Grapheme-color synesthesia interferes with color perception in a standard Stroop task

•Synesthetes activated the rostral cingulate zone (RCZ) in a synesthetic Stroop task.•Synesthetes showed a reduced interference effect in the standard Stroop task.•Synesthetes activated the RCZ to neutral stimuli in a standard Stroop task.•Data provide evidence for the automatic and involuntary nature of synesthesia.

This study examined the proposed automatic and involuntary nature of synesthetic experiences in grapheme-color synesthetes by comparing behavioral and blood-oxygen level dependent (BOLD) responses in a synesthetic and a standard version of the Stroop task. Clear interference effects in terms of slower reaction times and stronger BOLD responses in the rostral cingulate zone (RCZ) were found in synesthetes performing the synesthetic version of the Stroop task. Surprisingly, less interference was found in synesthetes compared with controls performing the standard Stroop task. This smaller interference effect, expressed as the difference in reaction time between incongruent and neutral stimuli, was explained in terms of experienced interference during the neutral condition of the Stroop task in synesthetes. This was confirmed by stronger BOLD responses in the RCZ for synesthetes specifically in the neutral condition. To the best of our knowledge, this is the first study to show different performance of synesthetes in a standard Stroop task and the presented data can be seen as strong evidence for the automatic and involuntary nature of synesthetic experiences.