Congruency effects in the retrieval task of the synesthesia Stroop, even for briefly learned letter-color associations

Grapheme-color synesthesia is a neurological phenomenon in which graphemes evoke specific color experiences. This study investigates whether the color-naming and retrieval tasks of the synesthesia Stroop produce congruency effects that reliably index the synesthetic experience compared to non-synesthete controls. Experiment 1 compared 18 associator grapheme-color synesthetes and 18 non-synesthete controls. The non-synesthete group memorized five letter-color pairings before performing both a color-naming and a retrieval tasks. While a congruency effect was observed in the color-naming task for synesthetes only, both groups demonstrated congruency effects in the retrieval task. Furthermore, associator synesthetes showed larger congruency effects during the color-naming compared to the retrieval task, a pattern originally attributed to projector synesthetes. Experiment 2 examined whether associator synesthetes would also exhibit congruency effects for non-synesthetic letter-color pairings. Following a brief exposure to a novel letter-color association, synesthetes exhibited a congruency effect solely in the retrieval task, mirroring the findings from non-synesthetes in Experiment 1. These results align with MacLeod and Dunbar's continuum of automaticity, suggesting that congruency effects emerge when retrieval processes are less automatized than color-naming ones. Moreover, the presence of congruency effects in both synesthetes and non-synesthetes indicates that the retrieval task may not differentiate synesthetic experiences from briefly memorized associations. These findings question the utility of the retrieval task of the synesthesia Stroop as an objective tool for distinguishing synesthetes from non-synesthetes. It also reinforces the need for refined paradigms that better capture the perceptual properties underlying synesthesia.