How Does Synesthesia Work?

Synesthesia is a phenomenon in which a presentation of a stimulus, or inducer, produces additional phenomenal experiences, or concurrents, for which no physical sensory inputs exist. For instance, the letter “A” may trigger an experience of red color (Hochel and Milán, 2008) even though there is nothing that can be established as objectively red in the stimulus. Rather, the experience of color red is created exclusively internally. Synesthetic mappings of the inducing stimulus and the concurrent experience are automatic and involuntary, unique for each individual, and generally stable over time (Wollen and Ruggiero, 1983; Baron-Cohen et al., 1987; but see Simner, 2012). The prevalence of synesthesia in the normal population is estimated to be around 1–2% (Simner et al., 2006).

Synesthesia is a unique subject for research particularly because of the additional phenomenal experiences, i.e., the concurrent qualia. How and why phenomenal experience arises is part of the “hard” problem in consciousness research (e.g., Chalmers, 1995). Many requirements that are necessary for us to be conscious of the world around us can be explained in terms of functioning, information processing, circuits or systems—for instance, the integration of information by a cognitive system, deliberate control of behavior, and the reportability of mental states (also, among others, named the “easy” problems of consciousness; Chalmers, 1995). This is the relatively easier problem. However, it has turned out much more difficult to explain the mechanisms of subjective experience or “what things feel like.” Examples of subjective experiences are for instance the experience of seeing red, or the experience of feeling the wind on your face on a sunny day. This aspect of consciousness is considered harder to account for.

In the present work we suggest several additional reasons why synesthesia has merit for research on consciousness. We first review the research on the dynamic and rapidly growing field of the studies of synesthesia to inform the reader of the current state of affairs. We pay specific attention to the role of semantics in synesthesia, which seems important for establishing synesthetic associations in the brain. We then propose that the interplay between semantics and sensory input in synesthesia can be helpful for the studies of the neural correlates of consciousness, especially when making use of ambiguous stimuli for inducing synesthesia. Finally, alterations of functional connectivity and physical network connectivity discovered in the brains of synesthetes can be useful for the studies of consciousness.