What is Synesthesia

The word “synesthesia” refers to a special sort of sensory experience that combines a specific sensation with something else. (British English spells this word “synesthesia.”)

Synesthesia is the conscious experience of systematically induced sensations that are not experienced by most people under comparable circumstances. Synesthesia occurs in many distinct forms that are experienced as familiar and ordinary only by a small minority of people. In the sound-to-color form of synesthesia, sounds are both heard and seen to have one or more visual attributes such as color, texture, shape, location, or movement. In the smell-to-sound form of synesthesia, odorants are experienced as both smells and auditory attributes such as pitch and loudness. Through conducting interviews with over a hundred persons having traits of synesthesia, we have discovered previously unknown forms of synesthesia.

A thorough understanding of synesthesia challenges widely held assumptions about the relation between perceptual experience and reality. This understanding fosters genuine appreciation for each person’s uniquely individual conscious experience, regardless of whether it involves synesthesia. As a founding board member of the American Synesthesia Association, Dr. Grossenbacher is committed to helping raise people’s awareness about synesthesia as an important dimension of human diversity.

Synesthesia is a perceptual experience that comes in many different forms of synesthesia. Letters of the alphabet may appear to have color, each number may appear to occupy its own position in the space around the synesthete’s body, sounds may trigger visual sensations, and so on. Each form of synesthesia has two parts to it. The concurrent is the sensation that is synesthetically triggered (such as color), and the inducer is that which induces the concurrent (such as letter of an alphabet).

People often think that their experience of synesthesia must be completely unique, yet it turns out that many people have similar experiences. Experiences of synesthesia fall into two categories. Synesthetic perception is synesthesia triggered by sensory inducers, such as hearing a sound. Synesthetic conception is synesthesia triggered by concepts, such as thinking about months in a year. We distinguish between 3 etiologies (origins) of synesthesia: developmental, acquired, and pharmacological (drug-induced).

Developmental synesthesia runs in families, cropping up in many but not all biologically related family members. At least one in a thousand adults may regularly experience synesthesia, and the prevalence could be as high as one in two hundred people. Developmental synesthesia is a gift that has been enjoyed by famous scientists, writers, artists and musical composers, and has not been associated with any disorder.

Unfortunately, it can complicate matters when someone happens to have developmental synesthesia in addition to some medical condition that impacts perception. Most people (including educators, counselors, clinicians, doctors, and other health professionals) do not know much about synesthesia. This ignorance makes it difficult to understand which parts of unusual sensory experience can be explained as synesthesia and which may be something else entirely. There have been cases in which a person’s medication has been inappropriately changed because they were not reducing the patient’s experience of “hallucinations” that were actually innocuous experiences of synesthesia.

Only very rarely does synesthesia appear to be a symptom of pathology. Acquired synesthesia is extremely rare, and has been shown to be caused by insults to the brain such as stroke and tumor.  In pharmacological synesthesia, the experience occurs due to some pharmacological agent (drug) having been introduced into the person’s body.

The Cortical Disinhibited Feedback Theory of Synesthesia claims that normally existing neural pathways in the brain are used in an atypical manner (Grossenbacher & Lovelace, 2001). This theory explains sound-to-color synesthesia as bottom-up activation in an auditory pathway (via feed-forward neural connections) reaching a cortical area of multisensory convergence, where two or more senses interact, and this neural activity then propagates down a visual pathway (via feedback neural connections). According to this theory, the reason that most people do not experience synesthesia is that these feedback connections are normally inhibited enough so that multisensory neural activation does not propagate down the visual pathway. Points in favor of this theory include simplicity (no abnormal anatomy is assumed), and consistency with existence of drug-induced synesthesia (which presumably does not involve a temporary growth of novel anatomical connections during the drugged state).

We are still finding out about how synesthesia happens. People willing to be interviewed about their experience with synesthesia may contact Dr. Grossenbacher (see below) if interested in participating in research.