Synesthesia is a neurological with a long history of research, but there is always more to discover. Synesthesia is a condition where multiple senses are interconnected more than usual. This means that a stimulus can trigger a response in another sense than what it would usually trigger. For example, sound-color synesthesia could cause someone to see yellow with ragged green edges when listening to a jazzy trumpet, or perhaps someone with grapheme-color synesthesia might argue fiercely that the number 7 is blue while 2 is orange. It could even be seeing shapes when smelling certain scents or feeling different sensations when hearing certain sounds. Synesthesia somehow makes connections where, normally, there aren’t any. With more recent research, old hypothesis for how synesthesia worked are being proven, disproven, or modified.

Theories

While synesthesia has had a fairly long history of acknowledgement and research, there have been periods where scientists have been less likely to continue research due to current scientific movements such as the rise of behaviorism between 1920s and 1940s, where the approach to psychological science became more observation-based rather than introspection-based. Since synesthesia is a very personal experience with no outward proof of its existence, it was largely dismissed. However, there have been long-standing theories on how and why synesthesia works for decades, with different ways of explaining how two senses become linked.

To explain why certain stimuli affect more than just their given sense, one theory suggests that increased neural connectivity between brain regions. These extra connections are thought to come from a lack of “neural pruning.” During an infant’s development in gestation, the infant’s brain has more connections between regions of the brain that are then “pruned” (the synapses are lost) due to less traffic of those pathways. This allows the brain to work efficiently through major pathways. However, this pruning may be less effective or perhaps non-existent in synesthetes, meaning that the extra connections cause a stimuli to affect multiple parts of the brain and create multiple sensations.

The neural pruning theory is closely related to another important theory, which is of the genetics behind synesthesia. Since synesthesia has been shown to run in families, there is thought to be a genetic component. This could very well go along with the neural pruning theory, as it could be a quirk of genetics which prevents the usual pruning from happening. For years, however, it was difficult to find any proof for this theory as no single genes which could cause the condition emerged. Despite this, there has been new discoveries on the genetic frontier. A research team from Max Planck Institute for Psycholinguistics selectively looked for genetic variants in families of synesthetes among only the genetic code that codes for proteins. This showed 37 genes that could predict a family member’s synesthesia, but the families tested did not share the same variants. There was no single “synesthesia gene” that would cause directly cause synesthesia. Interestingly, 6 of the 37 identified genes were to do with the development of neural connections in both visual and auditory portions of the brain. This discovery has far-reaching possibilities not just for synesthesia, but for other types of neurodivergence such as autism which are thought to come from similar causes of hyperconnectivity.

On the other hand, there are theories that suggest that synesthesia is more nurture than nature, even if genetics might have a role. This suggests that synesthesia might be like perfect pitch: a person with perfect pitch must have the genetic component, but early musical training and exposure plays a huge role in expressing the trait. In this theory, synesthesia is thought to occur due to childhood connections between sounds, sequences, colors, and sensations. This may be the reason why the letters A, B, and C are commonly thought of as red, blue, and yellow respectively. If these are commonly put together, a child might make these early connections and extend them further than others might. This is especially significant for grapheme-color synesthesia. Letters, numbers, months, and other sequences have to be taught, which makes it seem likely that synesthesia may come from early childhood associations. Other forms of synesthesia might seem more ingrained in a person’s experience, as these do not necessarily require teaching of human-made sequences. While synesthesia could be heavily influenced by nurture and childhood association, the hereditary nature does point to at least some genetic component in the mixture of why it occurs.

Points of Difficulty

There are several factors that make synesthesia continue to be mysterious. The vast range of types, all of which could have different genetic or associative causes, means that it might be harder to pinpoint direct factors in which people get synesthesia. Each type of synesthesia might have it’s own blend of each theory as its cause, since each one works differently and has different amounts of early learning required. Synesthesia is also not a widely known condition, and it can be difficult to gauge how common it really is. Estimates range from 1 in 20 to 1 in 25,000, and there are conflicting studies on whether it is more common in women or not. An under-reported male population of synesthetes could make it even harder to pinpoint the exact nature of the condition. Many synesthetes might not even realize that how they see the world is different. There are many personal stories of synesthetes who were shocked to find that other people did not experience the same connections that they did.

Like any other mysterious human condition, synesthesia remains largely unknown in its mechanisms. Still, with research technology constantly improving, it may not be too far in the future when we finally understand how synesthesia works and perhaps even why it continues to exist in our changing world.