Abstract

Action observation activates the observer's motor system. These motor resonance responses are automatic and triggered even when the action is only implied in static snapshots. However, it is largely unknown whether an action needs to be consciously perceived to trigger motor resonance. In this study, we used single-pulse TMS to study the facilitation of corticospinal excitability (a measure of motor resonance) during supraliminal and subliminal presentations of implied action images. We used a forward and backward dynamic masking procedure that successfully prevented the conscious perception of prime stimuli depicting a still hand or an implied abduction movement of the index or little finger. The prime was followed by the supraliminal presentation of a still or implied action probe hand. Our results revealed a muscle-specific increase of motor facilitation following observation of the probe hand actions that were consciously perceived as compared with observation of a still hand. Crucially, unconscious perception of prime hand actions presented before probe still hands did not increase motor facilitation as compared with observation of a still hand, suggesting that motor resonance requires perceptual awareness. However, the presentation of a masked prime depicting an action that was incongruent with the probe hand action suppressed motor resonance to the probe action such that comparable motor facilitation was recorded during observation of implied action and still hand probes. This suppression of motor resonance may reflect the processing of action conflicts in areas upstream of the motor cortex and may subserve a basic mechanism for dealing with the multiple and possibly incongruent actions of other individuals.

INTRODUCTION

In everyday life, we simultaneously deal with the actions of numerous agents that we must flexibly imitate, complement, or react to (Sartori, Bucchioni, & Castiello, 2013). Optimal interactions in such a crowded social world require fast, accurate, and dynamic representations of others' actions that involve motor resonance responses in the observers' brains (Rizzolatti & Craighero, 2004). Single-pulse TMS experiments have shown that action observation triggers a strictly congruent, muscle-specific facilitation of corticospinal excitability that reflects motor resonance (e.g., Fadiga, Craighero, & Olivier, 2005). Although mirror-like motor facilitation during action observation was initially thought to imply covert imitation, it has been demonstrated that it takes place independently of explicit instruction to rehearse the actions (Fadiga, Fogassi, Pavesi, & Rizzolatti, 1995) and to be modulated by subtle kinematic differences in the movements even when these differences cannot be recognized by the observers (Sartori, Bucchioni, & Castiello, 2012). Moreover, it even occurs during passive viewing of static images that imply body actions (Avenanti, Annella, Candidi, Urgesi, & Aglioti, 2013; Urgesi et al., 2010; Urgesi, Moro, Candidi, & Aglioti, 2006).

In a similar vein, behavioral studies have shown that viewing incongruent versus congruent dynamic action sequences (Kilner, Paulignan, & Blakemore, 2003; Brass, Bekkering, & Prinz, 2001; Brass, Bekkering, Wohlschläger, & Prinz, 2000; Stürmer, Aschersleben, & Prinz, 2000) or single frames that imply actions (Vogt, Taylor, & Hopkins, 2003; Craighero, Bello, Fadiga, & Rizzolatti, 2002; Brass et al., 2000; Craighero, Fadiga, & Rizzolatti, 1999; Craighero, Fadiga, Umiltà, & Rizzolatti, 1996) may affect the correct execution of the observer's movements. These visuomotor interference effects have been demonstrated to occur even if the observer ignores the action stimulus while performing a different task-relevant action (Vainio, Tucker, & Ellis, 2007). Furthermore, they are not suppressed by attentional modulation of the predictability of the prime–cue association obtained by varying the proportion of congruent and incongruent trials (Hogeveen & Obhi, 2013). It is worth noting that there is evidence that both motor facilitation, at the neurophysiological level, and visuomotor interference effects, at the behavioral level, are modulated by attention. For example, the electrophysiological indices of motor activation during action observation (i.e., suppression of beta band recorded from central sites) are amplified by the increasing task demands of active imitation or counting task with respect to passive observation, but they are present in all conditions (Muthukumaraswamy & Singh, 2008). In a similar vein, visuomotor interference effects are attenuated by the activation of self-related processing by mirror self-observation or judgment of evaluative personal statements, but are still present in all conditions (Spengler, von Cramon, & Brass, 2010).

Together, these studies suggest that motor resonance is elicited automatically, that is, in every instance in which a biological movement is perceived independently from the presence or absence of executive control, cognitive effort, intentionality, strategies, or focus of attention of the observer (Bargh, Schwader, Hailey, Dyer, & Boothby, 2012). Although these top–down processes are not necessary for motor resonance to occur, they can modulate its intensity, increasing or attenuating its manifestations. Currently, however, it is still unclear whether motor resonance requires observers' awareness of the perception of the movement or whether it also occurs in response to actions that are not consciously perceived. Varying the visibility of a stimulus has been widely used to manipulate perceptual awareness, as indexed by observers' ability to report the perception when they are prompted to do so (Van den Bussche, Van den Noortgate, & Reynvoet, 2009). It has been argued that phenomenal experience is not a direct reflection of perceptual processes and that the effect of consciously perceived stimuli may be different than that of unconsciously perceived ones (e.g., see Merikle, Smilek, & Eastwood, 2001; Merikle & Daneman, 1998, for reviews). For instance, masked visual presentations of prime stimuli that are associated with a lateralized motor response, including hand postures (Vainio & Mustonen, 2011), may affect the execution of incongruent versus congruent responses and modulate motor cortex activity (D'Ostilio & Garraux, 2012; Théoret, Halligan, Kobayashi, Merabet, & Pascual-Leone, 2004; Dehaene, Naccache, Le Clec'H, Koechlin, & Mueller, 1998). However, these masked visuomotor prime effects strictly depend on the selection of the motor response concurrently required by the explicit task (Eimer & Schlaghecken, 1998, 2003) and may reflect the influence of the masked primes on action execution processes rather than motor resonance representations.

On the other hand, automatic motor responses during passive observation of emotional face and body expressions have been reported even when stimuli are presented to the blind hemifield of hemianopic patients (Van den Stock et al., 2011; Tamietto et al., 2009) or to healthy individuals with masking procedures (Dimberg, Thunberg, & Elmehed, 2000). However, such automatic motor responses may be specific for stimuli with emotional valence and may not be involved in nonemotional action perception (Tamietto & De Gelder, 2010). In fact, mirror-like, muscle-specific facilitation of corticospinal excitability during observation of grasping actions was attenuated by presentation of the moving finger in partially shadowed illumination conditions (Sartori & Castiello, 2013); the grasping act, however, was still visible, and mirror-like motor facilitation was attenuated but not extinguished, thus leaving unquestioned whether motor resonance requires perceptual awareness.

In this study, to investigate the impact of the unconscious perception of nonemotional actions on motor resonance responses, we tested the modulation of corticospinal activity of individuals that passively observed masked hand action stimuli.

