Sense Switchup: Synesthesia
Sense Switchup: Synesthesia
Joanna Cheng Thomas Jefferson High School for Science and Technology
Imagine that every word you heard had a taste. That doesn’t make any sense, does it? However, it makes perfect sense to people with lexical-gustatory synesthesia. This condition is almost like a mix of the senses of sound and taste. People with this synesthesia might taste onions upon hearing the word “society”, and the name “Philip” might trigger a taste of bitter oranges [6]. Although those of us without synesthesia might find this unusual or even doubtful, synesthesia is a real condition that has intrigued scientists for centuries.
About 4% of people have synesthesia, a neurological condition in which stimulation of one sense leads to experiences in an unrelated sense [5]. There are over 100 recorded types of synesthesia, including lexical-gustatory synesthesia, in which the stimulation of the sense of sound leads to taste experiences. The most common variants of the condition are grapheme-color, in which letters and numbers are associated with colors, and color-auditory, in which sounds are associated with colors. Some synesthetes may see sunny yellows when presented with the letter C or pale blues with splashes of silver upon hearing their brother’s voice. Other rarer types of synesthesia include mirror-touch synesthesia, which causes people to feel sensations when they see other people being touched. Some synesthetes even assign specific personalities to numbers [6].
There are three features of synesthesia that distinguish it from simply imagination or visualization: automaticity, reliability, and consistency. First of all, synesthesia is automatic and cannot be controlled by the individual, unlike imagination. Secondly, a synesthetic response is reliable, meaning that it is sure to occur when a synesthete is presented with the stimulus. Even though certain memories in non-synesthetes can cause mental images or responses, these connections can fade or change with age, unlike synesthesia. Finally, synesthesia is consistent, meaning that a stimulus always produces the same response. A color-auditory synesthete who associates middle C with scarlet will likely do so for their entire life [4].
Underneath the large umbrella term of synesthesia, there are several general categories of synesthetes. Scientists have separated synesthetes into projectors and associators. Projectors see their synesthetic responses out in space, while associators experience their responses in their “mind’s eye”. For example, grapheme-color projectors will see the color of the letter projected onto the page, but associators will only see it in their head [4]. There are also lower synesthetes and higher synesthetes. In lower synesthesia, low-level, perceptual properties of the stimulus cause the sensations. For instance, a grapheme-color synesthete might not see the same color for an italicized and boldfaced ‘a’. However, in higher synesthesia, higher-level, conceptual properties produce the response, so any letter with the concept of ‘a’, whether bold, italicized, or capital, would cause the synesthete to see the same color [5].
Synesthesia encompasses a wide range of conditions, but many people who believe they are synesthetes are actually not, causing wide ranges in the estimated occurrence of synesthesia among humans. Scientists have developed various tests to confirm whether someone has synesthesia. The test-retest procedure is the most common one, in which a researcher tests an individual’s responses to certain stimuli and then retests the individual without warning a year later. If the responses largely match, the individual most likely has synesthesia. Synesthetes retested a year after the initial test had a 92.3% match in their responses, while non-synesthetes tested a week after the initial test scored a 37.6% match, establishing the consistency of synesthesia [4].
Another test to determine synesthetes involves a variation of the Stroop phenomenon, which describes a delay when people try to name the color of text that spells out a different color. For example, it is difficult for people to name the color blue when looking at the word “GREEN” written in blue ink. However, scientists slightly modified this test for synesthetes by presenting a stimulus with and without the synesthetically associated condition. For example, if a synesthete reported seeing purple when presented with the letter M, they would be presented with a purple M (the congruent condition) and a differently colored M (the incongruent condition) and asked to recognize the color of the letter. The results of the experiment showed that synesthetes responded more quickly in congruent trials than incongruent trials, while there was no time difference in non-synesthetes. This test confirms automaticity in synesthetes. Synesthetes responded more slowly in incongruent trials because the automatic synesthetic association in their brains interfered with the task [4].
Although this may all seem foreign if one is a non-synesthete, people without synesthesia also experience such seemingly arbitrary connections. For example, when asked to assign the names “Bouba” and “Kiki” to two shapes, one rounded and one jagged, Bouba was most often associated with the round shape and Kiki with the jagged one. Tests with non-synesthetes also discovered that subjects most often associated high pitched sounds with smaller objects, brighter and lighter colors, and high elevation. These associations are called crossmodal correspondences and are a weak form of synesthesia. The main difference between crossmodal correspondences and synesthesia is that crossmodal correspondences are a conceptual understanding that two things are associated, while synesthesia is a conscious experience of the association. Synesthesia is also more specific, while crossmodal correspondences are rather general [2].
