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Synaesthesia: The existing state of affairs

Matej Hochel and Emilio G. Milan

Departamento de Psicologa Experimental, Universidad de Granada, Granada, Spain

In synaesthesia one type of stimulation evokes the sensation of another, such as when hearing a soundproduces photismsthat is, mental percepts of colours. In the past, the idiosyncrasy of this phenom-enon, as well as the natural mistrust of scientists towards the subjective, consigned synaesthesia to theperiphery of scientific interest. However, the landscape has changed radically in the last two decades.

The labour of many researchers, inside as well as outside of cognitive neuroscience, has transformedsynaesthesia into a scientific reality whose existence can be demonstrated and studied empirically. Thepresent paper summarizes and reflects on our current knowledge concerning synaesthesia in all itsaspects (cognition, behaviour, neurology, genetics, and demographics).

Keywords: Synaesthesia; Cross-modal perception; Neurocognitive models of synaesthesia.

When R. listens to music, his mind generatescolours that do not exist in the real world asperceived by normal people. He perceives classi-cal music as dark brown, electronic music asmostly purple, and symphonic compositions asred (Milan et al., 2007). Undoubtedly, in amore distant past this kind of perception wouldhave been considered witchcraft, and not so longago R.s extraordinary ability to see music couldhave led him to an imposed stay in a mental insti-tution (Cytowic, 1993; Day, 2005). Even todaymany people with synaesthesia prefer not to talkabout their condition, trying to act normal inorder to avoid the common hostility towards theunusual and the unknown (Day, 2005). Theterm synaesthesia comes from Greek, meaning

joined perception (aesthesis translates percep-tion, and syn joined, together). In synaesthetesone type of stimulation evokes the sensation ofanother, such as when hearing a sound producesphotismsthat is, mental percepts of colours.Quite frequently this condition involves a trans-duction of a learnt semantic category (e.g.,letters, numbers, human faces, or days of theweek) into a sensory experience (e.g., perceptionof synaesthetic colour).

The first legitimate report on synaesthesia isattributed to Sachs in 1812 (cited in Krohn,1982; also in Dann, 1998), who described thecondition in himself and his sister as a part of aPhD dissertation on his albinism. In the nine-teenth century there might have been a dozen of

Correspondence should be addressed to Matej Hochel, Departamento de Psicologa Experimental, Universidad de Granada,

c/Campus de Cartuja s/n, 18071, Granada, Spain (E-mail: [email protected]).Edward M. Hubbard provided helpful comments on the manuscript. We would also like to thank three anonymous reviewers for

their comments. Matej Hochels research activities are supported by a scholarship from Agencia Espanola de CooperacionInternacional (AECI), Ministry of Foreign Affairs (Spain). This study was supported by grant (SEJ2006 09029) from theSpanish Ministry of Education and Science to E. G. Milan.

# 2008 Psychology Press, an imprint of the Taylor & Francis Group, an Informa business 93http://www.psypress.com/cogneuropsychology DOI:10.1080/02643290701822815

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way similar to normal mental imagery.Meanwhile, other synaesthetes speak about per-ceptions that are projected externally onto the eli-citing stimulus, appearing as a coloured aurasurrounding the synaesthetic inducer.

The idiosyncrasy and the subjective nature of

the phenomenon make synaesthesia difficult tofit into common scientific taxonomy.Furthermore, confusion arises from the impreciseuse of the term synaesthesia, which has beenapplied to a wide array of phenomena, rangingfrom idiopathic synaesthesia (a naturally occurringcross-modal perception), to altered states of con-sciousness caused by psychotropic substances, tometaphorical language, and even to visual artsand theatre performances (Cytowic, 1996, 2002).

Despite this phenomenological heterogeneity,

Cytowic (1996, 2002) tried to establish a seriesof diagnostic criteria in order to distinguishidiopathic synaesthesia from different psychologi-cal conditions (e.g., hallucinations and drug-induced experiences) and artistic extravagancies.Following Cytowic (2002), synaesthetic percep-tion is:

1. Involuntary and automatic.2. Consistent and generic.3. Spatially extended.4. Memorable.

5. Affect-laden.

Before providing a more detailed description ofthese features, it should be noted that the firsttwo (i.e., synaesthesia being involuntary and auto-matic, consistent, and generic) are probably theleast problematic and the most agreed upon inthe scientific community. The rest of Cytowicscriteria are more controversial, and, rather thanbeing diagnostic in the strict sense of the word,they represent features that are likely (but notnecessarily) to be observed, to a higher or lesserdegree, in the majority of synaesthetes.

The involuntary character of synaesthesia refersto the impossibility to manipulate synaesthetic

perception or to suppress it by will (e.g., Dixon,Smilek, Cudahy, & Merikle, 2000; Mattingley,Rich, Yelland, & Bradshow, 2001; Wollen &Ruggiero, 1983). When a graphemecoloursynaesthete sees a printed character (e.g., theletter R), simultaneously he will perceive a

colour halo surrounding the grapheme. Thefeeling is much different from that of a memorybrought to ones mind due to some kind of associ-ation with certain sensory aspects of the outerworld. While usually we are able to stop thinkingabout unpleasant memories, it is not possible tostop seeing, hearing, or smelling external stimuliunless you eliminate sensory input. The sameapplies to synaesthesia in the sense that it is vir-tually immune to any voluntary control.

Synaesthetes reports suggest that synaesthesia

is acquired very early during development and itlasts for a lifetime.1 Once established, synaestheticassociations remain unchanged; when asynaesthete is presented with a series of inducersacross multiple time points, he or she will experi-ence the same synaesthetic concurrentsin responseto the triggering stimuli. Studies report on consist-ency measures with testretest periods of weeks,months, or even years. For instance, Baron-Cohen, Wyke, and Binnie (1987) studied a caseof a synaesthete who experienced photisms in

response to spoken language. In a preliminaryinterview they asked E.P. to describe in detailthe colours she saw when listening to 103 differentauditory stimuli (words, letters, and numbers).After 10 weeks they did a retest. The participantsanswers were 100% consistent with respect to theprevious experimental session. In contrast, a non-synaesthete participant who was asked to associatecolours with the same inducers was far less accu-rate in her responses. (With a testretest periodof 2 weeks only, the measure of consistency wasless than 17%.) In brief, the connection betweeninducing stimuli and synaesthetic responses isextremely stable over time, and, as is shown later,it cannot be explained by memory performance.

1Typically synaesthetes claim they have been synaesthetic from as early as they can remember. No cases of spontaneous remissionof synaesthesia have been reported so far even though synaesthetic capacity can sometimes be lost as a consequence of cerebral trauma.(See Sacks, Waserman, Zeki & Siegel, 1988; Spalding & Zangwill, 1950.)

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In addition to its consistency and durability,there is another aspect of synaesthesia that makesit different from other phenomena such ashallucinations present in psychotic disorders.Synaesthetic responses are typically generic; theycorrespond to basic perceptual qualities such as

colour, texture, and fundamental visual forms,tactile sensations, and so on. Moreover, quiteoften the synaesthetic percepts, through theirassociation to the inducing stimulus, mayenhance the synaesthetes memory by serving asadditional memory cues. (For instance, numbercolour synaesthesia may help to remember tele-phone numbers.) Even though Cytowic (2002)argues that synaesthetic experience is never pictor-ial or laden with semantic content, it should benoted that there have been cases of lettercolour

synaesthetes who, when they hear a word (e.g.,table), may actually see the letters (T-A-B-L-E) spelled out in colour.

When synaesthetes describe their subjectiveexperience, they often speak about colours beingprojected onto a written character (graphemecolour synaesthesia) or about visual entities on ascreen situated a few inches in front of onesface (audio-visual synaesthesia). Cytowic (1993)reports the case of a synaesthete who experiencedtactile sensations in response to gustatory stimu-

lation. The participant used to alter the positionof his hands in order to better reach for thefeeling. All these aspects illustrate the spatialquality of synaesthetic sensations. However, thisfeature is less obvious if we consider those peoplewhose synaesthesia is more similar to visualimagery. (See associator synaesthetes further onin this paper.) This variety of synaesthesia is alsocompletely automatic but the ability to spatiallylocalize is uncertain, given that percepts are notprojected externally.