METHODS

Participants

Twenty-two healthy volunteers (11 women and 11 men) aged 19–34 years (mean = 23 years, SD = 4 years) participated to the experiment. Participants had normal or corrected-to-normal visual acuity and were all right-handed according to a standard handedness inventory (Briggs & Nebes, 1975). All participants were naive to the purpose of the experiment but received detailed information about the procedures. Participants gave written informed consent and received course credit for their participation in the study. The procedures were approved by the ethics committee of the Scientific Institute E. Medea (Bosisio Parini, Como, Italy) and were in accordance with the ethical standards of the 1964 Declaration of Helsinki. None of the participants had neurological, psychiatric, or other medical problems or any contraindications to TMS (Wasserman, 1998). There were no reports or observations of any discomfort or adverse effects during TMS.

Stimuli

Stimuli were color pictures taken with a digital camera during the execution of abduction movements of the right index or little finger. Pictures from eight models (four women and four men, aged 20–23 years) were used as stimuli to minimize habituation and loss of attention. For each model, three pictures of the hand were taken: (a) a picture in a resting position, (b) a picture with the index finger abducted (a movement that requires contraction of the first dorsal interosseous or first dorsal interosseus muscle [FDI] muscle), and (c) a picture with the little finger abducted (a movement that requires the contraction of the abductor digiti minimi [ADM] muscle). To preserve the appearance of naturalistic movement, each picture was taken while the model was moving the finger. Pictures were taken from an egocentric perspective in light of evidence of higher mirror-like motor facilitation in response to hand movements viewed from egocentric than allocentric perspective (Alaerts, Heremans, Swinnen, & Wenderoth, 2009; Maeda, Kleiner-Fisman, & Pascual-Leone, 2002). Light conditions were kept constant across the three images of each model and the remaining luminance differences were manually corrected using Corel Paint Shop Pro X (Corel, Inc., Mountain View, CA). A rotating mask was prepared by overlapping two identical star-like geometrical figures that were textured with a scrambled version of the hand pictures using the Bryce 3-D software (DAZ Productions, Inc., Salt Lake City, UT). To mask the displacement of the fingers to the left or the right side of the screen, the two star-like figures rotated in opposite directions at 0.63 Hz (i.e., 3° every 13.33 msec refresh event), with a velocity almost comparable to that of the abduction/adduction displacement of the finger (2.8–4.3° every 13.33 msec refresh event). This way, we were able to mask not only the perception of the form cues (hand shape) but also any motion cues induced by the apparent displacement of the index or little finger. We prepared two different versions of the mask that were presented randomly in different trials. In one version, the foreground figure rotated clockwise, whereas the background figure rotated counterclockwise and vice versa for the other version. Hand and mask stimuli were presented on a uniform background and subtended a central 6° × 8.5° region.

Procedure

Stimuli were presented in a subliminal masked priming paradigm (Figure 1). Each trial started with a fixation cross presented in the center of the screen for 500 msec and proceeded to the successive presentation of the three following hand stimuli: a sample hand for 250 msec, a prime hand for 53 msec, and a probe hand for 250 msec. Prime duration was selected to be just below the threshold of conscious perception (i.e., subliminal) on the basis of preliminary behavioral data on a different group of individuals that could not report the presence of hand primes presented in the same paradigm as in this study. Pictures showing relaxed or abducted fingers were used instead of video clips because the stimulus duration required by subliminal masked presentations is too short for allowing even two-frame movement sequences at the typical 25-Hz video frame rate. Nonetheless, previous studies (Avenanti, Annella, et al., 2013; Urgesi et al., 2010; Urgesi, Moro, et al., 2006) have provided evidence of muscle-specific, mirror-like facilitation in response to single frames depicting an implied action, thus supporting that our stimuli are adept to study motor resonance. The sample hand was always a still hand, whereas the prime and the probe stimuli could depict a still hand or a hand implying an abduction movement of the index or little finger. Prime presentation was forward and backward masked with the rotating star-like figure. The mask preceding the prime (forward mask) could rotate for 40, 80, 120, or 160 msec; the duration of the rotation was randomly selected for each trial and could not be predicted; random duration of the rotating forward mask was aimed at limiting anticipation of the time of prime presentation, which could be perceived by participants as a flickering of the mask. On the contrary, the mask following the prime (backward mask) presented a rotation of 120 msec, thus inducing constant backward masking effects. Notably, the rotation of the backward mask started from a point congruent to that expected from a continuous rotation of the star-like figure during the prime presentation time; this facilitated the perceptual fusion of the forward and backward mask rotation into a single, smooth movement, thus strengthening the possible masking effect. After a 2000, 2500, 3000, or 3500 msec delay following probe presentation (with delay duration randomly varying for each trial), a response screen was presented with the request to report whether a still or a moving probe (either index or little finger movement) was pictured in the preceding trial. In different trials, participants were asked: “Was the hand still or moving?”, “Was the hand moving or still?”, “Was the hand moving?”, or “Was the hand still?” In no trials were the participants asked to report whether the probe hand displayed movement of the index or little finger. The question was presented for 3000 msec on the left of the response screen, whereas two possible answers (still or moving; yes or no) were simultaneously presented on the right. Participants were instructed to report by means of a vocal response the position of the correct answer (up or down) based on the type of probe. The vertical position of the correct answer was randomly varied across trials. This procedure was chosen to prevent spurious priming effects on the size of motor-evoked potentials (MEPs) such as sublexical processes that are known to activate M1 (e.g., McGettigan et al., 2012).

Figure 1. 

Time line of the experimental trials. Trial structure: Each trial started with a fixation cross (500 msec). Subsequently, a still sample hand (250 msec), a prime hand (53 msec), and a probe hand (250 msec) were presented. The prime and the probe stimuli could depict either a still hand or a hand implying the abduction movement of the index or little finger. Prime presentation was forward and backward masked with a rotating star-like figure that was presented before the prime for 40, 80, 120, or 160 msec and continued after the prime for 120 msec. The trials ended with a blank screen, and participants were prompted to report the motion of the probe stimulus. TMS pulse timeline: A single TMS pulse was delivered at one of two moments during the trial, either 307 msec (early TMS delay) after prime onset (corresponding to 133 msec after probe onset) or 307 msec (late TMS delay) after probe onset.

Figure 1. 

Time line of the experimental trials. Trial structure: Each trial started with a fixation cross (500 msec). Subsequently, a still sample hand (250 msec), a prime hand (53 msec), and a probe hand (250 msec) were presented. The prime and the probe stimuli could depict either a still hand or a hand implying the abduction movement of the index or little finger. Prime presentation was forward and backward masked with a rotating star-like figure that was presented before the prime for 40, 80, 120, or 160 msec and continued after the prime for 120 msec. The trials ended with a blank screen, and participants were prompted to report the motion of the probe stimulus. TMS pulse timeline: A single TMS pulse was delivered at one of two moments during the trial, either 307 msec (early TMS delay) after prime onset (corresponding to 133 msec after probe onset) or 307 msec (late TMS delay) after probe onset.

