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Temporal binding effect in the action observation domain: Evidence from an action-based somatosensory paradigm
Consciousness and Cognition 60 (2018) 1–8
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Consciousness and Cognition journal homepage: www.elsevier.com/locate/concog
Temporal binding effect in the action observation domain: Evidence from an action-based somatosensory paradigm Roberta Vastanoa,
T
⁎,1
, Eliane Deschrijvera,1, Thierry Pozzob,c, Marcel Brassa
a
Department of Experimental Psychology, Ghent University, Henri Dunantlaan 2, 9000 Ghent, Belgium Centro di Neurofisiologia traslazionale, Fondazione Istituto Italiano di Tecnologia, c/o sezione Fisiologia Umana, Via Fossato di Mortara, 17-19, 44121 Ferrara, Italy c INSERM U1093 Cognition, Action et Plasticité Sensorimotrice, UBFC, Dijon 21078, France b
AR TI CLE I NF O
AB S T R A CT
Keywords: Temporal binding Implicit sense of agency Tactile stimulation Action observation Congruency-effect
Temporal binding is understood as an effect in which a temporal interval between a voluntary action and its consequent effect is perceived as compressed. It denotes an implicit measure of a sense of agency. When people observe someone else performing an action that generates an effect, temporal binding also takes place. We aimed to test whether the interaction between observed actions and tactile sensation influences temporal binding. Participants observed finger tapping movements (of a human or wooden hand), in parallel to receiving tactile stimulations on their fingertip. These stimulations were either congruent or incongruent with the tactile consequences of the observed movement. The finger tapping movement was followed by a tone. Participants estimated the intervals between the observed action and the tone. We found that temporal binding for observed actions depends on the congruency between the perceived touch and tactile consequences of observed actions restricted to intentional actors.
1. Introduction Several behavioral studies have shown that the temporal interval between a voluntary action and ensuing effect(s) is perceived as compressed (Cravo, Claessens, & Baldo, 2009; Engbert, Wohlschläger, & Haggard, 2008; Haggard & Clark, 2003; Haggard, Clark, & Kalogeras, 2002; Humphreys & Buehner, 2010; Moore & Haggard, 2008). In particular, when people have to estimate the onset time of their action and the consequent effect, a perceptual shift occurs between the action and its effect: the action is perceived as delayed with respect to its actual onset time, and its effect is instead anticipated in time. Some experimental evidence also shows that this temporal binding occurs without voluntary actions, merely when participants are asked to judge the time between two events (causeeffect) (Buehner, 2012; Buehner & Humphreys, 2009). However, when control conditions that compare actions versus non-actions are included, temporal binding seems to be dominated by a key component, which is the “intention”. In fact, since this temporal binding effect does not occur for involuntary or passive movements (Engbert et al., 2008; Haggard et al., 2002), it is named “intentional binding” and it denotes an implicit measure of sense of agency (SoA) (David, Newen, & Vogeley, 2008; Haggard et al., 2002; Synofzik, Vosgerau, & Newen, 2008): the feeling of controlling one’s own actions. The occurrence of this effect appears to partially depend on predictive mechanisms of the action (Moore & Haggard, 2008; Waszak, Cardoso-Leite, & Hughes, 2012). The neural motor system is assumed to generate a prediction of the sensory consequences of an action, which is then compared with its actual sensory consequences. If the actual action consequences and the predicted consequences match, the feeling of agency emerges because the effects ⁎
Corresponding author at: Department of Experimental-Clinical and Health Psychology, Ghent University, Henri Dunantlaan 2, 9000 Ghent, Belgium. E-mail address: [email protected] (R. Vastano). 1 These authors have equally contributed to this work. https://doi.org/10.1016/j.concog.2018.02.002 Received 18 September 2017; Received in revised form 30 January 2018; Accepted 6 February 2018 1053-8100/ © 2018 Elsevier Inc. All rights reserved.
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will be considered self-generated (Blakemore & Frith, 2003). Noteworthy, the motor system is not only active when we perform actions, but also when we observe someone else performing an action (for a review see Rizzolatti & Craighero, 2004). It was therefore argued that we may use similar internal models to predict sensory effects of our own and others’ actions (Kilner, Friston, & Frith, 2007). In fact, it has been shown that temporal binding occurs both when people perform actions and when they observe others’ actions (Poonian & Cunnington, 2013; Poonian, Mcfadyen, Ogden, & Cunnington, 2015). For instance, when participants had to judge the time elapsed between an observed action and the ensuing auditory effect, a time compression between these two events was found similar to self-generated actions (Poonian & Cunnington, 2013). Interestingly, the temporal binding effect was widely reduced when participants had simply to judge the time between two events without the involvement of an agent. This indicates that a stronger temporal binding effect in the action observation domain occurs only when people observe intentional agents performing goal-directed actions, and not when they merely observe a causal event. Often however, the sensory consequences of actions involve distal (i.e. auditory effects) and proximal (i.e. tactile feedback) action effects at the same time (Prinz, 1990; Ridderinkhof, van den Wildenberg, & Brass, 2014). When we perform actions, we do not only perceive distal action effects, such as an auditory consequence after a key press. Actions, like a key press, also involve proximal action effects, such as somatosensory effects. Importantly, such proximal sensory consequences are the most consistent sensory consequences we experience (Deschrijver, Wiersema, & Brass, 2015). When I press a key, I will always have a tactile sensation of the key press, while I will not necessarily have a visual or auditory percept. Recently, it has been demonstrated that tactile feedback are crucial for the occurrence of temporal binding (Zhao et al., 2016). By comparing situations in which participants performed actions generating an effect, a stronger temporal binding effect has been found when participants’ movement involved physical contact with a surface (i.e. a key) compared to the condition without physical contact (i.e. a laser system detected the participants’ finger movements to generate action effects). In the present study, we propose to test the influence of tactile congruency when participants merely observe a moving hand generating an effect while receiving tactile stimulation. Our aim was to investigate how the interaction of visual and tactile information influences the temporal binding in the action observation domain. In a previous study (Vastano, Pozzo, & Brass, 2017), we showed a modulation of the temporal binding effect based on a congruency manipulation. The observation of actions that were either congruent or incongruent with one’s own performed actions influenced the temporal binding effect. More specifically, observed incongruent movements led to an attenuation of temporal binding because of a perceived mismatch that influenced the intention-action-outcome link. Here, we focus on the congruency of actionoutcomes rather than on the congruency of action. We reasoned that if similar predictive processes underlie inferences of agency to oneself (i.e. “I caused that outcome”) and to other agents (i.e. “Somebody else caused that outcome”) (Poonian & Cunnington, 2013) and furthermore observers simulate tactile consequences of the observed actions (Avenanti, Bolognini, Maravita, & Aglioti, 2007; Deschrijver et al., 2015; Gazzola & Keysers, 2009; Lamm, Fischer, & Decety, 2007; Vastano et al., 2016; Voisin et al., 2011), perceiving a proximal sensory stimulus (i.e. tactile sensation) congruent with observed movements could influence the temporal binding effect. Therefore, we tested how the perception of tactile stimuli that are either congruent or incongruent with an observed action would modulate the temporal binding for observed actions. We adapted the action-based somatosensory congruency paradigm (Deschrijver et al., 2015). Participants observed a series of images showing index or middle finger tapping movements of a human agent (or a non-human agent), while synchronous tactile feedback was delivered on their index or middle fingertips. Within a random interval after the presentation of the tapping movement, an auditory effect was delivered. Participants were instructed to perform a time estimation task: they were asked to estimate the duration of the temporal interval between observed action and the ensuing effect (a tone). The tactile stimulation could be delivered either congruently (same finger) or incongruently (different finger) with the observed finger tapping movement. In the congruent condition, the tactile consequence of the observed movement and the actual perceived touch matched. In the incongruent condition, the actual perceived touch resulted in a perceptual mismatch with the observed tactile consequence. Based on the experimental evidence that the observers simulate the tactile consequences of observed movements when these match with the perceived touch (Deschrijver et al., 2015), we hypothesized that congruent tactile stimulation on the observers’ fingers should consequently boost the temporal binding between observed actions and the ensuing effects, compared to incongruent tactile stimulation. Finally, we expected no congruency effect on the temporal binding measure when participants observed a non-biological agent, because this agent could not trigger motor simulation in the observer (Ramnani & Miall, 2004; Tsai & Brass, 2007; Tsai, Kuo, Hung, & Tzeng, 2015). 2. Material and methods 2.1. Participants Thirty healthy volunteers (11 males, mean age: 19.4 years, SD: 1.6 years) were enrolled for the study. An a priori sensitivity power analysis (G∗Power 3 software (Faul, Erdfelder, Lang, & Buchner, 2007); revealed that our sample size (N = 30) is large enough to detect the significant effect for the Actor by Congruency interaction (of prime theoretical interest here) corresponding to an effect size smaller than the value classically considered as “medium” (f = 0.25, see (Cohen, 1988) with a statistical power of (1–β) = 0.80 (given α = 0.05, repeated-measure ANOVA, within factors, correlation between repeated measures = 0.6, number of groups = 1, number of measurements = 2. Note that a 2 × 2 interaction in a repeated measure ANOVA is mathematically equivalent to the main effect of a 2-level factor in a one-way rmANOVA performed on the difference scores computed for the other factor). All participants 2
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were right-handed, as assessed by the Edinburgh Handedness Inventory (Oldfield, 1971). All testing procedures were approved by local ethical committee of Ghent University. All participants gave written informed consent and received academic credits for their participation.
2.2. Stimuli, design and procedure We used the same stimuli as a previously published paper (Deschrijver et al., 2015). Subjects were seated at a distance of 60 cm from the computer monitor (refresh rate 60 Hz, with a resolution of 1920 × 1080). The visual stimuli consisted of images of a human or wooden right hand (640 × 380 pixels large and were centrally presented). Tactile stimulation was achieved by means of a custom made tactile stimulator (Dancer Design; www.dancerdesign.co.uk). We used two independently controlled piezo elements to deliver mechanical supratreshold ‘single tap’ stimuli to about 1 cm2 of the tip of the participant’s index and middle fingers of the right hand, consistent with the location of the observed action-based touch. To achieve this, an audio file containing a single sawtooth waveform drove the piezo-element of the tactile stimulator. We asked participants to keep their right hand on the stimulator in a rest position. To prevent participants from seeing their hand, it was covered with a cloth. Participants were instructed to observe the finger movements and to estimate the interval between the observed movement (finger tapping) and the ensuing tone in milliseconds. They were told that the interval was never longer than 1000 ms and their answer had to be in a range between 100 ms and 999 ms. Each trial started with a fixation cross (2000 ms) followed by an image of a right hand in a neutral posture from a first person perspective, corresponding to the participant’s own right hand for 1200 ms. This start frame was followed by another image of an index or middle finger in a tapping position inducing the impression of a tapping movement. Synchronized with the image showing the finger in a tapping position, the participant received a tactile sensation at either the index or the middle finger, delivered by the piezo elements. The observed finger tapping was followed by a tone (a pure tone 1000 Hz, 300 ms) delivered by means of headphones with a delay of either 300, 500 or 700 ms. After a random interval (between 0 ms and 600 ms), the following question was displayed on the screen.: “How long was the interval between the observed movement and the ensuing auditory effect?”. Participants were required to indicate the interval length with their left hand using the number pad. In order to prevent the disappearance of the image from acting as an anchor for the participants and thus potentially influencing their time estimation, the image of the finger in the tapping position stayed on the screen until the appearance of the question. Finally after an inter-trial interval (ITI) of 800 ms, the next trial started. Stimulus delivery and data acquisition were achieved by means of the program Presentation (Neurobs) and the delivered tactile stimulus was synchronized with the observed finger tapping movement. The design had three factors (Fig. 1): Actor (human vs. wood), Congruency (congruent vs. incongruent) and Intervals (300, 500 and 700 ms). In congruent trials, the observed hand executed a finger movement that would, when executed by one’s self, naturally lead to the tactile sensation at the corresponding finger of the participant. In incongruent trials, the observed hand movement was incongruent with the presented tactile sensation. In the experiment, a total of 264 trials were presented. Human and wooden hand trials were presented in separate blocks with a