Finally, a series of authors (Cytowic, 2002;Milan et al., 2007; Ramachandran & Hubbard,2001b; Ward, 2004) have emphasized therelationship of synaesthesia with emotion.Frequently synaesthetes claim experiencing plea-sant sensations that accompany synaesthetic per-ception and are sometimes similar to a Eureka!feeling (Cytowic, 2002). Occasionally,

synaesthesias can also be related to negative feel-ings, particularly when the synaesthetic perceptionis incongruent with the outer reality. (For example,when a graphemecolour synaesthete sees a letterprinted in a different colour from that of theassociated photism.) Certain types of synaesthesia

are directly related to the emotion. For instance,R., a synaesthete reported by Milan et al. (2007),experienced mental colours in response to faces,human figures, and visual scenes with emotionalcontent. Normally the colours experienced werecongruent with R.s emotional assessment of theperson or the visual stimulus in question. (R.often used the photisms to refine his opinionsabout people.) Very rarely, the synaestheticaura experienced by R. was not congruent withrespect to the personal relationship that

R. maintained with a personfor example, whena good friend of his had a very unpleasant greencolour. This kind of incoherency was extremelyuncomfortable to R. and was accompanied bynegative emotions. A few similar cases have beenreported in literature, one that is particularly inter-esting is a report by Ward (2004) about G.W., asynaesthete who experienced mental colours inresponse to human faces, known persons names,and affect-laden words.

Empirical demonstrations of synaesthesia

Initial scepticism around synaesthesia among psy-chologists, neurologists, and other professionalswas, at least in part, due to the lack of experimentalmethods that would allow for an objective demon-stration of the phenomenon. A few early exper-imental studies actually examined the hypothesisthat synaesthesia was a consequence of a classicalconditioning mechanism (Howells, 1944; Kelly,1934). With the intent to corroborate this propo-sal, they trained nonsynaesthete participants toassociate arbitrary pairs of tones and colours.Even though some participants were quite success-ful in this memory task, there was no evidence ofany type of accompanying colour perception(Rich & Mattingley, 2002), such as that reportedby synaesthetes. Behind these early attempts toempirically approach synaesthesia, it took

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decades for scientists to return to the subjectmatter and finally invite synaesthetes to participatein experimental studies that could confirm orrefute the genuineness of their condition.

As we mentioned before, one of the indicatorsthat strongly supports the reality of synaesthesia is

its constancy over time. In order to assess the con-sistency of a synaesthete, the participant is normallyshown a series of synaesthetic inducers (e.g., a gra-pheme for a graphemecolour synaesthete). His orher task is to simply report on the elicited synaes-thetic perception (for example, the colour of thephotism). (In more sophisticated designs, a standar-dized colour set or a software colour palette can beused to match the colours of the photisms more pre-cisely; e.g., Milan et al., 2007; Witthoft & Winawer,2006.) The experimental session is replicated days,

weeks, or even months later. Following this meth-odology, virtually all studies report consistencylevels that are very close to 100% (e.g., Baron-Cohen et al., 1987; Dixon et al., 2000; Mattingleyet al., 2001). The stability of synaesthetic associ-ations is maintained over time, even when assessedafter a time gap of several months (Baron-Cohen,Harrison, Goldstein, & Wyke, 1993).

However, the high consistency by itself does notreveal the nature of the underlying neurocognitivemechanisms. Not so long ago, the most common

explanation of synaesthesia was in terms ofmemory associations, possibly due to a learningexperience in early childhood (Ramachandran &Hubbard, 2001b, 2003a). In theory, a lexicalsynaesthete could have played with refrigeratormagnets with coloured letters and consequentlycould have developed powerful associationsbetween printed characters and specific colourhues. A better understanding of the nature ofsynaesthesia required converging evidence from anumber of sources: experiments designed todemonstrate that the experience of photisms isnot under voluntary control; perceptual exper-iments to demonstrate that synaesthetes doindeed experience synaesthetic percepts whenexposed to inducing stimuli; subjective reports tosupplement and inform these objective inquiries.

In the 1980s, a series of studies sought touncover whether synaesthesia was a perceptual or

a memory phenomenon. Most of these studies(as well as much of the current research) workedwith the most common graphemecoloursynaesthetes, employing modifications of theStroop task (Stroop, 1935). Standard design con-sisted in presenting a printed character that was

either of the same colour (congruent) or of a differ-ent colour from the synaesthetes photism (incon-gruent) and then asking the synaesthete to namethe print colour (e.g., Mills, Boteler, & Oliver,1999; Wollen & Ruggiero, 1983). Dixon,Smilek, Cudahy, and Merikle applied this samelogic in a more recent study (Dixon et al., 2000)with C., a synaesthete who reacted synaestheticallyto Arabic numerals. Stimuli consisted of a coloursquare (base line) or a coloured number whosehue was congruent or incongruent with respect

to C.s photism. The stimuli were presented on acomputer screen in random order. C.s task wasto identify and say the colour of the stimulus asfast as possible. As expected, C. was significantlyslower when responding to incongruent colours(797 ms, 2.8% error rate) than when respondingto the congruent colours (525 ms, 1.4% errors)and the base line (545 ms, 0.0% errors).

This result has been replicated in several studies(e.g., a study with 15 synaesthetes by Mattingleyet al., 2001), revealing clearly the automaticity of

synaesthetic perception. However, this kind ofresult does not demonstrate whether synaesthesiais a genuinely perceptual process or not.MacLeod and Dunbar (1988) trained nonsy-naesthete participants to associate black andwhite geometric shapes with the names of differentcolours. After thousands of trials, the participantswere tested on a Stroop task where the originalshapes were presented either in the same or indifferent colours with respect to the originalmatching. The results showed a clear interferencepattern that could only be explained as an effect ofthe previous excess of learning. These data suggestthat a simple associative mechanism, based on amemory performance, could be sufficient toexplain the reaction times of synaesthetes.Nevertheless, subjective reports do not fit withinthis hypothesis. Synaesthetes normally do notspeak in terms of remembering or imagining



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a colour hue, when exposed to an inducer stimu-lus. In their verbal descriptions they typicallymention a halo of specific colour tone (Smilek &Dixon, 2002), concrete tactile sensations(Cytowic, 1993), or tastes on the tongue(Ward & Simner, 2003). In other words, these

reports suggest that synaesthesia is much betterdescribed as a sensory phenomenon, and it cer-tainly cannot be seen as a high-level memoryassociation (Ramachandran & Hubbard, 2001a,2001b, 2003a).

In order to test this hypothesis it was necessaryto explore to what degree the synaesthetic colourslead to sensory effects observed with real colours.Ramachandran and Hubbard (2001a) studiedtwo synaesthetes (J.C. and E.R.) who experiencedphotisms upon observing letters and numbers. To

see whether this was a sensory process or not, theauthors set up number matrixes that, due to theirvisual characteristics, could be perceived as eithera vertical arrangement (a series of columns) or ahorizontal one (i.e., numbers grouped in rows).When a visual feature leads to formation of clus-ters that are perceived as wholes, it is assumedthat such a feature is genuinely perceptual (Beck,1966; Treisman & Gelade, 1980). For instance,if a series of neighbouring elements in a matrix isof a different colour from the rest, it will be recog-

nized as a distinctive group, standing out in thebackground. This phenomenon, termedperceptualgrouping, occurs in response to basic visual featuressuch as colour, shape, and orientation. In

Ramachandran and Hubbards design, the matrixelements were numbers that due to the similarityin shape between the number elements (e.g., the3s and 8s) could be perceived as organized in aspecific manner (for example, as rows inFigure 1, left). However, the authors chose the

matrix numbers for each synaesthetic participantin such a way that the grouping by synaestheticcolour would lead to a different visual organizationthan would the grouping by shape. In the stimulusrepresented in Figure 1, the elements in alternat-ing columns (3s and 7s, 8s and 0s) induced thesame synaesthetic colours for the synaestheteE.R. If the photisms behaved much like realcolours, they should overcome the horizontalorganization induced by the number shapes,leading to a perception of coloured vertical

columns (Figure 1, right). This is what actuallyhappened. While the control participants tendedto group the elements solely on the basis of gra-pheme shape, the synaesthetes reported perceivinggroupings based on their induced colours in90.97% (J.C.) and 86.75% (E.R.) of the trials.

