High School High School Mind, Brain, and Behavior

Color My Words: What is Synesthesia?

You all have a friend that organizes notes with different-colored pens and makes you feel embarrassed of your “scholar scribbles.” However, what if I tell you people can be that organized with colors without even thinking about it. I know that sounds odd, but hang on.

A representation of how I wish my notes could look like

Synesthesia (commonly known as “a union of the senses”) is any combination of two senses, including rarely mentioned ones like thermoception and chronoception, that combine to form a cross-modal perception of stimuli. In other words, synesthetes perceive stimuli with two non-related senses at the same time. There are approximately 80 or more different types of synesthesia, and some forms of synesthesia are more common than others. For brevity, I will refer to the most common form of synesthesia: ‘grapheme-color synesthesia,’ which associates letters and symbols with colors. (1). Since it is the most common type of synesthesia, data derived from grapheme-color synesthesia studies are commonly used to characterize most other forms of synesthesia.

There are two types of synesthetes present across all the 80 forms of synesthesia. The first type is the ‘projector’ synesthetes, which experiences cross-modal stimuli in the outside world. Projectors synesthetes can be subdivided into two categories: “surface-projectors,” who perceives cross-modal stimuli onto the inducing stimulus, and “space-projectors,” who perceives cross-modal stimuli onto an externalized space. The second type is ‘associative’ synesthetes, which experiences cross-modal stimuli inside their minds. Associative synesthetes can also be subdivided into two categories: “see-associators,” who perceives cross-modal stimuli inside their minds, and “know-associators,” who knows the nature of their cross-modal stimulus without having to project it.

Grapheme-color synesthetes, for instance, commonly perceive the letter “A” being red. If exposed to the same stimuli (let’s assume it is the letter “A” written in black ink), both types of projector synesthetes would see the color red. Surface-projectors synesthetes would see red inside the letter “A,” but space-projectors would see red popping out of the letter and into other spaces. If exposed to that same stimuli, both types of associative synesthetes would strongly associate the letter “A” with the color red, but they would not “see” the color red. See-associators would perceive the color red inside their minds, while know-associators would know the letter “A” is inherently red for them.
Associative synesthetes do not perceive stimuli the same way projectors synesthetes do, but that does not make their synesthesia fictitious. In fact, these differences are related to the ability to create a mental picture. Visual imagery varies between individuals, so the better an individual is at forming a mental picture, the more vivid their perception of cross-modal stimulus will be. Accordingly, a vivid perception correlates with projector synesthetes, while a vague perception correlates with associative synesthetes (2).


Now, let’s explain: how can people have mental colored pens?

The first type of synesthesia ever studied was “grapheme-color synesthesia.” It was first investigated by Francis Galton in 1880 when he tried to isolate its genetic component. Galton’s early studies’ data displayed synesthesia were more common in women than in men, which led researchers to believe the “synesthesia gene” might be carried in the X-Chromosome. However,  studies later confirmed synesthesia has no gender bias and that it is a recessive trait, which proves synesthesia cannot be present solely in the X-Chromosome. Statistics show synesthesia occurs in 1 of 2000 people, but this is sometimes considered an underestimate because 1 in 20 people experience grapheme-color synesthesia. To date, though, no specific loci have been associated with any kind of synesthesia.

Multiple studies of synesthesia took place over the course of the nineteenth century; however, a lack of known genetic components led most psychologists and neurologists to discredit synesthesia until the 1980’s, when a series of experiments proved it is a real sensory experience. In these experiments, scientists first tried to determine if there is a delayed reaction to external stimuli in synesthetes. For this, scientists used a “standard stroop paradigm,” which tests the ability of an individual to shift usual perceptions in favor of unusual ones. Scientists wrote the word “red” in red and green ink (the red pen version is concordant, but the green pen version is contradictory), and synesthetes were asked to say the color of the ink rather than the word red.

As expected, synesthetes are much slower than average to mention the ink color when it is contradictory because of the different colors they perceive coming from the letters “r-e-d.” For instance, synesthetes would commonly see the word red as a mix of the colors red (for r), yellow (for e), and gray (for d). When the word red is written in red ink, synesthetes have less trouble identifying it (even though they are still slower than average) because it is concordant and their brains process that easier. However, once the word red is written in green ink, synesthetes take even slower to identify the color green because it remains hidden under the colors red, yellow, and gray. This shows that synesthesia is involuntary and instantaneous.

In a separate study, synesthetes were shown hidden characters in a puzzle. Because synesthetes perceive different colors with different characters, five out of six synesthetes were able to identify the hidden character quicker than the six people in the control group. Alternatively, synesthetes have trouble identifying hidden characters when they perceive them to be the same color as the background they are placed in. This ultimately shows grapheme-color synesthesia is not only real, but it can also provide a cognitive advantage or disadvantage.

An example of a “Standard Stroop Paradigm.”After the study, scientists acknowledged synesthetes perceive letters and numbers different than most people; however, they were skeptical about the synesthetes’ perception memorability. Therefore, scientists asked synesthetes to describe the colors they associate with different letters of the alphabet every year. Surprisingly, 90% of individuals were consistent with their perceptions. But even more surprisingly, the colors synesthetes described were extremely specific. Because there are not many “basic” colors, synesthetes perceive different characters with different shades of colors that most people would name as the same color (like cobalt and blue). However, synesthetes are very specific with the shades of color they describe, marking an outstanding difference in their perception of each character (3).

Surprisingly, capitalization and fonts influence the colors synesthetes perceive. Synesthetes perceive lowercase letters as less saturated than uppercase letters. They also perceive vivid colors from a prototypical font (Times New Roman and Arial) but perceive stronger colors from “weird” fonts (Gothic). It is theorized that a capital letter or an unusual font hyper-stimulates graphene neurons because they take more energy to visualize and understand, leading to a stronger cross-modal stimuli perception.
Synesthetic associations are consistent in an individual, but not within groups of individuals, as different synesthetes perceive every character differently from each other. However, data shows synesthetic associations are not completely sporadic. For instance, the letter “A” is most likely to be red rather than blue or yellow among synesthetes. This is because our mirror neurons are activated when reading the letter “A,” leading us to “hear” the letter “A” in our minds; this process might also activate neurons that perceive colors with long wavelengths, such as red, inside the brains of synesthetes. It is theorized this occurs in the “number graphene area” and the “Color V4 Area” (both in the temporal and occipital lobe), which overlap in synesthetes’ brains when reading. Curiously, synesthetes only perceive the alphabet of their first language to be colored, since alphabets of languages learned later in life are mapped in a different region of the brain (4).

Grapheme-Color Synesthesia occurs in the overlap between the green and red areas.

In the footnotes, there is the link to a study done by Sean Day, a synesthesia researcher, in collaboration with MIT that proves cross-modal perceptions of grapheme-color synesthetes are not completely sporadic (5).
Different types of synesthesias have different cognitive advantages and disadvantages. In parts two and three of this article, the second and third most common types of synesthesia (“Space Sequence Synesthesia” and “Chromesthesia,” respectively) will be covered.

Sources and Footnotes:
(3) https://www.sciencedirect.com/science/article/pii/S0896627305008354
(5) http://web.mit.edu/synesthesia/www/trends.html

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