002, except the difference between location incongruent and both features congruent, which was a strong trend, p = .01, not significant after correction for multiple comparisons). In addition, the three incongruent

conditions did not differ from one another (all ps > .06), except for the both incongruent condition being significantly slower than the location incongruent condition (p < .0001). By contrast, controls showed no effect of congruency (all ps > .07). The exact p-values of all post-hoc comparisons for this critical interaction are reported in Supplementary Materials. For the shape task, we conducted the identical analysis with a between-participant factor of group (synaesthetes this website vs controls) and a within-participant factor of congruency (both features congruent, location incongruent, shape incongruent, and both features incongruent). The results revealed no significant main effect of group (F < 1.0, n.s.), a significant main effect of congruency EX 527 purchase [F(1.28, 15.44) = 4.47, p = .04, η2 = .27], and a significant group × congruency interaction [F(3, 36) = 3.95,

p = .01, η2 = .24; see Fig. 6b]. Post-hoc comparisons (Bonferroni corrected α-level: .008) showed that synaesthetes were significantly slower in the location incongruent, shape incongruent, and both features incongruent conditions than the both features Nintedanib (BIBF 1120) congruent condition (all ps ≦ .008). No other comparisons in the synaesthete group achieved significance (all ps > .05; except for location incongruent vs shape incongruent, p = .03, not significant after correction for multiple comparisons). Consistent with the colour task, controls show no effect of congruency (all ps > .4, except both congruent vs location incongruent, p = .048, not significant after correction for multiple comparisons). The exact p-values are reported in Supplementary

Materials. The same analyses on the error rate reveal, in the colour task, a significant main effect of congruency [F(2.13, 25.67) = 4.21, p = .02, η2 = .26]. Post-hoc tests show that error rate is significantly higher in the location incongruent condition (1.48%, p = .01) and marginally higher in the both features incongruent condition (3.42%, p = .08) than in the both features congruent condition (0%). In the shape task, there were no significant effects (all ps > .18). Auditory–visual synaesthesia, an unusual phenomenon in which sounds elicit visual experiences, is often mentioned anecdotally in scientific literature but has rarely been studied experimentally. The few studies that use objective measures focus on the reported colour experience (e.g., Goller et al., 2009; Ward et al., 2006). In the present study, we studied seven synaesthetes with consistent visual experiences of coloured geometric objects in space when listening to sounds.