Five plus two equals yellow

In synaesthesia, ordinary stimuli elicitextraordinary experiences. For examplewhen C., who is a digit–colour syn-

aesthete, views black digits, each numberelicits a photism — a visual experience of aspecific colour. It has been proposed thatsynaesthetic experiences differ from imageryin their consistency1, automaticity2,3 andreliance on external stimuli to induce them4.Here we demonstrate that C.’s photisms areboth consistent and automatic, but we findthat an externally presented inducing stimu-lus is not necessary to trigger a photism andthat simply activating the concept of a digitis sufficient.

As a measure of consistency, we asked C.to name the colour of her photisms elicitedby the digits 1 to 9 shown in random order,10 times each. C.’s pairings between digitsand colour names were 100% consistentacross repetitions.

To assess automaticity, we used a Strooptask. Colour-naming reaction times wererecorded for coloured squares (baselinecondition) or for digits displayed in coloursthat were either congruent or incongruentwith C.’s photisms (Fig. 1). Her mean reac-tion times for each condition were: congru-ent, 552 ms (error, 1.4%); baseline, 545 ms(error, 0.0%); incongruent, 797 ms (error,2.8%). An analysis of variance (F(2,410) 474.88, P * 0.001) and Tukey post-hocanalyses revealed that incongruent trialreaction times were significantly slowerthan baseline (P * 0.001) and congruenttrial reaction times (P * 0.001) — the lasttwo conditions did not differ.

C.’s large significant difference incongruent/incongruent reaction times (245ms) contrasts with the small, non-signifi-cant, congruent/incongruent differences(ranging from 111 to 17 ms) of eight non-synaesthetes tested using identical stimuli.Figure 1a shows the Stroop performanceof C. and of the non-synaesthete mostsimilar to C. in terms of baseline-trialcolour-naming reaction times. C.’s perform-ance indicates that the digits automaticallyelicited photisms that interfered with hercolour-naming on incongruent trials.

For C., viewing the digit 7 automaticallyinduces a yellow photism. To assess whetherC.’s photisms could be elicited withoutphysically showing her an inducing stimu-lus, we presented, in sequence, a digit (forexample, 4), an operator (for example, +), asecond digit (for example, 3), and a colourpatch. C. had to name the colour patchas quickly as possible, and then reportthe solution to the arithmetic problem.On each trial, the colour patch was either

congruent or incongruent with the photismassociated with the arithmetic solution. Forexample, 5 & 2 was followed either by ayellow patch, which is congruent with C.’sphotism colour for 7, or a red patch, whichis incongruent with C.’s photism for 7.To discourage C. from using the arithmeticsolution to predict the colour of the patch,we presented three times as many incongru-ent trials as congruent trials.

If automatic photisms can be inducedsimply by calculating the solution to arith-metic problems, then they should interferewith C.’s ability to name the colour ofthe patch on incongruent trials but noton congruent trials. Consistent with ourreasoning, C.’s colour-naming reactiontimes were significantly slower on incon-gruent trials (mean, 650 ms) than on con-gruent trials (mean, 414 ms), t(390) 419.48, P * 0.001. This large congruent/incongruent difference (236 ms) was 19.3standard deviations away from the meandifference (1 ms) obtained for eight non-synaesthetes. Figure 1b shows C.’s perfor-mance compared with the performance ofthe non-synaesthete who was most similarto C. in terms of overall reaction times.

C.’s large difference in congruent/incon-gruent reaction times (236 ms) indicatesthat automatic photisms were induced bythe arithmetic solution (for example, theyellow photism associated with the digit 7was generated in response to calculating5 & 2). This suggests that an external stim-ulus (for example, a physically presentnumeral 7) is not required to trigger a

photism. Rather, activating the concept of adigit by a mental calculation was sufficientto induce a colour experience. Thus,although C.’s photisms are both consistentand automatic, they do not require a physi-cal inducing stimulus to elicit them.

Stroop-type paradigms have shown thatphotisms are automatic and involuntary4.Brain imaging has demonstrated thatphotisms activate visual-association areasinvolving binding colour to form5. Cogni-tive research, brain imaging and even genet-ics (synaesthesia runs in families)6 all attestto the reality of synaesthesia. Although theneural architecture and neurodevelop-mental aspects of synaesthesia are notyet fully understood, the finding thatphotisms can be concept-driven may helpto guide our understanding of this rarephenomenon.Mike J. Dixon, Daniel Smilek, Cera Cudahy,Philip M. MerikleDepartment of Psychology, University of Waterloo,Waterloo, Ontario N2L 3G1, Canadae-mail: mjdixon@watarts.uwaterloo.ca

1. Baron-Cohen, S., Wyke, M. & Binnie, C. Perception 16, 761–767

(1987).

2. Mills, C. B., Boteler, E. H. & Oliver, G. K. Cogn. Neuropsychol.

16, 181–191 (1999).

3. Odgaard, E. C., Flowers, J. H. & Bradman, H. L. Perception 28,

651–664 (1999).

4. Cytowic, R. E. The Man who Tasted Shapes (Putnam, New York,

1993).

5. Frith, C. D. & Paulesu, E. in Synaesthesia: Classic and

Contemporary Readings (eds Baron-Cohen, S. & Harrison, J. E.)

123–147 (Blackwell, Cambridge, Massachusetts, 1997).

6. Bailey, M. E. S. & Johnson, K. J. in Synaesthesia: Classic and

Contemporary Readings (eds Baron-Cohen, S. & Harrison, J. E.)

182–210 (Blackwell, Cambridge, Massachusetts, 1997).

brief communications

NATURE | VOL 406 | 27 JULY 2000 | www.nature.com 365

Five plus two equals yellowMental arithmetic in people with synaesthesia is not coloured by visual experience.

Non-synaesthetePatient C.

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Figure 1 Colour-naming reaction times (and errors) for a

digit–colour synaesthete, C., and for representative non-synaes-

thetes. a, Digit Stroop task: on each trial, reaction times were

recorded for naming coloured squares (‘baseline’) or digits dis-

played in colours that were either congruent or incongruent with

C.’s photism colours. There were 144 trials of each type, random-

ly intermixed. b, Arithmetic Stroop task: on each trial, participants

were presented, in sequence, with a fixation cross, a digit, an

arithmetic operator, a second digit, and a colour patch. Partici-

pants named the colour patch as quickly as possible and then

reported the arithmetic solution. Colour patches were either con-

gruent (108 trials) or incongruent (324 trials) with C.’s photism

colours for the arithmetic solutions. Trial types were intermixed.

Further details are available from the authors.

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