Abstract
In colour–grapheme synesthesia, non-coloured graphemes are perceived as being inherently coloured. In recent years, it is debated whether visual processing of synesthesia-inducing achromatic graphemes is similar to that of chromatic graphemes. Here, we exploit the phenomenon of binocular rivalry in which incompatible images presented dichoptically compete for conscious expression. Importantly, the competition only arises if the two images are sufficiently different; if the difference between the images is small, the images will fuse into a single mixed percept. We show that achromatic digits that induce synesthetic colour percepts increase the incidence of binocular rivalry compared to achromatic non-digits that do not evoke such percepts. That is, compared to achromatically perceived non-digits, synesthesia-inducing digits increase the predominance of binocular rivalry over binocular fusion. This finding shows that the synesthetic colour experience can provide the conditions for promoting binocular rivalry, much like stimulus features that induce rivalry in normal vision.
Highlights
In grapheme–colour synesthesia, non-coloured graphemes may be perceived as inherently coloured
We found no general difference between synesthetes and controls: binocular rivalry is not generally different in synesthetes compared to controls
We hypothesize that the interaction between colour and digit was mainly driven by synesthetes: our hypothesis states that rivalry in achromatic digits should be selectively increased in synesthetes and not in controls
Summary
In grapheme–colour synesthesia, non-coloured graphemes (i.e., letters or digits) may be perceived as inherently coloured. Examples range from experiments on visual search (e.g., Ramachandran and Hubbard, 2001) and perceptual grouping (e.g., Kim et al, 2006), to Stroop-like interference effects and priming effects (e.g., Gebuis et al, 2009a,b; Mattingley et al, 2001). Results of these studies suggest that synesthesia-inducing achromatic stimuli can affect processing in a manner comparable to chromatic stimuli. Synesthetic hues appear to be impervious to brightness contrast, are differentially affected by simultaneous colour contrast (Nijboer et al, 2011), and fail to induce chromatic after-effects (Hong and Blake, 2008), suggesting that they are not identical to veridical colour at the earliest stages of visual cortical processing
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