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Abstract:
Introduction
The visual recognition of actions occurs at different levels (Jellema and Perrett 2006; Blake and Shiffrar 2007; Prinz 2013). At a kinematic level, an action can be described as the physical movement of a body part in space and time, whereas at a semantic level, an action can carry
various social meanings such as about the goals or intentions of an action. In the past decades, a substantial amount of neuroscientific research work has been devoted to various aspects of action recognition (Casile and Giese 2005; Blake and Shiffrar 2007; Prinz 2013). Still, the question at which level the representations for different social actions might be encoded and categorically ordered in the brain is largely left unanswered. Does the brain categorize different actions according to their kinematic similarities, or in terms of their semantic meanings? In the present study, we wanted to find out whether different
actions were ordered according to their semantic meaning or
kinematic motion by employing a visual action adaptation aftereffect paradigm as used in our previous studies (de la Rosa et al. 2014).
Materials and methods
We used motion capture technology (MVN Motion Capture Suit from XSense, Netherlands) to record different social actions often observed in everyday life. The four social actions chosen as our experimental stimuli were handshake, wave, punch, yopunch (fistbump), and each of the actions were similar or different with the other actions either in terms of their semantic meaning (e.g. handshake and wave both meant a greeting, whereas punch meant an attack and yopunch meant a greeting) or kinematic motion (e.g. the movement of a punch and a yopunch were both similar, whereas the movement of a punch and a wave were very
different). To quantify these similarities and differences between each action, a total of 24 participants rated the four different social actions pairwise in terms of their perceived differences in either semantic meaning or kinematic motion on a visual analogue scale ranging from 0
(exactly same) to 10 (completely different). All actions were processed into short movie clips (\2 s) showing only the joint movements of an actor (point-light stimuli) from the side view to the participants. Then, the specific perceptual bias for each action was determined by measuring
the size of the action adaptation aftereffect in each participant. Each of the four different social actions were shown as a visual adaptor each block (30 s prolonged exposure in the start, 3 x repetitions each trial) while participants had to engage in a 2-Alternative-Forced-Choice
(2AFC) task where they had to judge which action was shown. The test stimuli in the 2AFC task were action morphs in 7 different steps between two actions which were presented repeatedly (18 repetitions each block) and randomized. Finally, the previously obtained meaning and motion ratings were used to predict the measured adaptation aftereffect for each action using linear regression.
Results
The perceived differences in the ratings of semantic meaning significantly predicted the differences in the action adaptation aftereffects (p\0.001). The rated differences in kinematic motion alone was not able to significantly predict the differences in the action
adaptation aftereffects, although the interaction of meaning and motion was also able to significantly predict the changes in the action adaptation aftereffect for each action (p\0.01).
Discussion
Previous results have demonstrated that the action adaptation aftereffect paradigm could be a useful paradigm for determining the specific perceptual bias for recognizing an action, since depending on the adaptor stimulus (e.g. if the adaptor was the same action as in one of the test stimuli) a significant shift of the point of subjective equality (PSE) was consistently observed in the psychometric curve judging the difference between two different actions (de la Rosa et al. 2014). This shift of PSE is representing a specific perceptual bias for each recognized action because it is assumed that this shift (adaptation aftereffect) would not be found if there would be no specific adaptation of the underlying neuronal
populations recognizing each action (Clifford et al. 2007; Webster 2011). Using this paradigm we showed for the first time that perceived differences between distinct social actions might be rather encoded in terms of their semantic meaning than kinematic motion in the brain. Future studies should confirm the neuroanatomical correlates to this
action adaptation aftereffect. The current experimental paradigm also serves as a useful method for further mapping the relationship between different social actions in the human brain.
