EMG activity
There were no main effects or interactions of the factors Group F(1,31) = 1.6, p = 0.215; Task F (1,31) = 2.36, p = 0.135, and Hand F (1,31) = 3.18, p = 0.09, which indicate similar muscle tension in both groups and all experimental conditions. Significant differences were observed only in the Channel factor F (1,31) = 4.33, p = 0.045, np2 = 0.12. The electrode placed on the left forearm registered higher amplitude than the one that was placed on the right forearm (Channel-left M = 0.14mV, SE = 0.08mV; Channel-right M = 0.04mV, SE = 0.03mV) (Supplementary Fig. 1.).
Behavioral performance
The main effect of the factor Group was significant, F(1,38) = 4.44, p = 0.04, np2 = 0.10, indicating that the correctness was higher in the left-handed group (right-handed: M = 31.76, SE = 2.17; left-handed: M = 38.22, SE = 2.17). In addition, we found the significant interaction of the factors Task x Group, F (1,38) = 6.14, p = 0.018, np2 = 0.14, and the Bonferroni post hoc test revealed (p = 0.02), that the left-handers performed the VMI task better (M = 40.7, SE = 2.21) than the KMI task (M = 35.75, SE = 2.41). Also, in the VMI condition the left-handed subjects achieved significantly (p = 0.038) higher results (M = 40.7, SE = 2.21) than right-handers (M = 31.37, SE = 2.21) (Fig. 1). There were no main effects or interactions of the factors Task F (1,38) = 3.26, p = 0.078, and Hand F (1,38) =2.79, p = 0.1.
EEG activity
Central occipital cluster:
There were no main effects of the factors Group F(1,31) = 2.51, p = 0.119, and Task F (1,31) = 3.37, p = 0.068. However, we have noted significant main effects for factors Frequency F (1,53) = 24.57; p <0.001, np2 = 0,32, and Hand F(1,53) = 6.04; p = 0.017, np2 = 0,08. The alpha frequency (M = 1.24 dB, SE = 0.26 dB) had higher strength than beta (M = 0.18 dB, SE = 0.09 dB). Stronger signal was also recorded during the imaginary movement of the right hand (Left: M = 0.61dB, SE = 0.16 dB; Right: M = 0.81dB, SE = 0.17 dB). Moreover, we observed a significant Task x Group interaction F(1,53) = 11.52, p = 0.001, np2 = 0.18. The Bonferroni post hoc test showed that a significantly (p = 0.003) stronger signal was recorded during the VMI task in the left-handed group (KMI: M = 0.27 dB, SE = 0.26 dB ; VMI: M = 0.64 dB, SE = 0.22 dB). The results of the interaction between factors Frequency x Task x Group was also significant F(1,53) = 9.16, p = 0.004, np2 = 0.15, and post hoc comparisons made it clear that the difference between the KMI and VMI tasks is present only in the left-handed group (p < 0.001), for the 8 - 13 Hz frequency band (KMI: M = 0.62 dB, SE = 0.41 dB; VMI: M = 1.2 dB, SE = 0.35 dB) (Fig. 2).
Right parietal cluster:
There were no main effects of the factors Group F(1,54) = 0.49; p = 0.515, and Frequency F(1,54) = 0.429; p = 0.515. However, we have noted significant main effects for factors Task F(1,54) = 5.19; p = 0.026, np2 = 0.09, and Hand F(1,54) = 56.65; p <0.001, np2 = 0.51. Desynchronization was greater in the KMI condition (KMI: M = -1.01 dB, SE = 0.15 dB; VMI: M = -0.77 dB, SE = 0.13 dB), and during the motor imagery of the left hand (Left: M = -1.33 dB, SE = 0.16 dB; Right: M = -0.47 dB, SE = 0.12 dB). Moreover, we observed a significant Frequency x Hand interaction F(1,54) = 15.04, p <0.001, np2 = 0.21. The Bonferroni post hoc test showed a significantly (p < 0.001) stronger suppression in the alpha frequency band, during the motor imagery of the left hand (8-13 Hz: M = -1.49 dB, SE = 0.23 dB; 15-30 Hz: M = -1.15, SE = 0.12 dB). The results of the interaction between factors Frequency x Task x Group were also significant F(1,54) = 9.16, p = 0.004, np2 = 0.15, and post hoc comparisons showed that the difference between the KMI and VMI tasks is present only in the right-handed group (p < 0.001), for the 8 - 13 Hz frequency band (KMI: M = -1.14 dB, SE = 0.27 dB; VMI: M = -0.53 dB, SE = 0.25 dB) (Fig. 3).
Left parietal cluster:
There were no main effects of the factors Group F(1,59) = 0.04; p = 0.832; Task F(1,59) = 0.01; p = 0.938, and Frequency F(1,59) = 0.267; p = 0.607. We have noted only one significant main effect for factor Hand F(1,59) = 48.55; p <0.001, np2 = 0.45, and Frequency x Hand interaction F(1,59) = 17.97, p < 0.001, np2 = 0.23. Desynchronization was greater during the motor imagery of the right hand (Left: M = -0.58 dB, SE = 0.14 dB; Right: M = -1.45 dB, SE = 0.16 dB). Moreover, post hoc comparisons showed that during the left (p < 0.001), and right (p = 0.002) motor imagery of hand movement, suppression was stronger in alpha frequency band (Left8-13 Hz: M = -0.47 dB, SE = 0.19 dB, 15-30 HzM = -0.69 dB, SE = 0.1 dB; Right 8-13 Hz: M = -1.63 dB, SE = 0.23 dB; 15-30 HzM = -1.27 dB, SE = 0.11 dB) (Fig. 4.).