In a recent review published in Current Opinion in Neurology researchers Safran and Sanda (2015) discuss the complexities of a rare condition called synesthesia. Synesthesia is a unique perceptual phenomena that involves a mixing of sensory modalities. A person who has this condition experiences the world in ways that are very unusual from the average person. Stimulation in any sensory modality, such as sight or smell, elicits another perceptual experience in one or more unstimulated sensory modalities. Some individuals may experience a cross-over between sound and visual percepts. The sound of a cello for instance, may cause a person to envision brightly coloured shapes moving about in his or her visual field. This is not the same experience that we have when we hear a familiar meaningful song, or smell a certain fragrance, that triggers a memory. For a person with synesthesia, the experience is automatic and is not clearly associated with any particular memory. As well, the phenomena can involve a significant emotional component: For example, some individuals with synesthesia describe feelings of pleasantness or discomfort associated with certain percepts. Indeed, the perceptual experiences of synesthetes has been of interest to many over the centuries,  from philosophers curious about the nature of perception, to synesthetic artists who have influenced styles of art. For example, some have speculated that Vincent Van Gough was a synesthete. Neuroscientists have become fascinated with this rare condition and with advancements in techniques used in functional brain imaging they have begun to make some discoveries.

Safran and Sanda comment on the complexity of this condition and note that, currently, more than 60 different synesthesia variants have been described. According to their research, synesthesia can be developmental or acquired. Developmental synesthesia appears to be the most common form, and is said to have a genetic component. While acquired forms of synesthesia are typically associated with conditions such as autism spectrum disorder, chronic migraine, multiple sclerosis or thalamic stroke. The condition can also sometimes occur in the visually impaired, or following the use of certain psychotropic drugs. 

The most common form of synesthesia is referred to as grapheme-colour synesthesia. In this type, neutral numbers or letters automatically elicit a unique colour percept. For example, when viewing the letter ‘M’ an individual with colour-grapheme synesthesia might experience an intense red colour percept. The second most common form of synesthesia is known as time unit-colour synesthesia wherein colours are associated with days of the week or months of the year.  The third most common form is musical sound-colour synesthesia, wherein listening to certain musical sounds or pieces is associated with colourful percepts.


Although more research is needed to fully understand the condition, researchers have come to assume that multiple mechanisms underly synesthesia. For example, hyperconnectivity across cortical regions has been identified, as well as decreased inhibitory feedback from certain brain structures. On the other hand, some scientists have also found that certain individuals learned their synesthetic associations in early childhood.

Due to the highly idiosyncratic nature of the synesthetic experience, and due to the fact that not much is known about the condition, many physicians either believe their patients are confabulating their experiences or they mistakenly attribute features of synesthesia to other conditions such as schizophrenia or substance abuse. Because of the stigma associated with these latter conditions, some synesthetes choose to conceal their unusual experiences. Accordingly, the number of individuals with this “rare” condition might actually be higher than is reported in the literature.

Safran, A. B., & Sanda, N. (2015). Color synesthesia. insight into perception, emotion, and consciousness. Current Opinion in Neurology, 28(1), 36-44. doi:10.1097/WCO.0000000000000169