Life commonly teaches us that there are five senses: vision, hearing, touch, taste and smell. You’ve always known this. The task of differentiating these senses seems relatively easy. If you are looking for a principled way to set vision apart from hearing and smell, you might point to the different qualitative experiences these senses give rise to. The way the bacon looks has a certain qualitative feeling—it looks brown, burnt and crispy. The way it sounds has a different qualitative feel—it sizzles and pops as it cooks in the pan. The way it smells is characteristically bacon-y. Alternatively, you might differentiate the senses by pointing to the proper objects of perception. Vision deals with a certain spectrum of colored light. Hearing deals with vibrating sound waves. Smells deal with chemical particles wafting through the air. Finally, you might differentiate the senses by pointing to the different sense organs. The eyes are made for vision. The ears are made for hearing. The nose is made for smelling. However, cases of synesthesia makes differentiating the senses along these three lines, or some combination of them, a bit more complicated than it originally appears. The varieties of synesthesia are quite varied and, in principle, can combine any sensory, emotional or cognitive experiences. For instance, a sight-sound synesthete might combine visual experiences with auditory experiences, so that whenever they see a person’s face, they hear a particular ‘pinging’ sound or tune. A person might also have a combined experience of fear and color, so that when they see something scary, like a poisonous snake, their visual field becomes saturated with a blue-green experience. If we want to be able to differentiate the visual sense modality based on the qualitative feeling of visual experience, we should be troubled by cases of synesthesia. The qualitative feeling of vision for a sight-sound synesthete is quite different from the qualitative feeling of vision for a non-synesthete. Here the differentiating characteristic fails to point out a particular sense, since the qualitative feeling varies so widely between synesthetes and non-synesthetes. If we want to differentiate the senses along the lines of the objects of perception, cases of synesthesia create problems again, especially when we attempt to link certain objects of perception with certain sense organs or a certain type of qualitative feeling. If the objects of visual perception are a definite spectrum of light waves, how can we account for the blended experience of a fear-color synesthete? For a fear-color synesthete, a visual experience is apparently not created by the spectrum of colored light waves, but by an emotional state of fear. The fear-color synesthetic experience also does not have much to do with the eyes. For a fear-color...

One of the reasons that synesthesia is so fascinating among researchers is the implications it has for the crossmodal perception, or interactions between different senses. However, in recent research it has also been proposed that there is a type of sensory-to-motor synesthesia (Ramachandran & Hubbard 2001). This proposal has a great deal of explanatory power, especially in the evolution of language.The philosopher Noam Chomsky argued for a “universal grammar” among human beings. He suggests that the ability to learn languages is innate for human beings, and that there are properties that all languages have in common. (See more about “universal grammar” here). The problem for confirming this thesis lies in giving an evolutionary account of the structure and components of the universal grammar. There seems to be some empirical evidence that could strengthen this hypothesis. The “Bouba-Kiki effect” is a phenomenon originally observed by Wolfgang Köhler, but more recently analyzed by Ramachandran and Hubbard. The experiment involves two shapes, and asks subjects to designate each object with a particular name. They asked “Which of these shapes is bouba and which is kiki?” An overwhelming majority, 95%, named the figure on the left “kiki” and the figure on the right “bouba”. They argue that the reason for this inclination is that the sharp lines and edges of the figure on the left match the sharp sounds in “kiki”, while the rounded edges of the figure on the right match the auditory inflection of “bouba”. They argue that the proposed sensory-motor synesthesia can explain the “bootstrapping” of these two phenomenon in evolutionary history.In their paper, Ramachandran and Hubbard explore many other indications of this bootstrapping. One instance, originally noted by Charles Darwin, is the word “disgusting”. When we smell or taste something unpleasant, we scrunch up our noses and raise our hands towards our face. They explain that the olfactory bulb projects to the orbital-frontal cortex, and that disgust is mediated by the frontal lobes. They point out that the same behavior (and same area, the orbital-frontal cortex) is present when one observes behavior one finds morally disgusting as well. As mammals became more social, they may have used behavioral cues to discourage others from eating rotten food or coming near an awful smell. One can imagine that this sort of behavior could be used to discourage people from acting in a manner that they found immoral or unpleasant. From these examples, we can see how synesthesia and analysis of cross-modal perception can be useful in studying the origins of language and other components of human...

When I joined the St. Louis Synesthesia Research Team as an undergraduate student of philosophy with a profound interest in neuroscience, my knowledge of synesthesia was minimal at best. I knew that synesthetes experienced a “blending of the senses” but that was about it. So I did what any modern person would do, I started searching the Internet for links about synesthesia and synesthetes. One of the first things to catch my eye was the celebrities who have/had synesthesia, are thought to have had synesthesia, or claim to have synesthesia. From iconic actress/model Marilyn Monroe to blind musicianStevie Wonder (sound-color) to Nobel Prize-winning physicist Richard Feynman (grapheme-color), famous synesthetes can be found in a variety of different forms and professional fields, and synesthesia is found relatively often in artists and musicians. While there are a plethora of synesthetes out there to focus on, I will detail a two of my favorites and encourage you to search the web and see if any of your favorite celebrities are synesthetes! Patrick Stump, vocalist and musician The former lead singer of everyone’s favorite pop-punk band Fall Out Boy, Stump has multiple synesthesiae (grapheme-color and sound-color). He wrote in a 2008 blog post, “For instance the letter ‘F’ for me is green. When I see it written in black I obviously still notice that it is black but it ‘feels’ green. Or ‘S’ is red. Most of the alphabet and numbers from 1-10 have some sort of associated color to me. It’s ultimately totally trivial but I found it fascinating that this is a documented phenomenon and not just me being a weirdo.” Later, in response to a comment on his blog asking if he was a synesthete, Stump responded that he had grapheme-color synesthesia and a “less pronounced” form of sound-color synesthesia. It is interesting to note that Stump sees the ‘actual’ color of a grapheme but ‘feels’ the synesthetic color. This would seem to indicate that his grapheme-letter synesthesia is not projected into space and is consistent with many other synesthetes’ reports. Like many musicians, Stump reports having sound-color synesthesia, although less pronounced than his grapheme-color synesthesia. His multiple synesthesiae certainly make him even more interesting! Ludwig Wittgenstein, philosopher and mathematician The famous Austrian-British philosopher and logician’s synesthesia is a speculative case of grapheme-color synesthesia, stemming from a comment in his book Zettel where he said, “It’s just like the way some people do not understand the question ‘What color has the vowel A for you?” While this comment suggest he had at minimum a rudimentary understanding of what synesthesia was – highly uncommon for his time – many have taken it to suggest that he himself had synesthetic experience. Certainly, Wittgenstein was interested in qualitative phenomenon, and in Remarks on Colour, he...