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 dˈ 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 thatdˈ 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 dˈ at 0kg was
significantly higher than at 10kg (p<0.0001), 12.5kg (p=0.001),
and 15kg (p=0.004). Also, dˈ at 2.5kg was significantly higher
than at 10kg (p<0.001), 12.5kg (p=0.004) and 15kg (p=0.002).
Furthermore, dˈ at 5kg was significantly higher than 10kg
(p=0.006). Lastly, dˈ 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 dˈ 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 dˈ 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 dˈ 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 dˈ 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 —