It is believed that between 1 and 4% of the population is synesthetic, but recent articles suggest that the percentage could increase: Melero published a paper that found that, of a sample of 803 people, 14% were synesthetic . However, to corroborate this figure, studies must continue.
And what do you think the brain does when there is synesthesia? To answer Melero, he gives an example: let’s think about a grapheme-color synesthesia, in which a letter generates an experience of color, even if it is printed in black. In the brain, the area that processes the grapheme and the one that processes the color are located adjacently.