Perceptual Dissimilarity Analysis Distinguishes Grapheme‐Color Synesthetes from Nonsynesthetes

What is it about?

Synesthesia is a phenomenon in which perception of ordinary stimuli, such as written letters and numbers (collectively termed graphemes), is accompanied by involuntary, secondary experiences, such as colors. These synesthetic associations are arbitrary (i.e., there is no apparent reason why ‘A’ should evoke purple), idiosyncratic (i.e., different synesthetes may associate different colors with the same grapheme), and remain consistent over time. In this study, we used a novel perceptual similarity task with grapheme-color synesthetes, and a control group of non-synesthetes, to test whether synesthetes’ secondary color experiences influenced similarity judgments of visually presented graphemes. Participants iteratively arranged graphemes and, separately, the associated synesthetic colors in an online display, placing similar items close together and dissimilar items further apart. The resulting relative inter-item distances were used to calculate the dissimilarity between each pair of items in the set. We then measured the correlation between these dissimilarity estimates for graphemes and colors in synesthetes and in the control group. This correlation was significantly higher in synesthetes than non-synesthetes. Importantly, synesthetes’ correlations scaled with their score on the online Synesthesia Battery, a well-established test for synesthesia based on the consistency of synesthetic associations. By contrast, this relationship was not significant for non-synesthetes.

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Photo by Mika Baumeister on Unsplash

Photo by Mika Baumeister on Unsplash

Why is it important?

The novel similarity task quantitatively distinguished synesthetes from non-synesthetes, in a way that meaningfully reflected consistency, a key aspect of synesthetic experience. These results advance our understanding of synesthesia by enabling us to quantify the influence of concurrent color experiences on perception of the grapheme stimuli by reference to the strength of the correlation between the grapheme and color dissimilarity estimates. This finding might be useful for kinds of synesthesia that cannot be assessed via the online Synesthesia Battery; for example, lexical-gustatory synesthesia, in which non-food words nonetheless evoke taste experiences; or synesthesias involving personality, for example, where someone’s personality evokes color, or where a sequence (e.g., days of the week, or months of the year) evokes a personality. Another possibility is that this approach could be used as an additional way of checking the synesthetic status of individuals who do not pass the Synesthesia Battery test because, although their synesthetic associations are strong, they are not sufficiently consistent over the short term to pass conventional consistency-based testing.