What is synesthesia? Here I attempt to explain the phenomenon and some relevant theories in layman’s terms. So, what is it? According to some researchers:

  • “Synesthesia is a hereditary condition in which a triggering stimulus evokes the automatic, involuntary, affect-laden, and conscious perception of a sensory or conceptual property that differs from that of the trigger.” – Richard E. Cytowic Aka. The coupling of two or more modalities
    • Modality: a partial aspect of perception as a whole. One of the five senses.
    • Directionality: While commonly senses cross in only one direction, in very real cases, it can be bidirectional. People will hear colors and colors will have a sound.
  • “Synesthesia is commonly thought to be a phenomenon of fixed associations between an outside inducer and a vivid concurrent experience.” Krischner & Nikolic

What it’s not:

  • Crazy, attention-seeking, and prone to fantasy
  • Merely remembering childhood associations from coloring books or refrigerator magnets, which is why they imagined that A was read, or D was green
  • Engaging in metaphor that was no different that talking about warm or loud colors, sharp cheese, or bitter cold
  • Burned-out junkies suffering the residual effects of their assumed drug use

Fun facts:

  • 1812 – George Tobias Ludwic Sachs – First reported clinical case of synesthesia, George wrote a medical dissertation on himself claiming himself as being polymodal.
    • Polymodal: experiencing more than one type of synesthesia
  • In-depth research into the phenomenon of Synesthesia didn’t start until around 1980. It was orignially not studied because it was, at the time, no scientific, verifiable, or observable. It was too idiosyncratic and first person reports were not reliable. Through current research, however, these idiosyncratic elements can now be attributed to neural plasticity, genetic polymorphism, and environmental effects. Synesthesia also gained an iffy reputation because it was so romanticised by the arts. Behaviorism peaked between 1920 and 1940, which further pushed interest away from the phenomenon.

Knowing that synesthesia is the concurrent experience of one sense as a result of stimuli affecting another sense, we continue on to the formation:

  • First is the concept called “one shot synesthesias which are occasionally generated one off synesthetic experiences.” These one-shots share all the properties with the classical synesthetic associations except that they occur extremely rarely. They are quite literally one-shots, versus being the continuous condition that affects synesthetes throughout their lives. Through this, we can conclude that since one-off synesthetic experiences exist, synesthesia cannot be prewired at birth but instead dynamically develops after early childhood.
    • Because many synesthetic experiences deal with the processing of symbols, it only makes sense that a human would only be able to have synesthesia once the connections that allow the processing of things like language are formed.
    • This process occurs in most human infants around the age of 2.

Understanding the basics, we’ll get into the nitty gritty of things. We’ll briefly discuss one of the prominent theories behind synesthesia in this blog post: cross-activation theory. The cross-activation theory was first proposed by Ramachandran and Hubbard in a 2001 study and finds its basis in a process called synaptic pruning.

  • Synaptic pruning is exactly what its sounds like: pruning or cutting back extra synaptic connections in the brain that are no longer needed.

Cross-activation theory suggests that synesthesia occurs due to an increase in neural connection between two sensory modalities caused by the brain not pruning enough synaptic connections during early childhood to keep the different modalities entirely separate. In English: the brain doesn’t trim the neural connections between different areas of the brain that process different senses like it does in the average, neurotypical person. Their theory stems from the observation that the brain areas involved in letters and the brain areas involved in colors are actually right next to each other in the brain. Associating words and letters with colors is actually one of the most common forms of synesthesia, also called grapheme synesthesia.

I personally find this to be the most interesting and easily understood of the theories behind the condition and one that I could actually explain. I tried my best to put this into English so please let me know how I did!


  • Hubbard, E.M., Brang, D. & Ramachandran, V.S. The cross-activation theory at 10. J Neuropsychol 5, 152-77 (2011).
  • Hubbard, Edward M. “Neurocognitive Mechanisms of Synesthesia.” Neuron, Cell Press, 2 Nov. 2005,
  • Cytowic, R. E. (2018). Synesthesia. Cambridge, MA: The MIT Press.
  • Bradford, Alina. “What Is Synesthesia?” LiveScience, Purch, 18 Oct. 2017,

2 thoughts on “Synesthesia

  1. scampbellharris

    This post stuck out to me because it went hand in hand with what I talked about in my post. I discussed the idea that synesthesia is far removed from this simple idea we have of a taste reminding us of something. Thanks for outlining it so well!

  2. srussell

    I was really excited to see someone go in more depth with Synesthesia, and you did it really well! When we first talked about the topic in class, I immediately remembered something that I believe is Synesthesia for me. I have a really intense fear of spiders, and whenever I see them, I smell an EXTREMELY metallic smell, and I never knew why! I think it may be Synesthesia, though I’m not completely sure! I find it interesting that humans can connect two concepts or experiences that aren’t even related. I really like how you discussed the age at which people are able to connect symbols and ideas in this way is age 2, because I was also wondering what age people developed this ability at while I was doing research for my own article!

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