In another experiment with the samesynaesthetes, Ramachandran and Hubbard(2001a) used arrangements of randomly scatteredgraphemes, containing an embedded shape(square, rectangle, parallelogram, or triangle) that

consisted of a grouping of identical characters(Figure 2, left). The participants task was toobserve the figure for 1 second and then try toidentify the hidden shape. Nonsynaesthete

Figure 1.Normal participants usually perceive this number matrix as horizontally organized (left figure), due to the shape similarity betweenthe numbers 3 and 8. Lexical synaesthetes studied by Ramachandran and Hubbard (2001a) observed a vertical set-up, induced by a chromatic

pattern of the photisms (right). (Figure can be seen in colour online.)


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controls found the shape in 59.4% of trials. In con-trast, synaesthetes got it right in 81.25% of trials.The most frugal explanation is that photismsinduced by the graphemes led to a sensory segre-gation (pop-out) of the embedded shapes.When this experiment was replicated later byHubbard, Arman, Ramachandran, and Boynton(2005) with 6 synaesthetes, the results confirmedthat the synaesthetes performance was signifi-cantly better than that in the controls. However,

the synaesthetes were inferior to a second groupof control participants who were exposed toarrangements of actually coloured graphemes,such as in Figure 2, right. This suggests that pho-tisms are not as effective as real colours in so far asreaction-time performance is concerned.

Overall, the results of both experiments byRamachandran and Hubbard (2001a; see alsoHubbard et al., 2005) suggest that the nature ofthe mechanism underlying synaesthesia is sensoryand cannot be accounted for by simple memoryassociations or attributed to an excessive use ofmetaphoric language. In line with these data,Smilek, Dixon, Cudahy, and Merikle (2001)demonstrated that photisms could actually influ-ence visual perception. The researchers presentedan achromatic number character on a plain colourbackground, followed by a mask. When thenumber was presented on a background of the

same hue (a congruent condition), the performancewas significantly worse (88% correct) than that withthe incongruent condition (96% correct). In otherwords, the discrimination seemed to be more diffi-cult for the synaesthete when the photism projectedon the grapheme stimulus was of the same colour asthe background. The same result was obtained in avisual search task where the participant had to lookfor a specific grapheme among distractors(Figure 3). Again, her performance was impaired

(longer reaction time) for colour congruent trialscompared to the incongruent trials.The amount of available evidence suggests that

theories of synaesthesia in terms of childhoodmemories or metaphorical language are defini-tively out of place. Synaesthesia seems to be asensory-like process that can be accounted for byan anomalous communication between specificbrain regions, as we see later on. In the followingsections we examine the evidence concerning theincidence of synaesthesia in the population andexplore the different modalities of this peculiarneuropsychological condition.

Synaesthesia prevalence and subtypes

Synaesthesia is not a widespread condition. Eventhough estimations of prevalence have been chan-ging widely across different studies, until recently

Figure 2.The letter soup used in a visual search experiment by Ramachandran and Hubbard (2001b). Right figure represents the image asseen in synaesthetic colours. (Figure can be seen in colour online.)



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the consensus seemed to lie somewhere around 1 in2,000 (0.05%), as proposed by Baron-Cohen, Burt,Smith-Laittan, Harrison, and Bolton (1996). Thesame study estimated a proportion of 6 femalesynaesthetes for every male with the condition.Even though the latter estimation is in accordancewith quite a few other papers (e.g., Rich,Bradshaw, & Mattingley, 2005), it should benoted that the majority (if not all) of these studies

probably suffered from a sampling bias caused by ahigher willingness in women to inform about theirsynaesthesia (Simner et al., 2006; Ward & Simner,2005). In a more recent study, using opportunisticsamples (participants recruited from the commu-nities of Glasgow and Edinburgh Universities),Simner et al. (2006) did not find any significantgender asymmetry. Besides challenging previousreports on this account, the result could possibly,but not necessarily, undermine purported X-linkedinheritance of synaesthesia. (The subject of

synaesthesia genetics is discussed later on in thispaper.) Moreover, Simner et al.s assessment ofsynaesthesia prevalence yielded a considerablyhigher value (4.4%) than previously assumed.

Given the scope and the type of sampling employed,Simner et al.s data are likely to be the best estimatesto date.2

Besides the prevalence and the likely absence ofsex bias, there are additional demographic aspects ofsynaesthesia that ought to be mentioned. A series of

authors have reported high incidences of synaesthe-sia among people dedicated to artistic and/or crea-tive professions and hobbies (Dailey, Martindale, &Borkum, 1997; Domino, 1989; Galton, 1880/1997; Ramachandran & Hubbard, 2001b). Thisidea has been confirmed in a large-scale study byRich et al. (2005); 24% out of the 192 synaestheteswho participated in the research were professionalartists or had a career linked to the arts. (In com-parison, in the normal population of referenceonly 2% of people worked in the field of arts.)

Interestingly, some authors believe that there arespecific neurophysiological mechanisms that leadto above-average creative abilities in synaesthetes.As we discuss later, virtually all current neurocogni-tive models suggest an existence of some kind ofneural hyperconnectivity between specific regionsof a synaesthetes brain. Ramachandran andHubbard (2001a) proposed that if such hypercon-nectivity was more diffusely expressed, it couldlead to a greater propensity and opportunity forcreatively mapping from one concept to another.

However, up to date there have been only a fewempirical studies focused on creativity in synaesthe-sia (e.g., Domino, 1989; Sitton & Pierce, 2004),and more research will be necessary in order toconfirm (or refute) this proposal.

A large number of authors agree that synaesthe-sia is a familial condition (e.g., Baron-Cohen et al.,1996; Galton, 1880/1997; Rich et al., 2005). Richet al. reported that 36% of synaesthetesparticipating in this large-scale study informedhaving at least one biological relative withsynaesthesia. The authors also interviewed theparticipants about potential advantages and disad-vantages of synaesthesia. The majority ofsynaesthetes (71%) perceived their condition

Figure 3.The synaesthete C. was significantly slower when lookingfor a number grapheme that was presented on a background of the

same colour as the associated photism (e.g., a blue 4 on a bluebackground). When the background was incongruent (i.e., of adifferent colour from the photism of the target), the number of distractors did not influence C.s performance (Smilek et al.,

2001). (Figure can be seen in colour online.)

2It must be noted that even in this study (Simner et al., 2006), testing of certain subtypes of synaesthesia was excluded for ethicaland/or technical reasons: namely, the variants triggered by, or inducing, pain and emotional states.


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positively, claiming that synaesthesia enhancestheir memory skills, eases data organization, andprovides a source of mental pleasure and creativeinspiration. Approximately one third of thoseinterviewed mentioned some negative aspects,mainly synaesthesia being a source of confusion

due to incongruence between synaesthetic percep-tion and physical reality (e.g., when the meaning ofa word does not fit with the elicited photism).Lexical synaesthetes reported contradictory feel-ings caused by a negative disposition towardspersons whose names were perceived in negativemental colours. A small number of synaesthetescomplained about sensory overload and feelingsof discomfort because of being different.

On account of anecdotal reports it has oftenbeen assumed that synaesthetes tend to be bad in

arithmetic, have a poor sense of direction, and fre-quently have precognitive experiences such asdeja vu or premonitory dreams (Cytowic,1989). The data collected by Rich et al. (2005)support the suggestion concerning synaesthetespoor sense of direction. Conversely, synaesthetesdo not seem to be more likely to report precogni-tive phenomena than the general population.

As far as different subtypes of synaesthesia areconcerned, all the authors agree that the most fre-quent modality of synaesthesia is the one induced

by lexical stimuli, such as numbers, letters, and

words (Baron-Cohen et al., 1996; Cytowic,1993; Day, 2005; Rich et al., 2005; Rich &Mattingley, 2002; Simner et al., 2006). Forinstance, in the study by Rich et al. less than 2%of the interviewed synaesthetes did not experiencesynaesthesia in response to lexical stimuli (words,

phonemes, or graphemes) and only presentedother kinds of synaesthesia. If lexical stimuli arefurther subdivided into more specific categories,the days of the week turn out to be the most fre-quent synaesthetic inducers (Simner et al., 2006).On the other hand, nonlexical modalities ofsynaesthesia are much less frequent. Up to 50%of synaesthetes (estimate based on the data byDay, 2007) experience synaesthesia in more thanone sensory modality. (For example, lexicalsynaesthete can see colours when seeing,

hearing, or merely thinking of numbers andletters.) Even though for the majority ofsynaesthetes the synaesthetic sensation is colour(Day, 2005, 2007; Simner et al., 2006), there arereported cases of smell, tactile sensations, sound,taste, and proprioception as concurrent percepts.(See Figure 4.)

