Synesthesia—Are All Mondays Blue?

Does Monday have a specific color? What does the ABC song taste like? These may sound like strange questions, but for a small number of people, such questions may be very reasonable. This is due to a phenomenon called synesthesia. It originates from Greek, meaning “joined” (syn) and “sensations” (esthesia). We experience the environment through a range of senses. You may hear music with your ears or enjoy the color of a clear sky with your eyes. However, when someone has synesthesia, certain experiences seem to have an additional sensory dimension. So, on top of hearing music, a person with synesthesia may also see the song in front of them. The phenomenon is still not fully understood. However, there are several suggestions about its mechanisms. Synesthesia provides an insight into individual differences in how we experience the world we live in.


WHAT IS SYNESTHESIA?
When navigating the environment, our senses collect information. Sensory information is then processed by the brain to guide our actions and behaviors. We tend to assume that when we see an object, say an apple, everyone else sees and experiences the apple the same way. Philosophers have debated whether we can ever be sure about other people's experiences. Examples continually emerge indicating that people have di erent ways of experiencing the world. Synesthesia SYNESTHESIA A brain phenomenon in which a stimulus triggers a typical sensation, such as seeing the letter "A," followed by an unrelated sensation, like seeing the color yellow.
is one such example, demonstrating that sensory experiences can vary between observers. Synesthesia is derived from the Greek words "syn," meaning "joined," and "esthesia," meaning "sensations." Synesthesia is estimated to occur in about % of the population. Synesthesia can vary between people because there are many di erent types of synesthesia. One estimate is that there are at least di erent variations . Some people experience colors when they see letters [ ].
http://www. daysyn.com/ Types-of-Syn .html For others, weekdays have a specific color, for example Monday is red and Tuesday is yellow. There are also people who taste words.
In the example in which Monday is red, we call Monday the inducer,

INDUCER
The piece of information that causes a synesthesia experience, such as a letter, a number or a song. because it causes the synesthesia experience. We call red the concurrent, because it is the sensory experience that follows from CONCURRENT The unrelated sensation that comes as a reaction to certain stimuli, as such the color sensation that follows seeing a letter. thinking about Monday. A person who experiences synesthesia is commonly called a synesthete. A synesthete may have several types SYNESTHETE A person who experiences synesthesia. of synesthesia in varying degrees. In other words, there is a wide range of di erent experiences a synesthete may have.
If there is that much variation, how are we sure that all the examples are synesthesia? Not all synesthetes would associate the number with the same color, for example. Nevertheless, all types of synesthesia share some common features. One is that the associations seem stable over time. Imagine a person for whom the letter A is a particular hue of red. That person will report the same red hue when asked about it even weeks apart.
Figure shows drawings from Anneline, who is a synesthete. She made a drawing of her synesthesia for numbers and shapes back in ( Figure A). Seven years later, we asked her to redraw how she experiences numbers and shapes ( Figure B). Anneline has several forms of synesthesia, and among these, she experiences colors for numbers and shapes. Numbers are also associated with various sizes. Interestingly, this size di erence does not correspond to the numeric quantity. She would describe the number six as very annoying due to its small size, making it very di cult to see. However, she likes the number five because it is large, orange, and easy to see.
In addition to synesthetes' direct descriptions of their synesthesia, it is possible to demonstrate in the lab that these associations influence perception. Synesthetes process letters faster when the letters are shown in their synesthetic color. Synesthetes also have to use fewer Four di erent explanations for synesthesia have been suggested. The cross-activation theory, the disinhibited feedback theory, the FEEDBACK A process of information traveling from the associative area back to the sensory area.
re-entrant hypothesis, and the developmental learning hypothesis.

TOO MANY CONNECTIONS IN THE BRAIN
To understand the first explanation, we can think about the brain as a house. We first need to build the house. This happens before we are born and for the first couple of years of our lives. We say that the brain is maturating. But this is a strange house in the beginning. The builder is not sure how we will want to use various rooms. Therefore, he connects every electrical switch to several di erent outlets. What does it mean for the brain? It means that there are way too many connections between brain cells (called neurons) and between various areas of the brain. When people move into the house, they decide where to plug in the lamps, the TV, the toaster, etc. They call in an electrician and ask him to remove all the cables that connect switches to the wrong outlets. Now, when the people want to turn on the kettle, they know which switch to use-that switch now turns on the kettle only.
A similar process happens in the brain when children are about years old. A child's experiences of the world will decide which brain connections are useful. The useless ones will disappear while the useful ones will become stronger. We call this process pruning. In PRUNING A brain process in which connections between neurons that are not used are removed. At the same time, useful connections are strengthened.

kids.frontiersin.org
May | Volume | Article | the case of synesthesia, the cross-activation theory [ ] suggests that the electrician forgot one of the useless connections. Now, when people turn on their living room light, it also turns on the TV! In other words, some brain connections that were supposed to disappear have remained ( Figure A). When the synesthete hears music, the information goes to their auditory (hearing) system, as expected. However, it also activates parts of the visual system. So, that person may see shapes or colors every time they listen to a song. Figure   Figure (A) The cross-activation theory states that, in synesthesia, some useless brain connections are not pruned as they are in non-synesthetes, keeping unrelated brain areas connected (green lines). (B) The disinhibited feedback theory states that, in synesthetes, information travels back (green arrow) to parts of the brain. In non-synesthetes, this feedback is inhibited.
(C) The re-entrant hypothesis states that, when seeing a letter a synesthete recognizes, a feedback process (green arrow) will occur, but when seeing a letter she does not recognize, no feedback will be initiated.

