SYNESTHESIA REVISITED
A REVIEW OF RICHARD CYTOWIC'S SYNESTHESIA: UNION OF THE SENSES.
Noam Sagiv
Department of Psychology
University of California
Berkeley, CA 94720-5050
U.S.A.
noam@socrates.berkeley.edu
Copyright (c) Noam Sagiv 2003
PSYCHE, 9(16), December 2003
KEYWORDS: synesthesia, synaesthesia.
REVIEW OF: Richard Cytowic (2002). Synesthesia: Union of the senses. Cambridge, MA: MIT Press. ISBN: 0262032961
ABSTRACT: In the second edition of Synesthesia: Union of the Senses,Cytowic reviews the varieties of synesthetic experience, theories of synesthesia and in particular, possible neural bases. Neuroscience research relevant to his framework for understanding synesthesia is extensively reviewed as well as new research on synesthesia published since the publication of the first edition.
Interest in synesthesia has increased steadily in the past few years. Cytowic's books Synesthesia: Union of the Senses and The Man Who Tasted Shapes played a major role in this renaissance, as did the Psyche symposium on synesthesia. Thirteen years after the first edition was published, Cytowic re-evaluates his ideas on synesthesia, discusses further phenomenology and reviews new findings.
While the book is written primarily for neuroscientists, others will find interest at least in the extensive review of synesthetic sexperiences including "simple" sensory-sensory synesthesia (such a tasting shapes or seeing voices), synesthesia involving language or conceptual information (such as colored letters or days of the week), as well as less familiar phenomena such as the assignment of a personalities or moods to numbers. Of particular interest is a new chapter on the spatial extent of synesthesia including "number forms" and other mental maps in which a variety of information may be spatially organized, in an idiosyncratic yet consistent manner (e.g., peculiar time or number lines). Numerous descriptions in synesthetes' own words bring to life what could have been a rather tiresome grocery list of unusual phenomena otherwise. Moreover, these first hand accounts enable us to appreciate not only the similarities, but also some of the differences between synesthetes' experiences.
This detailed report is in contrast with a relatively impoverished report of the phenomenology typically found in the scientific literature. Take for example the common chromatic-graphemic synesthesia and consider what happens when such synesthetes see written letters or words: some of these individuals perceive colors projected externally with a well-defined spatial extent but others may report seeing the colors in their mind's eye. Furthermore, the color could be perceived on the letter surface (either seen letter or another "imagine" copy of the letter) or elsewhere. Thus we already have four different qualitatively different groups clustered together, before even beginning to consider potentially varying vividness of the synesthetic experience and additional synesthetic features that may be present, e.g., textures or perhaps affective components. Naturally, by clustering all these in one group without considering such substantial differences, we risk averaging out interesting effects (Smilek and Dixon, 2002).
Does Cytowic emphasize phenomenology at the expense of third-person approaches? He does discuss in length much data pertaining to the neurobiology of perception in general and specific related phenomena such as hallucination or temporal lobe epilepsy. However, to date, little direct evidence to support his (or anybody else's) neurobiological theory of synesthesia is available, but there is also a shortage of concrete suggestions of ways to directly test these claims. The motivation for his limbic theory of synesthesia appears to rely heavily on such experiential factors as synesthesia's memorability and the strong feelings some synesthetes have about sensory combinations that are either congruent or incongruent with their synesthetic pairing. Nevertheless, one cannot overemphasize the importance of phenomenology here. In this front, Cytowic's work is a step forward: Everybody may know what attention is, but relatively few experience synesthesia. Given that synesthetes have demonstrated surprise time and again when realizing others do not perceive the world as they do, it is likely that there are aspects of synesthesia we have not been informed about. This
will not change unless we ask more questions and examine closely synesthetes' reports in addition to recording their reaction times or cerebral blood flow.
