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Bryan Alvarez, Lynn Robertson; A “hole” new look at grapheme-color synesthesia. Journal of Vision 2009;9(8):46. doi: 10.1167/9.8.46.
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Grapheme-color synesthesia is a phenomenon whereby letters or numbers (graphemes) evoke the visual perception of color. In this experiment, seven synesthetes were primed for synesthetic color and spatial plane with letter-shaped holes cut from circular discs. Primes and probes appeared sequentially (8ms SOA) in either the same or different plane of 3D space (induced with stereoscopic glasses). Probes were congruent or incongruent to the color synesthetically induced by the letter-shaped primes. All synesthetes showed positive color priming effects, replicating previous studies and showing for the first time that synesthetic color can be induced by letter-shaped holes. Synesthetes also showed effects of plane; they were faster to name colors that appeared in the opposite plane of depth relative to the letter prime, a result likely due to the residual representation of the prime that lasts beyond the 8ms inter-stimulus interval and competes for representation with the probe when it appears in the same 3D location of space. This result was also seen in a separate experiment for non-synesthetes who viewed colored letter primes followed by colored probes, and contrasts with previous synesthetic research showing that the relationship of spatial plane to prime-probe pairs correlates with the vividness of visual imagery of synesthetes. Specifically, synesthesia was suggested to be supported by visual imagery and, when vivid, would suppress visual perception and perceptual binding, reducing prime-probe competition. In the current study, vividness of visual imagery did not correlate with RTs for any condition when synesthetic color was induced from letter-shaped holes; an important clue that actually furthers the idea that synesthesia is supported by visual imagery. Together, these findings suggest that grapheme-color synesthesia is induced from the shape of a letter, even when the shape itself has no surface properties, and acts similarly to non-synesthetic color priming.
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