A single TMS pulse was delivered at one of two moments during the trial, either 133 msec (early delay) or 307 msec (late delay) after the onset of the probe, corresponding to 307 msec after the onset of the prime in the early delay and 307 msec after the onset of the probe in the late delay. A blank screen was presented before the next trial to create interpulse intervals ranging from 11,713 to 11,833 msec (Brasil-Neto et al., 1992).

Participants were tested in a single experimental session lasting ∼75 min. They sat in a dimly lit room 57 cm away from a 21-in. CRT monitor (resolution: 1024 × 768 pixels, refresh frequency: 75 Hz) with their head positioned on a chin rest. Participants were instructed to pay attention to the sequence of hands presented on the screen but were informed only of the presence of the sample, mask, and probe hands. Thus, they were naive regarding the presence of the prime hand.

The different conditions were presented in a randomized order in eight blocks of 48 trials each. In half of the trials a still probe was presented, and in the other half a moving probe was presented. Half of the moving probe trials displayed an index finger movement, and the other half presented a movement of the little finger. Thus, the same numbers of moving and nonmoving trials were presented to avoid biasing the participants in the motion detection task. The TMS delay was randomly varied in each trial. In total, 384 MEPs were recorded from each muscle, with 192 still probe trials (32 trials for each of the 2 delays × 3 primes), 96 index finger movement probes (16 trials for each of the 2 delays × 3 primes) and 96 little finger movement probes (16 trials for each of the 2 delays × 3 primes). Two series of eight baseline MEPs were recorded at the beginning and at the end of the experimental session, during which participants were required to look at the screen, but no stimuli were presented.

After the experimental session, participants were prompted to report any discrepancies between the instructions given and their understanding of the trial structure during the experiment. This allowed us to test for spontaneous report of the prime presence. Following this report, the experimenter described the actual trial structure, including the presence of the prime. Participants were then asked for a confirmation of their previous report and were required to perform a control, forced-choice task to determine whether they could discriminate the type of prime after being informed of its presence. In this forced-choice task, participants were presented with the experimental trials, and after each trial, they were asked to press one of three keys to report whether the prime showed a still hand, a hand with the index finger abducted, or a hand with the little finger abducted. They were instructed to answer “by feeling” and to guess for every trial in which they did not notice the presence of the prime. The trial structure used was identical to that used during TMS, with the exception that the response screen was replaced by a request for a button press in the middle of the screen. This ensured that participants were faced with the same conditions as in the main experiment. No time limit was given for the participants' responses. The next trial started immediately after the response. All 384 trials of the experimental session were presented and randomized in two blocks of 192 trials each. The control task lasted approximately 4 min.

Electromyography Recording and TMS

To check for muscle specificity and for any modulation related to the different observational conditions, both FDI and ADM MEPs were simultaneously recorded. Surface Ag/AgCl disposable electrodes (1 cm diameter) were placed in a belly-tendon montage for each muscle and connected to a Biopac MP-36 system (BIOPAC Systems, Inc., Goleta, CA) for amplification, band-pass filtering (5 Hz to 20 kHz), and digitization of the EMG signal (sampling rate: 50 kHz).

A 70-mm figure-eight stimulation coil (Magstim polyurethane-coated coil) connected to a Magstim 200 Rapid (The Magstim Company, Carmarthenshire, Wales, UK) was used to perform focal TMS (maximum output = 2 T at coil surface, pulse duration = 250 μsec, rise time = 60 μsec). The coil was placed tangentially on the scalp, with the handle pointing backward and approximately 45° lateral from the midline such that coil was placed perpendicularly to the line of central sulcus (Di Lazzaro et al., 1998). The coil position was marked on the participants' scalp. The coil was held on the scalp by a coil holder with an articulated arm, and the experimenter continuously checked the position of the coil with respect to the marks and compensated for any small movements of the participant's head during data collection. The exact position of the coil varied for each participant on the basis of the optimal scalp position (OSP), which was defined as the position from which MEPs from both the FDI and ADM muscles with maximal amplitude were recorded. The OSP was detected by moving the coil around the motor hand area of the left motor cortex projection on the scalp and delivering single TMS pulses of constant intensity. The resting motor threshold, defined as the lowest TMS intensity able to evoke MEPs with amplitudes of at least 50 μV after 5 of 10 stimulations in the higher threshold muscle (ADM), was determined by holding the stimulation coil over the OSP. To record stable MEPs from both muscles, stimulation intensity during the recording session was 120% of the resting motor threshold and ranged from 50% to 84% (mean = 67%, SD = 8.46%) of maximum stimulator output. Importantly, the chosen scalp positions and stimulation intensities allowed us to record clear and stable EMG signals (10 MEPs of 10 TMS pulses) from both recorded muscles in all participants. During MEP recordings, the background EMG signal was continuously monitored, and when voluntary contractions of the recorded muscles were detected, participants were encouraged to fully relax their muscles. The peak-to-peak MEP amplitudes (in millivolts) were collected and stored on a computer for offline analysis.

Data Handling

Five participants were excluded from the analysis because they reported the presence of the prime spontaneously or after receiving a description of the actual trial structure. This procedure is weak to false positives (i.e., reports of seeing the prime after the explanation of its presence by participants that were actually unaware of the prime) but not to false negatives (i.e., participants that were aware of the prime failing to report its presence). The remaining participants were entirely unaware of the presence of the prime. The peak-to-peak amplitude of each MEP was calculated, and trials with background activities greater than 50 μV, amplitudes less than or equal to 50 μV, or amplitudes greater or less than ±2.5 SD from the mean were discarded. After this procedure, mean values were obtained from an average of 92.52% (SD = 7.31%) of the recorded MEPs per condition. The number of recorded MEPs did not vary across the different experimental conditions between the early and late delays (all Fs < 2.6), but there was a significant effect of muscle in the early delay MEPs; fewer MEPS from the little finger (mean = 91.87%, SD = 1.09%) than from the index finger (mean = 92.79%, SD = 1.02%) were used.

