All About Synesthesia

Synesthesia is a neurological trait possessed by approximately 4% of the general population.

For these people, perceiving a certain stimulus (sensory or conceptual) involuntarily and consistently triggers a second perceptual experience, typically via a different sense.

There are many different types, and people who have synesthesia (called synesthetes) usually have several at once.

The most frequent type is coloured sequence synesthesia, where elements forming part of learnt sequences of concepts such as letters, numbers, days or months evoke a colour perception. Grapheme-colour synesthesia, in which each letter and/or number is associated with its corresponding colour, is the most common type of coloured sequence synesthesia. Approximately 60% of synesthetes have this kind.

Auditory-visual synesthesia is another relatively common type, as about a third of all synesthetes have it. These are the people who can “see music”: sounds, timbres, musical notes or songs trigger visual perceptions. There are various subtypes of auditory-visual synesthesia: while some people see or feel a different colour on hearing each individual musical note, others get their synesthetic perceptions from timbre, visualising different geometric shapes for example for the sound of each musical instrument, while for other synesthetes general day-to-day sounds evoke colour, shape, position and movement. In fact, no two cases are alike.

Another relatively common type of synesthesia is spatial sequence, with subtypes including calendar synesthesia, where the concept of time units – hours, days, months, years, decades and so on – are perceived visually on a kind of mental map around them, which can shift its perspective depending on the current day or month, etc.

Other types of synesthesia clearly involve two different senses, such as olfactory-visual synesthesia, where smells evoke visual perceptions (colours and/or shapes), or auditory-tactile, where hearing certain sounds (those of different musical instruments, for example) consistently triggers tactile sensations on different parts of the body.

Some synesthetes see their colours or other synesthetic concurrents physically as if they were on a “screen” in front of them, while others (the vast majority) only perceive them in the mind’s eye or “know” they are there. The first type of synesthetes are called projectors or are said to have projective synesthesia, and the latter type are associators or have associative synesthesia. In both cases the visual concurrent that is seen or perceived is called a photism.

Synesthesia always involves an inducer and a concurrent. The inducer is the specific stimulus that triggers the synesthetic experience – hearing a certain musical note, for example. The concurrent is the additional perception that is triggered, which might be feeling the sensation of the colour dark blue when that specific note is heard. Other features of synesthesia are that the experiences are consciously perceived; involuntary or automatic; idiosyncratic (the same inducers are associated with different concurrents in each synesthete); consistent; memorable; and usually pleasant, with the vast majority of synesthetes enjoying the way they perceive their world. It is also true that their form of perception seems totally normal to them and they are often deeply shocked when they realise that most people do not see the world like they do. Of course, many of them have no idea that they have such a thing as synesthesia. But it’s never too late to realise and the “eureka moment” can come at any age.

 

Are synesthetes born or made? They’re born. Synesthesia is a hereditary trait and synesthetes are often aware of other family members who have it too. In 2018 some of the genetic variants responsible for the phenomenon were identified, although it seems clear that many different genes are involved, most of them still unknown. It is also a fact that not all people born with these genes actually express them, so there are even cases of identical twins where one is a synesthete and the other is not. There are also people with a very high “synesthetic disposition” – multiple types, strongly expressed – and others with a lower disposition, who might have only one or two types and only mild experiences with them. Synesthesia is rather more common among people on the autism spectrum (it has been estimated that as compared to a frequency of around 4% in the general population, 15%-20% of those people on the spectrum could have it). Up until about 15 years ago synesthesia was thought to be much more common in women than in men, but today’s rigorous study methods have disproven this theory, discovering that it is actually equally prevalent in both sexes. Another theory that has bitten the dust is that there is a higher incidence of left-handedness in the synesthete population: it is now known that this is not the case, although there do seem to be more ambidextrous or cross-dominant people than in the general population (although still a minority).

With regard to the neurological basis for synesthesia, the main theories that have been put forward are 1. the cross-activation (“neural pruning”) theory (Ramachandran and Hubbard, 2001), 2. the disinhibited feedback theory (Neufeld et al., 2012) and, recently, 3. the stochastic resonance model (Lalwani and Brang, 2019). None of these models have yet been conclusively proven as the definitive basis, and research is ongoing. There is a brief description of each theory on this page, with links to the original studies and further reading.

On the Tree website we only talk about developmental or natural synesthesia, which is present from birth and is the result of genetic expression, although induced or acquired synesthesia also exists, producing experiences that are similar to (but not exactly the same as) those of congenital synesthesia and which can be caused by brain injury, drug use – mainly hallucinogens – or other less common causes.  

 

While the first studies on synesthetic phenomena date back to the early nineteenth century, it was only in the 1990s that more mainstream research began to be dedicated to synesthesia and the general population became more aware of it. Some relevant researchers are Richard Cytowic, Vilyanur Ramachandran, Sean Day, Julia Simner, Jamie Ward, Simon Baron-Cohen, Lawrence Marks, David Eagleman and Anton Dorso, among others. Conferences and congresses are held worldwide, bringing together researchers, synesthetes and other people with an interest in the subject, and there are an increasing number of online resources for anyone who wants to read recent research, find out basic information and make contact or compare experiences with other synesthetes.

Articles for further reading

This recent scientific article (November 2021) by Jamie Ward gives a brilliant comprehensive overview of synesthesia in the state of the art, addressing all its main aspects.  

Here's a recent general article on synesthesia I can also recommend, by Sydney Perkowitz for Nautilus magazine.

If you're looking for a readable, easy-to-understand article about synesthesia that is fun while being scientifically correct, you'll like this one by Clare Jonas on her website That Thinking Feeling.

Here's another good article that touches on many different aspects of synesthesia: Exploring Synesthesia: A Journey into Neuroscience of Perception by Shelly Jones on the site Webmedy (2023).

In this video, "Synaesthesia Masterclass", Dr. Mary Spiller, a cognitive psychologist and lecturer at the University of East London, gives a good overview of what synesthesia is and what kind of tests can be done to determine it.

... and in a recent podcast in her Let's Talk Synaesthesia series, Maike Preissing interviewed leading scientist and synesthesia researcher Julia Simner, who described what it's all about. You can listen via this link and if you click on the Transcription tag you can also see an (automatically generated) written transcription too. 

This page last updated: 05 May 2024

This page is All About Synaesthesia