OOPS, WRONG WAY!
According to the second explanation, synesthesia is not due to di erences in the structure of the brain. Instead, this explanation hypothesizes that synesthesia comes from di erences in the way people process sensory information. Let us take the example of someone reading the letter A. Information first enters through the person's eyes, then it arrives at the visual area, at the back of the brain. This is called a sensory area. Here, the information is analyzed

SENSORY AREA
Brain areas that process information from our senses-sight, hearing, touch, etc.
in several steps. We can imagine it as an assembly line, in which each worker evaluates a specific aspect of the information, spotting whether it contains a straight line, if the image is moving, or if it is colored, for example. Once the image has been analyzed, the information moves on to be processed in the associative areas of the brain. There, the

ASSOCIATIVE AREAS
Brain areas in which information from the various senses is combined. More complex processes occur, such as understanding and reacting to what we read.
information from various sources is combined. Naming the letter, or listing words that start with A, are examples of associative tasks. Once the information has been processed by the associative areas, it travels back to the sensory area. This process is called feedback. However, since the information has already been treated there, it does not get passed on further-we say that it is inhibited. The disinhibited feedback theory [ ] suggests that, in synesthetes, the inhibition process may not occur correctly ( Figure B). Instead, the information gets analyzed in the sensory area one more time. Take the case of Anneline. Every time kids.frontiersin.org May | Volume | Article | an A is sent back to the visual sensory area, is it also labeled with the additional sensation of yellow.
The re-entrant hypothesis [ ] is a variation of this idea ( Figure C). In the disinhibited feedback theory, visual information is spontaneously sent back from the associative area to the sensory area. However, according to the re-entrant hypothesis, this feedback process is not spontaneous-it can happen only if the meaning of the letter is known. Take Anneline, who can read English and Danish. Now imagine we show her the Japanese grapheme?, which is equivalent to A. She will not get any colored response to it, as her brain does not know what it means. Aurore however can read Japanese. When she sees?, she experiences pink ( Figure C).

A LEARNING TRICK OF THE MIND
The three hypotheses presented so far try to explain how the brain of a person with synesthesia functions. They do not try to explain why only some people have synesthesia. You may have noticed that the inducers, like letters or months, are concepts that we do not know from birth. We learn about these concepts as we grow up, and they are not the same in every culture. For example, not all languages are written with the same symbols. This led researchers to wonder whether synesthesia is involved in the learning process. Learning every letter and being able to use them all is a di cult task for some people. So, it is possible that their brains came up with a trick. Their brains slowly started to associate the concept of the letter A to a specific color, to help them tell the di erence between letters. A similar process may occur for all types of synesthesia inducers. Researchers call this explanation the developmental learning hypothesis [ ].
SUMMARY Synesthesia gives people a particular experience of the world. The origin of synesthesia is still a mystery to researchers. However, great progress has been made, and we now know more about the di erent profiles of synesthetes. We also have some hypotheses regarding the brain mechanisms involved. Understanding synesthesia can help us understand the individual di erences in how our brain processes information. It could help us discovering new ways to overcome learning di culties. . doi: . /frym. .

CONFLICT OF INTEREST:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
COPYRIGHT © Zelazny and Sørensen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. LICEO SCIENTIFICO M. G. AGNESI, AGES: -Hi, we are the class AS from the Liceo Scientifico M.G. Agnesi, in Italy. We think this was a really good project and could be quite useful to some of us in the future. We found it very interesting because we learned things we did not know before: it gave us a chance to expand our curiosity and our knowledge.

AURORE ZELAZNY
I have always been fascinated by languages and the brain. I started o studying linguistics, then I became interested in how language works in the brain. As a synesthete myself, I wondered whether synesthesia is related to early language learning. I am interested in how synesthesia is involved in acquiring expert knowledge of new concepts. I work at the Center for Cognitive Neuroscience at Aalborg University in Denmark. *aurore@hum.aau.dk

THOMAS ALRIK SØRENSEN
I have a keen interest in memory and perception and how expertise influences perceptual processes. During my Ph.D. at the University of Copenhagen I started to investigate how short-term memory was influenced by various factors, which has since led to a more general interest in what shapes the way we experience our environments. Synesthesia is a particularly interesting phenomenon, as synesthetes clearly demonstrate a di erence in perceptual content compared to non-synesthetes, which may help us to better understand what drives individual di erences in perception. I am heading the Center for Cognitive Neuroscience at Aalborg University.