Fig. 1. Time line of the Experimental procedure. (a) Participants observed finger tapping movements of a Human or Wooden hand (visual stimuli); (b) in parallel they received tactile stimulation on their fingertips (tactile stimuli): tactile stimuli could be delivered on the finger corresponding to the observed finger (green arrows: congruent stimulation) or on the different finger (red arrows: incongruent stimulation). After the observed movement with a random interval (300, 500 or 700 ms) followed a tone (effect of observed action). At the end, participants judged the time elapsed between the observed action and the tone. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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counterbalanced order across participants. Within each block, 66 congruent and 66 incongruent trials were randomly presented, with an equal probability of index finger and middle finger movements. Each temporal interval included 22 trials. Self-paced breaks occurred after every 33 trials. The duration of the experiment was about 45 min. Before starting the experiment, 10 trials of each actor (human and wooden hand), were shown to participants, to allow them to familiarize themselves with the stimuli and to understand the causal link between the observed finger tapping and the auditory ensuing effect. 2.3. Statistical analysis A Shapiro–Wilk test indicated a normal distribution of data for all conditions (p > 0.5). We performed a repeated measures analysis of variance (ANOVA) on the time-judgment errors (i.e., the difference between the estimated intervals and the actual intervals), with within-participants factors Actor (2 levels: human and wooden hand), Congruency (2 levels: congruent and incongruent) and Intervals (3 levels: 300, 500, and 700 ms). We chose to use the time-judgment errors as a measure of the temporal binding effect on the base of previous works (Kühn, Brass, & Haggard, 2013; Poonian & Cunnington, 2013; Poonian et al., 2015; Vastano et al., 2017) in order to observe the tendency of the error to determine the temporal binding effect (e.g. negative time-judgment errors indicate that participants perceive the interval between the observed action and the tone to be shorter than it actually is (underestimation); positive time-judgment errors indicate that participants perceive the interval to be longer than it actually is (overestimation)). Significant effects found in the ANOVA were followed by Bonferroni post hoc test. Greenhouse–Geisser corrections were used to correct for sphericity violations. We also performed an ANOVA with block presentation (Human first – Wooden first) as categorical factor, in order to check if the order of presentation of blocks affected our results. Since the presentation order did not show significant interactions with any of the other variables (p > 0.05) this factor was dropped from analysis. 2.4. Results The ANOVA on time-judgment errors revealed a significant main effect of Congruency (F (1,29) = 7.24; p = 0.011; η2p = 0.19). The incongruent condition (M = −125 ms; SD = 86 ms) led to a significantly reduced underestimation (less time-judgment errors), compared to the congruent condition (M = −135 ms; SD = 88 ms). In addition, there was a main effect of Actor (F (1,29) = 4.99; p = 0.033; η2p = 0.14). The human hand condition led to a reduced underestimation (M = −117 ms; SD = 86 ms), compared to the wooden hand condition (M = −144 ms; SD = 98 ms). Most importantly, a significant interaction between Actor and Congruency was found (F (1,29) = 6.54; p = 0.016; η2p = 0.18). Post-hoc comparisons revealed larger underestimation (increased time-judgment errors) for the human hand, in congruent (M = −127 ms; SD = 84 ms) compared to incongruent condition (M = −107 ms; SD = 87 ms) (p = 0.007). This congruency difference was not present for the wooden hand (p = 1; congruent condition: M = −144 ms; SD = 97 ms; incongruent condition: M = −143 ms; SD = 94 ms) (Fig. 2). Unsurprisingly, there was a main effect of Interval (F (2,58) = 29.66; p < 0.001; η2p = 0.5), with a stronger underestimation of the action-effect interval at longer delays (300 ms: M = −70 ms, SD = 74 ms; 500 ms: M = −138 ms, SD = 96 ms; 700 ms: M = −183 ms, SD = 117 ms) (p < 0.001 for all conditions). In fact, it is well known that in time-judgment or time-reproduction tasks, subjective intervals are underestimated and proportionally this effect increases with the length of the interval (Caspar, Cleeremans, & Haggard, 2015; Humphreys & Buehner,
Fig. 2. Results. The graph shows the bar plot (mean of the time-judgment errors) for each experimental condition. *p < 0.05, error bars indicate within-subjects SEM (Morey, 2008).
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2009; Kühn et al., 2013). It has been argued that this result may be due to a slowdown of an “internal clock” in the prediction of the action outcomes that increases with interval length (Humphreys & Buehner, 2009, 2010; Wenke & Haggard, 2009). However, since the Interval variable was only used to create variability in this task and it did not interact with the other variables of interest, we do not address any further explanation in the discussion section. All others effects and interactions were not significant (p > 0.05). 3. Discussion Previous research has demonstrated a temporal binding between actions and ensuing effects when people execute their own actions as well as observe actions of others (Poonian & Cunnington, 2013; Poonian et al., 2015). The latter suggests that observers can make causal attribution over others’ actions by anticipating their sensory consequences. In the present study, we investigated whether temporal binding for observed behavior can be modulated by the congruency of tactile sensations. We show that when observers estimated the time between observed movements and the resulting auditory effects, a stronger subjective time compression (underestimation) between action and the auditory outcome was found when tactile stimulations were congruent with the tactile consequences of the observed action, compared to incongruent. Importantly, such a congruency effect was only found when participants observed movements executed by a human hand (and not when executed by a wooden hand). Our results suggest that the temporal binding effect is sensitive to tactile stimulation that is either congruent or incongruent to the observed movement. A recent study showed that proximal action effects (tactile feedbacks) are crucial for the occurrence of temporal binding (Zhao et al., 2016). In this experiment, the authors tested the temporal binding effect with and without tactile sensory feedback. In the condition with tactile feedback, participants pressed a key to generate a visual effect after a temporal interval. In the condition without tactile feedback, they developed a laser system to detect the participants’ finger movement, which generated a visual effect after a temporal interval without pressing a key. The results showed a stronger temporal binding effect when participants’ movement involved physical contact with the key compared to the condition without physical contact. Extending these effects, our results demonstrate the important role of tactile feedback for the temporal binding effect when passively observing behavior of others. More importantly, we found evidence that congruency of tactile feedback had a differential effect on temporal binding. This result could also be compatible with the forward models account of temporal binding (Moore & Haggard, 2008; Moore & Obhi, 2012) which claims that the temporal binding effect occurs as a result of a perceptual matching between the sensory feedback of performed actions and the predicted ones. Hence, if one assumes that action observation leads to the generation of a forward model (Kilner et al., 2007; Wolpert & Flanagan, 2001), when the observed action is congruent with the perceived tactile sensation, the observers could anticipate the distal action effects (auditory consequences) and the temporal binding effect is increased. As we expected, the congruency effect was only observed for a human agent (human-hand) and not for a non-human agent (wooden hand). This modulation driven by the human agent suggests that observers anticipate the tactile consequences exclusively when actions are performed from an intentional biological agent. In support of this interpretation, an electroencephalographic study, that used the same action-based somatosensory congruency paradigm (Deschrijver et al., 2015), showed that the human brain distinguishes between the human and wooden hands. Crucially, the authors showed that the brain differentiates sensory mismatch between simulated and felt touch in early neuronal processing. Furthermore, it is well known that action observation involves a motor simulation process of the observed movement (Gazzola & Keysers, 2009; Keysers & Gazzola, 2014; Rizzolatti & Craighero, 2004) and that tactile stimulation delivered during observed actions has been