The prevalence of synesthesia: The Consistency Revolution Oxford Handbooks Online The prevalence of synesthesia: The Consistency Revolution Donielle Johnson, Carrie Allison, and Simon Baron-Cohen Oxford Handbook of Synesthesia Edited by Julia Simner and Edward Hubbard Print Publication Date: Dec 2013 Subject: Psychology, Cognitive Neuroscience, History and Systems in Psychology Online Publication Date: Dec 2013 DOI: 10.1093/oxfordhb/9780199603329.013.0001 Abstract and Keywords We begin this chapter with a review of the history of synaesthesia and a comparison of what we consider to be either genuine or inauthentic manifestations of the phenomenon. Next, we describe the creation and development of synaesthetic consistency tests and explore reasons why assessing consistency became the most widely used method of confirming the genuineness of synaesthesia. We then consider methodologies that demonstrate synaesthesia's authenticity by capitalizing on properties other than consistency. Finally, we discuss how together, consistency tests and other methodologies are helping researchers determine prevalence and elucidate the mechanisms of synaesthesia. Keywords: synaesthesia, consistency, Test of Genuineness, prevalence A Brief History of Synesthesia Research Traditionally, the term synesthesia describes a condition in which the stimulation of one sensory modality automatically evokes a perception in an unstimulated modality (e.g., the sound of a bell leads the synesthete to experience the color pink; Baron-Cohen, Wyke, and Binnie 1987; Bor, Billington, and Baron-Cohen 2007; Marks 1975; Sagiv 2005). While this definition describes a cross-sensory association, synesthetic experiences can also be intrasensory (e.g., the letter “g” triggers a blue photism when read). The stimulus (bell or “g”) that triggers the synesthetic perception is referred to as an inducer, while the resulting experience or percept (blue) is called the concurrent (Grossenbacher 1997; Grossenbacher and Lovelace 2001). Not all inducers are sensory, however. Page 1 of 26 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: University of Cambridge; date: 12 July 2016 The prevalence of synesthesia: The Consistency Revolution Grossenbacher and Lovelace (2001) used the term synesthetic conception to refer to synesthetic experiences in which inducers are concepts. For example, in “time-space” synesthesia months take on locations in space, and this can occur whether the months are read, heard, or even thought about (Grossenbacher and Lovelace 2001). Throughout this chapter, when we use the term synesthesia we are referring to genuine, developmental synesthesia. Developmental synesthetes generally report having synesthesia for as long as they can remember and cannot provide an explanation or learning account of their experiences (Baron-Cohen et al. 2007). Genuine synesthesia (p. 4) does not arise voluntarily, nor is it learned through training, acquired through drug use, or induced (solely) by any neurological pathology.1 Learned or acquired associations (e.g., in those familiar with the number-color coding on cross-stitch needles) can be difficult to distinguish from genuine synesthesia. Over- rehearsal can lead to a high level of performance similar to synesthetes on tests designed to assess synesthesia (i.e., the synesthetic Stroop test; discussed in more detail later). Non-synesthetes have also been shown to experience learned or trained associations as automatically as true synesthetes do (Elias et al. 2003; Meier and Rothen 2009). This in turn has led some to suggest that synesthesia itself is merely learned. However, a learning account of synesthesia cannot explain why siblings with synesthesia reared in essentially identical environments (or at least exposed to the same alphabet teaching materials or children’s books) report different colors for the same inducer, or experience different variants of synesthesia altogether (Barnett et al. 2008). Metaphors, too, can seem synesthetic. It is not uncommon, for example, for non- synesthete artists and musicians to ascribe colors or other characteristics to pieces. A composer might refer to a musical piece as heavy and blue because of its tone or the mood it evokes, not because it literally triggers a heavy, blue percept for him or her. Synesthetic metaphors are not exclusively used by artists. Many people describe wine or cheese as “sharp” or “robust.” However, the use of metaphor and imagery should be distinguished from true synesthesia, in which percepts are induced automatically and involuntarily (Baron-Cohen and Harrison 1999; Simpson and McKellar 1955). Having established what we consider as synesthesia in this article, we will now review the recent history of research into this phenomenon. Despite its popularity in the late nineteenth century, by the mid 1930s there was a considerable reduction in the psychological literature on synesthesia (Marks 1975). The problem for researchers then and now is that synesthesia is an internal, subjective experience. Although some early studies incorporated objective verification (Calkins 1893, and see later), most relied on descriptions and reports given by synesthetes themselves. In the belief that it would restore objectivity to the field of psychology, the Page 2 of 26 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: University of Cambridge; date: 12 July 2016 The prevalence of synesthesia: The Consistency Revolution school of behaviorism, which emerged in the 1920s and 1930s, rejected the study of subjective, mental phenomena, or required rigorous independent evidence of their existence. One casualty of this was synesthesia research. With few or no objective tests to validate self-reports from synesthetes and thereby distinguish them from people with a vivid imagination or a faculty for generating striking pseudo-synesthetic metaphors, the condition was “[no longer] considered amenable to scientific investigation” (Harrison and Baron-Cohen 1997, 4). When behaviorism was supplanted by cognitivism in the latter half of (p. 5) the twentieth century, however, new methods were introduced to assess the genuineness of synesthesia. This ushered in a synesthesia “renaissance” characterized by rigorous empirical investigation which continues to this day. Since the earliest days of synesthesia research, we have moved beyond merely proving that synesthesia exists. Much of this progress can be attributed to advancements stemming from a relatively simple but powerful test developed (or re-discovered, see later) in the 1980s. This test allows researchers to quickly determine whether a person is a genuine synesthete, and ultimately opens the door for a more thorough examination of the condition. In this chapter we will describe the creation and development of this test, now known as the Test of Genuineness (TOG) and discuss reasons why its central tenet, assessing the consistency of synesthetes’ reports, became the first “gold standard” method for authenticating reported synesthesia (Rich, Bradshaw, and Mattingley 2005; Simner et al. 2006; Ward and Mattingley 2006). We will also consider recent examinations of synesthesia’s other features and show that together, consistency tests, behavioral tasks, and neuroimaging methods are helping researchers establish prevalence rates, shed light on neural and cognitive mechanisms underlying the phenomenon, and use synesthesia research to inform our understanding of typical cognition. Synesthetic Consistency and the First Test of Genuineness Early investigators meticulously recorded the experiences of synesthetes, primarily providing detailed case descriptions and reproducing drawings of reported percepts when possible. A comparison of early studies reveals differences of opinion regarding definitions (Phillips 1897), prevalence rates (Calkins 1895; Phillips 1897), heritability (Flournoy 1893), and what might be considered typical manifestations of the same form of synesthesia (e.g., what constitutes “normal” number-form visualizations for synesthetes; Galton 1883; Phillips 1897; see later). However, the constancy of synesthetic Page 3 of 26 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: University of Cambridge; date: 12 July 2016 The prevalence of synesthesia: The Consistency Revolution percepts is one point of agreement: for any particular synesthete, the synesthetic experience is typically the same for any given trigger (e.g., if the letter “A” is red for a synesthete, it will tend to always be red for that synesthete, over days, months, years, and even decades).2 In one early study, for example, Dresslar (1903) aimed to assess the stability of a young woman’s synesthesia over a period of 8 years. Whether she chose color words from a dictionary