Synesthesia (Greek syn “together,” and aisthesis, “perception,” the same word that gives us “aesthetics” and “anesthesia”) is a sensory blending across modalities.  In it, one simultaneously experiences two different, apparently unrelated, sensations as a result of stimulation in a single sensory system.  One might, for instance, experience sensations of color in response to certain auditory stimuli.  Its frequency has been estimated at one in every 25,000 people (1).
        This condition is difficult to study because a great deal of the work that has been done in the past does not concern true synesthesia, but rather merely metaphor.  This primarily comprises what Donnell and Duignan call “associated synesthesia,” in which cross-modal sensation may be artificially conceived, but are not actually experienced.  In keeping with humanity’s fondness for unified theories, there has historically been a great deal of interest in the possibility that there may be some absolute extrinsic correlates to our subjective experience.  The first recorded effort to associate sound and color was by Sir Isaac Newton, who, having discovered that white light consists of a discrete spectrum of colors, sought to find a way to correlate these colors with the seven-note keys in our musical system.  He is said to have associated red with C, orange with D, and so on through violet with B.  It is as a result of his efforts that the visible spectrum is said to have 7 colors (I mean, indigo? c’mon).  Many composers also theorized about synesthesia, taking to its logical extreme the belief that one can express certain thoughts and ideas with patterns of sound (2).  Liszt commonly used color terminology when describing his music, and Mozart described the key of A major as brilliant, “a pattern of many colors” (3, 4).  Even more extreme was Alexander Scriabin, whose composition Prometheus was performed in 1911 on a special instrument he had designed, a “light keyboard” that displayed certain colors along with the music (2).  Artists, too, such as Vasily Kandinsky, worked to fuse music with visual media.  He described his painitngs as “compositions” and “improvisations.”  His one-act opera, “The Yellow Sound” stipulated a specific mixture of color, light, dance and sound (1).
        Certain sensory pairings are more common than others.  Color seems to be the most prevalent sensation evoked.  The fewest reported cases are of smell and taste triggering synesthetic events, although one well-documented subject, MW, experienced tactile responses to taste, and others have perceived colors alongside odors.  Though it has been previously reported (2) that there is no clear-cut evidence as to whether individuals share consistent patterns of synesthetic responses, as the Accounts section of this paper indicates, there does not appear to be any such agreement.