Baron-Cohen, Bor, Billington, Asher, et al, 2007).
Despite the recent growth of interest, synaesthesia still requires further investigation to divulge the mysteries of its genetic components and neural mechanisms. Such inquiries will further human understanding of both normal and abnormal neurological development, including conditions such as autism, savantism (Baron-Cohen, Bor, Billington, Asher, et al, 2007), and dyslexia (Asher, Lamb, Brocklebank, Cazier, et al, 2009). It may also show how the brain incorporates sensory data into what is perceived and the role of heredity involved in this process (Asher, Lamb, Brocklebank, Cazier, et al, 2009).

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