Introduction

Certain otherwise normal individuals when presented with a stimulus in one sensory modality may vividly perceive additional sensations in unstimulated sensory modalities, a condition known as synesthesia. For example, a synesthete may experience a specific color whenever he or she encounters a particular tone (e.g., C-sharp may be blue) or may see any given number as always tinged a certain color (e.g., 5 may be green and 6 may be red). One long-standing debate in the study of synesthesia has centered on the issue of whether synesthesia is a perceptual or a conceptual phenomenon. To resolve this issue, we have conducted experiments on 11 grapheme-color synesthetes, with extensive testing on two of them (J.C. and E.R.). These results suggest that grapheme-color synesthesia is a sensory effect rather than a cognitive one (Ramachandran & Hubbard, 2001a).

We have previously reported that synesthetically induced colors are able to influence perceptual grouping, lead to perceptual pop-out, and can be experienced even when the inducing grapheme is not consciously visible (Hubbard & Ramachandran, 2001; Ramachandran & Hubbard, 2000, 2001a). Based on these results, we argued that synesthesia is a sensory phenomenon and that it may be caused by genetically mediated, persistent neural connections, causing cross-wiring between brain maps that process color (V4 or V8) and visual graphemes, which lie adjacent to each other in the fusiform gyrus (see also Hubbard & Ramachandran, 2003).

We identify two different subtypes of number-color synesthesia and propose that they are caused by hyperconnectivity between color and number areas at different stages in processing. “Lower synesthetes” may have cross-wiring (or cross-activation) within the fusiform gyrus, whereas “higher synesthetes” may have cross-activation in the angular gyrus. We then discuss the implications of the study of synesthesia for understanding other aspects of the human mind, such as creativity and metaphor. Finally, we discuss the implications of synesthesia for the philosophical problem of qualia.