Exp Brain Res (2011) 211:531–545 All analyses included trials in which both pictures depicted the same hand (ﬁrst: right hand; second: right hand) and trials in which the two pictures depicted different hands (ﬁrst: right hand; second: left hand). Results Reaction times Four participants were excluded due to error rates that were more than two SDs above average (8%). The remaining 22 participants had a mean age of 20.9 years (13 women, 18 right-handed). Only trials with correct responses were included in the analysis. We found the typical mental rotation pattern, that is, an increase in RTs with increasing angle of rotation (slope tested against zero) [t(21) = 7.6, p \ .001; see Table 1]. The comparison of slopes for the single and the joint-attention condition revealed a signiﬁcant difference. Slopes were considerably ﬂatter when both participant were jointly attending [t(21) = 3.7, p \ .001; see Fig. 2]. Intercepts differed signiﬁcantly [t(21) = 3.4, p \ .01]. Participants were slower at processing non-rotated stimuli in the joint-attention condition compared to the single- attention condition. Table 1 Slopes (ms/deg; per cent error/deg) and intercepts (ms; per cent error) for RTs and error rates of 1st PP trials in experiment 1, experiment 2 (separate for the cooperation and the competition group) and experiment 3 (separate for trials following 1st PP trials and trials following 3rd PP trials) Experiment and condition RTs Errors Slopes Intercepts Slopes Intercepts Experiment 1 Single Joint Experiment 2 Cooperation Single Joint Competition Single Joint Experiment 3 Previous 1st Single Joint Previous 3rd Single Joint 2.8 1.9 1.9 1.3 2.1 1.6 1.4 1.7 1.5 1.0 595 695 555 586 498 518 599 592 575 641 0.08 0.08 0.63 0.53 0.06 0.09 0.12 0.12 0.14 0.14 0.1 -0.5 -1.1 -1.3 1.3 -0.2 -5.1 -2.8 -4.8 -4.9 535 Fig. 2 Reaction times and linear ﬁts for 1st PP trials in both attention conditions of experiment 1. The single-attention condition is depicted in grey (squares), the joint-attention condition in black (triangles). The trend line for the single condition is depicted in grey, R2 = .99. The trend line for the joint-attention condition is shown in black, R2 = .95 Errors Error rates increased signiﬁcantly with increasing rotation [t(21) = 7.0, p \ .001]. No effect of attention on slopes was present in error rates [t(21) \ 1], nor was there any effect on intercepts [t(21) \ 1]. See Table 1 for intercepts and slopes of both attention conditions. Debrieﬁng session Participants indicated that they thought their behaviour and their performance had been unaffected by the other’s attention. None of the participants guessed that joint attention had affected their performance differentially depending on degree of rotation. When asked to guess in which way their performance might have been different in the joint-attention condition, approximately half of the participants indicated that they thought attending together had made them faster, whereas the other half of partici- pants guessed that attending together had made them slower overall. 123