Synesthesia is not a new phenomenon. It has appeared in the written literature for centuries and has piqued the interests of many critical thinkers, including philosophers, psychologists, psychiatrists, and theologians. It has now entered a different scope with new interest in the phenomenon by scientists who study vision, cognition, and the brain.
Unlike color generated from light waves or odors by chemical compounds, the color, smell, sound, taste, or touch that is experienced by synesthetes is generated by a physical stimulus that for most of us is entirely unconnected to its induced sensation (e.g., middle C invokes the sight of red: the shape of a ball invokes the taste of chocolate).
For instance, while wavelength induces color perception in both synesthetes and nonsynesthetes alike, additional inducers such as particular shapes or sounds can also evoke color perception for synesthetes.
The varied manifestations of synesthesia and its phenomenological nature have made it difficult to verify and study, and skeptics abound. However, recent scientific evidence, as indicated by the contributions to this volume, demonstrates that the question of its existence as a “real” phenomenon is no longer in doubt (although its prevalence remains debatable). Neuroscientific evidence using functional imaging techniques have shown brain activation in predicted areas that corresponds to synesthetes reported experiences (e.g., synesthetic color activates areas that normally respond to color).
Behavioral data have also substantiated the perceptual reality of synesthesia, and cognitive scientists have moved on to questions such as how synesthesia might be related to perceptual learning, perceptual organization, and attention and to what degree the mechanisms that support synesthesia operate similarly to or differently from mechanisms underlying non-synesthetic experience.
Synesthesia may have even wider implications. It might represent a basic mechanism for the development of metaphors, but in a more vivid form, and it may even question fundamental assumptions about the nature of biological systems. The chapters in this volume were solicited to stimulate thought across a wide spectrum, from computations that could produce such phenomenon to philosophical questions of functionalism that the existence of synesthesia may question.
Synesthesia has become more than a curiosity. It is a phenomenon for a larger part of the population than we originally thought. It is a real perceptual experience induced by stimulation that does not induce a similar experience for most individuals. It is a scientific puzzle, and it raises fundamental issues about how it may relate to “normal” perception and how brains must work such that they can generate such phenomena.
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