found to have a strong impact on somatosensory processing in observers thanks to motor simulation processes (Avikainen, Forss, & Hari, 2002; Deschrijver et al., 2015; Keysers et al., 2004; Rossi et al., 2002; Vastano et al., 2016; Voisin et al., 2011). In this vein, our results could also match with the motor simulation account. We suggest that the occurrence of the temporal binding effect in the action observation domain involves a motor simulation process when observing human but not non-human agents. This is consistent with previous research that found motor simulation processes to be restricted to the perception of biological agents (Ramnani & Miall, 2004; Tsai & Brass, 2007; Tsai et al., 2015). It is also important to note that the temporal binding effect has also been found in joint-actions (Obhi & Hall, 2011a). In this context, temporal binding was only found when participants interacted with an intentional agent but not with a computer (Obhi & Hall, 2011b). Although in our task, participants simply observed others’ actions, it could be possible that a similar “shared identity” is automatically formed only when the human actor-observer pair it is present and the touch felt matches with the observed movements of the human hand. While, we expected a modulation of temporal binding limited to the human hand, we found a global increasing of the temporal binding effect for the wooden hand compared to the human hand. Apparently, this result contrasts with Poonian and Cunnington’s study (2013), which found a strong temporal binding only when participants observed a human hand. However, in contrast to the current study, they compared the intentional agent with a no-agent condition where a key on the keyboard magically was moving downwards. There are several potential interpretations for this unexpected effect that we address in the following paragraphs. – Disembodied interpretation First, one could argue that, for the observer, proximal sensory consequences of observed actions are typically visual signals. Therefore, looking at a human hand performing finger tapping movements while tactile stimulations are delivered simultaneously on the observers’ hands, could cause a mismatch in the observers’ expectations, by generating, for instance, uncertainty about the consequent auditory effects by reducing the temporal binding effect. However, this would explain the reduced global temporal 5
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binding effect for the human hand, but not the congruency effect that we found. If visuo-tactile signals resulted in a mismatch, then no modulation of congruency would have been found, since the tactile stimulation could always result in a perceived mismatch. We believe that the integration between visual (observed finger-tapping movements) and the tactile signals boosted the observers’ ability to make causal attribution over others’ actions when a congruent tactile stimulation was delivered. Therefore, considering the lacking of congruency effect for the wooden hand, a possible interpretation would refer to a self-other distinction process. Participants would have perceived the wooden hand as a disembodied stimulus and focused completely on the link between the perceived tactile stimulation on their own fingers and the auditory effect to provide the time estimates. We propose that the visual stimulus representing a non-human agent to perform actions would have enhanced the global temporal binding effect because of the absence of an embodied sensorimotor simulation process. Consequently the perceived passive touch would have represented the only salient stimulus (independently if congruent or not with the visual stimulus) and facilitated the global time estimations. The absence of a congruency effect for the wooden hand is therefore explained by the fact that a disembodied visual stimulus did not interfere or facilitate the participants’ performance. – Intentional actor interpretation A temporal binding effect also has been shown during the observation of an artificial (Braun, Thorne, Hildebrandt, & Debener, 2014) or a robotic hand (Caspar et al., 2015). Although there is a difference between experimental designs (in our study the participants simply observed fingers movements, while in these studies participants had to execute a finger movement that resulted either in a congruent or incongruent movement of the observed artificial hand – positioned anatomically congruent with the participants’ hand), temporal binding effect also seems to be modulated by a causal relationship involving actions with their outcomes, which is not restricted to the use of one's own real hand, or to the observation of a human actor. Similarly, a recent study (Khalighinejad, Bahrami, Caspar, & Haggard, 2016) also showed an equal temporal binding effect in joint actions, both when participants acted with a human being and a robotic hand (but see Obhi & Hall, 2011b). Taken together, these findings show how this effect is sensitive to contexts when other agents are present. Although our experiment only involved observed actions, it could be possible that in this context, our participants perceived the wooden hand as an intentional actor, so we cannot fully exclude the influence of top-down beliefs on the time judgments. – Action/Effect onset time interpretation Finally, it is also conceivable that participants provided shorter time estimations for the wooden hand because they perceived the action onset of the human hand as occurring earlier than as for the wooden hand. Unfortunately the interval judgement procedure, although extensively used and well accepted to measure temporal binding effect (Cravo, Haddad, Claessens, & Baldo, 2013; Engbert et al., 2008; Engbert, Wohlschläger, Thomas, & Haggard, 2007; Kühn et al., 2013), does not allow for an investigation of the action onset independent from the effect. Therefore, we cannot directly compare the onset time of actions and effects regarding the human and wooden hand conditions.
4. Conclusions Overall, our findings reveal that tactile feedback delivered on the observers’ fingertips congruently with an observed goal-directed action of a biological agent led to a greater temporal binding effect between actions and ensuing outcomes. Such congruency effect was not found for non-biological agent. As a consequence of a motor simulation process, observers make causal attribution of others’ actions when the observed movements match with the felt touch. We therefore, propose a “mirror mechanism” of the temporal binding effect, exclusively when sensory matching occurs between self-sensations and observed actions made by biological agents. To our knowledge, our study represents some of the first experimental evidence that compares the temporal binding effect between human and non-human actors during action observation. Although there are several potential interpretations of the results regarding the main effect of the Actor, the interaction between Actor and Congruency effect provides evidence that the action-based somatosensory simulation may influence the temporal binding effect. Finally, we only tested temporal binding during action observation (human and non-human actor) and not during voluntary movements, therefore the influence of the causality between events that explains the temporal binding effect (Buehner, 2012; Buehner & Humphreys, 2009) cannot be fully excluded. Additional investigations need to be addressed to better understand the impact of non-human agents on the feelings of agency for observed actions, since contrasting results are present in the literature. One possibility would be to explicitly manipulate the participants’ beliefs by attributing (or not attributing) to non-human agents intentions or human-like abilities. Our results add a contribution to the literature on implicit sense of agency by showing how it is influenced by congruency effects, contexts and observed actors.
