A Book Review--Synesthesia: A Union of the Senses by Richard E. Cytowic, M.D.

        In the second edition of his book Synesthesia: A Union of the Senses, Dr. Cytowic draws on his intimate knowledge of the development and workings of the human brain, as well as his ample experience with synesthetic patients of various ages and backgrounds, to define the basic characteristics of the phenomenon, search for a probable location for the physiological integration of synesthetic percepts, and relate synesthesia to common neurological processes that allow each one of us to interpret our surroundings.  Along the way, Cytowic provides a current model of the architecture and connectivity of the brain and neural tissues, as well as some interesting examples of how synesthesia can inspire the creation of art and music.  
 

        The book begins with a brief history of synesthesia and of Cytowic’s introduction into this field of research, accompanied by descriptions of many of Cytowic’s patients and their unique symptoms.  This introduction is both interesting and very helpful for the reader who may be unfamiliar with the specifics of synesthesia.  The book then moves into several theories that have been proposed for the basis of the phenomenon, including linkage theories, abstraction theories, and Cytowic’s own favored hypothesis: that of polymodal combination at some intermediate level of signal processing, which would allow for experiences that were less concrete than the most basic, “one-to-one” sensory to perception associations, and yet less abstract than the highest-level of cognition in which one sensation can evoke different perceptions or meanings, depending on context (76, 82). 

        Cytowic goes on to describe numerous experiments used to analyze the cognitive and perceptive abilities of patients with synesthesia and similar neurological conditions (ex: people who experience eidetic imaging, release hallucinations, or temporal lobe epilepsy) under normal conditions and also (in some cases) under the influence of substances such as amphetamine and ethanol.  This particular section of the book can be a bit confusing for those who are unfamiliar with the above conditions or the particular neuropsychological tests that are mentioned, although Cytowic provides summaries after each series of analyses.  

        Throughout this experimental portion of the book, Cytowic repeatedly refers to what he considers to be the five diagnostic criteria for synesthetic percepts: that they be involuntary but elicited, spatially extended, consistent and discrete, memorable, and emotional (summarized on 67-70).  He also contrasts his more clinically-based approach to synesthesia research, focusing on patients’ descriptions of physical sensations, to those methods used by two other researchers whom he considers to join him in the forefront of his field: Simon Baron-Cohen, who is more of an experimentalist and Hinderk Emrich, a psychiatrist who focuses on the emotional aspect of synesthesia (64-65). 

        One interesting section of the book, and one that reminds me of some of the discussions from our class, is Cytowic’s overview of the current understanding of the brain’s architecture and function, including how this understanding has changed over the course of time.  In particular, he emphasizes how the initial ideas of linear information flow, localized areas of mental function, and neocortex-dominated hierarchy have been replaced by the more modern view of nonlinear information flow between all areas of the brain, with functions being distributed over wider areas (212-216).  Just as our class focused on the important role of the reafferent loop, corollary discharge signals, and surrounding chemicals in determining nervous system output, Cytowic reminds his readers that internal information exchange and non-electrical signals such as hormones can also affect perception (241). 

        Having reviewed the brain’s structure, Cytowic returns to the idea of intermediate-level polymodal combination, and suggests that this level of sensory integration takes place somewhere in the limbic system.  He argues that a person with synesthesia is able to essentially “intercept” the sensory information before it has been processed by the cortex, leading to perceptions that are more concrete (ex: a particular color always associated with a particular letter, no matter what the context) than the more semantically-abstract perceptions that can result from further processing, but also more complex (ex: involving multiple perceptions, and sometimes emotion) than the most basic levels of processing (349). 

        In terms of our class discussions, then, the synesthete’s “I-function” is able to access information, (and thus the synesthete is able to become aware of certain perceptions) at a level to which the rest of us do not consciously have access.  Cytowic cites the existence of the more common phenomenon of cross-modal association (ex: bright colors being associated with high-pitched sounds and sharp objects, dark colors being associated with low-pitched sounds, etc…) as evidence that certain stimuli, or perhaps at least certain intensities of stimuli, in terms of the rate of neuron firing, can affect more than one sense (276, 283).   Such cross-modal associations have been shown to be present in children as young as 3 ½ years.  One theory holds that all newborns are synesthetes, and then gradually lose synesthetic percepts while gaining the ability to make cross-modal associations (273).  From the topic of cross-modal associations, Cytowic briefly visits the physical basis of metaphor, and the connections between synesthesia and language, providing an enlightening look at how common metaphors such as love as “a journey” or the mind as “a machine” can bridge the gap between rational/physical and emotional, and between the objective and the subjective (279-282).  So, our class’s adopted metaphors of the neuron as a computer, the semi-permeable membrane as a battery, and the different areas of the nervous system as boxes in fact illustrated yet another unique ability of the human mind: the capacity to use language to connect objects and ideas that evoke similar thoughts and experiences.

        Towards the end of Synesthesia, Cytowic takes the time to discuss two synesthetes, Olivier Messiaen and David Hockney, who used their synesthetic percepts to guide their art (composing music and designing sets to accompany musical productions, respectively).  This section is incredibly fascinating, and the author’s conversation with Hockney reveals totally unique perspectives on, for example, the use of color (“It’s a special characteristic of color, that the more you see it the more there is…Light and dark is a factor too.  If it’s bigger, then you know it’s not dark.  It becomes something else…Blue has this quality to being spatial, which other colors do not.” (315)).

        Cytowic then uses the phenomenon of vision, including color constancy and color and brightness perception, to demonstrate that the brain does indeed put sensory information through a complex, multi-level modification process, in order to construct a picture that is not “really” there.  His argument is very similar to the one we made in class regarding the “picture in our head” or our “personal reality” versus what is real.  Cytowic comments that, when the lack of common percepts among synesthetes leads some people to doubt synesthesia’s existence, the question is not whether or not synesthesia is real, but “Real to whom?  To the questioner or the person that has it?” (8).

        Synesthesia is tied in some way to many of the topics that we considered in class this semester, including sensory perception, nervous system architecture, objective versus subjective experience, conscious versus subconscious, feedback, the construction of reality, and patterns of behavior.  As such, it is not surprising that some of Cytowic’s arguments parallel our own discussions.  Overall, Synesthesia: A Union of the Senses is a worthwhile read that explores the characteristics and possible origins of synesthesia, but also goes beyond this topic, making connections to a wide variety of biological, psychological, and cultural topics, and illustrating how our perceptions shape our interactions with the world.

Works Cited:

Cytowic, Richard E., M.D. Synesthesia: A Union of the Senses. 2nd ed. Massachusetts Institute of Technology 2002