Human beings are very metaphoric creatures. We love to juxtapose the experience of one sense with another. Whether it’s taste with touch – a sharp cheese, a smooth finish to a beer, or sight with sound – “I see what you’re saying,” or smell with sight – a putrid color.

For some people though, this kind of joining isn’t metaphoric, it’s how they experience the world. The phenomenon is called synesthesia, and while rare it’s very real. Synesthetics have one sense, or more, that responds to sensations from an unrelated sense. The most common form of synesthesia is called colored hearing. The people who have it get impressions of color, and sometimes of shape from the sounds that they hear. The responses may be for only a few specific kinds of sound – loud, sudden noises, for instance, or they may color every sound the synesthetic hears. Other sense pairings have also been found: taste with touch, sight with smell, and in one instance, sound with kinesthesia (body positioning). Synesthetics tend to have strong emotional responses to their experience, often pleasurable, though not always, and are distressed if their synesthesia is blocked in some way. They also tend to have extremely good memories, particularly in regard to their affected sense.

For many decades when neurology was first blooming as a modern science, synesthesia was not a recognized condition. It left no signs in the brain structures, so it couldn’t be seen on a CAT scan or an MRI. It produced no signs of disease or abnormality in neurological tests. The only evidence for synesthesia was the reports of the people who had it, and they were mostly dismissed as hallucinating or insane. Most synesthetics quickly learned not to report their odd way of seeing the world, lest they be considered crazy.

Then in 1980 a neurologist named Richard Cytowic began exploring synesthesia, starting with a neighbor who experienced flavors as shapes. Over the next several years he was able to establish that synesthesia had nothing to do with the imagination. Instead synesthetic experience seems to arise from the limbic system, well before conscious thought enters the picture. The limbic system is heavily involved in emotion and memory, which makes sense of the synesthetic’s typical emotional response and vivid memory. The limbic system and its connections to the cerebrum tend to act as a filter for the consciousness, sorting information from the senses, deciding what deserves to be brought to the attention of the conscious mind. That filtering function seems to provide the barriers that keep our senses discrete. In the synesthetic, the conscious mind seems to be accessing the sensory data at an earlier stage in the game, so that it arrives with all the additional impressions that would normally be filtered out.

At least one neurologist has opined that everyone is, or rather has been a synesthetic. Her belief is that babies may experience the world in this way until their developing cortex allows them to segregate sensory inputs. Some evidence for this comes from adults with newly acquired disabilities. Damage to a sensory system can lead to temporary synesthesia as another sense stimulates a response in the newly unused one. The newly blind, for instance, may suddenly experience colored hearing, with bolts or blots of color arising in response to sounds. The effect fades over time. If the source of synesthesia is indeed the filtering of data from the limbic system, then the synesthesia would be a result of the attempt to fill in the sudden blanks, and resultant confusion over what is important in the remaining sensory input. The fading would occur as the brain adjusts to its new state.

If this last hypothesis is true, then down deep in all of our memories is a repository of synesthetic experience. Could this be what gives us our inherent love of metaphor?