Conflict of interest Not declared.
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Contents lists available at ScienceDirect
Consciousness and Cognition journal homepage: www.elsevier.com/locate/concog
Temporal binding effect in the action observation domain: Evidence from an action-based somatosensory paradigm Roberta Vastanoa,
T
⁎,1
, Eliane Deschrijvera,1, Thierry Pozzob,c, Marcel Brassa
a
Department of Experimental Psychology, Ghent University, Henri Dunantlaan 2, 9000 Ghent, Belgium Centro di Neurofisiologia traslazionale, Fondazione Istituto Italiano di Tecnologia, c/o sezione Fisiologia Umana, Via Fossato di Mortara, 17-19, 44121 Ferrara, Italy c INSERM U1093 Cognition, Action et Plasticité Sensorimotrice, UBFC, Dijon 21078, France b
AR TI CLE I NF O
AB S T R A CT
Keywords: Temporal binding Implicit sense of agency Tactile stimulation Action observation Congruency-effect
Temporal binding is understood as an effect in which a temporal interval between a voluntary action and its consequent effect is perceived as compressed. It denotes an implicit measure of a sense of agency. When people observe someone else performing an action that generates an effect, temporal binding also takes place. We aimed to test whether the interaction between observed actions and tactile sensation influences temporal binding. Participants observed finger tapping movements (of a human or wooden hand), in parallel to receiving tactile stimulations on their fingertip. These stimulations were either congruent or incongruent with the tactile consequences of the observed movement. The finger tapping movement was followed by a tone. Participants estimated the intervals between the observed action and the tone. We found that temporal binding for observed actions depends on the congruency between the perceived touch and tactile consequences of observed actions restricted to intentional actors.
1. Introduction Several behavioral studies have shown that the temporal interval between a voluntary action and ensuing effect(s) is perceived as compressed (Cravo, Claessens, & Baldo, 2009; Engbert, Wohlschläger, & Haggard, 2008; Haggard & Clark, 2003; Haggard, Clark, & Kalogeras, 2002; Humphreys & Buehner, 2010; Moore & Haggard, 2008). In particular, when people have to estimate the onset time of their action and the consequent effect, a perceptual shift occurs between the action and its effect: the action is perceived as delayed with respect to its actual onset time, and its effect is instead anticipated in time. Some experimental evidence also shows that this temporal binding occurs without voluntary actions, merely when participants are asked to judge the time between two events (causeeffect) (Buehner, 2012; Buehner & Humphreys, 2009). However, when control conditions that compare actions versus non-actions are included, temporal binding seems to be dominated by a key component, which is the “intention”. In fact, since this temporal binding effect does not occur for involuntary or passive movements (Engbert et al., 2008; Haggard et al., 2002), it is named “intentional binding” and it denotes an implicit measure of sense of agency (SoA) (David, Newen, & Vogeley, 2008; Haggard et al., 2002; Synofzik, Vosgerau, & Newen, 2008): the feeling of controlling one’s own actions. The occurrence of this effect appears to partially depend on predictive mechanisms of the action (Moore & Haggard, 2008; Waszak, Cardoso-Leite, & Hughes, 2012). The neural motor system is assumed to generate a prediction of the sensory consequences of an action, which is then compared with its actual sensory consequences. If the actual action consequences and the predicted consequences match, the feeling of agency emerges because the effects ⁎
Corresponding author at: Department of Experimental-Clinical and Health Psychology, Ghent University, Henri Dunantlaan 2, 9000 Ghent, Belgium. E-mail address: [email protected] (R. Vastano). 1 These authors have equally contributed to this work. https://doi.org/10.1016/j.concog.2018.02.002 Received 18 September 2017; Received in revised form 30 January 2018; Accepted 6 February 2018 1053-8100/ © 2018 Elsevier Inc. All rights reserved.
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will be considered self-generated (Blakemore & Frith, 2003). Noteworthy, the motor system is not only active when we perform actions, but also when we observe someone else performing an action (for a review see Rizzolatti & Craighero, 2004). It was therefore argued that we may use similar internal models to predict sensory effects of our own and others’ actions (Kilner, Friston, & Frith, 2007). In fact, it has been shown that temporal binding occurs both when people perform actions and when they observe others’ actions (Poonian & Cunnington, 2013; Poonian, Mcfadyen, Ogden, & Cunnington, 2015). For instance, when participants had to judge the time elapsed between an observed action and the ensuing auditory effect, a time compression between these two events was found similar to self-generated actions (Poonian & Cunnington, 2013). Interestingly, the temporal binding effect was widely reduced when participants had simply to judge the time between two events without the involvement of an agent. This indicates that a stronger temporal binding effect in the action observation domain occurs only when people observe intentional agents performing goal-directed actions, and not when they merely observe a causal event. Often however, the sensory consequences of actions involve distal (i.e. auditory effects) and proximal (i.e. tactile feedback) action effects at the same time (Prinz, 1990; Ridderinkhof, van den Wildenberg, & Brass, 2014). When we perform actions, we do not only perceive distal action effects, such as an auditory consequence after a key press. Actions, like a key press, also involve proximal action effects, such as somatosensory effects. Importantly, such proximal sensory consequences are the most consistent sensory consequences we experience (Deschrijver, Wiersema, & Brass, 2015). When I press a key, I will always have a tactile sensation of the key press, while I will not necessarily have a visual or auditory percept. Recently, it has been demonstrated that tactile feedback are crucial for the occurrence of temporal binding (Zhao et al., 2016). By comparing situations in which participants performed actions generating an effect, a stronger temporal binding effect has been found when participants’ movement involved physical contact with a surface (i.e. a key) compared to the condition without physical contact (i.e. a laser system detected the participants’ finger movements to generate action effects). In the present study, we propose to test the influence of tactile congruency when participants merely observe a moving hand generating an effect while receiving tactile stimulation. Our aim was to investigate how the interaction of visual and tactile information influences the temporal binding in the action observation domain. In a previous study (Vastano, Pozzo, & Brass, 2017), we showed a modulation of the temporal binding effect based on a congruency manipulation. The observation of actions that were either congruent or incongruent with one’s own performed actions influenced the temporal binding effect. More specifically, observed incongruent movements led to an attenuation of temporal binding because of a perceived mismatch that influenced the intention-action-outcome link. Here, we focus on the congruency of actionoutcomes rather than on the congruency of action. We reasoned that if similar predictive processes underlie inferences of agency to oneself (i.e. “I caused that outcome”) and to other agents (i.e. “Somebody else caused that outcome”) (Poonian & Cunnington, 2013) and furthermore observers simulate tactile consequences of the observed actions (Avenanti, Bolognini, Maravita, & Aglioti, 2007; Deschrijver et al., 2015; Gazzola & Keysers, 2009; Lamm, Fischer, & Decety, 