When some people hear a high C note, they experience it as having a certain color. Others may have strong color experiences when touching different things. These are examples of the phenomenon known as synesthesia.
In the most common form of synesthesia, people have color experiences when looking at grey letters and numbers. But there are more surprising forms of synesthesia: lexical-gustatory (strong taste experiences when looking at letters), spatial time units/sequence-space (strong spatial experience when thinking about time units like the days of the week or the months of the year). And here is an especially odd-sounding one: swimming-style synesthesia (strong color experiences when seeing or thinking about a swimming style—breaststroke, crawl, butterfly, etc.).
It is widely agreed that synesthesia is intricately connected with unusual ways of exercising one’s mental imagery. Synesthetes tend to have more vivid mental imagery. Further, besides introspective reports, there is neuroimaging data that makes this connection: There is also a large body of data on the involvement of the early cortical areas of the relevant sense modality both in the case of mental imagery and in synesthesia. Synesthetes exercise their mental imagery differently from non-synesthetes, but to the extent that you have mental imagery (as the aphantasia literature shows, not everybody does), synesthesia is not too far away, either.
Now, we have an extra piece of evidence in favor of this connection. A recent set of findings show that synesthesia can be artificially induced in about half of non-synesthetes with only 5 minutes of sensory deprivation. When people who have never experienced synesthesia before are cut off from any kind of sensory stimulation for only 5 minutes, the result is that coming out of sensory deprivation, more than half of them experience some form of synesthesia.
In light of the similarities between mental imagery and synesthesia, this should not come as a surprise. We know that vividness of mental imagery comes on a spectrum. Some people don't seem to experience mental imagery at all—we even have a fancy label for this: aphantasia.
Some others have extremely vivid mental imagery—this is called hyperphantasia. Around the hyperphantasia end of the spectrum, things can go wild, and this is where synesthesia happens. But people who are closer to the middle or even the aphantasia end of the spectrum could be pushed to have synesthetic experiences if the circumstances are right.
Synesthesia is not some kind of weird condition most of us have nothing to do with. It is a much too close endpoint of a capacity most of us exercise constantly: mental imagery.
