Supplementary methods Auditory stimuli We used piano tones in the range of 264-528 Hz. The duration of each tone was 1 second and the tones were presented consecutively. Lower tones always preceded the higher ones. With these tone sequences 4 different intervals were designed (minor second, major second, minor third, and major third). In total 64 tone intervals were randomly presented 6 times (2 times no taste and 4 times taste conditions). Reaction time was measured as the interval from the end of tone two to the start of the button press. Indirect Stroop test In principle, two different strategies can be used to assess synesthesia with adapted Stroop-tasks: 1) manipulation of the inducer and registration of significant changes in the threshold or other perceptual aspects of the concurrent modality (here taste); 2) manipulation of the concurrent modality and registration of changes in perceptual aspects of the inducer. Although method 1 is the classical method for testing synesthesia, the second (indirect) method was applied for the following reasons:
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