The Man Who Tasted Shapes (paperback, Kindle), by Richard Cytowic, is the story of one of the neurologists who brought synesthesia back into the field of view of psychologists. Synesthesia is a neurological condition in which stimulation in one sense leads to involuntary (and consistent) experiences in another sense. (Famous artists believed to be synesthetes include Billy Joel, David Hockney, and Itzhak Perlman.) In some of the more common variants of synesthesia, individual letters on a printed page, or individual musical notes, are involuntarily experienced as having colors. While synesthesia was known and widely studied even in the 19th century, by the mid 20th century the predominant view was that synesthesia was not really interesting to investigate and that the phenomena that synesthetes experienced were just "in their heads". Unfortunately, before long, few doctors were even aware that synesthesia existed, despite the fact that estimates of its prevalance are generally agreed to be at least 1 in 2000. Thanks to the work of Cytowic and others, synesthesia is now something that gets covered in an intro psychology course (the one I took, anyway).
The first part of the book is sort of a memoir and a medical detective story, in which Cytowic— working with a couple of patients over a period of years— assembles clues about synesthesia, digs deep into the literature, and begins to conduct experiments to learn more about its genesis. While the dialogues do seem a bit overdramatized and redundant at times, it is a good portrait of how science is done. Not the technology part— the fMRI and other imaging devices are only incidental to the plot— but rather the imaginative part of science: synthesizing theories based on the mass of available evidence.
The conclusion of the "mystery" is a bit anticlimactic, since we still don't have a complete understanding of what mechanism is responsible for synesthesia, just a few tantalizing clues. Cross-talk between regions of the brain responsible for processing different senses has been implicated. In fact, all newborns are born synesthetic but lose that ability over time as connections in the brain are pruned. Interestingly, though, a substantial fraction of people appear to have latent synesthetic abilities. Studies have shown that the incidence of synesthesia among trained meditators is about one hundred times higher than the baseline prevalence! So it appears that synesthesia represents a natural background process of the brain that is simply unavailable to conscious view in a select few people.
This observation is a springboard for the second part of the book, which is a series of essays on "the primacy of emotion". Synesthesia is not an isolated phenomenon in the respect I mentioned: much of what goes on in our brains is not accessible to self-awareness except possibly under extraordinary circumstances. It now seems that the conscious mind is in fact not even in the driver's seat when it comes to planning and execution. (See: Bereitschaftspotential) The essays in this section are a reflection on the implications of this somewhat unsettling model of mind. I enjoyed some of the essays but found just as many of them to be completely opaque.
Despite its flaws I found this to be an enjoyable and eye-opening read. Recommended.