Discussion

The aim of this study was to test whether the weight estimation of an object moved in an observed action is influenced by the type of movement performed by the actor, namely a lifting movement produced mainly by a concentric contraction, or a lowering movement, requiring an eccentric contraction. Results of the Main experiment showed that, in the case of light boxes (e.g., 2.5kg and 5kg), discrimination sensitivity ( ) was significantly higher in Eccentric than in Concentric condition. This also appeared evident when considering the Mean d’ that in Light was significantly higher than in Heavy condition. Furthermore, this result was confirmed by the analysis of the psychometric function, which showed a significantly better performance in Eccentric than Concentric condition in correspondence of 2.5kg and 5kg. Differently, when the weights of the comparison stimuli were higher than the reference, was higher in Concentric than in Eccentric condition (as shown by analysis on Mean d’), and the upper asymptote of the psychometric function (AUP) was higher in Concentric than Eccentric condition. The Control experiment revealed that, in the case of Light stimuli (e.g., 2.5kg, 5kg), the difference between the effort that observers attributed to the actor when moving the comparison and the reference stimulus (ΔVAS Effort) was significantly higher in Eccentric than Concentric condition. No difference between conditions appeared when showing Heavy stimuli. At last, ΔVAS Effort in Light was significantly higher than in Heavy for both Concentric and Eccentric conditions. For the light boxes, the results of all the analyses indicated that the weight discrimination ability was better in the Eccentric than in the Concentric condition. This difference could be explained by the different perceptions of the effort reported by the observers when seeing the actor performing the eccentric movement, as shown by the result of the Control experiment. For the Light stimuli, the effort attributed by the observers to the actor when he/she moved the comparison stimuli was significantly lower from that associated to the reference box. This marked difference may have helped observers to discriminate the weight of the comparison stimuli from the weight of the reference stimulus. In a recent fMRI study by Casiraghi et al. (Casiraghi et al., 2019), the perceived effort reported by observers after watching an actress perform a grasping action at different percentages of her maximum voluntary force was shown to correlate with activity in cortical areas involved in sensorimotor and cognitive processes. In particular, a linear relationship was found between the BOLD signal evoked during action observation and the entity of the perceived effort in the postcentral gyrus, an area included in the somatomotor network and involved in processing proprioceptive and tactile representations of the manipulated object (Ebisch et al., 2008). Consistent with these results, the amount of force applied by an actor modulated the activation of the primary motor cortex of the observer (i.e., a higher level of force elicited higher cortical excitability in the observer (Alaerts, Senot, et al., 2010)). Based on these results, one could speculate that the observation of the actor’s movements may have elicited activity in a fronto-parietal network that was correlated with the effort the actor exerted to move the boxes, thus influencing the ability to discriminate the weight of the comparison stimulus from that of the reference stimulus. When observers were unable to distinguish the effort exerted by the actor in moving the comparison and the reference stimuli, as was the case for the Light boxes in the Concentric condition and the Heavy boxes in both conditions, the ability to discriminate the weight of the two boxes deteriorated. In fact, d’ was lower for the Light boxes in the Concentric than in the Eccentric condition. Then, considering the Heavy boxes, in the Eccentric condition, the individual d’ values at 10kg, 12.5kg, and 15kg were lower than those at 0kg and 2.5kg, whereas in the Concentric condition d’ at 15kg was lower than at 0kg. Again, in the Eccentric condition, the mean d’ was lower for Heavy than for Light and comparable to the value in the Concentric condition, and the upper asymptote of the psychometric function was lower than in the Concentric condition. All these data pointed out the deterioration of observers’ responses in Eccentric condition when observing the actor moving the heavy boxes. In a previous study of our group (Albergoni, Biggio, Faelli, Ruggeri, et al., 2023), we showed that the ability to discriminate the weight of a moved object was impaired with aging, and we discussed this result as related to the deterioration of the elderly’s strength. Thus, since the motor resonance mechanisms involved in action perception depend on the individual sensorimotor repertoire (Aglioti et al., 2008; Albergoni, Biggio, Faelli, Pesce, et al., 2023; A. Bisio et al., 2010; Petroni et al., 2010), the altered motor feature negatively affected the way older people perceived the movement. Furthermore, previous studies have shown that eccentric contractions are characterized by high force fluctuations (Christou & Carlton, 2002) and continuous adjustments in force level (Perrey, 2018). One could speculate that the high force fluctuations during movement execution may have translated into high uncertainty when observers were asked to judge the actor’s effort in moving the comparison versus the reference box (due to motor resonance mechanisms), and consequently into a deterioration in the ability to discriminate the weight of the two stimuli when the actor performed an Eccentric contraction involving the heavy boxes. One cannot reject the possibility that these differences resulted from the different cortical activation patterns elicited by the execution of eccentric and concentric contractions, which are transferred to motion perception. For example, higher brain activity during eccentric than concentric contractions has been described in a multimodal-associative brain network (Borot et al., 2018; Fang et al., 2001; Kwon & Park, 2011; Yao et al., 2014), which is also known to be involved in weight perception (Hamilton et al., 2006; Chouinard et al., 2009) and which may have played a role in the discrimination task. However, this explanation would not motivate the differences described here between Eccentric and Concentric contractions in Light but not in Heavy boxes. For this reason, we are inclined to discard this hypothesis. Another feature of the actor’s movement that might have helped the observers in estimating the weight of the object, and thus in weight discrimination, is the velocity of the end effector, namely the velocity of the hand moving the box (Bingham, 1987; Shim & Carlton, 1997). Although statistical analysis of the actor’s movement velocity cannot be done since there were only two actors, performing only one movement for each weight, it might be of interest to report that in Eccentric Condition, at the Light weights (0kg, 2.5kg and 5kg), maximum speed values (1.05 m/s, 0.88 m/s, and 0.82 m/s) were substantially different from those of the reference stimulus (7.5kg – 0.61 m/s), whereas these differences were not so pronounced in case of the Heavy weights (10kg - 0.64 m/s, 12.5kg - 0.57 m/s, and 15kg - 0.56 m/s). Therefore, the substantial difference between the maximum speed of the comparison and the reference stimuli in the Light weights may have helped observers to discriminate the box weights. In contrast, the minimal difference in the Heavy weights may have negatively affected observers’ ability in the discrimination task. Following this line of reasoning, in the Concentric condition, where the discrimination ability was terrible and, for the Light weights, also lower than that in the Eccentric condition, one would expect no substantial differences between the actor’s velocity in moving the comparison stimuli relative to that of the reference stimuli. Actually, data on the actor’s velocity is not clear, making it difficult to explain the results in the Concentric condition in terms of differences in the actor’s velocity. In conclusion, the results of this study show that the ability to evaluate the weight of the object involved in the observed action is influenced by both the type of contraction and the entity of the weight. The effort that observers attributed to the actor may partly explain these results. Future work assessing brain activation during the discrimination task when observing eccentric and concentric contractions may shed light on understanding the role of the different cortical areas in processing the information about the object involved in the different types of observed movement.