What is Synesthesia?
The word "synesthesia" literally means "sensations being experienced together (as a union)". It has been derived from the Greek words "syn", meaning "together (union)", and "aisthesis", meaning "sensation". 
Synesthesia is not a disease or an illness. It cannot be "acquired" or "learned" from any sources. It is a naturally occurring unique neurological condition or a phenomenon or an anomaly in which the experience of one sensation has an involuntary and cognitive effect on the experience of another sensation through a specific sensory pathway. 
Those individuals who experience the phenomenon of synesthesia are known as synesthetes, which comprise of around four percent of the world's population. In terms of gender demographics, the female synesthetic population is six times greater in number than the male synesthetic population because the gene coding for synesthesia is located on the dominant X chromosome. 
The phenomenon of synesthesia is studied keenly by psychologists and neuroscientists for the influence it might have on the individual's psychological perception of the world and the society in which we live.
Experiencing Synesthesia - The Mechanisms of Union of Sensations 
Synesthesia is a neurological phenomenon. The exact mechanism of the occurrence of synesthesia is under debate even today. Neuroscientists and psychologists have hypothesized that synesthesia may have an underlying neural and neurocognitive basis due to availability of neuroimaging data and neuroscientific principles.
The visual cortex is that part of the brain which helps in the perception of vision. The most important parts of the visual cortex are involved in processing of colour and motion. Cross-activation and cross-networking between the white-matter neurons present in these parts of the visual cortex of the brain may be responsible for the occurrence of the phenomenon of synesthesia. This has also been confirmed by researchers through the use a technique called Diffusion Tensor Imaging (DTI).
Lack of inhibition in the feedback pathways in the sensory and association areas of the brain means that information is transmitted in all possible directions, thus leading to various sensory areas in the brain involuntarily influencing one another. This is another hypothesis used for explaining the mechanism of occurrence of synesthesia. Single Nucleotide Polymorphisms (SNPs) leading to decreased feedback inhibition found to occur in families having synesthesia might also explain the genetic basis of the condition.
Various neuroimagery studies conducted through techniques such as PET and MRI have revealed that the brains of synesthetes possess an overly active colour (vision) processing function in the presence of auditory signals.
More research is required to study the effect of these overly active sensory functions on the neurotransmitters produced in the brains of synesthetes.
Causes of Synesthesia
It is difficult to pinpoint the exact causes of synesthesia. However, as per scientists, synesthesia may occur due to the following reasons -
- Differences in brain and neural development - Some scientists believe that every individual is born a synesthete. However, due to differences and variations in the continuous development of the brain and the neural network throughout infancy and childhood, some individuals end up experiencing multiple sensory inputs simultaneously. These individuals also process these sensory inputs much differently as compared to those individuals who cannot experience multiple sensations. 
- Genetic components - Synesthesia tends to run in families. Nearly 40% of synesthetes have a family history of the condition. However, the forms of synesthesia occurring within the same family vary from one person to another. In other words, within the same family, each individual is uniquely synesthetic. 
- Cross-networking between different parts of brain - Cross-communication between different parts of the brain results in random transmitting of brain signals and the creation of random uninhibited feedback pathways. This leads to two or more sensations being experienced simultaneously and involuntarily, causing an individual to become synesthetic. 
- Neural injuries and intake of hallucinogenic drugs - Neurologists have observed that temporary or permanent synesthesia has been found to occur in those individuals who have suffered injuries to various parts of the neurological system such as the spinal cord, brain, or other conditions. Also, intake of hallucinogenic drugs leads to experiencing (or "hallucinating") different kinds of sensations simultaneously. 
♦ Handedness - Synesthesia has been found to occur more frequently in left-handed individuals as compared to right-handed individuals. 
Forms of Synesthesia  
There are different types or forms of synesthesia. However, some forms are more frequently occurring than others -
Physically associating colours with numbers
Mentally visualizing objects as colours
Associating objects with certain sounds
Vision, Mental, Auditory
Associating colours with specific sounds, as well as abstract concepts such as time, months, etc
Associating certain numbers with a mental number map and other number-based objects, such as a clock
Ordinal Linguistic Personification (OLP)
Associating objects in an ordered sequence such as letters or numbers with specific personalities
Associating words with certain gustatory tastes
How to recognize a Synesthete? 
A synesthete can be recognized by the following signs of association -
♦ Instinctive and instantaneous association of the sensations of sounds, tastes, letters, and colours with one another. This "association" is not "thought out".
♦ Associated ("mental") as well as projected ("physical") sensations.
♦ Production of consistent associations, sensations, and projections. For example, the letter "A" is associated with the colour blue each time it is visualized.
♦ Associating names of individuals with colours, rather than the faces of those individuals.
♦ Creation of pleasant emotional reactions during the creation of a specific association or sensation.
Personality traits of a synesthete -
A synesthete is neurologically normal, and possesses above average intelligence.
However, the method of "perception" of various sensations in a manner that is extremely unique as compared to the majority of the (non-synesthetic) population causes these individuals to possess certain distinctive personality traits as listed below -
- Confused sense of direction
- Elephantine memory
- Prone to migraine attacks
- Creatively and artistically inclined
♦ More ticklish as compared to non-synesthetes
List of a few notable Synesthetes 
- Mary J. Blige
- Billy Joel
- Marilyn Monroe
- Geoffrey Rush
- Vincent van Gogh
- Stevie Wonder
- Jimi Hendrix
- Emily Dickinson
- Nikola Tesla
- Richard Feynmann
- Edvard Munch
Interesting Facts about Synesthesia  
♦ Number-colour synesthetes cannot associate any colours with Roman numerals. Therefore, is number-colour synesthesia restricted only to Hindu-Arabic numerals?
♦ Perceptual Synesthesia is observed in visually challenged individuals. These individuals associate objects with sounds.
♦ Variations in the spelling of a single word can trigger completely different associations with regards to taste. For example, "cat" could be associated with "chocolate", and "kat" could be associated with "wine".
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