According to Day (2005), there are two majorcategories of synaesthesia:

1. Cognitive synaesthesia: Photisms or other

synaesthetic perceptions are induced by

Figure 4.Relative frequency of synaesthetic inducers and concurrents (Source of data: Day, 2007). (Figure can be seen in colour online.)Note: Lexical stimuli include graphemes, digits, time units and written and spoken words.



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stimuli associated to symbolic meanings, trans-mitted within specific culture (graphemes, pho-nemes, peoples names, week days, etc.).

2. Synaesthesia proper: Stimuli of one sensorymodality are perceived simultaneously andinvoluntarily through an additional sensory

channel (e.g., seeing music).

This classification partially overlaps with the dis-tinction put forward by Marks and Odgaard(2005) who distinguish between an intramodalsynaesthesia (the inducer and the synaestheticresponse belong to the same sensory modality,e.g., when graphemes are perceived as coloured)and an intermodal synaesthesia (the stimulusinduces a concurrent in a different modality, e.g.,tactile sensations elicited by taste). However, the

overlap is only partial given that there have beencases of both intramodal and intermodal cognitivesynaesthesia.3 There are also synaesthetes withintramodal nonlexical synaesthesia for whomwhen observing shapes and visual scenes photismsare induced. For instance, the synaesthete R.,described by Milan et al. (2007), experienced thecolour red when watching the sky with clouds.This kind of synaesthesia would be clearly intra-modal. It does not seem to be related to any kindof culturally acquired semanticsthat is, it is not

the cognitive subtypebut neither is it a case ofsynaesthesia proper, as defined by Day. Takinginto account these nuances, we prefer to useMarks and Odgaards taxonomy, along withadditional clarification of the cognitive aspectsthat are sometimes difficult to determine. Forinstance, in the case of coloured music wemight be dealing with a culturally conditionedsynaesthesia, triggered by musical education, or itcould be the result of a more straightforwardtransduction of auditory input into visual

percepts of colour.Another important aspect of synaesthesia has todo with the mode of experiencing synaesthetic sen-sations. According to Dixon et al. (2004), there are

at least two qualitatively different varieties of visualsynaesthesia. Projector synaesthetes perceive theirphotisms as located in external space, usuallybeing projected onto the eliciting stimulus (e.g., agrapheme in lexical synaesthesia). On the contrary,associator synaesthetes observe synaesthetic colours

in their minds eye; there is no external projectionof the photisms. Dixon and colleagues discoveredthat these two groups differed not only in their sub-jective reports but also in their performance on amodified Stroop task. The experiment includedtwo tasks: to name the colour of a graphemepresented on a computer screen or to name thecolour of the photism triggered by the grapheme.The colour of the grapheme could be congruentor incongruent with respect to the photism. Theresults show that projector synaesthetes are

more sensitive to interference of the photismswhen naming the grapheme colour than viceversa. That is, the photisms somehow disrupt theability to name the grapheme colour, while thereal colour interferes little or not at all with theability to name the photism. On the other hand,the associator synaesthetes were faster atnaming the grapheme colour, and they showedthe same interference pattern in both tasks. Thisdouble dissociation in Stroop interference patternhas also been reported in a more recent study by

Ward, Li, Salih, and Sagiv (2007).Ramachandran and Hubbard (2001b, 2003b;Hubbard & Ramachandran, 2005) proposed analternative higher versus lower distinction, motiv-ated by differences in the level of representation ofthe inducing stimulus. In synaesthetes termed aslower, the photisms are elicited by specific percep-tual features of the inducer (e.g., the form of adigit), and they most probably occur in earlystages of perceptual processing. On the contrary,photisms of highersynaesthetes arise in responseto more abstract, conceptual aspects of theinducer (e.g., the meaning of a number), whichare processed by different brain areas. The matterof the level of representation of synaesthetic

3For instance, when hearing phonemes leads to the perception of photisms, we are dealing with a cognitive intermodal subtype ofsynaesthesia.


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triggers is indeed critical to the debate aboutthe underlying neurocognitive mechanisms.4 Onthe other hand, the distinction put forwardby Ramachandran and Hubbard seems to bemore problematic than Dixon et al.s (2004),because it is founded on theoretical, yet-to-be-

proved assumptions rather then phenomenologicalreports of synaesthetes (Ward et al., 2007). Someauthors (Dixon & Smilek, 2005; Hubbard &Ramachandran, 2005) speculated that the projec-torassociator distinction actually mapped on tothe lowerhigher dimension. However, a recentstudy by Ward et al. (2007) demonstrates thatthe two distinctions are most probably orthogonal.According to previous accounts on this issue,certain behavioural characteristics are expected ofhigher/associator or lower/projector synaesthetes,

respectively. For instance, the higher levelsynaesthesia may be particularly associated withordinal sequences (a conceptual level property);digits, roman numerals, spelled out numbers, ordice patterns should elicit the same synaestheticcolours in higher synaesthetes (but not lowersynaesthetes) because they represent the samenumber concept. On the other hand, in lower/projector synaesthetes the colours of wordsshould be more frequently derived from graphemicconstituents, provided that it is true that they are

sensitive to the external stimulus per se (i.e., thegraphemes) and not the underlying concept.First, Ward et al.s findings indicate that the pre-sence or absence of the lower or higher character-istics is independent of the projector/associatorclassification (assessed by first-person reports).Some of their associator participantspresented lower features, as well as higher fea-tures being observed in many projectorsynaesthetes. Moreover, the only reliable differ-ence (beyond phenomenological reports) betweenthe associators and the projectors was found intheir respective patterns of Stroop-type interfer-ence, first reported in the study by Dixon et al.

(2004). (Remember that projector synaestheteswere faster in naming photism colours than realcolours of graphemes; the opposite was observedin the associators.) According to Ward and col-leagues, this behavioural discrepancy results fromdiverse spatial frames of reference evoked during

synaesthesia. While the projectors employ anexternalized frame of reference defined relative tothe location of graphemic stimuli, the associatorsuse an internalized frame (their minds eye) orperhaps do not have any specific frame of referencewith respect to their photisms.5 Following thisproposition, the slower reaction times of theassociators in a photism naming task actuallyreflect an attentional effect. They need to shifttheir attention from one spatial location to a newone; the projectors attentional resources remain

in the same location during the task.In summary, the study by Ward et al. (2007)

brings in important new data and a novel approachto the problem of phenomenological andbehavioural differences, and it may have importantimplications for present neurocognitive modelsof synaesthesia. Even though the consensusregarding this issue is still to be sought, it is clearthat interindividual variability could accountfor a number of inconsistencies observed acrossvarious experimental studies (Hubbard &

Ramachandran, 2005; Ward et al., 2007). Forexample, Palmeri, Blake, Marois, Flanery, andWhetsell (2002) demonstrated a strong sensorysegregation effect with a synaesthete W.O. in avisual search task (the participants task was todetect a grapheme target among distractors). In asimilar experiment with 14 synaesthetes(Edquist, Rich, Brinkman, & Mattingley, 2005),only 2 participants showed facilitation in visualsearches in comparison to the control group ofnonsynaesthetes, and they did not present asensory pop-out pattern that had appeared inPalmeris results. In theory, it is possible that theparticipant in the Palmeri et al. study was of a

4It is not clear whether this classification reflects a dichotomic dimension, which corresponds to qualitative differences betweenthe synaesthetes, or whether there is a continuum with cases laying halfway between the lower and the higher synaesthesia.

5Ward et al. (in press) suggest that the projector associator dichotomy needs to be further subdivided, in order to include alltypes of frames of reference reported by synaesthetes. See the original paper for a full account on this topic.



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projector or lower type, while there were nosuch synaesthetes in the latter experiment. Giventhe heterogeneity of the phenomenon ofsynaesthesia, particular caution is necessary whendesigning and analysing data. If individual dataare averaged indiscriminately, the obtained

results and their interpretation are likely to be dis-torted. Frequently, single-case designs and/orinterindividual comparisons between synaesthetesare highly recommended research strategies(Smilek & Dixon, 2002).

Preconscious and sensory or conscious andconceptual?