2007; Vastano et al., 2016; Voisin et al., 2011), perceiving a proximal sensory stimulus (i.e. tactile sensation) congruent with observed movements could influence the temporal binding effect. Therefore, we tested how the perception of tactile stimuli that are either congruent or incongruent with an observed action would modulate the temporal binding for observed actions. We adapted the action-based somatosensory congruency paradigm (Deschrijver et al., 2015). Participants observed a series of images showing index or middle finger tapping movements of a human agent (or a non-human agent), while synchronous tactile feedback was delivered on their index or middle fingertips. Within a random interval after the presentation of the tapping movement, an auditory effect was delivered. Participants were instructed to perform a time estimation task: they were asked to estimate the duration of the temporal interval between observed action and the ensuing effect (a tone). The tactile stimulation could be delivered either congruently (same finger) or incongruently (different finger) with the observed finger tapping movement. In the congruent condition, the tactile consequence of the observed movement and the actual perceived touch matched. In the incongruent condition, the actual perceived touch resulted in a perceptual mismatch with the observed tactile consequence. Based on the experimental evidence that the observers simulate the tactile consequences of observed movements when these match with the perceived touch (Deschrijver et al., 2015), we hypothesized that congruent tactile stimulation on the observers’ fingers should consequently boost the temporal binding between observed actions and the ensuing effects, compared to incongruent tactile stimulation. Finally, we expected no congruency effect on the temporal binding measure when participants observed a non-biological agent, because this agent could not trigger motor simulation in the observer (Ramnani & Miall, 2004; Tsai & Brass, 2007; Tsai, Kuo, Hung, & Tzeng, 2015). 2. Material and methods 2.1. Participants Thirty healthy volunteers (11 males, mean age: 19.4 years, SD: 1.6 years) were enrolled for the study. An a priori sensitivity power analysis (G∗Power 3 software (Faul, Erdfelder, Lang, & Buchner, 2007); revealed that our sample size (N = 30) is large enough to detect the significant effect for the Actor by Congruency interaction (of prime theoretical interest here) corresponding to an effect size smaller than the value classically considered as “medium” (f = 0.25, see (Cohen, 1988) with a statistical power of (1–β) = 0.80 (given α = 0.05, repeated-measure ANOVA, within factors, correlation between repeated measures = 0.6, number of groups = 1, number of measurements = 2. Note that a 2 × 2 interaction in a repeated measure ANOVA is mathematically equivalent to the main effect of a 2-level factor in a one-way rmANOVA performed on the difference scores computed for the other factor). All participants 2
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were right-handed, as assessed by the Edinburgh Handedness Inventory (Oldfield, 1971). All testing procedures were approved by local ethical committee of Ghent University. All participants gave written informed consent and received academic credits for their participation.
2.2. Stimuli, design and procedure We used the same stimuli as a previously published paper (Deschrijver et al., 2015). Subjects were seated at a distance of 60 cm from the computer monitor (refresh rate 60 Hz, with a resolution of 1920 × 1080). The visual stimuli consisted of images of a human or wooden right hand (640 × 380 pixels large and were centrally presented). Tactile stimulation was achieved by means of a custom made tactile stimulator (Dancer Design; www.dancerdesign.co.uk). We used two independently controlled piezo elements to deliver mechanical supratreshold ‘single tap’ stimuli to about 1 cm2 of the tip of the participant’s index and middle fingers of the right hand, consistent with the location of the observed action-based touch. To achieve this, an audio file containing a single sawtooth waveform drove the piezo-element of the tactile stimulator. We asked participants to keep their right hand on the stimulator in a rest position. To prevent participants from seeing their hand, it was covered with a cloth. Participants were instructed to observe the finger movements and to estimate the interval between the observed movement (finger tapping) and the ensuing tone in milliseconds. They were told that the interval was never longer than 1000 ms and their answer had to be in a range between 100 ms and 999 ms. Each trial started with a fixation cross (2000 ms) followed by an image of a right hand in a neutral posture from a first person perspective, corresponding to the participant’s own right hand for 1200 ms. This start frame was followed by another image of an index or middle finger in a tapping position inducing the impression of a tapping movement. Synchronized with the image showing the finger in a tapping position, the participant received a tactile sensation at either the index or the middle finger, delivered by the piezo elements. The observed finger tapping was followed by a tone (a pure tone 1000 Hz, 300 ms) delivered by means of headphones with a delay of either 300, 500 or 700 ms. After a random interval (between 0 ms and 600 ms), the following question was displayed on the screen.: “How long was the interval between the observed movement and the ensuing auditory effect?”. Participants were required to indicate the interval length with their left hand using the number pad. In order to prevent the disappearance of the image from acting as an anchor for the participants and thus potentially influencing their time estimation, the image of the finger in the tapping position stayed on the screen until the appearance of the question. Finally after an inter-trial interval (ITI) of 800 ms, the next trial started. Stimulus delivery and data acquisition were achieved by means of the program Presentation (Neurobs) and the delivered tactile stimulus was synchronized with the observed finger tapping movement. The design had three factors (Fig. 1): Actor (human vs. wood), Congruency (congruent vs. incongruent) and Intervals (300, 500 and 700 ms). In congruent trials, the observed hand executed a finger movement that would, when executed by one’s self, naturally lead to the tactile sensation at the corresponding finger of the participant. In incongruent trials, the observed hand movement was incongruent with the presented tactile sensation. In the experiment, a total of 264 trials were presented. Human and wooden hand trials were presented in separate blocks with a
Fig. 1. Time line of the Experimental procedure. (a) Participants observed finger tapping movements of a Human or Wooden hand (visual stimuli); (b) in parallel they received tactile stimulation on their fingertips (tactile stimuli): tactile stimuli could be delivered on the finger corresponding to the observed finger (green arrows: congruent stimulation) or on the different finger (red arrows: incongruent stimulation). After the observed movement with a random interval (300, 500 or 700 ms) followed a tone (effect of observed action). At the end, participants judged the time elapsed between the observed action and the tone. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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counterbalanced order across participants. Within each block, 66 congruent and 66 incongruent trials were randomly presented, with an equal probability of index finger and middle finger movements. Each temporal interval included 22 trials. Self-paced breaks occurred after every 33 trials. The duration of the experiment was about 45 min. Before starting the experiment, 10 trials of each actor (human and wooden hand), were shown to participants, to allow them to familiarize themselves with the stimuli and to understand the causal link between the observed finger tapping and the auditory ensuing effect. 