Synaesthesia – is everything connected?

7. April 2015
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While Synaesthesia has been known to exist for centuries, a recent surge in interest from researchers has led to a veritable boom: With national newspapers praising its potential benefits, the race is on to unravel the mysteries behind this fascinating trait.

Spotlight on Synaesthesia

Although our impression of the world around us is shaped by sensory experiences, the fact that we are equipped with the same basic perceptual tools does not necessarily mean that we process the incoming data in quite the same way. Synaesthesia – a condition in which stimulation of one sensory domain also triggers experiences in other, non-stimulated domains – is a prime example for just how much perceptual experiences can vary between individuals.

In recent years, interest in Synaesthesia has grown rather rapidly, with a whole ‘market’ developing around the subject. One can now take courses to ‘learn’ how to become a Synaesthete, and there are numerous books and websites on the topic.

However, Synaesthesia is neither merely an acquired association between a stimulus and a percept in one or more sensory domains, nor are these inducer-response pairings general or random. While more than 60 types of Synaesthesia are known, affected individuals typically exhibit one particular variant only, and the relationship between inducing stimulus and response is similarly stable and specific across the lifespan.

Mad, bad, or enlightened?

Although the growing interest in the subject might suggest that Synaesthesia is a relatively recent phenomenon, the condition has been known to exist for more than a century. In fact, the first documented case dates back to 1812 – and was published in Latin.

However, back then, Synaesthesia was far from being thought of as a ‘cognitive asset’. On the contrary – Synaesthetes who spoke about their experiences were labelled ‘mad’, a stigma whose repercussions can be felt to this day, and are reflected in the reluctance of affected individuals to speak openly about their experiences.

Nowadays, Synaesthesia is commonly divided into two categories – developmental and acquired. While acquired forms of Synaesthesia are typically associated with psychotropic drug ingestion, neurological disease and/or structural damage, developmental variants are not categorized as a disorder, but are accepted as an individual cognitive variant in the normal population.

Looking for clues in the brain and beyond

Developmental Synaesthesia is of great interest to neuroscientists, as it challenges ideas about sensory perception and how it is shaped in development. Nevertheless, it is as yet unclear why this trait exists in certain individuals, and not in others.

One factor which is almost certain to have a bearing is genetics – Synaesthesia tends to run in families. However, a genetic predisposition is not sufficient to explain how and why Synaesthesia develops in the individual. For instance, while the hereditary element may to a certain extent explain why the trait exists in a given individual, it is unlikely that the particular type of Synaesthesia he or she develops is exclusively determined by genetic factors. The fact that consistency in Synaesthetic experiences is only established once conceptual knowledge about the inducer has been acquired seems to corroborate this view. For instance, the most common type of Synaesthesia – the grapheme-colour variant – involves colour percepts which are elicited by specific letters of the alphabet. This illustrates that inducing stimuli are often culturally salient, making it highly unlikely that genetics are responsible for the development of such a stimulus-response pairing.

Do the brains of Synaesthetes differ significantly from those of other people? Developmental Synaesthesia has been associated with a number of structural and functional changes – for example, grapheme-colour Synaesthesia has been linked to increased structural connectivity in occipito-temporal and parietal regions. However, it is not clear whether these differences are the cause of synaesthetic experiences, or simply a result thereof.

Another possibility is to look for structural clues in cortical connections and morphology. It has been suggested that congenital alterations in thalamic circuitry may be involved in developmental Synaesthesia, since the maturation of primary sensory areas is highly dependent on thalamic input. Thus, congenitally anomalous sensory input could lead to abnormal synaptic pruning, resulting in connectivity changes. This theory is supported by reports of differences in connectivity between the thalamus and cerebral cortex in the brains of grapheme-colour Synaesthetes.

Becoming a Synaesthete

What, then, is acquired Synaesthesia, and is it fundamentally different from its developmental counterpart? This question is even more salient given the growing interest in ‘deliberately’ acquiring Synaesthesia.

However, what we typically refer to when speaking of ‘acquired’ Synaesthesia is a condition that reflects the consequences of neurological disorders, drug ingestion or structural damage. For instance, patients have been known to develop synaesthetic traits following thalamic stroke – a particularly intriguing finding, as thalamic connections are suspected of playing a significant role in developmental Synaesthesia, as discussed above.

Synaesthetic experiences also often occur in patients who suffer from sensory deprivation. For instance, blind patients frequently report experiencing simultaneous visual percepts – such as bright colours or ‘photisms’ – on hearing an auditory stimulus. It has been suggested that this phenomenon is most likely caused by cross-activation of the deafferented cortex.

Arts and the Synaesthete

Why do we tend to think of Synaesthesia as a desirable trait? Perhaps reason is that it is often associated with heightened creative potential. It is thought to pre-dispose towards artistic and creative occupations, and its incidence among artists and art students is notably high. But perhaps the main factor swaying public opinion in favour of Synaesthesia is the many acclaimed artists who purportedly exhibit(ed) this trait: Kandinsky experienced ‘visions’ of lines and colours which were elicited by the sound of musical instruments, while Carol Steen emphasizes the bright colours she perceives as a consequence of her Synaesthesia in her artworks. Even van Gogh is rumoured to have been a Synaesthete, based on an anecdote about some early piano lessons, and his recollections of associating the keys of the piano with perceptions of specific colours.

While many factors are still unclear – including the question whether or not Synaesthesia can be acquired deliberately through training – what we do know is that studying its underlying mechanisms may afford as a unique insight into how our senses develop, and how they shape our unique view of the world around us.

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