What is it like to be tasting words and hearing colours? Imagine being able to taste a single word. Do you get a certain colour in mind when you hear a certain word? Or have you ever experienced the same feelings as the other person? If you can’t, you are not among the 2-4% of the population that suffers from Synesthesia, a perceptual phenomenon in which stimuli presented in one modality spontaneously trigger sensations in a different modality. Any two senses can be linked by synesthesia. This condition affects celebrities such as Billie Eilish, Kanye West, Beyonce, Alessia Cara, Ramin Djawadi, Billy Joel, Finneas O’ Connell and Pharrell Williams.
Synesthesia (from the Greek words “synth” = “together” and “ethesia” = “perception”) is a neurological disorder in which a single input connects two or more senses, allowing the person to perceive it in several ways. When someone talks, for example, a synesthete can hear the speech and link it with a colour, taste, smell, and physical texture. It is a condition that can emerge in childhood or arise later in life.
Types of Synesthesia:
There are about 60 different ways a synesthete’ senses might be connected. Some synesthetes, for example, see words as a flavour, while others may identify different personality attributes with each of the alphabet’s 26 letters. Additionally, while synesthesia is most generally thought to be a relationship between two sensations, there are some of its kinds that involve three or more senses.
Synesthetes can be divided into two categories according to their underlying synesthesia. The first type is Projective synesthesia, in which the synesthete hears, sees, feels, smells, or tastes the second experience generated by the first stimulus. Associative synesthesia is the second major category. Synesthetes in this category experience a link between a stimulus and a sense through which it is not ordinarily received.
Most common types of Synesthesia:
- Grapheme-colour synesthesia: Letters and days of the week get associated with specific colours.
- Auditory-tactile synesthesia: A condition in which sounds cause physical sensations.
- Lexical-gustatory synesthesia: Hearing a certain word leads to an experience of a specific taste.
- Chromesthesia or sound-colour synesthesia: Condition in which a sound causes you to see a specific hue, sometimes even a shape or movement
- Mirror-touch synesthesia: When a person has the identical physical sensation as the person in front of them.
The Neural basis:
Synesthetes had different brain area activation than non-synesthetes, according to neuroimaging studies such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) scans. The brain foundation of grapheme-colour synesthesia has been intensively investigated using both psychophysical testing and functional imaging. Simple achromatic graphemes have been shown to stimulate both grapheme areas and colour area V4 (a part of the visual cortex that responds to colours more strongly than grey-scale stimuli) in synesthetes’ brains, supporting the idea that synesthetic colours are sensory. Anatomical differences in the inferior temporal lobe near regions related to grapheme and colour processing have also been observed in synesthetes, including increased fractional anisotropy (reflecting increased white matter or coherence of white matter) and increased grey matter volume, according to several studies. Synesthesia is thought to be caused by an overabundance of brain connections between related modalities, known as cross-modal associations. These connections, according to scientists, are conceivable due to lower neuronal pruning across neighboring linked regions in the fetus.
Is Synesthesia heritable?
While a genetic cause for synesthesia is unknown the condition runs in families, with 40% of synesthetes reporting a first-degree relative with the condition. Synesthesia is highly transmissible from father to offspring, according to pedigree investigations. Previous investigations on the prevalence of synesthesia indicated a substantial gender discrepancy, with a 6:1 ratio of female synesthetes to men, implying that synesthesia is an X-linked disorder. When Brang and Ramachandran looked at the link between synesthesia like hallucinogenic experiences and the serotonin 2A receptor gene on chromosome 13, they concluded that it could be caused by overexpression of this gene, resulting in increased receptor density.
In light of these contradictory findings, research into the genetics of this lesser known phenomenon is still in its early stage. Understanding the fundamental mechanisms for transmission would necessitate considerably bigger sample sizes and variants of the disorder.
Research prospects:
In a nutshell, synesthesia is a highly heritable phenomenon linked to a variety of cognitive benefits, suggesting a possible explanation for why this condition has survived evolutionary forces. It can now help us better understand cognitive and perceptual processes in the general population by revealing both the mechanism behind the phenomena and the reasons for its selection. To fully comprehend this illness and its relationship to normal cognition, scientists from various fields of biology will need to contribute both technically and conceptually, It can also provide crucial clues to understanding some of the physiological principles that underpin some of the most enigmatic yet prized characteristics of the human mind.
References:
- American Psychological Association
- Maurer D, Ghloum JK, Gibson LC, Watson MR, Chen LM, Akins K, Enns JT, Hensch TK, Werker JF. Reduced perceptual narrowing in synesthesia. Proc Natl Acad Sci U S A. 2020 May 5;117(18):10089-10096. doi: 10.1073/pnas.1914668117. Epub 2020 Apr 22. PMID: 32321833; PMCID: PMC7211996.
- Safran AB, Sanda N. Color synesthesia. Insight into perception, emotion, and consciousness. Curr Opin Neurol. 2015 Feb;28(1):36-44. doi: 10.1097/WCO.0000000000000169. PMID: 25545055; PMCID: PMC4286234.