Sample 1 (N-back before rMT): There was no significant difference in rMT between men and women (t(138) = 0.23, P = 0.82). The results of our univariate ANOVAs for 0-back-and 3-back-accuracy or -d' performance indicated no significant main effects of rMT or gender and no significant interaction between rMT and gender. However, we found significant interactions of rMT and gender on 2-back-accuracy (F(1,136) = 8.85, P = 0.003) as well as 2-back-d' (F(1,135) = 11.36, P = 0.001), which survived Bonferroni correction (six comparisons, see Table 1). Therefore, in post hoc analyses, 2-back correlations with rMT were calculated separately for men and women. In women, no significant correlations were found between any of the 2-back performance measures and rMT (2-back-accuracy r(66) = 0.17, P = 0.16; 2-back-d': r(66) = 0.21, P = 0.1). In men, rMT was negatively correlated with 2-back-accuracy (r(70) = −0.31, P = 0.008) as well as 2-back-d' (r(69) = −0.34, P = 0.003, see Fig. 1), meaning that men with lower rMT showed a higher N-back performance. Moreover, regressing out mean N-back reaction times, 0-back performance, age, VAS for anxiety, STAI, daily intake of stimulants (caffeine, nicotine, alcohol), motivation, degree of dexterity or daytime of experiment, each separately, did not affect these negative correlations between rMT and 2-back performance in men.
Potential smaller variances within the 0-back or 3-back condition compared to 2-back condition may obscure the detection of significant correlations with rMT. Comparison of variance indicated a trend toward less variance for 0-back-d' compared with 2-back-d' (F(1,140) = 3.2, P = 0.07), as well as for 3-back-d' compared with 2-back-d' (F(1,140) = 2.93, P = 0.09, see Fig. S1). Furthermore, 2-back performance shared only little variance with 0-back performance (r2 ≤ 0.15) and only a moderate variance with 3-back performance (r2 ≤ 0.35) (see Table S1).
Sample 2 (rMT before N-back): To investigate the possibility that the observed negative correlation between rMT and 2-back performance was due to the procedure of sample 1, meaning that working memory performance affects the subsequent measurement of the rMT, we also assessed rMT before the working memory task in an additional sample. The results of our univariate ANOVAs indicated no significant interactions between rMT and sex on N-back performances (P > 0.24). But we observed a significant main effect of rMT on 2-back performance (accuracy: F(1,45) = 4.2, P = 0.047, β = −0.29; d': F(1,44) = 6.49, P = 0.013, β = −0.36, see Table S2) indicating also in this sample a negative correlation between rMT and 2-back performances (see Fig. S2). Comparison of variance indicated significantly less variance for 3-back-d' compared with 2-back-d' (F(1,94) = 7.36, P = 0.008 see Fig. S3). Furthermore, 2-back performance shared only little variance with 0-back performance (r2 ≤ 0.05) and only a moderate variance with 3-back performance (r2 ≤ 0.37).