For each participant and each condition, the mean MEP amplitude for each muscle and for each TMS pulse delay was expressed as percent change from the mean value of the baseline MEPs of that muscle (baselines were collapsed across the MEPs recorded at the beginning and at the end of the experimental session). This procedure allowed us to obtain an MEP ratio index of motor facilitation, hereafter referred to as the MEPratio, which takes into account interindividual differences in baseline corticospinal excitability and allowed improving normal distribution of the variables as checked with the Kolmogorov–Smirnov test for normality. The levels of the prime and probe for each experimental trials were coded into the following categories based on the correspondence between the muscle from which MEPs were recorded and the muscle that drives the observed movement: (a) still:still hands for both muscles; (b) related:index finger movements for FDI MEPs and little finger movements for ADM MEPs; (c) unrelated:little finger movements for FDI MEPs and index finger movements for ADM MEPs. It is worth noting that behavioral studies of visuomotor priming effects (e.g., Kilner et al., 2003; Brass et al., 2000; Stürmer et al., 2000) tend to code the conditions according to the congruency between the prime and the probe actions. However, because the critical measure in this study is the facilitation of the corticospinal excitability of specific muscle, which action is shown and not only whether prime and probe pairs are congruent versus incongruent is crucial for the effect. Thus, in keeping with previous single-pulse TMS studies of visuomotor interaction (Catmur, Mars, Rushworth, & Heyes, 2011), we coded the prime and probe conditions in terms of whether they showed a static hand or an implied action that was related versus unrelated to the motor role of the recorded muscle. Furthermore, because the action (i.e., index finger abduction) that is related to the FDI motor role is unrelated to the ADM motor role, and vice versa, expressing conditions in terms of related/unrelated actions, instead of index/little finger abduction, allowed us to directly test somatotopic motor facilitation (i.e., greater motor facilitation during observation of actions whose execution requires the recorded muscle vs. observation of static hand and of actions that do not involve the recorded muscle), independently of which specific action and muscle were involved.

The MEPratios for each condition were entered into two separate 2 × 3 × 3 repeated-measures ANOVAs, one for each TMS delay, with muscle (FDI vs. ADM), prime (still vs. related vs. unrelated), and probe (still vs. related vs. unrelated) as within-subject variables. Separate analyses for the two TMS delays were performed to avoid the spurious effect of the context in which the pulse was delivered: In the early condition, the pulse was delivered while the probe was still on the screen, whereas in the late condition, the pulse was delivered after the probe offset, thus confounding possible comparisons between delays. Post hoc multiple, pairwise comparisons were performed using the Duncan test. A significance threshold of p < .05 was set for all statistical analyses. Effects sizes were estimated using the partial eta-square measure (ηp2). Data are reported as the mean ± SEM.

We expected that, if motor facilitation is independent of perceptual awareness, muscle-specific motor facilitation should be obtained in response to both probe and prime action stimuli (main effects of prime and probe). Conversely, if the motor representation of observed actions is dependent on conscious processing, we expected to obtain motor facilitation in response to the probe but not the prime (main effect of probe). Finally, the masked action prime may not trigger motor facilitation per se, but it may modulate the response of the motor system to consciously perceived actions. In this case, we would expect an effect of the prime–probe congruence only for implied action probes that call for motor representations, that is, only when the probe shows a movement related to the recorded muscle (prime × probe interaction). Thus, the presentation of congruent versus incongruent prime–probe pairs should be specific for trials in which the probe shows an action that is related to the motor role of the recorded muscle (i.e., related prime and related probe pairs vs. unrelated or still prime and related probe pairs) but not when the probe shows an action that is not related to the motor role of the recorded muscle (i.e., related prime and unrelated probe pairs vs. unrelated or still prime and unrelated probe pairs). This would provide evidence that unconscious perception of the prime affected muscle-specific, mirror-like motor facilitation.

RESULTS

Behavioral Data

Analysis of the behavioral responses of participants in the TMS session revealed that they paid attention to the stimuli and successfully discriminated whether the probe was moving or not (the mean accuracy in each condition ranged from 89.89 ± 2.66% to 96.69 ± 1.43%). No differences were obtained between the different experimental conditions (all Fs ≤ 2.22), suggesting that participants were equally accurate in both early and late delay trials, regardless of the prime or probe. In a similar vein, a signal detection analysis (Macmillan & Kaplan, 1985) showed that participants had high sensitivity levels for detecting the probe motion when it was preceded by both still hand (d′ = 4.07 ± 0.24; one-sample t test against 0: t(16) = 17.014, p < .001) and implied action primes (d′ = 3.57 ± 0.19; one-sample t test against 0: t(16) = 18.62, p < .001), with the difference between the two conditions not reaching the significance threshold, t(16) = 1.781, p = .094. On the other hand, no response bias was obtained when the probe was preceded by still (ln(β) = −0.21 ± 0.63; one-sample t test against 0: t(16) = −0.34, p = .616) and implied action primes (ln(β) = 0.71 ± 0.38; one-sample t test against 0: t(16) = 1.82, p = .087); no difference was obtained between the two prime conditions, t(16) = −0.909, p = .151.

The 17 participants who were entered into the analysis reported that they were unaware of the presence of the prime in the TMS session. Their overall discrimination accuracy in the post-TMS session was 39.22% (SD = 10.03%), which was not significantly different from chance (two-tailed one-sample t test against 33.33%, t(16) = −1.895, p = .076, Cohen's d = 0.47). Although such overall comparison showed that participants' discrimination ability tended to be higher than that expected by guessing, effect size was small. More importantly, inspection of the mean accuracy values for each prime–probe pair (Table 1) revealed that such an effect was driven by apparently accurate responding when the prime and the probe actions were congruent but not when the probe depicted an action incongruent with the prime or a neutral, still hand. Indeed, a 3 × 3 repeated-measures ANOVA of the participants' prime discrimination accuracy with Probe (still vs. index finger abduction vs. little finger abduction) and Prime (still vs. index finger abduction vs. little finger abduction) as within-subject variables showed that participants' response to the prime was modulated by the type of probe: the main effect of Probe was significant, F(2, 32) = 3.82, p = .033, ηp2 = 0.1926, and was further qualified by the interaction between Probe and Prime, F(4, 64) = 12.32, p < .001, ηp2 = 0.4351. Post hoc tests showed that, in still probe trials, participants were more accurate in discriminating still primes than primes showing index (p = .006) or little finger abduction movements (p < .001). The last two conditions did not differ from one another (p = .347). Furthermore, in the trials with probes showing an index or little finger abduction, participants were more accurate in identifying primes showing a movement congruent with the probe than they were for primes showing a still hand (index finger probes: p < .001; little finger probes: p = .013). Participants also tended to be more accurate for congruent than incongruent probe-prime movements, but this difference reached significance only for the index (p < .001) and not for the little finger probes (p = .062). No difference was obtained between incongruent movement and still hand primes (index finger probes: p = .369; little finger probes: p = .461). Together, these results suggest that, even when participants were informed about the presence of the prime and were actively pursuing its identification, they tended to report the action showed by the probe and not that shown by the prime. This explains greater accuracy levels for congruent prime–probe pairs versus incongruent ones and marginally above-chance overall accuracy levels. These results corroborate the participants' subjective reports showing that the masked action primes were not consciously perceived and could not be explicitly discriminated in a forced-choice task.

Table 1. 

