Why Do Some People Associate Letters and Numbers with Colours (Synesthesia), but Others Don’t?
Imagine reading a book and instinctively seeing each letter in a specific colour, or looking at the number 5 and perceiving it as green, even though it is printed in black ink. For some people, this experience is not imaginary—it is a real, automatic perception known as synesthesia.
Synesthesia is a neurological phenomenon in which the stimulation of one sensory pathway involuntarily triggers another, leading some individuals to associate letters, numbers, sounds, or even emotions with specific colours. But why do some people experience synesthesia while others don’t? Is it genetic, learned, or a quirk of brain wiring? This article explores the science behind synesthesia, its possible causes, and how it affects perception.
What Is Synesthesia?
Synesthesia is a rare condition that affects about 1-4% of the population and comes in various forms. Some of the most common types include:
Grapheme-colour synesthesia – Letters and numbers are perceived in specific colours.
Chromesthesia – Sounds trigger the perception of colours.
Lexical-gustatory synesthesia – Words evoke taste sensations.
Spatial-sequence synesthesia – Numbers, dates, or months are visualized in spatial arrangements.
For those with synesthesia, these perceptions occur automatically and consistently. If someone sees the letter “A” as red, they will always see it that way.
What Causes Synesthesia?
1. Neurological Cross-Wiring in the Brain
One of the leading theories suggests that synesthesia occurs due to increased connectivity between sensory regions of the brain. The human brain has specialized areas for processing different types of sensory input.
The fusiform gyrus, located in the temporal lobe, is responsible for recognizing numbers, letters, and faces.
The visual cortex processes colour, shape, and spatial information.
In synesthetes, these regions appear to be more interconnected, allowing information from one sense to “spill over” into another. Research using fMRI scans has shown that when synesthetes view letters or numbers, their colour-processing areas (V4) activate, even though no colours are actually present.
This cross-activation theory suggests that in most people, these brain regions function separately, while in synesthetes, they remain more connected, creating unconscious sensory blending.
2. Genetic Influences: Is Synesthesia Inherited?
Studies suggest that synesthesia tends to run in families, indicating a possible genetic component. While no single “synesthesia gene” has been identified, research suggests that multiple genes may influence brain connectivity, increasing the likelihood of synesthetic perception.
A 2013 study by Asher et al. found that synesthesia is more common among first-degree relatives of synesthetes, suggesting a hereditary component. However, not everyone with synesthetic relatives develops synesthesia, indicating that both genetic and environmental factors likely play a role.
3. Early Childhood Learning and Experience
Another theory suggests that synesthesia might develop as a result of early childhood exposure to colours and patterns.
Many children play with colour-coded educational toys (e.g., alphabet blocks with letters in different colours).
If a child strongly associates letters or numbers with specific colours early on, this pairing might become ingrained in their neural pathways.
Over time, the associations may become automatic, leading to lifelong synesthetic perception.
This would explain why different synesthetes report different colour-letter associations—the pairings may be influenced by early life experiences rather than universal brain wiring.
4. Synesthesia and Creativity: A Cognitive Advantage?
Studies suggest that synesthetes are more likely to be artists, musicians, and writers, indicating a possible link between synesthesia and enhanced creativity.
Kandinsky, Van Gogh, and Liszt reportedly had synesthesia, experiencing colours when hearing sounds.
Synesthesia might enhance memory and imagination, as cross-sensory experiences make information more engaging and memorable.
A 2006 study found that synesthetes tend to score higher on creativity and visual-spatial tasks than non-synesthetes.
This suggests that synesthesia may not be a disorder but rather a different way of perceiving the world, possibly offering cognitive benefits.
Why Don’t Most People Experience Synesthesia?
For the majority of people, sensory processing remains compartmentalized, meaning different sensory areas of the brain function separately. Scientists believe that brain development during childhood determines whether these areas stay distinct or become cross-connected.
In most people, neural pruning occurs during early childhood, reducing unnecessary neural connections.
In synesthetes, some of these extra connections may persist, leading to cross-sensory experiences.
This means that all humans may start with synesthetic potential, but for most, the brain “unlearns” these associations over time.
If synesthesia is caused by extra neural connections, it’s possible that with training, non-synesthetes could develop synesthetic experiences, though not as naturally as those born with it.
Can Synesthesia Be Learned or Induced?
While natural synesthesia is believed to be involuntary, some research suggests it may be trainable to a certain extent.
Studies on "acquired synesthesia" show that with long-term exposure, people can develop temporary synesthetic associations.
Researchers have trained participants to associate numbers with colours, and over time, their brains started responding similarly to natural synesthetes.
Certain psychedelic substances, such as LSD and psilocybin, can temporarily create synesthetic-like experiences, further suggesting that these connections exist in everyone’s brain to some degree.
While true synesthesia cannot be easily taught, these findings suggest that the brain is more flexible than we think, and with enough exposure, it might be possible to create artificial synesthetic experiences.
Simply Put
Synesthesia is a fascinating example of how perception varies from person to person. While some experience automatic, involuntary associations between letters, numbers, colours, and sounds, others process these elements separately.
Current research suggests that synesthesia arises from a mix of genetics, brain structure, and early learning experiences. While most people do not experience it naturally, studies show that artificial synesthetic associations can be developed with training.
Ultimately, synesthesia highlights how the brain is capable of unique and unexpected connections, offering a glimpse into the diversity of human perception.
