This study investigates the origins of specific letter-colour associations experienced by people with grapheme-colour synaesthesia. We present novel evidence that frequently observed trends in synaesthesia (e.g. A is typically red) can be tied to orthographic associations between letters and words (e.g., "A is for apple"), which are typically formed during literacy acquisition. In our experiments, we first tested members of the general population to show that certain words are consistently associated with letters of the alphabet (e.g. A is for apple), which we named index words.Sampling from the same population, we then elicited the typical colour associations of these index words (e.g. apples are red) and used the letter  index word  colour connections to predict which colours and letters would be paired together based on these orthographic-semantic influences. We then looked at direct letter-colour associations (e.g., A  red, B  blue…) from both synaesthetes and non-synaesthetes. In both populations, we show statistically that the colour predicted by index words matches significantly with the letter-colour mappings: that is, A  red because A is for apple and apples are prototypically red. We therefore conclude that letter-colour associations in both synaesthetes and non-synaesthetes are tied to early-learned letter-word associations.

This study used grapheme-colour synaesthesia, a neurological condition where letters evoke a strong and consistent impression of colour, as a tool to investigate normal language processing. For two sets of compound words varying by lexical frequency (e.g., football vs lifevest) or semantic transparency (e.g., flagpole vs magpie), we asked 19 grapheme-colour synaesthetes to choose their dominant synaesthetic colour using an online colour palette. Synaesthetes could then select a second synaesthetic colour for each word if they experienced one. For each word, we measured the number of elicited synaesthetic colours (zero, one, or two) and the nature of those colours (in terms of their saturation and luminance values). In the first analysis, we found that the number of colours was significantly influenced by compound frequency, such that the probability of a one-colour response increased with frequency. However, semantic transparency did not influence the number of synaesthetic colours. In the second analysis, we found that the luminance of the dominant colour was predicted by the frequency of the first constituent (e.g. rain in rainbow). We also found that the dominant colour was significantly more luminant than the secondary colour. Our results show the influence of implicit linguistic measures on synaesthetic colours, and support multiple/dual-route models of compound processing.

In this paper, I present arguments and suggestions for the improvement of the scientific study of synaesthesia, and particularly grapheme-colour synaesthesia in relation to psycholinguistic research, although the principles I advocate can be easily adapted to any subfield of synaesthesia study. I postulate that the current state of research on synaesthesia in general, and on grapheme-colour synaesthesia in particular, suffers from a lack of exploratory evidence and essential groundwork upon which to build hypothesis-testing studies. In particular, I argue that synaesthesia research has been artificially bounded by assumptions about the nature of synaesthetic experiences, which constrain both the questions that researchers ask and the way in which they go about answering those questions. As a specific example, I detail how much of the current research on grapheme-colour synaesthesia is built to accommodate two major assumptions about the nature of colours for letters and for words—assumptions which I will contend are not universally true, and the exceptions to which point to a much richer and heterogeneous understanding of synaesthetic experience than current research practices capture. The top-down predetermination of what is important or meaningful to measure, and what is not, has subsequently impeded a full understanding of what synaesthesia is and how it works. I argue that these assumptions must be carefully addressed and evaluated, both for the particular case of grapheme-colour synaesthesia and for the field as a whole, to move towards a holistic and fruitful understanding of synaesthesia as a phenomenon and as a tool to study language, thought and perception. To that end, I propose specific recommendations for synaesthesia researchers to solidify and expand their understanding and to capture the actual experience of synaesthetes. This article is part of a discussion meeting issue ‘Bridging senses: novel insights from synaesthesia’.

In recent decades, synaesthesia – a condition characterised by a ‘joining of the senses’ – has grown in popularity as a subject of research. As the field expands from describing the condition to applying it to other areas of human cognition, a fundamental assumption must be addressed: Are the experiences of synaesthetes generalisable to non-synaesthetes as well? This article argues that not only can synaesthesia provide useful insights into the general functioning of language, but it is a useful and promising tool to investigate a variety of outstanding scientific questions.

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