State of Science

Synesthesia Research

The Brain, Genetics, development and training of synesthesia.

Synesthesia has become a well researched field in science. Psychologists and neurologists are dedicated to the topic of synesthesia.

The number of scientific publications of synesthesia research has been rising constantly over the past years. We understand more and more about synesthesia. But as science goes, the more questions that are answered, the more new questions arise.

What we know for sure, synesthesia is not a disorder. Synesthesia is not a disease. It is a gift!

In this article, we summarize the current state of synesthesia research. What are the disputes and facts nowadays? And where will synesthesia science go in the future?

You will learn about the brain and synesthesia and the underlying mechanisms of synesthetic perceptions. We will ask the question: why and how does synesthesia develop. Can synesthesia be really learned, as some scientists recently proposed?


Discover Your Synesthesia


A brief history of synesthesia

The first mention of synesthesia was in the late 19th century. In the beginning of the 20th century, the interest on synesthesia faded out. Now in  the last century, research has picked it up again.

Three rough phases in synesthesia science can observed.


Scientific origins – describing the phenomenon of Synesthesia

In the beginning, around the start of the 20th century, research consisted mainly of describing the synesthetic experiences of synesthetes. Most reports were based on single cases. It was sometimes rather more poetic than scientific.


Synesthesias’ renaissance – Proving that it is real

In the late 80’s , Richard Cytowic picked up on the topic of synesthesia again. The scientific community was rather sceptical towards synesthesia at this time. It was assumed that synesthetes just had consumed drugs. Or that synesthesia is rather a product of fantasy, or mere associations.

On the merge of this millennia, synesthesia science started to use empirical experiments to study synesthesia and not to rely solely on verbal, single case reports of synesthetes. New technology arose and new neuroimaging techniques became available, allowing deeper insights into the brain. In this phase it was shown that synesthesia is a real neuropsychological phenomenon and not a mere product of fantasy.


Synesthesia 3.0 – Synesthesias’ ascent

Nowadays, as science heads forward, new questions arise. Why is there something like synesthesia? What are the benefits of synesthesia? Can Synesthesia be trained and acquired? How does Synesthesia develop? What can we learn from synesthesia in terms of perception, evolution of language and the function of the brain?


Synesthesia on the Brain – neural bases

Researchers have debated the neural mechanisms of synesthesia since the earliest days of synesthesia research. The question is how do synesthetes and non-synesthetes differ in aspect of brain structure, brain function and brain development.

In a nutshell and said simply: different areas in the brain are neurologically, more strongly connected. If one area is activated, eg due to the stimulation of music, other areas are activated too.


The different sensory areas in the brain are stronger connected in synesthetic brains.
The different sensory areas in the brain are stronger connected in synesthetic brains.


Synesthetic brains may be wired differently

In the past 15 years there has been a steady evolution of the technical possibilities for neuroimaging studies. EEG, PET, fMRI and diffusion tensor imaging are among the most prominent techniques to research the structural and functional differences of synesthetic brains and those of non-synesthetes.

Most of the studies that were done were focusing on sound -> color synesthesia and grapheme -> color synesthesia.

The prevailing synesthesia hypothesis are the disinhibited feedback theory or the cross activation model hypothesis.  If you want to learn more about those, read more here.


Neuroimaging and synesthesia

Some studies have shown that when a synesthete hears a letter or number, that the color area in the brain (the visual cortex) is activated too. For non-synesthete this region was not activated, even if there had been a synesthetic training beforehand. Color-Grapheme synesthesia may be among the most frequent type of synesthesias because the area of color recognition and the area of grapheme processing are located close together in the brain.


Grapheme synesthesia hearing a letter or a number co activated the color cortex
The presentation of a letter or a number evokes an activation of the color cortex (left).


A “hyperconnectivity” between auditory areas and the color areas has also been found. This means, the auditory cortex was stronger connected with the visual cortex for synesthetes.

During different neuroimaging studies, there has also been a region detected which plays a role in synesthetic experiences. This region is called parietal lobe. It is “responsible” for attention and and involved in the process of “perceptual binding”.

In other words, in order to perceive synesthetic experiences, the perceptions need to be “bound together” through attention in this area.

This is, in essence, what Synesthesia Meditation is about: to shift attention to synesthetic experiences and sustain it there.

Some studies propose structural differences in the brain. One study proposes that synesthetes possess a higher neural connectivity between different areas of the brain.

It was also found that synesthetes show an increased grey and white matter density in certain parts of the brain. So synesthesia seems to be a beneficial trait. Synesthesia is not a disorder.

So it looks like that there are structural differences in the brains of synesthetes.

But is this the whole story?


More research is needed

There have been many studies that discovered differences in the brain between synesthetes and non-synesthetes. But other studies did not find these differences.

