Guidelines for using a synesthesia Stroop

A variation of the Stroop (1935) task has been used to attest grapheme-color synesthesia. Stimuli are displayed in colors congruent or incongruent to the synesthetic experience, resulting in a congruency effect, in both an ink-naming and a retrieval task (Dixon, 2004). Originally, only digit stimuli were used, however, letters are now often included (e.g., Bor et al, 2014). Experiment 1 investigates whether letter stimuli can cause orthographic priming (e.g., R primes “red”) which could confound results on both tasks. Such a congruency effect due to orthographic priming was indeed found in synesthetes in conditions incongruent to their synesthetic experiences as well as in non-synesthetes, in both the ink-naming and retrieval tasks. Secondly, no previous study has neither evaluated whether the retrieval task truly distinguishes between synesthetes and controls. In Experiment 2, genuine synesthesia is compared to memorized associations in synesthetes and controls. We find a congruency effect in the retrieval task for genuine synesthesia, but also for memorized associations in both groups. The ink-naming task, however, produces congruency effects isolated to genuine synesthesia. The present study demonstrates that the synesthesia Stroop should focus on the ink-naming task, avoiding letter stimuli that could cause orthographic priming.