The phenomenology of synaesthesia

This article supplements our earlier paper on synaesthesia published in JCS (Ramachandran & Hubbard, 2001a). We discuss the phenomenology of synaesthesia in greater detail, raise several new questions that have emerged from recent studies, and suggest some tentative answers to these questions. I: Robustness of Synaesthetic Effects It was Francis Galton (1880) who first reported the condition called synaesthesia. He noticed that a certain number of people in the general population, who are otherwise completely normal, seemed to have a certain peculiarity: they experience sensations in multiple modalities in response to stimulation of one modality. For example, musical notes might evoke distinct colours; F might be red and C blue. Or the printed number 5 always ‘looks’ green, whereas 2 looks red. Recent evidence suggest that synaesthesia is a genuine sensory phenomenon, not a high-level memory association (Ramachandran & Hubbard, 2001a,b). This raises several new questions regarding the robustness of the colours evoked by specific graphemes. Do physical changes in the number affect the perceived colour? And, to what extent are the grapheme–colour correspondences universal, i.e., seen in a majority of synaesthetes? (a) Does it matter whether the letters are upper or lower case? We usually find that upper and lower case letters evoke the same colours, although the lower case letters were usually less saturated, or were ‘shiny’ or ‘patchy’ compared to the upper case letter, perhaps because upper case is learnt earlier. There are exceptions to this. For example, in ‘Sarah’ most letters followed the rule, but E had completely different colours for upper and lower case (‘E’ was green, while ‘e’ was red). (b) Does the font of the number or letter affect the colours? Prototypical fonts like Times Roman or Arial normally give the most vivid colour for that letter, but on occasion a ‘weird’ font like Gothic actually evokes a stronger colour. We suggest that such fonts might serve as ‘hyper-normal’ stimuli Journal of Consciousness Studies, 10, No. 8, 2003, pp. 49–57 Correspondence: Center Brain and Cognition, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0109, USA. Email: vramacha@ucsd.edu that evoke even larger responses from the grapheme neurons than a more ‘prototypical’ font might. This would be an example of the peak-shift effect, which causes seagull chicks actually to prefer pecking at a long stick with three stripes to pecking at a real beak (see Ramachandran & Hirstein, 1999). (c) What if the number is presented in the ‘wrong’ colour? This produces a slight delay in naming the colour; the induced colour delays the ability to report the real colour. This effect — Stroop interference — shows that the colour associations are automatic (Dixon et al., 2000; Mattingley et al., 2001), but it does not necessarily show that it is sensory or perceptual (Ramachandran & Hubbard, 2001a,b). We have noticed that it also often produces a strong visceral discomfort like ‘nails scratching a blackboard’. Perhaps the ‘hyperconnectivity’ genes cause an excess of connections between sensory and limbic structures like the amygdala, so that even a trifling discord produces a disproportionately large abhorrence (see Cytowic, 1989/2002; Ramachandran & Hubbard, 2001a). We are testing this theory by monitoring galvanic skin response (GSR), which indexes emotional arousal. (d) Does a given number evoke the same colour across different synaesthetes? Synaesthetic colour associations remain stable in any given synaesthete, even when tested over intervals of up to one year (Baron-Cohen et al., 1993). But does the same grapheme tend to evoke the same colour across different synaesthetes? The answer is no. One synaesthete might see A as red, another might see it as green (Day, 2001). But the associations are not random either. There is a higher chance that A will be red than that it will be (say) blue or yellow. Although such trends have been noticed before no explanation was offered. We suggest that it may reflect the manner in which phonemes (in higher synaesthetes — see below) are mapped near the TPO junction in a systematic topographic manner, which in turn would make certain types of cross-activation more likely than others (e.g. front vowels might activate long wavelengths). Similarly, graphemes might be mapped in ‘form space’ in the fusiform in such a way that certain colour correspondences with colour neurons in V4 are more likely than others. A systematic search for such correlations has yet to be attempted and the few already undertaken have yielded ambiguous results (Day, 2001; Shanon, 1982). An analogy with the periodic table of elements might be appropriate. Initial attempts to classify elements produced a certain non-random clustering of properties (e.g. the alkaline metals vs the halogens) but no rhyme or reason could be discerned until Mendeleev noticed that when arranged according to atomic weights, the properties tended to repeat. When discrepancies emerged Mendeleev actually insisted that the empirical data on atomic weights was wrong, and later research has vindicated his view. Indeed he was even able to predict the existence — and properties — of missing elements. We believe it is only a matter of time before analogous correlations and patterns emerge for the rules of cross-activation in synaesthesia. For instance, it may not initially be obvious why the arbitrary vowel sequence AEIOU should map in a non-arbitrary manner to a certain sequence of colours (and we have seen some hints of this 50 V.S. RAMACHANDRAN AND E.M. HUBBARD