This is more than a memory linking a certain perfume to one of your partners. In synesthesia, some people, for example, perceive colors when musical notes are played. One very common form of synesthesia is the grapheme-color synesthesia, in which letters or numbers (graphemes) are highlighted with particular colors. This is still a mysterious phenomenon, reported as vivid and realistic, and no one can say if this is real or just a byproduct of some other mental processes, such as the memory.

A new research led by Danko Nikolic, from the Max Planck Institute for Brain Research in Frankfurt, Germany, used the Stroop to verify the veracity of these experiences. The subjects had to name the color of the font in which a word naming a color was printed. For instance, if the word "blue" was marked in red ink, the participant would say "red", a process requiring some mental training.

When a person sees a certain color, specific neurons in the visual occipital cortex area (in the rear part of our brain) are turned on. These specific neurons will turn off if a color from the opposite end of the spectrum is presented. Neurons turned on by the color blue will shut off when the opposite, yellow, is perceived.

Nikolic presented grapheme-color synesthetes with their five most color sensitive letters or numbers. The color of the letter or number was either the same (congruent), different but not completely on the opposite color association (incongruent independent), or on the opposite side of the spectrum of the linked color (opponent incongruent).

The team then assessed the time interval required by the subjects to name the color of the grapheme. Indeed, it was quite difficult for individuals with grapheme-color synesthesia to deliver a rapid answer in case of opponent incongruent colors, compared to cases of independent incongruent colors. Congruent colors just sped the naming of the colors.

In another approach, the team discovered that the color-opponency system was not linked to memories.

The same subjects were presented images of objects normally associated with a color (a lemon, for example, for yellow). But many objects were presented in weird, unexpected colors. Reaction intervals in this approach were much less hampered by the color change and matched the reaction speed of normal control subjects, who did not experience synesthesia. It clearly resulted that synesthetic colors are real, not linked to memory activity.