2.3. Statistical analysis A Shapiro–Wilk test indicated a normal distribution of data for all conditions (p > 0.5). We performed a repeated measures analysis of variance (ANOVA) on the time-judgment errors (i.e., the difference between the estimated intervals and the actual intervals), with within-participants factors Actor (2 levels: human and wooden hand), Congruency (2 levels: congruent and incongruent) and Intervals (3 levels: 300, 500, and 700 ms). We chose to use the time-judgment errors as a measure of the temporal binding effect on the base of previous works (Kühn, Brass, & Haggard, 2013; Poonian & Cunnington, 2013; Poonian et al., 2015; Vastano et al., 2017) in order to observe the tendency of the error to determine the temporal binding effect (e.g. negative time-judgment errors indicate that participants perceive the interval between the observed action and the tone to be shorter than it actually is (underestimation); positive time-judgment errors indicate that participants perceive the interval to be longer than it actually is (overestimation)). Significant effects found in the ANOVA were followed by Bonferroni post hoc test. Greenhouse–Geisser corrections were used to correct for sphericity violations. We also performed an ANOVA with block presentation (Human first – Wooden first) as categorical factor, in order to check if the order of presentation of blocks affected our results. Since the presentation order did not show significant interactions with any of the other variables (p > 0.05) this factor was dropped from analysis. 2.4. Results The ANOVA on time-judgment errors revealed a significant main effect of Congruency (F (1,29) = 7.24; p = 0.011; η2p = 0.19). The incongruent condition (M = −125 ms; SD = 86 ms) led to a significantly reduced underestimation (less time-judgment errors), compared to the congruent condition (M = −135 ms; SD = 88 ms). In addition, there was a main effect of Actor (F (1,29) = 4.99; p = 0.033; η2p = 0.14). The human hand condition led to a reduced underestimation (M = −117 ms; SD = 86 ms), compared to the wooden hand condition (M = −144 ms; SD = 98 ms). Most importantly, a significant interaction between Actor and Congruency was found (F (1,29) = 6.54; p = 0.016; η2p = 0.18). Post-hoc comparisons revealed larger underestimation (increased time-judgment errors) for the human hand, in congruent (M = −127 ms; SD = 84 ms) compared to incongruent condition (M = −107 ms; SD = 87 ms) (p = 0.007). This congruency difference was not present for the wooden hand (p = 1; congruent condition: M = −144 ms; SD = 97 ms; incongruent condition: M = −143 ms; SD = 94 ms) (Fig. 2). Unsurprisingly, there was a main effect of Interval (F (2,58) = 29.66; p < 0.001; η2p = 0.5), with a stronger underestimation of the action-effect interval at longer delays (300 ms: M = −70 ms, SD = 74 ms; 500 ms: M = −138 ms, SD = 96 ms; 700 ms: M = −183 ms, SD = 117 ms) (p < 0.001 for all conditions). In fact, it is well known that in time-judgment or time-reproduction tasks, subjective intervals are underestimated and proportionally this effect increases with the length of the interval (Caspar, Cleeremans, & Haggard, 2015; Humphreys & Buehner,
Fig. 2. Results. The graph shows the bar plot (mean of the time-judgment errors) for each experimental condition. *p < 0.05, error bars indicate within-subjects SEM (Morey, 2008).
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2009; Kühn et al., 2013). It has been argued that this result may be due to a slowdown of an “internal clock” in the prediction of the action outcomes that increases with interval length (Humphreys & Buehner, 2009, 2010; Wenke & Haggard, 2009). However, since the Interval variable was only used to create variability in this task and it did not interact with the other variables of interest, we do not address any further explanation in the discussion section. All others effects and interactions were not significant (p > 0.05). 3. Discussion Previous research has demonstrated a temporal binding between actions and ensuing effects when people execute their own actions as well as observe actions of others (Poonian & Cunnington, 2013; Poonian et al., 2015). The latter suggests that observers can make causal attribution over others’ actions by anticipating their sensory consequences. In the present study, we investigated whether temporal binding for observed behavior can be modulated by the congruency of tactile sensations. We show that when observers estimated the time between observed movements and the resulting auditory effects, a stronger subjective time compression (underestimation) between action and the auditory outcome was found when tactile stimulations were congruent with the tactile consequences of the observed action, compared to incongruent. Importantly, such a congruency effect was only found when participants observed movements executed by a human hand (and not when executed by a wooden hand). Our results suggest that the temporal binding effect is sensitive to tactile stimulation that is either congruent or incongruent to the observed movement. A recent study showed that proximal action effects (tactile feedbacks) are crucial for the occurrence of temporal binding (Zhao et al., 2016). In this experiment, the authors tested the temporal binding effect with and without tactile sensory feedback. In the condition with tactile feedback, participants pressed a key to generate a visual effect after a temporal interval. In the condition without tactile feedback, they developed a laser system to detect the participants’ finger movement, which generated a visual effect after a temporal interval without pressing a key. The results showed a stronger temporal binding effect when participants’ movement involved physical contact with the key compared to the condition without physical contact. Extending these effects, our results demonstrate the important role of tactile feedback for the temporal binding effect when passively observing behavior of others. More importantly, we found evidence that congruency of tactile feedback had a differential effect on temporal binding. This result could also be compatible with the forward models account of temporal binding (Moore & Haggard, 2008; Moore & Obhi, 2012) which claims that the temporal binding effect occurs as a result of a perceptual matching between the sensory feedback of performed actions and the predicted ones. Hence, if one assumes that action observation leads to the generation of a forward model (Kilner et al., 2007; Wolpert & Flanagan, 2001), when the observed action is congruent with the perceived tactile sensation, the observers could anticipate the distal action effects (auditory consequences) and the temporal binding effect is increased. As we expected, the congruency effect was only observed for a human agent (human-hand) and not for a non-human agent (wooden hand). This modulation driven by the human agent suggests that observers anticipate the tactile consequences exclusively when actions are performed from an intentional biological agent. In support of this interpretation, an electroencephalographic study, that used the same action-based somatosensory congruency paradigm (Deschrijver et al., 2015), showed that the human brain distinguishes between the human and wooden hands. Crucially, the authors showed that the brain differentiates sensory mismatch between simulated and felt touch in early neuronal processing. Furthermore, it is well known that action observation involves a motor simulation process of the observed movement (Gazzola & Keysers, 2009; Keysers & Gazzola, 2014; Rizzolatti & Craighero, 2004) and that tactile stimulation delivered during observed actions has been found to have a strong impact on somatosensory processing in observers thanks to motor simulation