Watercolor illusion induced by synesthetic colors.

1 Introduction Synesthesia, which literally means a mixing of the senses, takes on various forms, but the most common is the experience of color when viewing achromatic alphanumeric characters (Day 2005; Rich and Mattingley 2002). Called`color-graphemic synesthesia', this condition has attracted great interest recently, in part because of its potential implications for understanding neural plasticity and cortical modularity (Grossenbacher and Lovelace 2001). The colorful, mystifying verbal descriptions offered by color-graphemic synesthetes suggest that synesthetic colors are as salient and as perceptually real as the experiences enjoyed by non-synesthetes when viewing real-colored objects. Moreover, there is a growing body of evidence in support of this equivalence, evidence showing that synesthetic colors behave like real colors on a variety of perceptual tasks. It is known, for example, that color-graphemic synesthetes can segregate a figure from its background on the basis of synesthetic colour (Palmeri et al 2002; Ramachandran and Hubbard 2001a). Color-graphemic synesthetes also tend to group achromatic letters of the same synesthetic color when viewing apparent-motion animations (Ramachandran and Hubbard 2005) and when viewing multi-element binocular rivalry displays (Kim et al, forthcoming). In addition, color-graphemic synesthetes exhibit a synesthetic`Stroop' effect: they are slower at naming the color of ink in which words are printed when those ink colors are inconsistent with the individual's associated synesthetic colors (Dixon et al 2000; Odgaard et al 1999). In our laboratory, one telltale indicator of the perceptual reality of synesthetic colors was provided by the discovery of a synesthetic McCollough effect. As described by McCollough (1965), people experience an orientation-contingent color aftereffect following prolonged, sequential adaptation to orthogonally oriented gratings differing in color. We adapted two color-graphemic synesthetes alternately to horizontal and to verticaìgratings', the contours of which were composed of achromatic letters whose spacing clearly defined orientation of the contour. The letters themselves for the two gratings were selected so as to generate synesthetic colors of red and green for horizontal and vertical, respectively. This adaptation regime failed to produce any hint of a McCollough effect in non-synesthetic observers, which is not surprising. In the two synesthetes, however, it produced the same pattern of illusory, opponent colors as those produced by adaptation to real-colored gratings (Blake et al 2005). Unfortunately, the McCollough effect requires an extended period of adaptation, and the effect itself tends to be rather weak (ie the illusory colors are highly desaturated). Moreover, this synesthetic McCollough effect is remarkably enduring: one …