Accuracy (Percent Correct Responses) in Discriminating the Prime during the Postexperimental Control Task

Accuracy in the Prime Discrimination Task (%)


Prime
Still
Index Finger
Little Finger
Probe Still 66.91 ± 1.62 36.76 ± 1.54 26.1 ± 1.21 
Index finger 26.47 ± 1.56 66.91 ± 1.84 16.91 ± 0.9 
Little finger 27.21 ± 1.11 34.56 ± 1.99 54.41 ± 1.8 
Accuracy in the Prime Discrimination Task (%)


Prime
Still
Index Finger
Little Finger
Probe Still 66.91 ± 1.62 36.76 ± 1.54 26.1 ± 1.21 
Index finger 26.47 ± 1.56 66.91 ± 1.84 16.91 ± 0.9 
Little finger 27.21 ± 1.11 34.56 ± 1.99 54.41 ± 1.8 

Mean (SEM) accuracy values for each combination of prime and probe presentation are shown. “Still” indicates the presentation of a still hand stimulus; “Index finger” and “Little finger” indicate the presentation of hand with the index or little finger abducted, respectively.

Corticospinal Excitability

The MEPs recorded from each muscle at the beginning and at the end of the experimental session were entered into a 2 × 2 repeated-measures ANOVA with Muscle (FDI vs. ADM) and Session (pre- vs. post-TMS) as within-subject variables. A main effect of Muscle was found, F(1, 16) = 6.12, p < .001, ηp2 = 0.5705: MEPs recorded from the FDI (1.811 ± 0.473 mV) were greater than MEPs recorded from the ADM (0.883 ± 0.131 mV, p = .025). The main effect of Session and the interaction between Muscle and Session were not significant (all Fs < 1, ηp2 < 0.01).

Table 2 shows the raw MEP amplitudes (in millivolts) in the different experimental conditions. Inspection of Table 2 reveals that MEP amplitude at both TMS delays was higher during the observational task than at baseline, independently of whether participants were looking at a still hand or a hand performing a movement that was either related or unrelated to the motor role of the recorded muscles. This was confirmed by dependent-sample t tests (one-tailed) comparing the average of raw MEPs during all observation conditions at each TMS delay and for each muscle. Indeed, FDI MEP amplitude was higher during observation than at baseline when TMS was delivered at either early (2.067 ± 0.511 mV; t(16) = 2.8, p = .006) or late (2.039 ± 0.513 mV; t(16) = 2.52, p = .011) delay from stimulus onset; the same comparisons with baseline for the ADM muscle were only marginally significant (early: 1.078 ± 0.195 mV; t(16) = 1.69, p = .055; late: 1.066 ± 0.192 mV; t(16) = 1.61, p = .064). In a similar vein, dependent-sample t tests (two-tailed) showed that no difference was obtained between MEP amplitudes recorded at early and late TMS delays during the observation conditions for either FDI, t(16) = 0.88, p = .39, or ADM, t(16) = 0.9, p = .38, muscle. In summary, during observation of hand stimuli, we found an overall increase of corticospinal excitability that was not specifically associated to implied actions as it also occurred for still hand images. This is in keeping with previous studies showing that body part observation activates the motor cortex more than baseline trials in which participants either keep the eyes closed or fixate at a blank screen (e.g., Raos, Kilintari, & Savaki, 2013; Borgomaneri, Gazzola, & Avenanti, 2012; Hodzic, Muckli, Singer, & Stirn, 2009; Schütz-Bosbach, Mancini, Aglioti, & Haggard, 2006).

Table 2. 

Mean ± SEM Raw Amplitudes (in mV) of MEPs Recorded from the FDI and ADM Muscles in Each Experimental Condition

Probe
Prime
Raw Amplitude of MEPs (mV)
Early Delay
Late Delay
FDI
ADM
FDI
ADM
Still Still 2.08 ± 0.51 1.07 ± 0.2 2.12 ± 0.52 1.06 ± 0.19 
Still Related 2.03 ± 0.5 1.05 ± 0.18 1.99 ± 0.52 1.01 ± 0.18 
Still Unrelated 1.95 ± 0.46 1.1 ± 0.2 1.99 ± 0.5 1.07 ± 0.2 
Related Still 2.11 ± 0.52 1.13 ± 0.21 2.11 ± 0.54 1.14 ± 0.2 
Related Related 2.11 ± 0.52 1.09 ± 0.2 2.18 ± 0.52 1.1 ± 0.2 
Related Unrelated 2.12 ± 0.53 1.05 ± 0.18 2.01 ± 0.49 1.06 ± 0.19 
Unrelated Still 2.04 ± 0.54 1.07 ± 0.21 2.01 ± 0.52 1.06 ± 0.2 
Unrelated Related 2.05 ± 0.51 1.04 ± 0.19 2.01 ± 0.51 1.03 ± 0.18 
Unrelated Unrelated 2.1 ± 0.53 1.11 ± 0.2 1.93 ± 0.52 1.07 ± 0.2 
Probe
Prime
Raw Amplitude of MEPs (mV)
Early Delay
Late Delay
FDI
ADM
FDI
ADM
Still Still 2.08 ± 0.51 1.07 ± 0.2 2.12 ± 0.52 1.06 ± 0.19 
Still Related 2.03 ± 0.5 1.05 ± 0.18 1.99 ± 0.52 1.01 ± 0.18 
Still Unrelated 1.95 ± 0.46 1.1 ± 0.2 1.99 ± 0.5 1.07 ± 0.2 
Related Still 2.11 ± 0.52 1.13 ± 0.21 2.11 ± 0.54 1.14 ± 0.2 
Related Related 2.11 ± 0.52 1.09 ± 0.2 2.18 ± 0.52 1.1 ± 0.2 
Related Unrelated 2.12 ± 0.53 1.05 ± 0.18 2.01 ± 0.49 1.06 ± 0.19 
Unrelated Still 2.04 ± 0.54 1.07 ± 0.21 2.01 ± 0.52 1.06 ± 0.2 
Unrelated Related 2.05 ± 0.51 1.04 ± 0.19 2.01 ± 0.51 1.03 ± 0.18 
Unrelated Unrelated 2.1 ± 0.53 1.11 ± 0.2 1.93 ± 0.52 1.07 ± 0.2 

“Still” indicates the presentation of a still hand stimulus; “related” indicates the presentation of a stimulus that depicts an abduction of the finger controlled by the recorded muscle (shown in column); “unrelated” indicates the presentation of a stimulus that depicts an abduction of the finger not controlled by the recorded muscle.

Modulation of Motor Facilitation

Previous analysis of raw MEP amplitudes showed that, as compared with viewing a blank screen, observation of hand images increased corticospinal excitability. Because mirror-like motor facilitation is indexed by a greater activation during observation of dynamic (or implied action) bodies or body parts as compared with still bodies or body parts (Urgesi, Candidi, Fabbro, Romani, & Aglioti, 2006; Urgesi, Moro, et al., 2006; Fadiga et al., 2005), in the successive analysis, we tested how corticospinal excitability was modulated according to the hand image that was presented. We, thus, expressed MEP amplitude of each muscle as percent change (MEPratio; Figure 2) from that muscle baseline, averaging the baseline MEPs collected at the beginning and at the end of the experimental session. This gave a motor facilitation index of how much the motor cortex was facilitated during observation of hand images as compared with baseline. The MEPratio values for the two TMS delays were entered into two separate 2 × 3 × 3 (Muscle × Prime × Probe) repeated-measures ANOVAs to test whether motor facilitation was greater during observation of related actions than still hands and unrelated actions when depicted in either the probe or the prime.