processes (Avikainen, Forss, & Hari, 2002; Deschrijver et al., 2015; Keysers et al., 2004; Rossi et al., 2002; Vastano et al., 2016; Voisin et al., 2011). In this vein, our results could also match with the motor simulation account. We suggest that the occurrence of the temporal binding effect in the action observation domain involves a motor simulation process when observing human but not non-human agents. This is consistent with previous research that found motor simulation processes to be restricted to the perception of biological agents (Ramnani & Miall, 2004; Tsai & Brass, 2007; Tsai et al., 2015). It is also important to note that the temporal binding effect has also been found in joint-actions (Obhi & Hall, 2011a). In this context, temporal binding was only found when participants interacted with an intentional agent but not with a computer (Obhi & Hall, 2011b). Although in our task, participants simply observed others’ actions, it could be possible that a similar “shared identity” is automatically formed only when the human actor-observer pair it is present and the touch felt matches with the observed movements of the human hand. While, we expected a modulation of temporal binding limited to the human hand, we found a global increasing of the temporal binding effect for the wooden hand compared to the human hand. Apparently, this result contrasts with Poonian and Cunnington’s study (2013), which found a strong temporal binding only when participants observed a human hand. However, in contrast to the current study, they compared the intentional agent with a no-agent condition where a key on the keyboard magically was moving downwards. There are several potential interpretations for this unexpected effect that we address in the following paragraphs. – Disembodied interpretation First, one could argue that, for the observer, proximal sensory consequences of observed actions are typically visual signals. Therefore, looking at a human hand performing finger tapping movements while tactile stimulations are delivered simultaneously on the observers’ hands, could cause a mismatch in the observers’ expectations, by generating, for instance, uncertainty about the consequent auditory effects by reducing the temporal binding effect. However, this would explain the reduced global temporal 5
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binding effect for the human hand, but not the congruency effect that we found. If visuo-tactile signals resulted in a mismatch, then no modulation of congruency would have been found, since the tactile stimulation could always result in a perceived mismatch. We believe that the integration between visual (observed finger-tapping movements) and the tactile signals boosted the observers’ ability to make causal attribution over others’ actions when a congruent tactile stimulation was delivered. Therefore, considering the lacking of congruency effect for the wooden hand, a possible interpretation would refer to a self-other distinction process. Participants would have perceived the wooden hand as a disembodied stimulus and focused completely on the link between the perceived tactile stimulation on their own fingers and the auditory effect to provide the time estimates. We propose that the visual stimulus representing a non-human agent to perform actions would have enhanced the global temporal binding effect because of the absence of an embodied sensorimotor simulation process. Consequently the perceived passive touch would have represented the only salient stimulus (independently if congruent or not with the visual stimulus) and facilitated the global time estimations. The absence of a congruency effect for the wooden hand is therefore explained by the fact that a disembodied visual stimulus did not interfere or facilitate the participants’ performance. – Intentional actor interpretation A temporal binding effect also has been shown during the observation of an artificial (Braun, Thorne, Hildebrandt, & Debener, 2014) or a robotic hand (Caspar et al., 2015). Although there is a difference between experimental designs (in our study the participants simply observed fingers movements, while in these studies participants had to execute a finger movement that resulted either in a congruent or incongruent movement of the observed artificial hand – positioned anatomically congruent with the participants’ hand), temporal binding effect also seems to be modulated by a causal relationship involving actions with their outcomes, which is not restricted to the use of one's own real hand, or to the observation of a human actor. Similarly, a recent study (Khalighinejad, Bahrami, Caspar, & Haggard, 2016) also showed an equal temporal binding effect in joint actions, both when participants acted with a human being and a robotic hand (but see Obhi & Hall, 2011b). Taken together, these findings show how this effect is sensitive to contexts when other agents are present. Although our experiment only involved observed actions, it could be possible that in this context, our participants perceived the wooden hand as an intentional actor, so we cannot fully exclude the influence of top-down beliefs on the time judgments. – Action/Effect onset time interpretation Finally, it is also conceivable that participants provided shorter time estimations for the wooden hand because they perceived the action onset of the human hand as occurring earlier than as for the wooden hand. Unfortunately the interval judgement procedure, although extensively used and well accepted to measure temporal binding effect (Cravo, Haddad, Claessens, & Baldo, 2013; Engbert et al., 2008; Engbert, Wohlschläger, Thomas, & Haggard, 2007; Kühn et al., 2013), does not allow for an investigation of the action onset independent from the effect. Therefore, we cannot directly compare the onset time of actions and effects regarding the human and wooden hand conditions.
4. Conclusions Overall, our findings reveal that tactile feedback delivered on the observers’ fingertips congruently with an observed goal-directed action of a biological agent led to a greater temporal binding effect between actions and ensuing outcomes. Such congruency effect was not found for non-biological agent. As a consequence of a motor simulation process, observers make causal attribution of others’ actions when the observed movements match with the felt touch. We therefore, propose a “mirror mechanism” of the temporal binding effect, exclusively when sensory matching occurs between self-sensations and observed actions made by biological agents. To our knowledge, our study represents some of the first experimental evidence that compares the temporal binding effect between human and non-human actors during action observation. Although there are several potential interpretations of the results regarding the main effect of the Actor, the interaction between Actor and Congruency effect provides evidence that the action-based somatosensory simulation may influence the temporal binding effect. Finally, we only tested temporal binding during action observation (human and non-human actor) and not during voluntary movements, therefore the influence of the causality between events that explains the temporal binding effect (Buehner, 2012; Buehner & Humphreys, 2009) cannot be fully excluded. Additional investigations need to be addressed to better understand the impact of non-human agents on the feelings of agency for observed actions, since contrasting results are present in the literature. One possibility would be to explicitly manipulate the participants’ beliefs by attributing (or not attributing) to non-human agents intentions or human-like abilities. Our results add a contribution to the literature on implicit sense of agency by showing how it is influenced by congruency effects, contexts and observed actors.
Conflict of interest Not declared.
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