If you fix your gaze at the central cross inFigure 5A, you can discern the number 5 located

at your visual periphery. However, if the samenumber is surrounded by distractors (Figure 5B),it is virtually impossible to identify (Bouma,1970). This effect, termed as crowding, is aresult of attention overload due to the distractors.Quite contrary to the data obtained with normalparticipants, two synaesthetes studied byRamachandran and Hubbard (2001b), who weretested on the same perceptual crowding task,easily identified the invisible number. Asreflected in their verbal reports, they were able to

deduce the number identity through the photismcolour, elicited by the grapheme. This result isanalogous to that observed in nonsynaesthetic par-ticipants when the target is actually coloured(Kooi, Toet, Tripathy, & Levi, 1994) and suggests

that the synaesthetic colour percept occurs on apreconscious level, before the crowding effecttakes place.

Evidence provided by Smilek et al. (2001) goesin the same direction. The synaesthete C. carriedout a number identification task that consisted in

naming a number briefly presented on a computerscreen. When the graphemes were placed on abackground of a different colour from the associ-ated photism (i.e., incongruent condition), C.sresponses were significantly faster than in acolour-congruent condition. It is hard to thinkthat the photism would influence the numberidentification in such a way, if it occurred afterthe conscious recognition of the grapheme.

However, not all the evidence is consistent withthe preconscious hypothesis. Mattingley et al.

(2001) subjected 15 lexical synaesthetes to twopriming tasks. In the first one, they presented agrapheme during 28 or 56 ms and immediatelyfollowed by a mask, in order to eliminate consciousidentification of the character. Participants taskwas to name the colour of a square patch thatappeared on the screen right after the maskedstimulus (the prime). If photisms had occurredbefore conscious recognition of the masked char-acter, they would have influenced the colour-naming task, facilitating colour identification

when the grapheme photism and the colourpatch were of the same hue. Nevertheless, noneof the participants showed any synaestheticpriming effect, but they did present classicalpriming in a letter identification task. (In asecond task they had to identify another graphemepresented after the masked character.) Asexpected, synaesthetes reaction times were fasterwhen the target grapheme and the prime wereidentical. This demonstrates that even thoughthe grapheme presentation time of 28 or 56 mswas sufficient to produce unconscious classicalpriming, most likely it did not lead to photisminduction. On the basis of these findings, theauthors suggested that synaesthetic colors typi-cally arise only for inducers that are representedat conscious levels of visual processing (Rich &Mattingley, 2002). Furthermore, in a morerecent experimental series Mattingley, Payne,

Figure 5. The crowding task, applied to synaesthetes byRamachandran and Hubbard (2001b), suggests that theinduction of photisms precedes the conscious identification of

graphemes.


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and Rich (2006) also demonstrated that when theprimes are presented long enough so as to produceconscious percepts of colour, reducing the amountof attention available to process the synaestheticinducer considerably reduces the priming effects.Again, this seems to indicate that conscious, selec-

tive attention to the stimulus might play animportant role in synaesthesia.

The controversy of whether or not consciousidentification of synaesthetic inducers is necessaryfor synaesthesia to occur is closely related to adebate concerning the perceptual versus conceptualnature of synaesthetic triggers. While some suggestthat the recognition of the lexical stimulus as such isa necessary condition for photisms to occur, otherstudies seem to go in the opposite direction, defendingthat synaesthetic colours are a direct response to

perceptual features of the inducer. At least insome synaesthetes, photisms produce effectstypically observed with real colours: They canprovide an input to apparent motion perception(Ramachandran & Azoulai, 2006; Ramachandran &Hubbard, 2002), produce McCollough colour after-effect (Blake, Palmeri, Marois, & Chai-Youn,2005), or be influenced by changes in physical attri-butes of the inducer, such as contrast, eccentricity(with respect to central vision), and frequency rateof alternately presented graphemes (Ramachandran

& Hubbard, 2001a, 2001b). On the other hand, thefact that graphemecolour synaesthetes experiencephotisms with rare typographic fonts or even withhandwriting points to the importance of cognitiveinterpretation of the graphemes more than just theirvisual features per se.

So then are we dealing with a sensory phenom-enon or not? First, we propose that it is necessaryto make further progress in differentiatingbetween lower and higher as well as projectorand associator synaesthetes (Ward et al., inpress). In order to overcome the inconsistenciesit is essential to take into account and to controlthe presence of different varieties of synaesthesiain experimental designs. Available data suggestthat the degree of dependence upon either percep-tual or conceptual aspects is not the same for allsynaesthetes, and it may also vary across differentstimulus modes (e.g., spoken words vs. written

words) in the same synaesthete. Secondly, it isnecessary to take into consideration the differentspatial frames of reference evoked by synaesthetes,as suggested by Ward et al. Finally, it should benoted that dichotomic categorizations of cognitiveprocesses (as unconscious vs. conscious, perceptual

vs. conceptual, or pre- vs. postattentional) arepossibly too simplistic to account for synaesthetesbehavioural data (Ramachandran & Hubbard,2005). Even in those rare cases of graphemecolour synaesthetes who show typical sensory pat-terns in their behavioural performance, it is logicalto expect at least some influence of conceptual pro-cesses. It is hard to believe that graphemecoloursynaesthesia, whose origin is intimately related tothe acquisition of reading skills, is a purelysensory process, independent of lexical knowledge.

In fact, the synaesthetic colour of ambiguousstimuli (e.g., the O in 9O89 and LOVE)typically changes in accordance with the surroundinglexical context (Dixon, Smilek, Duffy, Zanna, &Merikle, 2006; Ramachandran & Hubbard, 2003a).The matter to be solved is whether this and othersimilar effects are the consequence of a top-downmodulation, as proposed by Ramachandran andHubbard (2001b), or whether the photisms onlyarise once the grapheme has been identified, as putforward by Mattingley et al. (2001).

Synaesthetic brain

The first study of the synaesthetic brain, carriedout by Cytowic and Wood (1982), supported thesuspicion that neuronal functioning ofsynaesthetes was different from that of a normalperson. Unfortunately, unexpected complicationsarouse due to the fact that the only subject ofthis investigation (M.W.) showed additionalanomalous cerebral activity even when he did notexperience synaesthesia. This fact made datainterpretation quite problematic; the scientificcommunity would have to wait more than adecade for new and finer neuroimaging data.

In 1995 Paulesu et al. seated 6 female withauditory lexical-chromatic synaesthesia in a posi-tron emission tomography (PET) scanner.During the exposition of the synaesthetes to



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spoken words, neural activity was observed invisual areas (posterior-inferior temporal cortexand parieto-occipital junction). This was to beexpected if, as hypothesized, synaesthesia was aperceptual phenomenon rather than a result ofhigh-level associations. Surprisingly, activity in

the area responsible for colour processing inhumans (V4/V8) did not reach statistical signifi-cance. In any case, Paulesu et al.s study provideda first clear-cut demonstration showing that thecerebral functioning of the synaesthete partici-pants was different from that of normal, controlparticipants. The nonsynaesthetes visual areasgave no response to the same auditory stimulation.

Posterior studies used finer technology, whichallowed making further progress in the understand-ing of functional neuroanatomy of synaesthesia

possible. Weiss, Shah, Toni, Zilles, and Fink(2001) worked with R.S., a lexical-chromaticsynaesthete who experienced photisms in responseto the names of people he knew. When R.S. wasbeing exposed to such stimuli, functional magneticresonance imaging (fMRI) revealed a bilateralactivity in the area V4, an activation of the extra-striate visual cortex, as well as the retro-splenialcortex (the region usually associated to personfamiliarity). Nunn et al. (2002) subjected lexical-chromatic synaesthetes to an experiment where

they presented spoken words, pseudowords (i.e.,nonsense syllables), and single pure tones. AnfMRI scan showed increased activity in the lefthemisphere colour area (V4/V8) when listeningto words, but not when listening to pseudowordsor tones. Nunns design included a control groupof nonsynaesthetes who received extensive trainingin associating specific words with colour hues. Incontrast to the synaesthetes, these participants didnot present any activity in colour regions whenimagining colours in response to the words. Theresults agree with previous research, showing noactivity in visual areas V1 or V2, and suggest thatprocessing in earlier visual areas is not necessaryto experience visual synaesthesia. The only studynot in line with the aforementioned hypothesiswas published by Aleman, Rutten, Sitskoorn,Dautzenberg, and Ramsey (2001), who detectedactivity in the V1 area of a lexical-chromatic

synaesthete exposed to auditorily presented words.However, this seems to be an isolated result thatonly reached marginal levels of significance.

