Synesthesia is a “neurological-based condition in which stimulation of one sensory or cognitive pathway leads to an automatic, involuntary, experiences in a second sensory or cognitive pathway.” For example, a scent could trigger a color, or a number could trigger a color. In both cases, neither experiences are related but they are automatically triggered. According to neurologist and synesthesia specialist Dr. Richard Cytowic’s book published in 2002, there are four diagnostic criteria for synesthesia:

  1. Synesthesia is involuntary and automatic.
  2. Synesthetic perceptions are generic and consistent.
  3. Synesthesia is memorable.
  4. Synesthesia leaves behind emotion or feeling.

Those who experience synesthesia are referred to as synesthetes.


Currently, there are at least 63 types of synesthesia. The most common forms of synesthesia are grapheme-color synesthesia, sound-color synesthesia, and number form synesthesia.

Grapheme-color synesthesia is a type of synesthesia where a grapheme, a number or letter, triggers a color. For example, the letter ‘Z’ could trigger the color brown.

What colors the synesthete sees in a given set of graphemes is called a synesthete’s photism.

By Mysid, Wikimedia Commons

This image is a grapheme-synesthete’s photism of the word ‘synesthesia’ and the numbers 0 to 9.
Although photisms among grapheme-synesthetes vary, some synesthetes find commonalities in some letters. For example, the letter ‘A’ is mostly seen as red among grapheme-color synesthetes.

Sound-color synesthesia is a phenomenon that occurs when a sound (i.e. music, someone’s voice, noise) stimulates a color. For example, a song could trigger a sound-color synesthete’s  Like grapheme-color synesthesia, trends among sound-color synesthetes are similar. For example, a louder tone produces a brighter color and a softer tone produces a darker color. Alexander Scriabin, a famous Russian composer in the 1800s, was known to have sound-color synesthesia. His photisms of keys is shown in a color wheel below.

MegaMatic, Wikimedia Commons

Number form synesthesia is a type of synesthesia where a mental map of  numbers are automatically and involuntarily appears. Research has shown that number form synesthesia is caused by the “cross-activation” in parts of the parietal lobe that are responsible for number and spatial cognition. Francis Galton, a famous eugenecist, explorer, inventor, and half-cousin of Charles Darwin, studied a patient with number form synesthesia. This research by Galton can be found in the book The Visions of Sane Persons. This is the photism of Galton’s subject:

Galton, Wikimedia Commons

These are the most common forms of synesthesia. The other types of synesthesia that have been identified are:

  • Personification
  • Lexical–> gustatory
  • Time units-colors
  • Grapheme Personification (OLP)

A more comprehensive list can be found here.