Figure 2. 

Effects of conscious and unconscious observation of actions. MEPratio (% of baseline) of MEPs recorded from the FDI and ADM muscles in the different prime and probe combinations. MEPs recorded from FDI and ADM are collapsed because the same pattern of results was obtained for the two muscles (see text). The prime and probe types were coded according to whether the hand depicted a still hand (still), a finger abduction movement that was related to the recorded muscle (related), or a finger abduction movement that was not related to the recorded muscle (unrelated). Error bars indicate the SEM, asterisks indicate significant comparisons, and @ indicates marginally significant comparisons.

Figure 2. 

Effects of conscious and unconscious observation of actions. MEPratio (% of baseline) of MEPs recorded from the FDI and ADM muscles in the different prime and probe combinations. MEPs recorded from FDI and ADM are collapsed because the same pattern of results was obtained for the two muscles (see text). The prime and probe types were coded according to whether the hand depicted a still hand (still), a finger abduction movement that was related to the recorded muscle (related), or a finger abduction movement that was not related to the recorded muscle (unrelated). Error bars indicate the SEM, asterisks indicate significant comparisons, and @ indicates marginally significant comparisons.

The ANOVA for the early TMS delay condition yielded no significant effect (all Fs < 1.1, p > .35), suggesting that motor facilitation at this early delay from stimulus onset was not specific for the action depicted in the images. The ANOVA for the late TMS delay condition yielded significant main effect of Probe, F(2, 32) = 5.6776, p = .008, ηp2 = 0.2619. The main effect of Prime was not significant, F(2,32) = 1.5, p = .239, ηp2 = 0.086, but a significant interaction between Prime and Probe was found, F(4, 64) = 3.23, p = .017, ηp2 = 0.168, suggesting that motor facilitation during observation of muscle-related probe actions was modulated by masked prime actions. The three-way interaction was not significant, F(4, 64) < 1, ηp2 < 0.05, suggesting comparable patterns of results for the two muscles when movements that were related versus unrelated to their motor roles were observed. Simple effect analysis of mirror-like motor facilitation in response to related as compared with still and unrelated probes showed significant effects for still, F(1, 16) = 9.09, p = .008, and related, F(1, 16) = 11.76, p = .003, prime trials but not for unrelated prime trials, F(1, 16) < 1. Thus, the observation of a probe stimulus implying a finger movement induced a significant facilitation of the corticospinal representation of the muscle involved in the execution of that movement when the probe was preceded by a still (neutral) prime and by a related (congruent) prime. In contrast, no mirror-like facilitation was obtained when the probe was preceded by unrelated primes, showing comparable motor facilitation for related than still and unrelated probes. Pairwise post hoc analysis of the interaction between the probe and prime hands revealed no differences among the three prime types in the unrelated and still probe conditions (all ps > .102). Conversely, when the probe was related to the recorded muscle, MEPratios in the unrelated prime trials (117.34 ± 10.89%) were lower than those in the related prime trials (130.9 ± 13.08%, p = .006) and marginally lower than the MEPratios in the still prime trials (126.66 ± 11.77%, p = .059); the related and still prime trials did not differ from each other (p = .308). Indeed, MEPratios in the related probe-unrelated prime condition were not significantly different from MEPratios in the still and unrelated probe conditions regardless of the prime type (all ps > .319). Because motor facilitation for still hands and unrelated actions reflects nonspecific activation in response to viewing body parts, no evidence of mirror-like motor facilitation was obtained when related movement probes were preceded by primes showing a movement that is unrelated to the motor role of the recorded muscle. Thus, presentation of an unrelated prime suppressed motor facilitation to a level that is expected in response to observation of body parts independently of movement information.

DISCUSSION

In this study, we provide evidence that motor resonance, as indexed by greater motor facilitation in response to related than static hands and unrelated actions, is not elicited by unconscious action perception but requires perceptual awareness. Nevertheless, its expression in response to consciously perceived actions is modulated by unconscious perception of incongruent actions. We tested corticospinal excitability while participants passively observed static images of either still or moving hands preceded by masked primes of either the same or different movements. Our results show the following: (i) observation of the action probe, which was consciously perceived, engendered a somatotopic activation of the cortical representation of the muscles actually involved in the same action when compared with observation of a still hand or of unrelated actions; (ii) subliminal perception of the action prime did not increase motor facilitation more than observation of a still hand, suggesting that actions need to be consciously perceived to evoke mirror-like motor facilitation (motor resonance); and (iii) unconscious priming of actions unrelated to the motor role of the recorded muscle interfered with mirror-like motor facilitation (motor resonance) in response to consciously perceived probe actions that are related to the motor role of the recorded muscle.

Conscious Perception of Implied Actions Activates the Motor System

Several studies have shown that passive observation of dynamic displays (Tomeo, Cesari, Aglioti, & Urgesi, 2013; Avenanti & Urgesi, 2011; Avenanti, Bolognini, Maravita, & Aglioti, 2007; Urgesi, Candidi, et al., 2006; Fadiga et al., 2005) and static images of bodies in action (Avenanti, Annella, et al., 2013; Urgesi et al., 2010; Proverbio, Riva, & Zani, 2009; Urgesi, Moro, et al., 2006) triggers somatotopic activation of the motor system and, thus, facilitates the corticospinal representation of those muscles involved in the perceived actions; this effect is thought to reflect motor resonance. Hence, motor resonance is involved in the extrapolation of dynamic information from actual as well as implied action stimuli. Although static images were used in this study, these images were presented in a sequence of three snapshots (i.e., sample, prime, and probe) rather than in a single static frame. This sequence may have contributed to the apparent motion perception if the mask served as a temporal occluding object and the successive hand postures were amodally completed into a continuous perception of movement (Shiffrar & Freyd, 1993). If true, this interpretation is in keeping with the notion that the amodal completion of occluded actions may involve automatic motor resonance processes (Avenanti & Urgesi, 2011; Orgs, Bestmann, Schuur, & Haggard, 2011; Urgesi et al., 2010).

