Results

Main Experiment: Sensitivity analysis (dˈ)

Table 1 shows the discrimination sensitivity at each box weight in the two conditions. These data are also plotted in Figure 2A, which shows how the difference between the two conditions was modulated by the weight of the observed box. In fact, the discrimination sensitivity was higher in the eccentric condition only when the lighter boxes were shown, whereas it was better in the concentric condition when the heavier boxes were shown. The results of the Wilcoxon test on showed significant differences between Concentric and Eccentric for 2.5kg (z=2.02, p = 0.043) and 5kg (z=2.63, p=0.008). No difference between the two conditions was observed for 0kg, 10kg, 12.5kg, and 15 kg (p>0.05). Friedman test showed a significant effect of weight in both Concentric (χ2(5)=28.2, p<0.0001) and Eccentric (χ2(5)=66.8, p<0.0001) conditions. In the Concentric condition, the post hoc analysis revealed that at 0kg was significantly higher than at 2.5kg (p=0.003), 5kg (p=0.003), 10kg (p=0.007), 12.5kg (p=0.002) and 15kg (p=0.03). In Eccentric Condition, the post hoc showed that at 0kg was significantly higher than at 10kg (p<0.0001), 12.5kg (p=0.001), and 15kg (p=0.004). Also, at 2.5kg was significantly higher than at 10kg (p<0.001), 12.5kg (p=0.004) and 15kg (p=0.002). Furthermore, at 5kg was significantly higher than 10kg (p=0.006). Lastly, at 15kg was significantly higher than 10kg (p=0.02). No additional significant differences were revealed. Data are given in Table 1 and represented in Figure 2A. — Table 1 here — ANOVA performed on LHmean showed a significant Amount-of-weight effect (F(1,34)=24.9, p<0.001, ƞ2=0.19) and a significant interaction Condition*Amount-of-weight (F(1,34)=30.6 , p<0.001, ƞ2=0.11). In Light, observers had a higherLHmean in Eccentric than Concentric condition (Concentric: 2.59±0.11, Eccentric: 3.03±0.06; p<0.0001). The opposite was observed in Heavy where LHmean was higher in Concentric than in Eccentric condition (Concentric: 2.43±0.10, Eccentric: 2.11±0.12; p=0.014). In the Eccentric condition, post hoc analysis revealed that LHmean was significantly higher in Light than in Heavy (Light: 3.03 ± 0.06, Heavy: 2.11 ± 0.18; p<0.001). No difference was found between Light and Heavy in Concentric condition (Figure 2B). The statistical analysis on mean dˈ did not show a significant difference between Conditions (Concentric: 2.47±0.03, Eccentric: 2.53±0.03; t(34)=0.763, p=0.45).

Main Experiment: weight discrimination ability

The graphical representation of the psychometric functions of the two conditions is displayed in Figure 2C. The psychometric curves represent the probability that the observer judges the weight of the comparison box to be heavier than that of the reference in the two conditions. It can be deduced from the figure that the ability to discriminate load was better in the eccentric condition for the lighter boxes, whereas when the weights of the comparison stimulus were heavier, this parameter was better in the concentric condition. The comparison between Heavier probability at each box weight showed a significant main effect of Condition at 2.5kg (Concentric: 0.08 [0.00, 0.17]; Eccentric: 0.00 [0.00, 0.08]; z=2.72, p=0.007), and at 5kg (Concentric: 0.17 [0.00, 0.33]; Eccentric: 0.08 [0.00, 0.13]; z=2.88, p=0.004) (Figure 2A). No difference between Concentric and Eccentric was found in JND (Concentric: 1.76 [0.59, 2.63] kg, Eccentric: 1.76 [0.95, 2.60] kg), ALOW (Concentric: 0.00 [0.00, 0.03], Eccentric: 0.00 [0.00, 0.03]), and threshold (Concentric: 7.46 [6.89, 7.66] kg, Eccentric: 7.54 [7.24, 8.06] kg). The result of the statistical analysis on AUP showed that it was significantly higher in Concentric (0.95 [0.89, 1.00]) than in Eccentric condition (0.88 [0.80, 0.98]) (z=2.49, p=0.021) (Figure 2D). — Figure 2 here —

Control Experiment

Figure 3A shows the effort that the observer attributed to the actor when performing the action, which increased as the weight of the box increased. Figure 3B shows the difference in the perception of effort, expressed as an absolute value, between each comparison stimulus and the reference stimulus, while Figure 3C shows the same parameter but averaging the lightest and heaviest boxes with respect to the reference. From panels B and C, it can be deduced that this difference was greatest when observing the lightest boxes in the eccentric condition. The results of ANOVA on ΔVAS Effort revealed a significant effect of Box-Weight (F(5,60)=102; p<0.0001; η2=0.65), a significant effect of Condition (F(1,12)=10.1; p=0.008; η2=0.09), and a significant interaction Box-Weight*Condition (F(5,60)=3.02; p=0.017; η2=0.10). The post-hoc analysis conducted on the Condition effect showed a significantly higher ΔVAS Effort in Eccentric than in Concentric at 2.5kg (Concentric: 30.8±2.7, Eccentric: 40.1±2.1, p=0.07) and at 5kg (Concentric: 14.6±2.2, Eccentric: 29.1±2.9, p<0.001) (Figure 3B). The post-hoc analysis computed on Box-Weight revealed significantly higher values in Concentric condition at 0kg (46.6±2.3) than at 2.5 (30.8±2.7, p=0.005), 5kg (14.6±2.3, p<0.0001), 10kg (15.5±1.5, p<0.0001), 12.5kg (17.3±2.7, p<0.0001), and 15kg (20.3±2.2, p<0.0001). Likewise, at 2.5kg ΔVAS Effort was significantly higher than at 5kg (p=0.0001), 10kg (p=0.0003), and 12.5kg (p=0.003). Within Eccentric condition, ΔVAS Effort was significantly higher at 0kg (49.6±2.2) than at 5kg (29.1±2.9, p<0.0001), 10kg (14.6±2.1, p<0.0001), 12.5kg (18.4±2.9, p<0.0001), and 15kg (21.5±2.0, p<0.0001), ΔVAS Effort was also significantly higher at 2.5 (40.1±2.1) than at 10kg (p<0.0001), 12kg (p<0.0001) and 15kg (p<0.0001). In the end, significantly higher values of ΔVAS Effort were found at 5kg than at 10kg (p=0.002). ANOVA performed on LHmean ΔVAS Effort showed a significant Amount-of-weight effect (F(1,12)=102.8, p<0.0001, ƞ2=0.65), a significant effect of Condition (F(1,12)=8.80; p=0.012; η2=0.13) and a significant interaction Condition*Amount-of-weight (F(1,34)=5.30, p=0.040, ƞ2=0.14). Considering Light, ΔVAS Effort was higher in Eccentric Condition than in Concentric (p=0.014). No difference between Concentric and Eccentric was found in Heavy weights. In the Eccentric condition, post hoc analysis revealed that LHmean ΔVAS Effort was significantly higher in Light than in Heavy (Light: 39.6±1.7, Heavy: 17.4±1.9; p<0.0001) and the same result was observed in Concentric (Light: 29.7±2.0, Heavy: 17.7±1.7; p=0.001) (Figure 3C). — Figure 3 here —