The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017


Erica Goode wrote in the New York Times, “Most people experience the sensory world as a place of orderly segregation. Sight, sound, smell, taste, and touch are distinct and separate. With synesthesia, in which the customary boundaries between the senses appear to break down, sight mingles with sound, or taste with touch” (1999). The word synesthesia derives from the Greek words syn meaning together and aesthesis meaning perception. Synesthesia is a perceptual condition whereby sensations from one sense are simultaneously perceived together by one or more additional senses in unusual ways. A person who has this condition is referred to as a synesthete. Some synesthetes hear, smell, or taste in color; some taste shapes; some see people surrounded by color; and others perceive written digits, letters, and words in color. Examples of this “joined perception” are seeing the number 7 as being purple or the sound of your mother’s voice as being green; seeing a yellow border around your best friend; seeing units of time as shapes; and feeling music as a tickling sensation on the back of your neck.

History and Indicators

Synesthesia has been known to exist for the past 300 years and was thought to be quite rare, mainly because people with the condition maintained silence about their experiences when they realized theirs were not typical. There are probably many people who have the condition but do not know it; estimates range from 1 in 200—300 to 1 in 100,000 people having some variation of the condition. Besides synesthesia, synesthetes have some other similarities: they tend to be female with three times as many women as men in the United States having synesthesia, while the number jumps to eight times as many women as men in the United Kingdom; they tend to be left-handed; they are of normal to above average intelligence and have normal standard neurological exams. Synesthesia runs in families and seems to be a dominant trait, possibly residing on the X-chromosome. The writer Vladimir Nabokov (1899—1977) and the physicist Richard Feynman (1918—1988) reputedly were synesthetes, as is the singer Mary J. Blige (1971—).

More Common Types of Synesthesia

It is estimated that there are more than 70 different types of synesthesia. These depend on the way that the synesthete’s brain processes sensory information. Rarely, synesthetes experience more than one type of synesthesia. Following are a few of the most common types:

1.Grapheme-color synesthesia—Individuals with this type of synesthesia experience letters or numbers as a color.

2.Chromesthesia—These synesthetes associate sounds with colors.

3.Lexical-gustatory synesthesia—These synesthetes experience words as taste.

4.Auditory-tactile synesthesia—Synesthetes with this synesthesia experience sensations on their bodies when hearing certain sounds.

5.Spatial sequence synesthesia—Individuals with spatial sequence synesthesia experience sequences at various different points in space.

Further Research

Synesthesia presents an intriguing problem because logically it should not be a product of the human brain, where the evolutionary trend has been for increasing anatomical separation of function. Studies have confirmed that synesthesia is biological, automatic, and apparently unlearned, distinct from both hallucination and metaphor. Researchers ascribe synesthesia as “crossed wires” in the brain. Neurologist Richard E. Cytowic (1952—) hypothesized in the 1980s that the condition’s cause rested in the limbic system, the more emotional and primitive part of the brain. His case studies and book written in 1993, The Man Who Tasted Shapes, brought attention to the condition and prompted psychologists and neuroscientists to research various hypotheses. Research teams found that synesthetes’ experiences are consistent across time. They also established that synesthesia is concretely measurable in the brain, using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Other studies demonstrate that synesthetic perception occurs involuntarily and interferes with ordinary perception. Studies have also shown an interesting effect in the cortex. When colored-hearing synesthetes hear certain words, they display activity in the areas of the visual cortex associated with processing color. Nonsynesthetes do not show activity in these areas, even when asked to imagine colors or to associate certain colors with certain words (Nunn et al., 2002).

It is unclear which parts of the brain are involved in synesthesia. It is proposed that synesthetes’ brains are genetically equipped with more connections between neurons, which causes the usual limited sensory communication to break down. Other speculations are that we all may be born with abundant connections but most of us lose those connections as we mature. Modern behavioral, molecular, genetic, and brain-imaging tools hold exciting promise for uncovering how synesthesia operates and for better understanding how the brain normally organizes perception and cognition.

Synesthesia may reveal something about human consciousness. No one knows how we bind all of our perceptions together into one complete whole, one complete concept. The additional perceptions that synesthetes have add to the concept. Studying synesthesia might help us understand the nature of human cognition and perception and how the concept of similarity is embedded within the nervous system. Equally important is the idea that synesthesia can bring unique abilities to a creative person, which might help bring significant contributions to the world.

Carolyn Johnson Atwater

See also: Auditory-Tactile Synesthesia; Grapheme-Color Synesthesia; Lexical-Gustatory Synesthesia; Mirror-Touch Synesthesia; Spatial Sequence Synesthesia

Further Reading

Cytowic, Richard E. (2003). The man who tasted shapes. Cambridge, MA: MIT Press.

Goode, Erica. (1999). When people see a sound and hear a color. New York Times. Retrieved from

Nunn, J. A., L. J. Gregory, M. Brammer, S. C. Williams, D. M. Parslow, M. J. Morgan, … J. A. Gray. (2002). Functional magnetic resonance imaging of synesthesia: Activation of V4/V8 by spoken words. Nature Neuroscience, 5(4), 371—375.