Introduction
Have you ever wondered what color tastes like? What sound notes taste sweet? Unbelievable questions? Not so, at least for the people who have this wonderous ability.
Synesthesia comes from joining two different words ‘syn’ and ‘esthesia’. Latin origin ‘syn’ means together, while ‘esthesia’ comes from Greek, meaning sensation or perception to give the literal meaning as combination of perception. In biological terms, synesthesia is an involuntary stable condition when a stimulus fires more than one sensory organ; for example a sound triggers both auditory and gustatory systems, thus giving the sound a specific taste. In this ability, any of the five senses can be correlated to one another. Sound can trigger a vision, color can have taste, the taste can have color and many more such combinations. The people who have such unique abilities are called synesthetes.
Past and Present
Historically this phenomenon was thought to be very rare but this might be because even though this was known as far back as the 18th century, it was popularized as a scientific field in neuroscience only in the past 20 years.
The first time synesthesia was reported in medical literature was by Thomas Woolhouse in 1710 when he referred to a case of a blind man who could see sound-induced color visions.
It is estimated that only 2-4% of the population are said to have synesthesia, though it is not confirmed. This may be because of the social stigma that the synesthetes are often subjugated since their young age. Many of them were considered an outcast amongst ‘normal’ individuals. They were often ridiculed when they tried to explain their many senses and eventually kept their uniqueness to themselves.
Types and Variation
There are different forms of synesthesia from simple and restricted to complex and elaborate. For example, only instrumental music will elicit a vision of color in restricted synesthesia, while in elaborate synesthesia stimulation of one sense, triggers a response in the rest four senses. Like a sound would trigger the perception of touch, smell and vision.
There are currently 60 reported variants of synesthesia. Most common ones of them are coloured numbers and letters (grapheme coloured synesthesia). Most synesthesia has color as a concurrent in many synesthesias such as coloured hearing (chromesthesia) or colored olfaction/taste etc.
Most of the synesthesia recorded are of coloured grapheme. Color appears to have a larger share in common synesthesia recorded such as chromesthesia (sound elicit sight), coloured time units, colored odor, colored pain, colored taste, colored temperature etc. while it is rare for smell or taste for either be stimuli or response in synesthesia. This is just the tip of the iceberg in types of synesthesia. There are calendar synesthesia, audio motor synesthesia, swimming style synesthesia and so much more and with so many possibilities of permutations and combinations that it is really hard to comprehend the extent of synesthesia.
The How and Why
The question now arises is why does it happen? Is it a mistake, some wrong cross-wiring in the brain ?
Though there is no particular answer, there are some possibilities and theories that try to shed some light on this incredible superpower.
How and Why
Study showed that 40% of synesthetes have a first degree relative with the same condition while another study reported a first degree family of synesthetes having a history of autism, dyslexia and attention deficit disorder implying that synesthesia runs in the families.
Pedigree analysis from these studies revealed that there is high transmittance to offspring from parents. Within the family, different individuals can have a different form of synesthesia while an individual may often have more than one form of synesthesia.
Whole- genome linkage scan and family-based linkage analysis results suggested a possibility of the polygenic origin ( involving multiple origins/genes) of synesthesia. This means that the ability is not because of one mutation or gene but involves high complexity from the genetic level itself.
Brain Anatomy and Function
With this unique power of synesthetes, it is a given that their neural networks must be different from the ordinary individuals.
Synesthetes are predominantly left- handed, consistent with their anomalous cerebral activity. They are known to exhibit certain mathematical deficiency (allochiria, lexical to digit transcoding), some are even dyscalculic even when they generally have overall high intelligence.
Brain structure studies revealed that synesthetes have increased white and grey matter density. The increased matter is often in regions, specific for a specific subtype of synesthesia, for example in grapheme color synesthetes increased white matter in parietal and temporal regions close to the grapheme area.
Similarly, taste synesthetes have differences in their anatomy of auditory and gustatory areas. Many such pieces of research compiled results in the same belief that synesthetes have altered brain anatomy, but whether this altering is due to synesthesia or is synesthesia a product of altered anatomy is not known.
Theories
Since a lot is still unknown, many scientists put forth their own theories to explain this extraordinary phenomena. Currently, there are two main theories proposed for synesthesia:
Theory 1: The activation through connectivity mechanisms
Proposes that connections activate neurons that results in the synesthetic
experience.
Two main theories that fall under this category are:
1. Cross activation theory
Cross activation theory proposed by Ramachandran and Hubbard in 2001, says that specific synesthesia is a result of an excess neural connection between associated regions that typically give rise to this phenomenon; for example sound giving rise to color.