Virtually all research points to the importanceof the colour processing area (V4/V8) for the gen-eration and/or experiencing of photisms (Sperling,

Prvulovic, Linden, Singer, & Stirn, 2006). Inorder to explore the degree of involvement ofthis as well as other brain regions, Hubbard et al.(2005) combined an fMRI scan with behaviouralmeasures (a crowding task and a synaestheticpop-out task, described earlier in this paper) in astudy with 6 graphemecolour synaesthetes and6 control participants. When they compared thebehavioural data and the fMRI data, they founda significant correlation between the degree ofactivation in visual areas (particularly the hV4)

and the performance in the crowding task. Theresults were correlated between the two beha-vioural tasks, too. In other words, thosesynaesthetes who reached the best scores in thecrowding and the pop-out tests also presentedhigher neuronal activity in the colour area (V4)when exposed to synaesthetic inducers(Figure 6). This result is in line with the ideathat synaesthetes, including those who have thesame kind of synaesthesia, are a heterogeneousgroup with important interindividual differences.

It is possible that the activation of visual areas isstronger in those synaesthetes who perceive theirphotisms as externally projected. Interestingly, ina recent study Rouw and Scholte (2007) reporton structural differences in neural connectivitybetween associator and projector synaesthetes.First, by means of diffusion tensor imaging(DTI) technology the authors have shown thatsynaesthesia is associated with increased (or morecoherent) connectivity in white matter structureat various locations in the brain (inferior temporalcortex, parietal and frontal cortex). In addition,they observed that graphemecolour synaestheteswith the strongest increased DTI signal in theinferior temporal cortex (adjacent to the fusiformgyrus) were projectors rather than associatorsynaesthetes.

In summary, the bulk of neuroimaging evidenceindicates that at least in some synaesthetes, the


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experience of photisms resembles the perception ofreal colours, on both the subjective and the neuro-logical levels. Yet, as more recent studies suggest

(Hubbard et al., 2005; Rich et al., 2006; Rouw& Scholte, 2007), the implication of visual areasin synaesthesia is not equally strong across differ-ent individuals. Moreover, according to the afore-mentioned study by Rouw and Scholte (2007),white matter hyperconnectivity is present notonly in inferior temporal visual areas but also inparietal and frontal cortices. More researchwould be needed to uncover the relationshipbetween these structural abnormalities and thebrain activation that leads to synaestheticperception.

Origins of synaesthesia and currentneurocognitive models

Before presenting the principal neurocognitivemodels, it is imperative to mention a series ofdata regarding the purported genetic background

of synaesthesia. Despite the relative rarity ofstudies exploring the incidence and the heritabilityof synaesthesia, most researchers agree that idio-

pathic synaesthesia tends to run in families (e.g.,Baron-Cohen et al., 1996; Galton, 1880/1997;Rich et al., 2005). This fact alone points to a poss-ible genetic cause of the synaesthetic condition.Earlier investigations about incidence ofsynaesthesia suggested an X-chromosome-linkeddominant trait (Bailey & Johnson, 1997), with apossible increase in male mortality that wouldexplain the higher frequency of this condition infemales (Baron-Cohen et al., 1996; Rich et al.,2005; Ward & Simner, 2005). However, thelatter proposition, based on the skewed female-to-male ratio, has been called into question by arecent study by Simner et al. (2006), who reportan almost perfectly equal proportion of female-to-male synaesthetes (1.1:1). (In fact, Ward &Simners, 2005, study with 85 synaesthetic familiesdid not find any evidence of the purported malemortality.) Yet, even if the absence of sex bias is

Figure 6.The participants of this neuroimaging experiment were exposed to achromatic grapheme stimuli. An fMRI output shows ventralviews of two inflated brains. The V4 area, responsible for colour processing, is indicated in purple, and the grapheme area is indicated in blue.

In both participants there is activation in the grapheme area but only the synaesthete shows activity in the V4. From Individual differencesamong grapheme-color synaesthetes: Brain-behavior correlations, by E. M. Hubbard, A. C. Arman, V. S. Ramachandran, and G. M.Boynton, 2005,Neuron, 45, 975985. Copyright 2005 by Elsevier. Reprinted with permission. (Figure can be seen in colour online.)



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confirmed, the higher incidence of synaesthesiaamong relatives provides a strong argument infavour of the genetic influence. The heritabilityof synaesthesia just might be more complex thanpreviously thought. For instance, Smilek et al.(Smilek, Dixon, & Merikle, 2005; Smilek et al.,

2002) have already reported two cases of monozy-gotic twins (two twin sisters and two twin broth-ers) who are discordant for synaesthesia.6 Thesefindings seriously question previous suggestionsthat synaesthesia is simply an X-linked dominanttrait. If the genetic case for synaesthesia iscorrect, it means that the penetrance of the geno-type for synaesthesia is probably incomplete(Smilek et al., 2005) and not sufficient by itselfas a cause of the synaesthetic condition. (The dis-covery of male identical twins discordant for

synaesthesia also contradicts a previous speculationin Smilek et al., 2002, based on a study of femalemonozygotic twins, that discordance of synaesthe-sia in identical twins is due to X-inactivation.)

In view of the evidence gathered thus far, theinvolvement of genetics in the development ofsynaesthesia still remains an open issue. First, whatis the mechanism of genetic transmission ofsynaesthesia, and, second, how does the hypoth-esized gene (or genes) affect the neuroanatomy,giving rise to this sensory alteration. In relation to

the second question, there are at least threemodern theories regarding the neuronal mechanismthrough which a normal sensory stimulation leads tothe experience of photisms. Because of the relativelyhigher prevalence (and availability) of grapheme-chromatic synaesthetes, all three models presentedhere have, in principle, been suggested to takeaccount for this modality of the phenomenon.

Synaesthesia and crossed wiresThe core idea of all current neurocognitive models

of synaesthesia is a postulation of some kind ofanomalous communication in the brain. If specificcortical areas, whose connectivity is limited in a

normal brain, established active connections, thiscould produce phantom sensations typical forsynaesthesia (Baron-Cohen et al., 1993). Theunderlying mechanism might be similar to theone present in amputee patients. The reorganiz-ation of the cortex following amputation frequently

gives rise to tactile sensations on a nonexistent limb(Ramachandran, Rogers-Ramachandran, &Stewart, 1992). This phenomenon inspiredRamachandran and Hubbard, leading them todevelop alocal cross-activationmodel that accountsfor the emergence of photisms in lexical-chromaticsynaesthetes (Ramachandran & Hubbard, 2001b,2003b; Hubbard & Ramachandran, 2005). Whenstudying graphemecolour synaesthesia, theauthors realized that the colour area (V4) and thevisual area responsible for grapheme identification

were located in the same brain regionthe fusiformgyrus. If neurons from these areas started to com-municate, the expected subjective output would bean experience of colours upon seeing graphemes.7

The question is: Why and how would these inter-connections emerge between cortical areas thatprocess completely different aspects of visual input?

In the course of cerebral maturation, the humanbrain goes through a phase of refinement inthe formation of functional neural circuits,where a number of redundant and/or unnecessary

pathways are eliminated through a process termedaxonal pruning. If a genetic mutation caused afailure of pruning in specific cortical regions, theseprenatal connections would persist in the adultbrain, eventually leading to unusual sensory altera-tionsthat is, synaesthesia. For instance, in thecase of graphemecolour synaesthesia the neuronalconnections between the V4 and the grapheme areawould be more numerous (less completely pruned)than in nonsynaesthetes. The fact that these twobrain areas are close to each other increases the like-lihood of reciprocal neuronal communication. Inaddition to the evidence demonstrating the impli-cation of V4 in lexical-chromatic synaesthesia, the

6It must be noted that in both studies the authors did not check for (or do not report on) the presence of subtypes of synaesthesiaother than the grapheme colour variety.

7In theory, the opposite result is also conceivablethat is, the participant could experience graphemes in response to colours. Seethe discussion by Hubbard and Ramachandran (2005), where they argue why such a possibility is much less plausible.