Importantly, the different delays of TMS stimulation allowed us to test modulation of motor facilitation during observation of still, related, and unrelated actions after a comparable delay (307 msec) from prime (early delay) and probe (late delay) presentation. Thus, we could test muscle-specific, mirror-like facilitation separately for unconsciously and consciously perceived stimuli. MEG studies (Nishitani, Avikainen, & Hari, 2004; Nishitani & Hari, 2000) have found that the responses to action observation peak first in the visual occipital areas (at approximately 118 msec from the onset of the stimulus) before subsequent activation of the STS, inferior parietal lobule, inferior frontal areas, and, finally, M1 at approximately 300 msec. Similarly, a recent TMS study (Barchiesi & Cattaneo, 2012) found that muscle-specific effects in M1 were observed only 250 msec after action stimuli onset. Therefore, any direct motor response to unconscious prime perception (i.e., main effect of prime) was likely to occur in the early TMS delay, because in that case processing of the prime action, but not of the probe action, was likely to have reached the motor cortex. In contrast, we did not find any modulation of the corticospinal excitability for the unconsciously perceived prime at the early TMS delay independently of the type of probe presented nor did we find any effect of prime presentation when the probe depicted a still hand or an unrelated action, thus ruling out that unconscious prime action perception increased motor facilitation in a muscle-specific fashion more than observation of a still hand. Conversely, the late TMS delay, when the TMS pulse was delivered after 307 msec from probe onset, allowed us to show that muscle-specific motor responses are facilitated for the consciously perceived probe. In summary, our finding that motor facilitation in response to consciously perceived probe actions was obtained at the late TMS delay corroborates the timing of cortical activations reported in previous MEG and TMS studies and suggests a late involvement of M1 in mapping observed (implied) actions.

Motor Resonance Requires Perceptual Awareness

The main aim of this study was to test whether perceptual awareness is an essential condition for the emergence of motor resonance responses, measured as a facilitation of corticospinal excitability during observation of actions that are related to the motor role of the recorded muscle as compared with observation of a still hand or of an action that is unrelated to the motor role of the recorded muscle. The forward and backward masking procedure allowed us to present the prime for a relatively long time (53 msec) but nonetheless prevented conscious perception by most participants (17/22 individuals). Indeed, even when these participants were explicitly informed about the presence of the prime and were required to discriminate whether it depicted a still hand or an index or little finger abduction, their discrimination abilities were at chance. This suggests that manipulation of the voluntary control of the attentional focus did not affect the perception of the prime, because prime discrimination was not possible even when participants were actively pursuing it. This does not exclude, however, that presentation of the probe stimulus may have contributed to prevent conscious perception of the prime with automatic attentional shift. Whether the locus of prime perception disruption was at a perceptual or attentional level, our paradigm ensured that participants remained completely unaware of the prime action.

In striking contrast with the muscle-specific response to the probe, no motor facilitation was measured in response to the presentation of the prime (independently of the probe type and TMS delay). The absence of motor responses to masked action primes cannot be ascribed to the interferential effect of probe movement presentation because M1 responses to the prime in the still hand probe conditions were also absent. Nor can this absence reflect that the presentation of either static or implied action probes completely suppressed the early processing of the masked prime stimulus in the visual system, thus preventing the result of this processing to be forwarded to the motor system. Indeed, we have evidence that the prime was processed to a certain extent and the result of this processing affected the activation of the motor system, because the presentation of incongruent action primes suppressed the motor resonance to implied action probes (see below). Furthermore, it is also unlikely that the absence of motor responses to masked action primes was because of inappropriate timing of the TMS pulses; indeed, in both the early and late TMS delay conditions, corticospinal excitability was tested after the 250 msec threshold for the establishment of muscle-specific motor resonance responses to the prime (Barchiesi & Cattaneo, 2012). Given our experimental settings, however, we cannot exclude the possibility that masked action stimuli may have modulated motor cortex activity at earlier phases of stimulus processing, for example, in the 60–90 msec time window in which a nonspecific activation of the motor cortex in response to action stimuli has been reported (Lepage, Tremblay, & Théoret, 2010). However, because this early activation lacks muscle specificity, it is unlikely to reflect motor resonance. Together, our data suggest that motor resonance requires awareness of the action stimuli.

Action stimuli with an emotional valence are known to activate the motor system in conditions of reduced visibility or when participants are not aware of the stimuli. Emotional face and body expressions induce fast facial mimicry responses (Tamietto et al., 2009; i.e., EMG preactivation of the muscles involved in the same expressions) and motor and premotor cortical activations (Van den Stock et al., 2011) even when presented in the blind hemifield of hemianopic patients or in masked presentation conditions in healthy individuals (Dimberg et al., 2000). Such affective facial mimicry responses, however, are not specific for the observed body parts because they also occur in response to whole-body emotional expressions and lack the muscle-specific properties of motor resonance responses. Thus, these responses seem to be mediated by the activation of subcortical structures in response to emotionally valenced stimuli and are not involved in nonemotional actions (Tamietto & De Gelder, 2010).

Unconscious Presentation of Incongruent Actions Inhibits Motor Resonance

Although the subliminal presentation of masked prime actions did not trigger motor facilitation per se, it affected the activation of the motor cortex in response to consciously perceived actions. In other words, masked prime actions that were unrelated to the motor role of the recorded muscle interfered with motor facilitation in response to probe actions that were related to the motor role of the recorded muscle. This evidence of implicit effects of unconscious action perception on motor resonance suggests the dissociation between visual awareness and the perceptual effects of masked action primes (Van den Bussche et al., 2009). Thus, in keeping with the finding that unconscious prime perception may affect motor execution processes during visuomotor masked priming tasks (Eimer & Schlaghecken, 1998, 2003), we show evidence that masked primes can affect motor resonance.

The suppression of motor facilitation during action observation is unlikely to involve a specific exogenous inhibition of motor activation induced by the abrupt interruption of stimulus presentation, as reported in visuomotor masked priming tasks (Eimer & Schlaghecken, 1998, 2003). Indeed, motor facilitation was suppressed for incongruent presentations of prime and probe movements, whereas visuomotor masked primes tend to inhibit the execution of congruent actions. Because no facilitatory or inhibitory responses to the prime were obtained with neutral (i.e., still hand) probes, the interaction between the probe and prime does not seem to be because of the orienting of spatial attention to the one or to the other side of the space. Accordingly, previous single-pulse TMS studies have shown that motor facilitation during action observation is based on mapping the specific muscle involved in the actual execution of the action rather than on the spatial compatibility between the model's and observer's body part (Alaerts, Van Aggelpoel, Swinnen, & Wenderoth, 2009; Urgesi, Candidi, et al., 2006). Furthermore, behavioral studies have shown that visuomotor interference effects occurred when the observed and executed movements involved different hands (i.e., left vs. right hand; Catmur & Heyes, 2011; Bertenthal, Longo, & Kosobud, 2006; Brass et al., 2000) or the same hand viewed from different perspectives (egocentric vs. allocentric perspective; Bortoletto, Mattingley, & Cunnington, 2013) and, thus, occurred in the opposite sides of the space. Even when visuomotor interaction was affected by the spatial compatibility between the executed and observed movement, this resulted into inversion of the effect (i.e., facilitation for incongruent actions) rather than into its suppression (Vainio & Mustonen, 2011). As such, the suppression of mirror-like (i.e., muscle-specific) facilitation obtained in this study is unlikely to be because of the effects of spatial attention. In a similar vein, it seems untenable a pure attentional account for the suppression of motor resonance to the probe, according to which the presentation of the incongruent prime may have diverted the focus of attention away from processing the probe action. Indeed, on the one hand, the prime was not perceived consciously and could not be discriminated even when the focus of attention of the observer was explicitly focused on its presentation, thus ruling out a role for voluntary control of attention. On the other hand, although prime presentation may have influenced probe processing by causing automatic attentional shift, the results of the online task suggest that participants could easily detect the probe action independently from the action shownby the prime, thus ruling out that the prime disrupted conscious perception of the probe at an extent capable of explaining the complete suppression of mirror-like motor facilitation. Accordingly, previous studies showed that attentional modulation of action perception may modulate, but not suppress, motor resonance (Muthukumaraswamy & Singh, 2008). Thus, it is likely the effects of unconscious perception of prime actions on motor resonance to probe actions were at least partially independent from attentional modulation processes.

