Consistency of synesthetic association varies with grapheme familiarity: A longitudinal study of grapheme-color synesthesia

How "diagnostic" criteria interact to shape synesthetic behavior: The role of self-report and test-retest consistency in synesthesia research

In the past few decades, researchers have established synesthesia as a genuine phenomenon, identified its characteristics (in particular, its automatic, specific and consistent nature), and developed "gold standard" inclusion criteria for research: synesthetes are participants that self-report synesthetic experiences and have consistent (beyond a "cutoff" score) inducer-to-concurrent pairings. While this approach has significantly advanced scientific progress, it can confuse interpretation of research findings due to its inherent circularity: consistency will always appear to be a defining characteristic of synesthesia so long as it is also an inclusion criterion for synesthesia studies. Here, we aim to clarify the relationship between self-report and consistency in "diagnosing"1 synesthesia. In four experiments, we find that: (1) the optimal consistency cutoff score differs across languages; (2) self-reported synesthetes that "fail" consistency tests can still behave like synesthetes - to our knowledge the first objective evidence that "inconsistent synesthesia" is a genuine phenomenon; (3) Using self-report as the sole inclusion criterion does not significantly change the effect size of two measures of synesthetic behavior (the synesthetic Stroop and synesthetic color Palette); and (4) Consistency influences Stroop effect size in self-reported synesthetes only, but influences the Palette in both synesthetes and non-synesthete controls. We conclude that (in certain cases) self-report alone is a sufficient diagnostic criterion for synesthesia, and that synesthesia studies can increase explanatory power by using raw consistency scores as a covariate in analyses, rather than as an inclusion criterion.

Induced awareness of synesthetic sensations in synesthetically predisposed "Borderline Non-synesthetes"

A long-standing issue concerning synesthesia is whether the trait is continuous or discontinuous with ordinary perception. Here, we found that a substantial proportion of non-synesthetes (>10 % out of >200 unselected participants) spontaneously became aware of their synesthesia by participating in an online survey that forced them to select colors for stimuli that evoke color sensations in synesthetes. Notably, the test-retest consistencies of color sensation in these non-synesthetes were comparable to those in self-claimed synesthetes, revealing their strong though latent synesthetic dispositions. The effect was absent or weak in a matched control survey that did not include the color-picking test. Therefore, the color-picking task likely provided the predisposed "borderline non-synesthetes" with an opportunity to dwell on their tendency toward synesthesia and allowed their subconscious sensations to become conscious ones. The finding suggests that the general population has a continuum of synesthetic disposition that encompasses both synesthetes and non-synesthetes.

Consistency and strength of grapheme-color associations are separable aspects of synesthetic experience

Consistency of synesthetic associations over time is a widely used test of synesthesia. Since many studies suggest that consistency is not a completely reliable feature, we compared the consistency and strength of synesthetes' grapheme-color associations. Consistency was measured by scores on the Synesthesia Battery and by the Euclidean distance in color space for the specific graphemes tested for each participant. Strength was measured by congruency magnitudes on the Implicit Association Test. The strength of associations was substantially greater for synesthetes than non-synesthetes, suggesting that this is a novel, objective marker of synesthesia. Although, intuitively, strong associations should also be consistent, consistency and strength were uncorrelated, indicating that they are likely independent, at least for grapheme-color synesthesia. These findings have implications for our understanding of synesthesia and for estimates of its prevalence since synesthetes who experience strong, but inconsistent, associations may not be identified by tests that focus solely on consistency.