At the neurobiological level, the fundamental question according toCytowic is – "where is the link?". He opposes the widely-held believe that additional cortico-cortical connections underlie "cross-talk" between the senses in synesthesia. Instead, as mentioned earlier, he proposes that the link is limbic, i.e., synesthesia is subserved by existing connections in transmodal brain areas rather than supplementar connections between unimodal cortices. Somewhat reluctantly, he does admit that unimodal cortical areas may be necessary for experiencing the synesthetically evoked percept itself, although some of the recent findings available to support this idea did not make it into the book (Aleman et al., 2001; Nunn et al., 2002). New techniques such as diffusion-weighted magnetic resonance imaging may help resolve the matter. Nevertheless, even if future findings will favor the idea that synesthesia relies on existing transmodal nodes rather than new connections, one key question remains unanswered: What is the difference between synesthete and non-synesthete brains? A central idea in Cytowic's framework for understanding the neurobiology of synesthesia is hemispheric differences. He proposes that synesthesia is basically a left hemisphere function resulting from delayed left-hemisphere development. This idea is consistent with non-standard lateralization seen in synesthetes. But Cytowic does not explain how synesthetes differ sfrom other individuals with non-standard brain lateralization.
Cytowic's grand view of brain function and cognition is thought provoking: For the most part, he does not view synesthesia as an abnormality. The idea that infants may experience some form of synesthesia or at least sensory-confusion in the first few months of life (Maurer, 1997) is generally accepted as plausible, i.e., that synesthesia may be a normal phase of early sensory development. But when it comes to adults, most researchers view synesthesia as an abnormality at some level (breakdown of modularity, defective pruning of cortico-cortical connections, failure of differentiation, and so on). Cytowic proposes an intriguing alternative. He views synesthesia as a state in which a fundamental mode of cognition is merely brought to the surface or exposed in some individuals or situations (cf. Merleau-Ponty, 1962; Shanon, 2002). According to Cytowic, synesthesia is "a normal brain process that is prematurely displayed to consciousness in a minority of individuals" (p. 2). Regrettably, this noteworthy idea remains rather vague throughout the rest of the book. It should be noted that it is in line with results showing some systematic cross-sensory
correspondences in normal perceivers (e.g., pitch and brightness; e,g, Marks, 1974). Still, the complex and apparently arbitrary nature of some cross-sensory correspondences in synesthetes remains largely unaccounted for.
The neurobiological study of synesthesia is only in its infancy. We have very few answers and many more questions at this point. Although at times Cytowic appears more confident than he should be, the second edition of Synesthesia: Union of the Senses remains a valuable source for anyone interested in synesthesia.
REFERENCES
Aleman, A., Rutten, G.J., Sitskoorn, M.M., Dautzenberg, G., & Ramsey, N.F. (2001). Activation of striate cortex in the absence of visual stimulation: an fMRI study of synesthesia. Neuroreport, 12,
2827-30.
Marks, L.E. (1974). On associations of light and sound: The mediation of brightness, pitch, and loudness. Journal of Psychology, 87, 173-188.
Maurer, D. (1997). Neonatal synaesthesia: Implications for the processing of speech and faces. In S. Baron-Cohen & J.E. Harrison (Eds.) Synaesthesia: Classic and contemporary readings. (p. 182-207). Oxford: Blackwell Publishers.
Merleau-Ponty, M. (1962). Phenomenology of Perception. New York: Routledge & Kegan Paul. p. 228-229.
Nunn, J.A., Gregory, L.J., Brammer, M., Williams, S.C.R., Parslow, D.M., Morgan, M.J., Morris, R.G., Bullmore, E.T., Baron-Cohen, S., & Gray, J.A. (2002). Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words. Nature Neuroscience, 5, 371-5.
Shanon, B. (2002), The Antipodes of the Mind: Charting the Phenomenology of the Ayahuasca Experience. Oxford: Oxford University Press.
Smilek, D. & Dixon, M. (2002). Towards a Synergistic Understanding of Synaesthesia: Combining Current Experimental Findings With Synaesthetes' Subjective Descriptions. Psyche, 8.
