This post is brought to you by a long line of google searches, links to other articles and me getting distracted by yet another cool fact and then not being able to decide how to focus this post into a reasonable length. Initially, I planned to write about synesthesia (i.e. a really cool phenomenon that I’ll explain soon but which essentially causes people to have unusual overlapping sensory experiences) and how it relates to other disorders and phenomenons linked to neuroplasticity and incorrect wiring of the brain. But then, during my research, I found an article discussing how advertisers use “culturally synesthetic” associations (the associations between various sensory stimuli (for examples, colors and flavors) that are largely common across all individuals in a society) to influence our perceptions and feelings about their products. Then, because this didn’t sound like enough neurobiology for me, I started looking up the theories behind how synesthesia actually occurs in the brain, and somehow came across the phenomenon of mirror-touch synesthesia in which a person observes another person touching something or being touched and feels like they themselves are also being touched which was super interesting and has a lot of implications for our understanding of empathy, consciousness, and the brain.

I got distracted once again and found a list on Wikipedia of all the famous people who are believed to have/have had synesthesia (the list overwhelmingly consists of artists, musicians, and composers, and includes Kanye West, Marilyn Monroe, Van Gogh, and Hans Zimmer). This got me to think about the ways in which you can use psychology and knowledge about perception to create tests that can be used to determine whether someone is an actual synesthete or not.

By this point (with 20+ tabs open and still no real focus) I realized I had found far too many sources and topics to be condensed into a single post. Long story short, what I’ve learned is that there are just too many cool things to study and learn about synesthesia that it is impossible to cover everything in a post that anyone would actually want to read in its entirety, so I’ve decided to force myself to focus on just one thing: current scientific theories about how synesthesia comes to occur in the brain (since this is supposed to be a blog about neuroscience). (Don’t worry, I’ll link all the cool articles I found at the bottom in case anyone is interested in learning more!)

First, I’ll start off by better explaining what synesthesia is and the many different ways in which it manifests. Synesthesia is a condition that causes involuntary ‘blending’ of the senses. Essentially, this means that when a synesthete experiences a particular sensory input, another experience occurs alongside in a different sense. For example, some synesthetes experience a taste in their mouth when they hear a given word, or see shapes, movement and colors when they detect certain smells. There are about as many ways in which synesthesia can manifest as there are ways to pair any sense with another (some estimates suggest as many as 80 variations). However, some forms, such as grapheme-color (letters have different color associations), chromesthesia (sounds cause colors to be experienced), and special sequence synesthesia (seeing numerical sequences such as dates or months as points in space around them) are much more common than others. Synesthesia is estimated to occur in 3.7% of the general population, although it is likely that this number is higher due to the fact that many people don’t realize they have it until someone convinces them that, for instance, other people don’t usually see a specific color when they think of each letter.

As synesthesia is a very personal and individual experience and can be difficult to observe in another person (although it is important to note that sometimes the associations can be so overwhelming and intrusive that it can be difficult for some synesthetes to function normally, especially in highly stimulating environments such as crowds or public spaces), it may leave you wondering how you could ever prove that someone does or does not have synesthesia; after all, couldn’t they just have memorized a color for each letter and be faking it? How can you determine that the associations are actually involuntary and consistent across time? There are a variety of tests that use existing knowledge about perception to test whether the person is actually having a perceptual experience or not.

For example, a common test for grapheme-color synesthesia is to present a person with a picture full of several black numbers or letters, which consist of mostly one number with a few instances of another number randomly sprinkled in (see picture to the left). A person without synesthesia sees a bunch of black shapes and has difficulty picking out which numbers are different from the rest, but a person with synesthesia will automatically associate colors with the numbers and thus will be able to quickly pick out the numbers that are ‘different colored’ (see picture to the right). This test can be effective for diagnosing people who have very strong synesthetic associations between letters/numbers and colors, but I don’t think it is necessarily as effective for people who sort of just ‘feel like’ a certain letter is supposed to be a certain color. Other more abstract forms of synesthesia such as sound-color or touch-flavor are more difficult to definitively prove, but another common method that is used to test for many types of synesthesia is to have a person report their synesthetic associations and then retest them again later to see if they are able to report the same associations. For example, in 1987, a group of researchers tested a word-color synesthete by asking her to describe the color that 100 random words triggered. A year later they randomly tested her again and she was able to report the same associations with 90% accuracy, while a group of non-synesthetic controls performing the same tests only two weeks apart only showed 20% consistency.

Ok, now that I have convinced you that synesthesia is real and can be scientifically verified in a variety of ways, you might be wondering about the underlying neurobiology of it all. The first insights came when Paulesu et al. (1995) used positron-emission tomography (a method of imaging brain activity), to show that word-color synesthetes, when presented with auditory words but not when presented with pure tones, exhibited activation in various regions of the brain not active in non-synesthetes doing this task. These regions included many involved in integration of color with shape and in verbal tasks which require attention to visual features of objects to which words refer, but notably did not include lower visual processing areas. This suggests a measurable, underlying neurobiological difference in the brains of synesthetes.

Despite evidence that it has a biological cause, there is still no definitive understanding of how synesthesia originates in the brain. There are countless theories, some involving genetics, increased myelination in certain axons (essentially causing signals in the brain to travel faster and more easily across various regions), or more molecular factors affecting inhibitory processes in some neuronal pathways, which results in those signals passing backwards from multisensory areas to single-sensory areas in a way that they are not normally allowed to. Research has shown that some types of synesthesia are associated with genetic factors, even locating specific regions of DNA that might be implicated in colored sequence synesthesia. Other research suggests that, more specifically, a genetically driven overabundance of neural connections in the brain leading to unusual wiring between multiple sensory areas might be the cause.

While there is still much we don’t understand, what we do know is that synesthesia is an extremely complex and fascinating phenomenon, the study of which has implications for a broad array of other areas of neuroscience including perception, consciousness, the effects of psychedelic drugs, disorders like schizophrenia, learning and memory, the list goes on and on. Further research is needed in order to more fully understand synesthesia, but maybe we can one day unlock the secrets to its development so that everyone can experience it. Wouldn’t you like to see beautiful colors or taste chocolate when you listen to your favorite song?

P.S. Fun fact: while talking to my roommate about all the things I was learning about synesthesia while writing this blog post, we realized that she actually probably has some kind of grapheme-color and number form synesthesia! ( “But you don’t like, see the days of the week arranged on a shape or automatically associate Friday with the color purple??”)

For your future reading pleasure:

• An explanation of the various ways used to test synesthesia, including an online test: https://synesthesia.com/blog/synesthesia-tests/
• How “cultural synesthesia” is used in advertising: https://www.theguardian.com/science/2014/apr/27/benefit-synaesthesia-brain-injury-mental-decline
• The Mirror-Touch Synesthetes Who Can Literally Feel Your Pain: https://broadly.vice.com/en_us/article/mb7ndy/the-mirror-touch-synesthetes-who-can-literally-feel-your-pain
• Evidence that Van Gogh might have had some forms of synesthesia? https://www.psychologytoday.com/us/blog/sensorium/201308/vincent-van-gogh-was-likely-synesthete
• Wikipedia’s list of people who are believed to have synesthesia (no promises on its actual accuracy): https://en.wikipedia.org/wiki/List_of_people_with_synesthesia
• Examples and prevalence of all the different types of synesthesia that have been observed: http://www.daysyn.com/Types-of-Syn.html