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authors make reference to the discovery of theaforementioned connections in foetal monkeys.Kennedy, Batardiere, Dehay, and Barone (1997)found neuronal pathways between inferior temporalregions and the V4 area in the prenatal macaquebrain. Thanks to the process of axonal pruning, in

adult animals the proportion of neural afferencesfrom higher areas to V4 is reduced radically.Ramachandran and Hubbard (2003b; Hubbard &Ramachandran, 2005) propose that in humans agenetic mutation could lead to defective axonalpruning in the fusiform gyrus and consequently toanomalous connectivity in the adult brain. Thus,synaesthetic photisms would be the result of thecolour area being activated through connectionsoriginating in the area responsible for graphemeprocessing.

It should be noted that there are alternativetheoretical models that fall within the cross-wiring scheme as well. Rich and Mattingley(2002) proposed that in graphemecoloursynaesthetes unique functional connections werepresent at later levels of the processing hierarchy:concretely between the module of letter recog-nition and the modules of colour categorizationand colour imagery. Even though the lack of ana-tomical specificity makes this hypothesis difficultto test using neuroimaging techniques, recent

data by Rich et al. (2006) give some support tothe latter proposal. In their experimental seriesinvolving colour imagery tasks as well as expositionto synaesthetic inducers, the authors observed thatsynaesthetic colours were associated with fMRIactivity of the left medial lingual gyrus in gra-phemecolour synaesthetes. This area is knownto be involved in tasks requiring the retrieval ofcolour knowledge, such as naming the colour ofan object (e.g., Martin, Haxby, Lalonde, Wiggs,& Ungerleider, 1995). On the other hand, no sig-nificant activity in the V4 area was registeredduring photism experiences. Apparently theresults contradict earlier neuroimaging investi-gations (Hubbard et al., 2005; Nunn et al., 2002;Sperling et al., 2006) that emphasized the role ofthe V4/V8 area for the generation of synaestheticcolours. On the other hand, it is likely that inter-individual differences observed in synaesthetes on

behavioural and phenomenological levels mayalso account for the differences in brainbehaviourcorrelates (Hubbard et al., 2005). Indeed, it wouldhave been interesting if Rich et al. (2006) hadreported on whether their experimental participantswere projector or associator synaesthetes. It is poss-

ible that for the associators the cross-activationinvolves higher visual areas related to more abstractattributes of colours, while the projectors synaes-thetic experience relies on areas activated earlierduring visual processing.

Disinhibition of top-down connectionsNot all of the authors agree with the necessity ofassuming anomalous neural pathways in synaesthe-sia. Grossenbacher and Lovelace (2001) affirm thatto account for synaesthetic perception it is sufficient

to consider the functioning of a (structurally)normal brain. As the authors correctly point out,synaesthetic experiences have been reported byotherwise nonsynaesthetic individuals who wereunder the influence of hallucinogens. The proces-sing of sensory input progresses through a seriesof hierarchically organized modules. After goingthrough various processing stages along dedicatedpathways, the signals progressively converge untilreaching a multimodal area. According to theauthors, in synaesthesia neural signals of the

synaesthetic inducer (e.g., a sound) travel along aseparate processing pathway before reaching a mul-timodal crossroad where they meet signals ofother sensory modalities. However, in addition tothese afferent (feedforward) connections, thehuman brain also presents connections whereinformation can travel backwards in the processingchain. Normally the feedback from themultimodal convergence zone is restricted to thepathway in which afferent (feedforward)information has arisen (Rich & Mattingley,2002). For instance, top-down visual signalsshould only influence processing in the correspond-ing (lower level) visual areas, while other directionsof information flow are inhibited in order to avoidneuronal noise and processing anomalies.What Grossenbacher and Lovelace propose isthat a failure of such inhibition could lead tothe activation of an otherwise independent



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neural pathway, generating synaesthetic percep-tion.8 More specifically, when the signal of a synaes-thetic inducer reaches the zone of multimodalconvergence (after going through all of the unimo-

dal processing modules), it can travel backwardsalong the pathway of the synaesthetic modalityand arrive at the level where the concurrentpercept is generated. (See Figure 7.) In summary,Grossenbachers model defends the view thatsynaesthesia is the result of a disinhibited feedbackoriginating in a multimodal cortical area such asthe superior temporal sulcus. (The aforementionedstructure shows connections with unimodal areas,and, at the same time, it is responsive to specific per-ceptual features of several sensory modalities.)

The idea that synaesthesia occurs in a relativelylate stage of sensory processing has received

some empiric support from event-related potentialsresearch. The authors emphasize the fact thatdifferences in brain activity in synaesthetes andnonsynaesthetes are not observed until 200 milli-

seconds after the inducing stimulus onset (Schiltzet al., 1999). Moreover, synaesthetic perception issometimes experienced during psychotropichighs in normal participants (Cytowic, 1993;Ramachandran & Hubbard, 2001b), and it can alsobe inducedby posthypnotic suggestions in highly sus-ceptible nonsynaesthetes (Fuentes, Cohen-Kadosh,& Catena, 2007). While it is true that these factsseem to show that the existence of anomalousanatomical connections is not a necessary conditionfor synaesthesia to occur,9 it should also be pointedout that the recent study by Rouw and Scholte(2007) actually reports on increased white matter

Figure 7.Scheme of Grossenbachers and Lovelaces model. Adapted from Grossenbacher & Lovelace (2001). (Figure can be seen in colouronline.)

8A similar mechanism was proposed earlier by Armel and Ramachandran (1999) to explain acquired synaesthesia, observed in apatient who became blind as a consequence of suffering retinitis pigmentosa.

9Nonetheless, Ramachandran and Hubbard (2005) argue that the phenomenon of congenital synaesthesia can differ from the drug-induced experiences. Superficial similarity of the phenomena should not be straightforwardly interpreted in terms of identical neuronalmechanisms.


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connectivity in graphemecolour synaesthetes,favouring the hyperconnectivity accounts ofsynaesthesia.

Reentrant processing modelThe reentrant processing model (Myles, Dixon,

Smilek, & Merikle, 2003; Smilek et al., 2001) is,in some ways, a hybrid scheme that combinescertain aspects of the aforementioned proposals.The authors base their model on the fact thatvisual information flows both forwards (bottom-up) and backwards (top-down) along the visualprocessing pathways. When a lexical coloursynaesthete observes an achromatic grapheme, thecorresponding neural signals from the retina firstarrive at lower visual areas. Subsequently they areprocessed by a shape-processing area (the posterior

fusiform gyrus) before finally reaching the area incharge of the analysis of the meaning (the anteriorfusiform gyrus). According to Smilek et al., the acti-vation of photisms in projector synaesthetes is aresult of cyclic feedback communication from theshape and the meaning processing areas to thecolour region V4. As an illustration, let us considera case of red photism induced by an achromaticletter B. When the curves and the lines makingup the letter are being processed by the striatecortex and the posterior fusiform gyrus, simul-

taneously signals originating from these areastravel to the anterior fusiform gyrus, where themeaning of the grapheme is analysed. In the

beginning, before completion of the shape analysis,these signals are not sufficient to give way to con-scious recognition of the letter. However, theauthors propose that even a partial activation ofthe meaning can activate a red percept in thecolour area (V4). More specifically, prior to the con-

clusion of the form and the meaning analyses, theanterior region of the fusiform gyrus will communi-cate with the colour area through top-down connec-tions, activating the representation of the colour red.At the same time, this activation of red in the V4will strengthen the activation of the meaning ofB in the anterior fusiform, through bottom-upconnections. (See the scheme of the model depictedin Figure 8.)

In this way, the neural signals will travel in acyclical manner until they give rise to a complete

conscious perception of a red B. This mechanismalso accounts for the fact that the context affectingthe interpretation of a grapheme (e.g., the O in9O89 and SOUL), also determines the synaes-thetic colour of the stimulus (Dixon et al., 2006).

Given the fact that the top-down modulation isa general feature of human brain functioning, atthe moment it is not possible to experimentallydissociate the processes proposed by the localcross-activation model (Hubbard & Ramachandran,2005) and the reentrant processing model. In order

to account for the contextual effects in photisms,Ramachandran and Hubbard (2001b) suggest thatthe same top-down mechanisms that are present in

Figure 8.Schematic depiction of a photism activation mechanism, following the model by Smilek et al. (2001). The meaning of B denotes amore abstract representation of the letter that is independent of font or case. (Figure can be seen in colour online.)