There are also skeptics about this state of understanding of the brain of synesthesia. A review paper stated, that the described structural differences between two groups (synesthetes and non-synesthetes) could be related to random differences. Claims of differences in the brain of synesthetes and non-synesthetes may be based on statistical errors, questionable methodological choices or low statistical power.

Dismissing all scientific studies is kind of bold, but an interesting point of view.

Maybe synesthesia is not really a neurological phenomenon but based on associations created early in childhood?

Well, let’s take a look at the different hypotheses on how synesthesia may have formed.


Why does synesthesia develop?


Neonatal synesthesia theory

The cause of synesthesia and its developmental bases remain unclear. One of the theories explaining the cause of synesthesia is the neonatal synesthesia theory. The development of synesthesia may have happened very early in life, when we were newborns.

In early life, brain development is characterized by two different processes. First, the brain of infants creates an exuberant amount of neural connections and synapses. Second, the stimulated connections are strengthened and unused connections are be pruned away. “Use it or lose it” is a commonly saying for such neuronal processes.

There is evidence that infants perceive synesthesia-like perceptions due to the “excess” of neuronal connections between areas of the brain.


Synesthesia: a product due to decreased pruning of connections?

The “Neonatal Synesthesia Theory” states, that all infants experience synesthesia. But for non-synesthetes, those synesthesia-like connections are pruned away. In other words, some connections are eliminated as the brain grows and matures. Due to less pruning of neuronal connections, synesthesia as we know it, may form.

This process could leave some synesthetic remnants that can be found in every brain. For example, also for non-synesthetics, higher pitches are perceived brighter than lower pitches. Interestingly, people that are blind from birth also show synesthesia-like activations of the visual cortex.

So the neonatal synesthesia theory says, that for synesthetes, there was less pruning in the brain and therefore, some stronger connections remained. How does this theory fit into the genetics of synesthesia?

Genetics of synesthesia

Synesthesia runs in families and hence there is a genetic cause to it. In early days of synesthesia research, it was assumed that there are more females with synesthesia (around 8:1). Many cases of a mother inheriting synesthesia to daughters were known. But only a few reported on fathers passing it over to sons. The idea was, that “the synesthesia gene” may be connected to the X-Chromosome.

More recent studies reported that the ratio between men and women may be equal or slightly in favor of women. The foregoing assumption that females more commonly have synesthesia may have been based on bias. Women are more likely to engage with synesthesia and participate in synesthesia studies.

The X-Chromosome hypothesis was dropped. Especially after a study that genetically screened families with synesthesia and families without. There have been several genes found on different chromosomes that are candidates for the “synesthesia genes”.

Synesthesia genes are associated with brain development and autism

Interestingly, the genes that were detected are all involved in the development of the brain. Some of these genes were thought to be integral in embryo development. They also have been associated with neuronal “migration”, how the neurons form their path in the brain. These genes may also influence the neuronal differentiation. This supports the neonatal hypothesis that synesthesia develops in the early stages of life, as a baby.

Some genes that were found to potentially be causing synesthesia are associated with autism. It is not a surprise that synesthesia may be more common among  people in the autism-spectrum. Interestingly a study found increased neuronal connectivity in the brain of autistics. An increased connectivity in the brain was also found within synesthetes.

Savantism is connected to autism. Recent studies suggest that savantism is, in some form, a combination of synesthesia and autism. A famous example herefor is Daniel Tammet. He learned the Icelandic language within a week and memorized the number Pi to more than 20 000 digits.

There is a study in the pipeline that aims to decode the genetics of synesthesia. More exciting outcomes are awaited.

However, genetics may not be the only cause of synesthesia. There have been reports of monozygotic twins that differed in their synesthetic abilities.

What else could then influence the development of synesthesia?


Does synesthesia develop due to learning challenges in childhood?

In the last paragraphs we talked about the brain development of neonatal children, synesthesia on the brain and the genetics. It could be concluded that some people have “synesthesia genes” that alter the process of brain development in early days leading to “synesthetic brains” and hence synesthetic abilities. Everybody happy?

Not really. It could be more complicated. Recently, there was a novel hypothesis formulated that has a different approach.


Synaesthesia and learning are connected

Often synesthetic experiences involve perceptions that were important in childhood. Such as letters, musical notes, numbers, weekdays and years. Synesthetes have the ability to use their synesthetic experiences as mnemonic devices. In other words, they can exploit synesthetic colors while learning novel abstract categories, such as letters.

This hypothesis proposes that synesthesia arises, at least partially, because of cognitive demands while learning in childhood. So, synesthesia in adults could be a “fossilized trace” of past learning challenges “of which synesthesia was helpful”.

This hypothesis has been supported by a study which found, that 6 year old school children are less consistent with their color letter associations than those at 11 years. Synesthetic colors may not already be present in very early childhood, it needs time to develop. This is in contrast to the reports of many synesthetes claiming that synesthetic colors have been present since they can remember , since early childhood. Though now It looks that synesthetic associations need time to establish.


