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provided by Frontiers - Publisher Connector OPINION ARTICLE published: 26 September 2013 doi: 10.3389/fpsyg.2013.00651 Synesthesia, at and near its borders

Lawrence E. Marks 1,2* and Catherine M. Mulvenna 3

1 John B. Pierce Laboratory, Department of Environmental Health Sciences, Yale School of Public Health, New Haven, CT, USA 2 Department of Psychology, Yale University, New Haven, CT, USA 3 Child Study Center, Yale University School of Medicine, New Haven, CT, USA *Correspondence: [email protected] Edited by: Michael Banissy, Goldsmiths University of London, UK Reviewed by: O. Deroy, University of London, UK Keywords: synesthesia, cross-modal correspondence, cross-modal imagery, cross-modal memory, empathic perception, hallucination, Doppler illusion

INTRODUCTION sensory (cross-modal) autobiographical Cross-modal correspondences often In synesthesia, experiences in one domain memory, empathic perception, hallucina- reflect alignments between bipolar dimen- evoke additional experiences in another, tion, and the Doppler illusion. Do any or sions, such as higher pitch being associated as when musical notes or letters of the all of the six constitute forms of synesthe- with greater lightness, greater bright- alphabet evoke colors. Both the domains sia? The answer depends, we suggest, on ness, higher vertical location, and smaller and their pairings are diverse. Indeed, the framework for characterizing synes- size (e.g., Karwoski et al., 1942; Wicker, Day’s (2013) recently tabulated 60 types thesia. Consequently, after describing the 1968; Marks, 1974, 1989; Ward et al., of synesthesia, each referring to a differ- six phenomena, we sketch three frame- 2006). Several auditory-visual correspon- ent combination of inducing and induced works that differ in how they characterize dences reveal themselves in young children domains. The domains conjoined through these phenomena relative to prototypical (Marks et al., 1987; Mondloch and synesthesia may belong to different sense forms of synesthesia. Several investigators, Maurer, 2004) and infants (Lewkowicz modalities, as in music-color synesthesia, taking different perspectives and coming and Turkewitz, 1980; Walker et al., 2010; but may also belong to the same modal- to different conclusions, have already con- Haryu and Kajikawa, 2012), as well as in ity: In grapheme-color synesthesia, seeing sidered possible relations to synesthesia in denizens of disparate cultures: Members printed letters or numbers evokes color three of the six: cross-modal correspon- of a remote, semi-nomadic, preliterate experiences. dence (Martino and Marks, 2001; Deroy desert-tribe in southern Africa, having vir- In music-color and grapheme-color and Spence, 2013); cross-modal imagery tually no contact with Western culture, synesthesia, the inducing stimuli are (Craver-Lemley and Reeves, 2013; Spence nevertheless overwhelmingly matched perceptual, reflecting culture-specific cat- and Deroy, 2013); and empathic percep- lightergraycolorstohigher-pitched egories (notes of the Western musical tion (Fitzgibbon et al., 2010; Rothen and tones—thereby revealing pitch-lightness scale, letters of the alphabet) learned by Meier, 2013). correspondence (Mulvenna, 2012). synesthetes and non-synesthetes alike. The tendency for non-synesthetes to Synesthesia may be triggered not only SIX AT THE BORDERS: SYNESTHESIA’S perceive similarities between experiences by sounds, tastes, smells, and pains, but FAR AND NEAR KIN? in different domains, despite the absence also by more complex signals: words(e.g., CROSS-MODAL CORRESPONDENCE of secondary qualia, has been called Simner, 2007), emotional states (e.g., Cross-modal correspondences pervade “synesthetic thinking” (Karwoski et al., Ward, 2004), and even personalities not only several forms of traditional 1942) and “weak synesthesia” (Martino (e.g., Novich et al., 2011). Analogously, synesthesia but also, importantly, the expe- and Marks, 2001), consistent with the the domains of synesthetic responses too riences of individuals typically deemed notion that cross-modal correspondence can range widely. The composer Rimsky- non-synesthetic (Marks, 1975, 1978; reflects general perceptual and cogni- Korsakoff “saw” the key of D-major as Spence, 2011). Even non-synesthetes per- tive processes. Further, by capitalizing on golden (Myers, 1914), while a grapheme- ceive high-pitched vs. low-pitched sounds cross-modal correspondences, synesthesia personification synesthete reported, “Ts to resemble bright vs. dark colors—the too presumably capitalizes on these gen- are generally crabbed, ungenerous crea- resemblances evident in various tasks of eral processes of perception and cognition tures” (Calkins, 1893; p. 454). Other cross-modal comparison (Marks, 1975; (e.g., Karwoski et al., 1942; Marks, 1978; phenomena, however, such as cross- Ward et al., 2006). Where music-color Ward et al., 2006). modally evoked images or memories, synesthetes see brighter colors in high- are not typically considered examples of pitched notes (e.g., “gold, yellow and CROSS-MODAL IMAGERY synesthesia. white moving ...like a rippling stream”: Compared to cross-modal correspon- In this article, we briefly describe half a Mulvenna and Walsh, 2005; p. 399), people dence, which can lack induced qualia, dozen illustrative cases that border on tra- lacking the induced qualia of synesthe- cross-modal imagery is nearer, phe- ditional forms of synesthesia: cross-modal sia nevertheless recognize cross-modal nomenologicaly, to prototypical synes- correspondence, cross-modal imagery, similarities. thesia. In cross-modal imagery, as in