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normal people can explain the influence of semanticcontext on the photism colour. Specifically, if thecontext is able to skew the perception ofthe graphemeshapeby a top-downinfluence from higher regions tothe grapheme area, it makes sense to assume that, insynaesthetes, the latter region will consequently

alter the activation of synaesthetic colours in theV4. In principle, the reentrant processing mechanismproposed by Smilek et al. (2001) would lead to thesame behavioural output. In the future it will benecessary to reach a deeper level of specificity inboth models, so that it will be possible to either corro-borate or refute the respective proposals. At the sametime, as Hubbard and Ramachandran (2005) havepointed out, the two models may not be mutuallyexclusive. It is perfectly plausible that a combinationof both mechanisms (the local cross-activation and

the disinhibited feedback from the meaning analysisareas) is present in some synaesthetes. In addition,we should also consider the possibility that differentways of experiencing synaesthesia in projector andassociator synaesthetes imply diverse neurocogni-tive mechanisms. In fact, the aforementioned studyby Rouw and Shoulte (2007) demonstrates that pro-jectorsshowed strongerstructural connectivityin theinferior temporal cortex than do associators.

Final notes

Along this review of current scientific knowledgerelating to the phenomenon termed synaesthesia,we have tried to analyse a few central points thathave marked the development of synaesthesiaresearch. First, we have seen that this peculiar con-dition is not the result of childhood memories, andits origin is probably due to a genetic alteration.Second, we have observed that the synaestheticsensations can lead to sensory effects where pho-tisms can increase the efficiency with which gra-phemes can be detected in perceptual crowdingtasks, visual search, and perceptual groupingtasks. Third, we have analysed behavioural andneuroimaging evidence, concluding thatsynaesthesia shares many aspects with normalperception on both behavioural and neurologicallevels. And finally, after revising the most influen-tial neurocognitive models, we may conjecture that

the root of synaesthesia is in some sort of anoma-lous communication between specific brain areas,which could be caused by abnormal cross-wiring in the sensory cortex, by top-down path-ways disinhibition, or both. What then are thelimitations of our present knowledge, and where

is synaesthesia research heading? First of all, it isworth saying that the stage of verifying thereality of synaesthesia seems to be finally over. Inour opinion, the scientists involved in the studyof synaesthesia successfully demonstrated beyondany reasonable doubt that synaesthesia is authenticand that the subjective reports of synaesthetes areto be taken seriously. Even so, the investigationinto the neurophysiological background of thiscondition is still in its beginnings. In order tomake further progress in our understanding of

synaesthesia, it will be necessary to expand thelimits of existing research in several directions.

To begin with, it should be pointed out that agreater part of the hypotheses and models are builtupon the basis of data proceeding from studieswith lexical synaesthetes (Grossenbacher &Lovelace, 2001; Hubbard & Ramachandran,2005). Even though this is understandable giventhe relatively high frequency of this subtype ofsynaesthesia, it would be imperative to broadencurrent experimental paradigms with the aim of

exploring to what degree other varieties of this con-dition are similar to the lexical-chromatic synaesthe-sia on behavioural as well as neurological levels.

Furthermore, as we emphasized before in thistext, it is vital to take into account individual dis-crepancies in the intensity and the phenomenolo-gical quality of the synaesthetic experience. If thetheoretical division of synaesthetes into lower/higher and projector/associator subtypes isfurther confirmed, it will have important impli-cations for the experimental designs, for theinterpretation of the data, and, possibly, for therefinement of the neurocognitive models.

On the other hand, the term synaesthesia hasbeen applied to a broad range of phenomena: idio-pathic synaesthesia, drug-induced anomalous per-ception, artistic expositions involving the couplingof sound and light, and so on. Perhaps the impre-cision with which the word synaesthesia is used


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actually reflects certain aspects of the truth. It isnot an accident that various authors defend thatsynaesthesia research can bring new insights intoour understanding of the neurological bases ofmetaphor and language (Cytowic, 2002; Marks,1978; Maurer, Pathman, & Mondloch, 2006;

Ramachandran & Hubbard, 2001b). Subjectivereports reveal striking parallels between thesynaesthesia and cross-modal associations reportedby nonsynaesthetes. For instance, Ward,Huckstep, and Tsakanikos (2006) demonstratedthat the associations of sounds to colours insynaesthesia showed the same pattern of corre-spondence between luminosity and tone pitch asin cross-modal associations of normal persons.Ramachandran and Hubbard (2003b) note thatpeople frequently tend to form the same synaes-

thetic associationsfor example, in the case ofassociating particular visual shapes to sounds(Kohler, 1929). (See Figure 9.) The authorsbelieve that specific regions of the human brainhave an innate capacity to extract common,abstract properties from otherwise unrelateddomains (Ramachandran & Hubbard, 2005),such as the concept of jaggedness present inboth the kiki sound and the shape depicted inFigure 9 on the left. If an elementary aptitudefor cross-modal abstraction was in fact hard-

wired in our nervous systems, the synaestheticphenomenon could be understood as a moreenhanced (and more perceptual) version of this

natural property of the human mind. In any case,hypotheses concerning the relationship betweensynaesthesia and the cross-modal language andmetaphor still remain on highly speculativegrounds. Does the similarity of these phenomenato the congenital synaesthesia extend beyond

superficial aspects? In theory, synaesthesia, in abroader sense, could encompass a great variety ofconditions ranging from lower synaesthesia tomore associative synaesthesias, to conceptuallytriggered synaesthesias, to emotional synaesthe-sias, eventually up to the level of multimodal artis-tic expression and, possibly, cross-modalmetaphor. Future research should respond to thequestion of where the frontiers lie between thesephenomena as well as to whether we are dealingwith qualitatively different conditions or if it is

only a matter of degree. Part of the answer mayarise from the neuroanatomical research, whichshould elucidate whether the synaesthetes (andwhat types of synaesthetes) really present unusualneuronal connectivity or, on the opposite side,whether the phantoms in their brains show updue to the disinhibition of normal cerebralmachinery.

As a final point of reflection, we believe it isworth mentioning the importance of synaesthesiafor the progress of our understanding of subjective

experience. The synaesthetic phenomenon consti-tutes a peculiar puzzle for cognitive science for onesimple reasonit is a phenomenological experi-ence that has a very low occurrence. Unlike dis-orders caused by brain injuries, it is a heritableand highly stable, life-long condition.Moreover, synaesthesia is unlike schizophrenia(which is as likely as heritable as synaesthesia) inthat it is not bothersome to the person who experi-ences it; it occurs in otherwise normal individuals.What attracts the attention of both the generalpublic and the experts is the fact that thesynaesthetes claim to see things that normalpeople cannot perceive. If, upon waiting for a ped-estrian crossing you declare observing a red lighton a semaphore, there will be no cognitive scien-tist running to subject you to a Stroop task in orderto corroborate that you really see what you claim tosee. Oddly enough, from the synaesthetes point of

Figure 9. Who is Kiki and who is Bouba? These figuresdemonstrate that humans do not associate shapes to images in anarbitrary manner. Almost 100% of the interviewed participantsrelated kiki sound with the left figure and the bouba soundwith the right figure (Kohler, 1929). (Figure can be seen incolour online.)



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view the beginning of synaesthesia research wasidentical to the aforementioned situation.Numerous professionals tried to answer questionslike: Is it true what you state, and do you actuallyperceive an achromatic B in red? In otherwords, cognitive science invested time and efforts

in confirming the genuineness of subjective experi-ence, achieving the same data that the synaestheteshad already provided through their own words. Ifwe were to completely believe synaesthetes verbalreports, then apparently science has done muchwork in vain. However, in our opinion, thiseffort in addition to empirically demonstratingthe veracity of the introspection comprises alesson for psychology in relation to the scientificunderstanding of subjective experience. Possiblyfor the first time empiric research has had to

cope directly with the matter of qualia, trying toanswer a question analogous to the renownedriddle proposed by Nagel (1974): What is it liketo be a bat? (What is it like to be a synaesthete?)The development of our understanding ofsynaesthesia, accompanied by significant improve-ments in methodology, also amounts to an import-ant advancement in the study of the first-personexperience per se. In order to make further pro-gress in this direction, it will be essential tocombine behavioural and neurological evidence

with subjective reports (Smilek & Dixon, 2002)and to convert the first-person viewpoint into anintegral part of neuropsychological research.

Manuscript received 26 February 2007Revised manuscript received 7 November 2007

Revised manuscript accepted 14 November 2007First published online 5 February 2008

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