Introduction

For persons with synesthesia, ordinary stimuli can elicit extraordinary experiences. For C.S., pain triggers a perception of the color orange, and for M.W., the taste of spiced chicken is pointy (Cytowic, 1993). When H.G. hears a person speak, the voice he hears elicits a multisensory experience of both color and taste. For example, the first author's voice induced the color brown and evoked a taste experience that was “hard to describe, but somewhat like syrup!” All of these extraordinary experiences reflect variants of synesthesia—a union of senses. For synesthetes like C.S., M.W., and H.G., the inducing stimulus leads to experiences that cross modalities (e.g., for H.G., sounds trigger colors and tastes). For others, the inducing stimulus and the synesthetic experience occur in the same modality. For example, in grapheme-color synesthesia, viewing black digits or letters induces a conscious experience of color.

In our laboratory, we have learned much about this extraordinary condition by asking different synesthetes to give detailed subjective descriptions of their experiences (Smilek & Dixon, 2002). When C., a 21-year-old grapheme-color synesthete, is shown black digits or letters (e.g., 1, 3, 7, B, H, Z) in addition to seeing the black graphemes, she claims that she also perceives colored overlays that appear on top of each digit and each letter. Each grapheme has its own highly specific color (called a photism), and viewing a specific grapheme (e.g., a 7) invariably elicits the perception of that particular photism. As C. relates, “It is difficult to explain . . . I see what you see. I know the numbers are in black . . . but as soon as I recognise the form of a 7 it has to be yellow.” Although for a given synesthete the mappings between graphemes and colors remain consistent over time (i.e., for C., a 7 has been yellow ever since she can remember), these mappings tend to be different for every synesthete. For example, when J. is shown a black 7, rather than seeing yellow, her overlay is “purple with a hint of blue, darkly lit up from behind as though there were a brownish red LED behind.”

Two general principles emerge when considering the grapheme-color pairings for different synesthetes (Table 52.1). The first is that the for almost all of the synesthetes we have interviewed (and all those who have been described in the literature), the digit 0 induces a colorless photism appearing to different synesthetes as either white, black, gray, or clear. (Similar correspondences have been noted for the letter O; Baron-Cohen, Harrison, Goldstein, & Wyke, 1993.) The second principle is one of diversity: other than 0 and O, the grapheme-color pairings appear to be idiosyncratic. These two principles highlight a theme of this chapter, namely, that there are important individual differences among synesthetes, yet in the face of such diversity, there are crucial commonalities. We argue that one key aspect of synesthetic experience, namely, where a photism is experienced, allows two important subgroups of grapheme-color synesthetes to be identified.

Based on our interviews of more than 100 synesthetes, we have learned that they can differ markedly in where they experience their photisms. For C., J., and P. D., their photisms appear “out there, on the page,” as though a transparency bearing a colored number were superimposed over the black grapheme. We refer to synesthetes like these as projectors, an allusion to their photisms being perceived in external space (see also Cytowic, 1989, 1993). For other synesthetes, the synesthetic color is described as appearing “in my mind's eye” or “in my head” (e.g., synesthete G.S., described by Mills, Boteler, & Oliver, 1999). For these synesthetes their experience can be likened to that of a nonsynesthete viewing a black-and-white picture of a stop sign. We “know” the stop sign is red, but do not project this color onto the picture. We refer to synesthetes like G.S. as associators because their descriptions of their experiences are more reminiscent of a strong semantic association between a grapheme and a color as opposed to a perceptual experience of color. We believe that this novel distinction between projector and associator synesthetes can be extremely important both in evaluating previous research and in devising new experiments.

Here we describe a number of recent experimental investigations from different laboratories designed to further our understanding of synesthesia. Although the research that is reviewed pertains to investigations of grapheme-color synesthesia, many of the principles that emerge are relevant to cases of cross-modal synesthesia where the inducer is in one modality and the synesthetic experience is in another modality. Our main conclusions are the following: (1) photisms are an automatic consequence of viewing alphanumeric characters, (2) projector synesthetes can be empirically distinguished from associator synesthetes by using methods based on the Stroop effect, (3) under certain presentation conditions (e.g., masking, visual search), the photisms experienced by projector synesthetes alter the ease with which black graphemes can be identified, and (4) both the graphemic form and the meaning of alphanumeric characters contribute to determine the color of projected photisms.