The complexity of synesthetic connections has bewildered scientists for centuries, and the neural basis of the condition has remained largely debated. The Infantile Synesthesia Hypothesis states that all infants are synesthetes, but lose this ability as they age. There are several points of evidence for this theory, the first being that high neural and synaptic connectivity between sensory cortices in the brain of infants is inhibited or removed during development. Also, sensory cortices are less specialized in infants, so the senses are less separated from each other. This hypothesis predicts that synesthetes may possess genetic factors that can prevent the increased connectivity in the brain from being inhibited or removed during development.
An ongoing debate today is whether synesthesia occurs because of reduced inhibition of neural pathways or increased connectivity in the brain. To support the connectivity hypothesis, scientists have found that synesthetes have a larger gray matter density and, therefore, a greater number of neurons in certain parts of their brain [6]. For example, grapheme-color synesthetes have elevated structural connectivity in areas of the brain involved in color processing. However, scientists have also discovered that there is evidence of auditory-visual connections in infants, which are inhibited at the age of about one and a half years old [1]. This discovery, along with the finding that certain drugs can induce temporary synesthesia, support the reduced inhibition model instead. Since the synesthesia experience occurs soon after drug intake, the short time frame does not allow for new pathways to be made in the brain as in the increased connectivity model. Instead, these drugs likely uninhibit normally inhibited pathways, allowing more communication between sensory cortices [5].
Although the above theories discuss the development of synesthesia from birth, not all synesthetes obtain the condition from a young age. Developmental synesthesia, the most common form, occurs early on in childhood. Acquired synesthesia, which occurs unnaturally, develops as a result of brain injury or some other disruption to the brain. Scientists have recorded cases of acquired auditory-visual synesthesia in people with brain damage, likely due to deafferentation, or disrupted connections, between sensory cortices in the brain. Induced synesthesia is temporary and caused by hallucinogenic drugs. However, these drugs do not always induce synesthesia, and the experiences are highly variable. Scientists believe that the mechanism behind drug-induced synesthesia is also deafferentation [1].
Synesthesia brings with it many benefits, the main one being an increased memory, since synesthetes have two sets of cues available to help them memorize. For example, grapheme-color synesthetes can memorize the actual words and numbers themselves as well as the colors induced by the text [6]. There was a case of a five-fold synesthete, whose senses were all connected, that had nearly limitless memory and could memorize long speeches and complex equations with little loss of memory. However, if grapheme-color synesthetes memorize sequences where the colors of the letters are incongruent to their own synesthesia, then there is a drop in performance. In addition, synesthetes only perform better when they memorize stimuli that induce their synesthesia. For example, color-auditory synesthetes would not perform better than non-synesthetes in memorizing letters. [3]. However, synesthesia can pose problems. Lexical-gustatory synesthetes, who experience tastes when hearing words, may have attention issues when surrounded by a lot of noise. Furthermore, colored alphabets used in classrooms may provide learning problems for children with grapheme-color synesthesia [5].
The full truth behind this complex sense-switchup, which has intrigued humans for centuries, has yet to be unveiled. With tasteful words, colorful sounds, and personified digits, the large umbrella of synesthesia covers the experiences of many people, and its mechanisms promise to unlock deep secrets of how our brain processes senses.
References
[1] Afra, P. (2015). Auditory synesthesias. In Handbook of Clinical Neurology (Vol. 129, pp. 389-407). Elsevier. https://doi.org/10.1016/B978-0-444-62630-1.00022-6 (Original work published 2015)
[2] Brang, D., & Ramachandran, V. S. (2020). How do crossmodal correspondences and multisensory processes relate to synesthesia? In K. Sathian & V. S. Ramachandran (Eds.), Multisensory perception (pp. 259-281). Elsevier. https://doi.org/10.1016/C2016-0-03465-4
[3] Gosavi, R. S., & Hubbard, E. M. (2020). How synesthesia may lead to enhanced memory. In K. Sathian & V. S. Ramachandran (Eds.), Multisensory perception (pp. 301-317). Elsevier. https://doi.org/10.1016/C2016-0-03465-4
[4] Mylopoulos, M. I., & Ro, T. (2013). Synesthesia: A colorful word with a touching sound? Frontiers in Psychology, 4. https://doi.org/10.3389/fpsyg.2013.00763
[5] Simner, J. (2012). Synesthesia. In V. S. Ramachandran (Ed.), Encyclopedia of human behavior (2nd ed., pp. 571-577). Elsevier. https://doi.org/10.1016/B978-0-12-809324-5.06589-5
[6] Ward, J., & Simner, J. (2020). Synesthesia: The current state of the field. In K. Sathian & V. S. Ramachandran (Eds.), Multisensory perception (pp. 283-300). Elsevier. https://doi.org/10.1016/C2016-0-03465-4