The present single-pulse TMS study is not informative regarding the possible neural site of the interaction between subliminal and supraliminal action processing. We can speculate that such interaction likely involves an area or set of areas that have direct or indirect connections to the primary motor cortex and that process both supra- and subliminal presentations of actions, thus explaining the effects of the masked action prime. Considering the pattern of results, two possible scenarios can be hypothesized. On the one hand, the facilitation induced by supraliminal probe actions and its suppression by subliminal incongruent prime actions may be because of separate mechanisms operating within the motor system. A possible candidate is the ventral premotor cortex, which is strongly connected to M1 (Prabhu et al., 2009) and seems to be the main source of the motor resonance facilitation responses observed in M1 (Avenanti, Annella, et al., 2013; Avenanti, Candidi, & Urgesi, 2013; Avenanti et al., 2007). There is evidence of pyramidal tract neurons in the macaque ventral premotor cortex (Kraskov, Dancause, Quallo, Shepherd, & Lemon, 2009; but see a similar finding in M1; Vigneswaran, Philipp, Lemon, & Kraskov, 2013) that may exhibit one of two main patterns of activity: whereas one pattern consists in facilitation of the hand motor neurons that mirrors the observed action and corresponds to what is generally called motor resonance, the other consists in a number of neurons being suppressed during action observation. This suppression is thought to prevent overt movements during action observation, but because the same neurons showed inhibitory responses in a variety of conditions (e.g., grip with object, grip without object, concealed grip) it may be that this inhibitory mechanism is less selective with regard to the specific action observed when compared with classical mirror facilitation. Importantly, facilitation and suppression neurons seem to be part of two parallel mechanisms in the ventral premotor cortex, and their possible differential sensitivity to perceptual awareness and attentional modulation may explain the pattern of results obtained in this study in terms of facilitation of corticospinal excitability. Indeed, assuming that suppression neurons may respond to observed actions independently of perceptual awareness, suppression of motor activity in response to the prime and/or probe actions may remain unnoticed in a gross measure of corticospinal excitability as done with TMS-evoked motor potentials. Nevertheless, the suppression of their activity after presentation of two incongruent actions, which is likely stronger as compared with presentation of a single action, may contribute to suppress the expression of facilitation neuron activity in response to consciously perceived probe actions.

On the other hand, the suppression of motor facilitation for incongruent prime–probe actions may stem specifically from the processing of the conflict between two incongruent action representations in areas beyond the motor cortex but directly or indirectly connected to it. The detection of a conflict in these areas may generate a signal that is sent to the motor system to suppress motor resonance in response to consciously perceived actions. Unconscious processing of the conflict between different action-related cues may involve several cortical and subcortical regions (D'Ostilio & Garraux, 2012). For example, the pre-SMA is involved in exerting control over voluntary actions during response conflict (Nachev, Wydell, O'neill, Husain, & Kennard, 2007) and may also subserve the suppression of incongruent motor resonance responses. Furthermore, controlling the automatic tendency to imitate other's actions, as tested in the visuomotor priming paradigm, involves activation in the ventromedial pFC and the TPJ (Spengler, Brass, Kühn, & Schütz-Bosbach, 2010; Brass, Ruby, & Spengler, 2009). These areas are implicated in higher-level mentalizing abilities, such as the sense of agency, perspective taking, and self-referential processing (Brass et al., 2009) and seem to be also involved in preventing the coding of others' movements to be translated into self-movements. Finally, inhibitory afferents to M1 via corticostriatal pathways play a role in masked visuomotor priming effects (D'Ostilio & Garraux, 2012; Bowman, Schlaghecken, & Eimer, 2006) and may also be involved during action observation (Marceglia et al., 2009) and, especially, for suppressing unwanted motor resonance responses.

In conclusion, we show that, although motor resonance in response to nonemotional actions is automatic, it requires the perceptual awareness of actions and operates at a conscious level of motor representation. Although the presentations of subliminal actions do not activate the motor cortex, they suppress motor resonance responses to incongruent actions that are consciously perceived. We suggest that this suppression reflects the processing of action conflicts in areas upstream of the motor cortex and may subserve a basic mechanism for dealing with the multiple and possibly incongruent actions of other individuals. Motor resonance responses, indeed, seem to endow the social brain with the ability of simulating anticipatory representations of unfolding actions ahead of their realization and in the absence of complete perceptual information from the environment (Avenanti, Candidi, et al., 2013; Avenanti & Urgesi, 2011; Wilson & Knoblich, 2005). Such representations, however, need to be flexibly updated on the basis of upcoming information, to take into account not only actual deployment of actions but also their future phases and to be modulated by the social context toward imitative or complimentary responses (Sartori et al., 2013). The results of this study suggest that modulation of motor resonance according to the social context (in this case incongruent actions) may also occur at an implicit level and independently from the observer's perceptual awareness.

Acknowledgments

This research was supported by grants from Istituto Italiano di Tecnologia SEED 2009 (Prot. No. 21538; to C. U.), from the Ministero Istruzione Università e Ricerca (Progetti di Ricerca di Interesse Nazionale, PRIN 2009; Prot. No. 2009A8FR3Z; Futuro In Ricerca, FIR 2012, Prot. N. RBFR12F0BD; to C. U.), and from Istituto di Ricovero e Cura a Carattere Scientifico “E. Medea” (Ricerca Corrente 2012, Ministero Italiano della Salute; to C. U.).

Reprint requests should be sent to Cosimo Urgesi, Department of Human Sciences, University of Udine, via Margreth, 3, I-33100 Udine, Italy, or via e-mail: cosimo.urgesi@uniud.it.

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Author notes

*

These authors contributed equally to the study.