I found out pretty recently that one of my friends has synesthesia which is a blending of the senses which causes input to one sense e.g. vision to cause firing of other senses e.g. hearing! In one common form of synesthesia, known as or color-graphemic synesthesia, letters or numbers are perceived as coloured. In other forms of synesthesia numbers, days of the week and months of the year evoke personalities or musical notes can evoke colours. In number form synesthesia, numbers, months of the year, and/or days of the week have precise locations in space, e.g. a number line may be ever present in the air infront of the person and 20 may be further away than 12. Over 60 types of synesthesia have been reported, but only a fraction have been evaluated by scientific research. Even within one type, synesthetic perceptions vary in intensity and people vary in awareness of their synesthetic perceptions. Because they're usually born with synesthesia they can be completely unaware of their perceptual differences until they are made aware of it
Now researchers are getting closer to understanding the mechanics behind this phenomenon. In this study Reading in 2 colours at once they explored the fact that synesthetes report seeing both the original colour of the word (i.e. black) at the same time as the synesthetic colour!!!
Synesthesia is honestly the only disorder people are jealous of (me included, I'm very jealous)!