Synesthesia: crossed wires or free association?
For millennia, philosophers have mused over the nature of perception, how closely it mirrors “reality” and whether different people might, quite without knowing it, subjectively perceive the world in very different ways. We might agree that an apple is “red”, but is the quality of your experience of its redness the same as mine? This has seemed an almost impossible nut to crack, but the fascinating condition of synesthesia provides a stark example where the quality of subjective experience is very definitively and demonstrably different. This may be due to genetic variants which affect the functional segregation of specialized circuits in the brain which makes synesthesia even more interesting.
The following is a book review I recently wrote for the American Journal of Human Biology, of: Wednesday is Indigo Blue: Discovering the Brain of Synesthesia. By Richard E. Cytowic and David Eagleman. X + 309 pp. Cambridge, MA: MIT Press. 2009.
For people with synesthesia, particular stimuli automatically and involuntarily elicit a characteristic secondary percept or feeling. Thus, hearing particular sounds may stimulate the very real perception of colored shapes in the visual field, tasting flavors may induce the tactile sensation of specific objects, and different textures may induce specific emotions. Curiously, in addition to basic sensory stimuli, many learned categories such as numbers, letters, words, calendar units, musical notes or chords can act as inducers. Indeed, colored letters and numbers (grapheme-to-color synesthesia) and the association of numbers with specific positions in space (numbers forms) are by far the most common types of synesthesia. The particular pairings, between inducing stimuli and concurrent, secondary percepts are quite idiosyncratic, though very consistent and stable over time in each individual. As such, they form an enduring and essential component of the qualitative experience of perception in synesthetes.
Cytowic can rightly claim some of the credit for bringing synesthesia – a once popular subject that languished while the strict doctrine of behaviorism held sway in psychology – back to the attention of the scientific community and the general public. Here he parlays his extensive personal research and experience with synesthetes into a wonderfully detailed description of the phenomenology of this condition.
The personal accounts of synesthetes, teased out by probing questioning, richly convey a sense of what the experience of synesthesia feels like, in as much as that is possible to convey to people who are not synesthetes. One synesthete who perceives shapes in response to sounds says: “The shapes are not distinct from hearing them – they are part of what hearing is”. These accounts, including descriptions of various synesthetes’ efforts to convey the nature of the experience through art, graphically illustrate the diversity and idiosyncrasy of synesthetic experiences. They are counterbalanced by data derived from the extensive and systematic characterization of large numbers of synesthetes by Eagleman and colleagues, and other researchers, which provide an informative and detailed overview of the condition across the population. One of the amazing findings of recent years is just how common this condition is – exact figures vary but are on the order of one to four percent of the population.
The condition often runs in families and most synesthetes report that it “has always been there”. Synesthesia has therefore attracted a great deal of recent interest as a possible case where mutation of a gene or genes results in altered organization or function of circuits mediating perception. The use of clever psychophysical experiments has verified objectively the existence of an extra percept as part of the experience of people with synesthesia. More recently, neuroimaging studies of synesthetes with sound-to-color or grapheme-to-color synesthesia have directly demonstrated activation of visual areas subserving color perception in response to aurally presented words or visually presented, achromatic graphemes. The hypothesis has thus emerged that the condition results from cross-activation of one specialized cortical area by another, normally separate one.
There are two major mechanistic explanations: the first, that there are extra axonal connections between the relevant cortical areas in synesthetes; the second is that such connections exist in all people, but are disinhibited in synesthetes. The authors consider both models as plausible explanations but advance several arguments in favor of the latter. These are based mainly on the premise that synesthetic experiences represent an extension of normal processes of multisensory integration. This resemblance seems quite superficial. In particular, while multisensory effects are ubiquitous in normal perception, they tend to be modulatory rather than driving; that is, stimulation in one sensory modality can alter the perceived timing or spatial location of sensory information in another modality (as in the ventriloquist effect) but does not generally trigger a totally new percept. In addition, recent studies have found that synesthetes do not generally show greater multisensory integration of this sort.
More pertinent evidence in favor of the disinhibition model is the fact that even non-synesthetes have a tendency to map stimuli from one sense to another (e.g., matching higher pitched sounds with brighter colors) and also that some hallucinogens can induce a pseudo-synesthetic state. The authors suggest that such mechanisms obviate the need for any extra connections in synesthetes, though why they think this would be a difficult situation to engender is not clear. There is substantial phenotypic variation in brain circuitry in the normal population, which is highly heritable, and now many examples of genetic lesions in humans that alter nervous system circuitry with behavioral consequences. A model of innate structural differences thus seems equally plausible, and even parsimonious (see review by Bargary and Mitchell).
The subtitle of the book promises to “discover the brain of synesthesia”. Exploration of this topic is largely postponed to chapter nine, but is foreshadowed so frequently in the rest of the book that I admit I skipped ahead to read it. I confess to being disappointed that the discussion remains at a somewhat metaphorical level, with abstract models of connections between cortical areas driving activity above a threshold of conscious perception. Much more realistic and detailed schemes of cortical connectivity could have been considered that would place important constraints on the models proposed to explain synesthesia.
Nevertheless, if this section leaves one somewhat unsatisfied this only serves to illustrate how much we have to learn about how the brain works and how much synesthesia, as an exception to the normal rules, will likely tell us. Whether the genes involved primarily influence the structure or function of cortical circuits, synesthesia will provide an important model to reveal the principles and processes controlling the dynamic balance between segregation and integration of cortical areas.
In particular, the authors highlight the fact that synesthesia presents a tractable paradigm of the important interaction between nature and nurture. Innate genetic effects on circuitry must interact with the experience-dependent processes that lead to the specialization of cortical areas for specific types of stimuli (such as letters and numbers, for example). Such interactions are likely to be crucial in many other psychological or psychiatric conditions such as autism and dyslexia, where cascading effects of a primary mutation over cognitive development determine the eventual phenotype. This book thus provides both an engaging window into the subjective world of synesthesia and a thoughtful introduction to the broader scientific issues it raises.
Mitchell, K. (2010). Book review: Wednesday is Indigo Blue: Discovering the Brain of Synesthesia American Journal of Human Biology DOI: 10.1002/ajhb.21039