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
(dˈ ) 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, dˈ 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.