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synesthesia, stimulation in one modal- touched may produce an analogous tactile simple recurring shapes and patterns, or ity may arouse mental images in another, sensation—often called “mirror touch” ‘form constants (Klüver, 1966), which although cross-modal imagery exhibits (e.g., Banissy and Ward, 2007). Although apparently reflect patterns of neural acti- greater voluntary control (e.g., Karwoski the inducing stimuli come from another vation in visual cortex (Ermentrout and and Odbert, 1938). Synesthetic responses modality—typically, vision or hearing— Cowan, 1979). commonly arise automatically,without the mechanisms underlying empathic No longer considered explicitly patho- requiring effort and being under relatively perception presumably rely on an under- logical, hallucinations are now generally little control (e.g., Mattingley et al., 2001). lying within-domain equivalence: where treated as independent perceptual phe- By comparison, some non-synesthetes implicitly-recognized sensations evoke nomena (Romme and Escher, 1989;see can voluntarily conjure up images, for sensory experiences of the same or similar Strauss, 1969). Auditory verbal hallucina- example, imagining colors while listen- kind, perhaps through merging constructs tions (hearing voices), for instance, occur ing to music (Karwoski et al., 1942). In of “self” and “other.” in about 13% of adults in the general some instances, there may be an espe- Possibly related to empathic pain is the population (Beavan et al., 2011), and cially intimate connection between visual couvade syndrome, which refers to a set of their presence does not correlate signifi- imagery and prototypical sound-color empathic symptoms (such as nausea, vom- cantly with psychopathology (Johns et al., synesthesia. Karwoski and Odbert (1938) iting, and abdominal pain) reported by the 2002; Sommer et al., 2010). Although the inferred that a small subset of their sub- partners of pregnant women. The couvade strongest predictor of psychopathology in jects experienced visual imagery that could syndrome appears to be fairly common, hallucinations is distress over their con- be modulated by music—a phenomenon having a reported prevalence of about tent or possible basis (Romme and Escher, that seems more automatic (less volun- 22% (Lipkin and Lamb, 1982), roughly 1989; Chadwick and Birchwood, 1994; tary) than typical visual imagery, albeit less five times that of traditional synesthesia Beavan et al., 2011), distress is rarely asso- automatic (more voluntary) than synes- (Simner et al., 2006). ciated with synesthetic experience. thesia. Perhaps music-modulated imagery A feature that distinguishes empathic bears an especially close connection to perception from traditional forms of THE DOPPLER ILLUSION traditional music-color synesthesia. synesthesia is the very characteristic that Day’s (2013) table listing 60 types of makes empathy empathic—the intrin- synesthesia includes only one type that is SENSORY AUTOBIOGRAPHICAL MEMORY sic equivalence between the emotional explicitly intra-modal, namely, grapheme- (PROUST PHENOMENON) qualities of the inducing and induced colorsynesthesia.Wenotehereanother Marcel Proust (1922) famously described experiences. In traditional forms of synes- phenomenon in which sensory experi- the floods of detailed, sensory memories thesia, however, as when sound evokes ences in one domain induce experiences from childhood evoked by tasting a tea- visual color or shape, the inducing and in the same modality, the “Doppler illu- soaked madeleine. Sensory, autobiograph- induced sensations not only reflect differ- sion,” reported by Neuhoff and McBeath ical memory of this sort has been dubbed ent domains but are also usually related (1996): When a tone increases continu- the “Proust phenomenon” in honor of more abstractly, even “metaphorically” ously in intensity over time (as though the eponymous author. In the Proust phe- (Rothen and Meier, 2013). a sound-emitting source were approach- nomenon, odors or flavors in particu- ing at constant velocity) but maintains a lar evoke strong sensory-based memories HALLUCINATION constant sound frequency, observers nev- of associated events experienced in child- A hallucination is a “percept-like expe- ertheless report hearing the tone’s pitch hood (Chu and Downes, 2000). Proustian rience which (a) occurs in the absence to increase as loudness increases. Neuhoff memory resembles traditional synesthe- of appropriate stimulus, (b) has the full and McBeath dubbed the illusory increase sia, but also differs from it—resembling force or impact of the corresponding in pitch the “Doppler illusion” because synesthesia in the automatic manner in (real) perception, and (c) is not amenable a sound source approaching at constant which sensory experiences evoke memory to direct and voluntary control by the velocity will produce, at the observer’s images, but differing in the episodic char- experiencer” (Slade and Bentall, 1988; location, an elevated, albeit constant, acterofthememories.Inthisregard,the p. 23). Several of these attributes also char- sound frequency (the physical Doppler sensory qualia of traditional synesthesia acterize synesthesia. Hallucinations may effect). But might we not also call the seem more “semantic” than “episodic.” involve any of the senses (Ohayon, 2000) Doppler illusion a case of intra-modal and are easily distinguished as “percep- (loudness-pitch) synesthesia? EMPATHIC PERCEPTION: PAIN, TOUCH, tions not confirmed by others” (Ohayon, COUVADE SYNDROME 2000; p. 154). Some types of halluci- SYNESTHESIA: CONTINUOUS, In several respects, empathetic per- nations, though not all, may fall near DISCRETE, PLURALISTIC? ception strongly resembles synesthesia. the borders of synesthesia. Thus, synes- If, as the term implies, synesthesia is first In empathic pain, seeing or hearing thetic experiences commonly include col- and foremost a “conjoining of experi- evidence of another person’s pain or ors and shapes (Day, 2013). Analogously, ences,” then one might construe several discomfort produces analogous pain or “simple hallucinations,” often triggered by or perhaps all six of our cases as exam- discomfort (e.g., Jackson et al., 2005). migraines or hallucinogens (Ermentrout ples of synesthesia. If, on the other hand, Similarly, seeing another person being and Cowan, 1979), commonly include synesthesia is defined more narrowly, for