2. The disinhibition or re-entrant theory
Disinhibition or reentrant theory proposed in 2001 believed that the stimulus doesn’t give rise to synesthetic experience directly, but it is caused by the feedback signals from higher-order associative regions ( an integration area between the sensory inputs and motor outputs. ) to the primary sensory region (input area) .
Disinhibition theory hypotheses that synesthetic experience is elicited after deep processing of the stimuli unlike Cross activation theory, which stipulates that regions that are stimulated directly affect the region that gives concurrent sensation.
Theory 2: Synesthetic experiences arise from semantics
A new recent theory that proposes the role of semantics in synesthesia. How the flow of information through brain regions dictate the synesthetic experiences. This theory implies that the brain can make quick changes depending upon how the stimulus is perceived. For example, a study showed that depending upon how the subject perceives the symbol as 5 or S, the concurrent color experienced by the subject changes.
The concept of semantics only works, if one believes in the role of learning and memory, in shaping synesthesia. Many research papers have introduced the role of learning and memory, in affecting synesthesia.
Studies have shown that synethesthes cannot recall the time when their synesthesia began or their experience differed. They always believed that a particular synesthetic experience is always triggered by the same stimuli.
But according to a study conducted on grapheme-color synesthetes ( they can see alphabets in color) aged between 6 to 7 years olds and again when they were aged between 10 and 11, there was an increase in consistent color association from 35% of letters to 70% of letters. That means more than 50% of letters did not have a unique color association between ages 6 and 7. Now, consider the case of pain-color synesthesia, in which different categories of pain induces a different sensation of color. But categorizing pain from pinch or slap required learning. Pain is learned through experience, electrophysiological studies revealed that infants cannot distinguish different types of pain. The pain, therefore, is categorized slowly as the child develops. Some synesthetes, the associated sensation is that from childhood toys or schooling. Like ‘A’ was written in red color in the alphabet book or refrigerator magnetic letters. The learning can also influence indirectly, like the similar shape of letters like E and F, evokes similar color. Not only shapes but phonetics, similar- sounding words can have similar colors. Synesthesia thus develops slowly influenced by surrounding, learning and memorizing.
Unifying idea: Ideasthesia
This led to the development of ‘Ideasthesia’ which translates to sensing concepts. Ideasthesia negates the theory that synesthesia is present from birth, rather synesthesia gets formed as the individuals learn and assign meaning to the stimulus.
It also proposes that synesthesia helps in understanding the world better. Not only it can explain the occurrences of common synesthesia such as grapheme color synesthesia, since letters, shapes, words and numbers are few things a child learns first. Ideasthesia also rationalized the point that synesthetes chose their different concurrence by learning from their external sources or internal sources.
Advantages and Needs
Synesthetes tend to use their synesthesia to memorize, learn and understand novel concepts. In some synesthesia like grapheme color synesthesia, synesthetes show better detection of colors in a low-level visual test and are generally better at discerning similar colors, giving them an edge over non-synesthetes.
Synesthesia's role in creativity has still not been established even though there is a higher prevalence of synesthesia among artists and most of the synesthetic individuals spend more time in creative ideas.
But this unique ability has one major problem especially in the case of trumas. It is hard to get over any traumatic experience, but for synesthetes it is harder than normal. Due to traumatic accidents, stress, or depression, some shapes or colors may get ingrained with the memory of the accident. This might be a problem for synesthetes who experience color in their everyday life. Antidepressant drugs also are known to mute synesthesia, which is a tremendous loss for synesthetes who experience synesthesia all through their life, pretty much like for non-synesthetes who lost the sense of taste. Hence, psychological needs of synesthetic individuals must take into account factors that might influence negatively on their mental health.
Can non-synthesthetes ever experience this?
Several researches have been conducted for non-synesthetes to learn synesthesia by training them for a specific amount of time every day for a few months. The training can include reading novels with coloured letters to hypnotism for an association of numbers to colors. And the results were astonishing as there were several positive cases for many non-synesthetes who could now experience synesthesia similar to that of a synesthetic individual. These individuals were also made to undertake a ‘synesthetic Stroop task’ and the results were quite similar to that of synesthetes.
Conclusion
Such a unique ability might be due to mutated genes that remained as it wasn’t detrimental or it developed in children because of its usefulness in learning, memorisation or finding an alternative solution to difficult tasks during their educational period. Nonetheless, we have come a long way from unbelieving this phenomenon and characterizing it as a disorder to believing it as a different ability. Yet there is still a social stigma against them and we need to go a long way before we can fully understand synesthesia and could devise a solution for their unique problems.
If anyone would like to view one of the synesthetic experience, there is an excellent youtube video: "What's It Like To Hear Colors? - A VR 360° Synesthesia Experience"