Zoom on synesthesia
This is a non-pathological cerebral phenomenon affecting 4 to 6% of the population. Synesthesia is generally specific to each individual, but it is defined as the association between a particular stimulus and an additional sensory perception. In fact, the name synesthesia gathers the ancient Greek words for union "syn" and sensation "aesthesis". This phenomenon has been known for a long time, for example Isaac Newton associated musical notes with the hues of the color wheel in 1704, and the first medical description of a synesthesia case was made by Oliver Sacks in 1812.
To date, more than 60 different types of synesthesia have been identified, involving associations between the senses of taste, hearing, vision, and smell. The more common have been studied the most: grapheme-color synesthesia, association between colors and numbers or letters (64% of synesthetes), chromesthesia (15%), and spatial sequence synesthesia (10 to 15%). This last type is the association between a series of numbers or letters for example, and a precise spatial arrangement.
But what are the causes of this phenomenon? Studies have shown that genetic factors are important and could define anatomical-functional factors involved in synesthesia. The neurobiological substrate of synesthesia is still not well known, but two main hypotheses have been made: the first suggests that structural particularities would be involved, and the other states that synesthetes would have connections with functional particularities between the cerebral areas. In addition, environmental and cultural factors are also important. Many grey areas remain, but a better understanding of this phenomenon will undoubtedly lead to a better understanding of normal human cognition.