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example, by requiring it to include qualia contains a cornucopia of distinct sub- daysyn.com/types-of-syn.html (Accesed May and to arise automatically and consis- categories, lacking common denomina- 17, 2013). tently, then fewer cases would make the tors but perhaps linked one to another Deroy, O., and Spence, C. (2013). Why we are not all synesthetes (not even weakly so). Psychon. Bull. cut. Recently, the first author outlined along the lines suggested by Wittgenstein’s Rev. 20, 643–664. doi: 10.3758/s13423-013-0387-2 three theoretical frameworks—monism, (1953) notion of family resemblance. Eagleman, D. M. (2012). Synaesthesia in its pro- dualism, and pluralism—that, in differ- Critical, in our view, to choosing tean guises. Br.J.Psychol.103, 16–19. doi: ent ways, characterize how synesthesia amongst frameworks is characterizing 10.1111/j.2044-8295.2011.02020.x could relate to borderline perceptual and the role of phenomenal experience in Ermentrout, G. B., and Cowan, J. D. (1979). A mathematical theory of visual hallucina- conceptual phenomena like the six just defining synesthesia; many investiga- tion patterns. Biol. Cybern. 34, 137–150. doi: described (Marks, 2011, 2012). tors judge this role to be significant 10.1007/BF00336965 Synesthetic monism refers to the notion (see, e.g., the exchange among Cohen Fitzgibbon, B. M., Giummarra, M. J., Georgiou- that synesthesia may appropriately be con- Kadosh and Terhune, 2012; Eagleman, Karistianis, N., Enticott, P. G., and Bradshaw, J. sidered a spectrum or continuum. Using 2012; Simner, 2012a,b). Monism in par- L. (2010). Shared pain: from empathy to synaes- thesia. Neurosci. Biobehav. Rev. 34, 500–512. doi: this framework, traditional forms of synes- ticular relies substantially on the notion 10.1016/j.neubiorev.2009.10.007 thesia, such as music-color and grapheme- that phenomenal experience plays a Haryu, E., and Kajikawa, S. (2012). Are higher- color, serve as prototypes, residing at the central, and ineluctable, role in char- frequency sounds brighter in color and smaller high end of the continuum, with weaker acterizing synesthesia. Alternatively, in size. Auditory-visual correspondences in 10- month-old infants. Infant Behav. Dev. 35, 727–732. forms, such as cross-modal correspon- jettisoning phenomenology may be con- doi: 10.1016/j.infbeh.2012.07.015 dence, residing toward the low end. In ducive to pluralistic frameworks that Jackson, P. L., Meltzoff, A. N., and Decety, J. the present examples, music-modulated rely on mechanism-based distinctions (2005). How do we perceive the pain of others. imagery, hallucinations, empathic percep- amongst multiple forms of synesthe- A window into the neural processes involved tions, Proustian evoked memories, and sia. And jettisoning phenomenology in empathy. Neuroimage 24, 771–779. doi: 10.1016/j.neuroimage.2004.09.006 the Doppler illusion might lie at vari- may be especially conducive to dualistic James, W. (1890). The Principles of Psychology. ous loci between the ends of the synes- frameworks that rely on a mechanism- Vol. 1. New York, NY: Henry Holt. doi: thetic spectrum—although the differences based distinction between synesthesia 10.1037/10538-000 amongst them suggest that the hypothe- and borderline phenomena—perhaps Johns, L. C., Hemsley, D., and Kuipers, E. (2002). A sized spectrum is multidimensional. akin to distinguishing mechanistically comparison of auditory hallucinations in a psy- chiatric and non−psychiatric group. Br. J. Clin. The other two frameworks both dis- between rhinovirus-induced sniffles and Psychol. 41, 81–86. doi: 10.1348/014466502163813 tinguish sharply between synesthesia pollen-induced seasonal nasal allergies. Karwoski, T. F., and Odbert, H. S. (1938). 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