Abstract
Individuals with grapheme-colour synaesthesia experience anomalous colours when reading achromatic text. These unusual experiences have been said to resemble ‘normal’ colour perception or colour imagery, but studying the nature of synaesthesia remains difficult. In the present study, we report novel evidence that synaesthetic colour impacts conscious vision in a way that is different from both colour perception and imagery. Presenting ‘normal’ colour prior to binocular rivalry induces a location-dependent suppressive bias reflecting local habituation. By contrast, a grapheme that evokes synaesthetic colour induces a facilitatory bias reflecting priming that is not constrained to the inducing grapheme’s location. This priming does not occur in non-synaesthetes and does not result from response bias. It is sensitive to diversion of visual attention away from the grapheme, but resistant to sensory perturbation, reflecting a reliance on cognitive rather than sensory mechanisms. Whereas colour imagery in non-synaesthetes causes local priming that relies on the locus of imagined colour, imagery in synaesthetes caused global priming not dependent on the locus of imagery. These data suggest a unique psychophysical profile of high-level colour processing in synaesthetes. Our novel findings and method will be critical to testing theories of synaesthesia and visual awareness.
Highlights
Studying colour perception epitomises the challenge of understanding the mechanisms that underpin the contents of consciousness – how is the subjective experience of colours created from variations in the wavelengths of light? Grapheme-colour synaesthesia provides a unique window into the mechanisms by which the brain creates colour
Perceiving a normal colour cue led to a bias towards seeing opponent colour during the subsequent rivalry display when cue and target appeared in the same location (relative to chance, t(5) = −6.69, p = .001, mean difference (Mdiff) = −38.83, confidence intervals (CI) [−53.75, −23.91], dunbiased = 2.30, negative indicating a bias away from the cued colour), but this was not the case when they were in opposite locations (relative to chance, t(5) < 1, Mdiff = −1.50, CI [−14.45, 11.45], dunbiased = 0.10), replicating prior findings of local, retinotopically-constrained chromatic habituation (Brascamp, Knapen, Kanai, Van Ee, & Van Den Berg, 2007; Pearson et al, 2008)
Synaesthetic colour cues had a facilitative effect: perceiving a synaesthetic colour cue led to a bias towards seeing the matching colour in subsequent rivalry stimulus when cue and target were in the same location (relative to chance, t(5) = 4.19, p = .009, Mdiff = 15.17, CI [5.86, 24.47], dunbiased = 1.44) and this seemed likely to occur when they were in opposite locations, albeit with less certainty (t(5) = 2.57, p = .05, Mdiff = 12.33, CI [0.01, 24.66], dunbiased = 0.88)
Summary
Grapheme-colour synaesthesia provides a unique window into the mechanisms by which the brain creates colour. Studying colour perception epitomises the challenge of understanding the mechanisms that underpin the contents of consciousness – how is the subjective experience of colours created from variations in the wavelengths of light? People with this unusual condition have involuntary colour experiences triggered by reading achromatic letters and numbers. Attempts to characterise the nature of synaesthetic colour have been faced with the classic difficulties of studying conscious experience. Other ‘proxy’ measures of synaesthesia, such as the synaesthetic congruency/Stroop effect, reflect the involuntary nature of synaesthetic colour rather than its qualia per se (for review of relevant evidence, see Chiou & Rich, 2014; Mattingley, 2009)
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