Dr. Christopher Sinke, Department of Psychiatry, Social Psychiatry, and Psychotherapy at Hannover Medical School: Yes, there are people known as synesthetes who experience certain sensory impressions mixing together. The most widely studied and most common form of Synesthesia is grapheme-color synesthesia, which is the association of letters, numbers, or symbols with colors. It occurs in about 1-4% of the population. Other forms are much rarer. In general, synesthesia is categorized according to its trigger (inducer) and what is triggered (concurrent). In this case, letters trigger a color perception, meaning that people see colored letters even though they are actually printed in black.

The definition of synesthesia includes that the perceptions are triggered automatically and that there is a stable link between the trigger and the triggered. A certain letter therefore always triggers the same color Perception. In addition, the links between letters and colors are idiocratic, meaning that each synesthete has an individual link and associates the letters with different colors.

A synesthesia test or verification of synesthesia is performed by having the test subject select the color perception for each grapheme on the computer, presenting the graphemes randomly and repeatedly. The consistency of the selection for each grapheme is then examined (see https://synesthete.ircn.jp/home). Non-synesthetes are usually unable to remember many grapheme-color combinations and will therefore assign different colors to the same letter. Synesthetes, on the other hand, are much more consistent in their statements in this task because they “see” the color. In addition, graphemes are presented in a specific color and the test subject must say under time pressure whether or not this is their color for this grapheme. At this point in the test at the latest, any Memory aids collapse because the information cannot be retrieved from memory so quickly, allowing these tests to clearly distinguish synesthetes from non-synesthetes.

There are two different explanations for this phenomenon. It is generally assumed that in grapheme-color synesthesia, the areas of the brain responsible for letter and color perception are activated simultaneously. One explanation assumes that there is a direct connection between these areas of the brain. The letter and color areas are located quite close to each other in the brain. The idea is therefore that this proximity could lead to additional connections.

Another explanation assumes that the phenomenon arises from feedback processes, since the same symbol, such as an I, can have different colors depending on the context (for example, when presented with letters or numbers). Once a grapheme has been processed, the information is fed back from the Parietal lobe to the color area, thereby activating it.

Synesthetes differ from non-synesthetes in their visual imagination. Basically, there is a wide range of visual imagination, from people who see hardly anything when they imagine something to people who have a very detailed visual imagination. Synesthetes tend to be among those with a very pronounced visual imagination.

In a study, we investigated the multimodal processing of different senses in synesthetes. We played them certain word sounds, such as ABA, while simultaneously presenting a person forming the word AGA with their lips. Non-synesthetes then connect what they hear with what they see and say that the person has spoken the word ADA, i.e., a sound that lies between the original sounds. This is known as the McGurk effect.

Synesthetes, on the other hand, do not connect what they see and hear as strongly and are less likely to report perceiving “ADA.” Their senses therefore do not seem to work together as well. It could therefore be that grapheme-color synesthesia is a strategy for better connecting the different sensory impressions. This is because cooperation between sight and hearing is fundamental, especially when learning to read. The connection of a letter with a color, combined with strong visual imagination, would thus be a mnemonic device for learning to read, which becomes more solid over time. A study of schoolchildren has shown that synesthesia is a process. In first grade, for example, there were children with only three fixed grapheme-color connections, while in second grade there were already five, six, or even seven connections. However, there are not many studies in this area.

Recorded by Stefanie Flunkert