Learning foreign languages may trigger synesthesia

A recent study compared the prevalence of synesthesia between Canadians and Czech students. It seemed, that synesthesia was more prevalent within the Czech group. The authors argued, that this may be because Czech children need to learn secondary foreign languages (such as English) in school while the Canadians do not, in the same extent. It is concluded that the challenge of learning secondary languages may promote the development of synesthesia. But this is also kind of a shaky conclusion, given that there is only one study claiming this. Additionally there is not yet an agreement on the prevalence of synesthesia and diagnosing synesthesia is tricky.

So, is there more than just the genetic component that influences the formation of synesthesia? Is the pure learning challenge for children enough to obtain synesthetic perceptions? What about synesthetic experiences, that are not necessarily bound to learning challenges? Is it a combination of genetics and learning?

Well, I know this is kind of confusing.

On the one hand we said, synesthesia has a genetic origin that leads to increased neuronal connectivity in the brain. On the other hand, Synesthesia may just be a result of learning challenges in childhood.

Don’t these two hypotheses contradict each other?

Good thing that there is a new study that promises to bring it together.


Synesthesia: A combination of increased neuronal connectivity and learning?

This new study had an interesting approach. It did not examine synesthetes. The researchers were rather interested in the non-synesthetic relatives of synesthetes. As we know, synesthesia runs in families.


Non-synesthetic family members could achieve synesthetic experiences

Those non-synesthetic relatives had to read books over the course of several weeks where some frequent letters (such as “A” and “E”) were written always in the same color. After and before the training, the researchers compared the brain structure and performances during several tests of those relatives of synesthetes to other non-synesthetes from non-synesthetic families.

What they found was pretty interesting.

Even before the training, the non-synesthetic relatives showed an overall  increased structural connectivity of white matter in areas in the the parietal lobe. The parietal lobe is the area of attention and sensory binding.

After the training the non-synesthetic members of synesthetic families had to undergo more scanning and some stroop tests. The researchers found differences in “atypical” activation in the “angular gyrus” compared to non-synesthetes from non-synesthetic families. So the non-synesthetic relatives behave like synesthetes.


The brain structure changes with synesthetic training

The angular gyrus, the area in the brain where the differences were found, is kind of a “multi sensory hub” important for “language, number processing and spatial cognition, memory retrieval,  and attention”

Well, this is not so easy to digest. So let’s oversimplify and wrap it up.

In families with synesthetes, not everybody has synesthesia. But it looks like those non-synesthetic family members have the neuronal predisposition in the form of increased neuronal connectivity in certain areas of the brain. Synesthesia “develops” when those non-synesthetic relatives are confronted intensively with colored letters over a longer time.


Synesthesia: a mix between genetics and learning

So it seems  that synesthesia has a genetic cause and is a result of increased neuronal connectivity. But this alone does not yet develop synesthesia. Learning components seem to also be essential to develop synesthesia.




But what about non-synesthetes of non-synesthetic families?


Benefits and Synesthesia Training for non-synesthetes.

Advantages of having synesthesia are known. Another question is about if it is possible to acquire synesthesia through training. Can also non-synesthetes benefits of synesthesia?

In one study, the participants learned letter color association over a course of 9 weeks with 30 minutes per day. After this period, they went through different cognitive and synesthetic tests. Interestingly, the participants reported synesthesia-typical perceptions after the training and passed synesthesia tests. They showed cognitive improvements and scored even higher in IQ tests. This results were remarkable, though after the training stopped those synesthetic experiences faded away over time.

It has been proposed, that synesthesia training may help older people slow down the decline in cognitive functions that occur in early stages of Alzheimer. Or that it could help patients to recover from brain injuries.



Synesthesia research is evolving. It is an interesting time! In the beginning, 100 years ago, studies were based solely on single case reports. With emerging neuroimaging techniques and empirical studies about synesthesia, researchers can now show that synesthesia is a real neurological phenomenon. In the future science may ask the question more often: how can we all benefit of synesthesia?

Synesthesia may develop in early childhood during brain development, as a result of increased neuronal connectivity. But it also seems that synesthetic experiences need time to form and that learning influences this development. The formation of synesthesia may be a combination of genetics, brain development, neuronal connectivity, atypical activation of the brain and learning challenges in childhood.

Even though structural differences in the brain of synesthetes compared to non-synesthetes were found, more standardized methods and research are needed to compare results among different studies. A newer genetic screening with larger groups of synesthetes and non-synesthetes is in the pipeline. More prevalence studies are needed and more research based on how training synesthesia can be beneficial.

For more information about the science on synesthesia, check also out our book section.

Find out for yourself. It may be time to let go of these theories and take 10 minutes out of your day for a mindful synesthetic exercise to become aware of your own synesthesia.