PSYCHOLOGICAL SCIENCE
Research Article
THE PERCEIVED ONSET TIME OF SELF- AND
OTHER-GENERATED ACTIONS
Andreas Wohlschläger,1 Patrick Haggard,2 Benno Gesierich,3 and Wolfgang Prinz1
1
Max-Planck-Institut für Psychologische Forschung, München, Germany; 2University College London, London,
United Kingdom; and 3Università degli Studi di Ferrara, Ferrara, Italy
Abstract—Awareness of actions is partly based on the intentions accompanying them. Thus, the awareness of self- and other-generated
actions should differ to the extent that access to own and other’s intentions differs. Recent studies have found a brain circuit (the mirrorneuron system) that represents self- and other-generated actions in an
integrated fashion. This system does not respond to actions made by
nonagents, such as machines. We measured the estimated onset time of
actions that subjects either executed themselves or observed being executed by someone else or by a machine. In three experiments, the estimates of the machine actions always differed from those of self- and
other-generated actions, whereas the latter two were indistinguishable. Our results are consistent with the view that intentions are attributed to others but not to machines. They also raise the interesting
possibility that people attribute intentions to themselves in the same
way as they do to others.
Voluntary action is fundamental to human existence. The traditional
concept of free will views action as starting within the mind of the individual. Neuroscientific studies have identified a specific role of frontal
motor areas in the generation of voluntary actions. Kornhuber and Deecke
(1965) identified a Bereitschaftspotential (readiness potential) over frontal
brain regions, beginning at least 1 s before a voluntary action, and growing gradually to a clear maximum just before movement. Later studies
identified the supplementary motor area as a key structure in the internal
generation of actions (Goldberg, Kwan, Borrett, & Murphy, 1984; Tanji,
2001) and a likely source of the readiness potential (Kornhuber & Deecke,
1965). These studies all point to the frontal motor areas as a key circuit for
willed actions.
Additional studies have investigated the relations between neural
preparation of action and conscious awareness (“free will”). Libet, Gleason, Wright, and Pearl (1983) found that the readiness potential preceding voluntary action began 300 to 500 ms before subjects became aware
that they intended to move, in apparent contradiction to the traditional
Cartesian concept of free will. The connection between frontal brain activity and conscious experience of action was confirmed by Fried et al.
(1991). When they stimulated frontal cortex intracranially at low intensity, their patients reported an urge to move a specific body part. Higher
intensities of stimulation evoked actual movements of the same body
part. These studies present voluntary actions as a specific brain function linked to subjective mental life. We call this the privacy position.
In contrast to this subjective tradition, there is increasing evidence
that a common brain circuit is used both to control object-oriented actions and to represent homologous actions of others. Thus, mirror neurons in the monkey premotor cortex fire both during grasping and
Address correspondence to Andreas Wohlschläger, Max Planck Institute for
Psychological Research, Cognition and Action, Amalienstraße 33, D-80799
München, Germany; e-mail: [email protected].
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Copyright © 2003 American Psychological Society
when the monkey observes a human or conspecific make a similar grip
(Gallese, Fadiga, Fogassi, & Rizzolatti, 1996). However, these neurons are silent when kinematically similar actions are performed using
a tool. Functional imaging also confirms activation of a parietal premotor network in both the control and the observation of action in humans (Rizzolatti, Fadiga, Matelli, et al., 1996). This evidence suggests
that the neural representation of voluntary action is not private and
subjective, but integrates across agents, and is thus intrinsically social
(Gallese, 2000). Moreover, observing the actions of others can directly
influence the action system of the observer. Fadiga, Fogassi, Pavesi, and
Rizzolatti (1995) found increased excitability of the motor cortex to
transcranial magnetic stimulation when subjects observed an action (see
also Aziz-Zadeh, Maeda, Zaidel, Mazziota, & Iacoboni, 2002; Maeda,
Kleiner-Fisman, & Pascual-Leone, 2002). We use the term integrationist to refer to this tradition in which action representation is seen as
comparable across agents.
Recent evidence from both human performance (Craighero, Fadiga,
Rizzolatti, & Umiltà, 1999; Craighero, Fadiga, Umiltà, & Rizzolatti,
1996) and neurophysiological (Gallese et al., 1996; Rizzolatti, Fadiga,
Gallese, & Fogassi, 1996) studies supports an integrationist position.
But, to our knowledge, no psychophysical work has compared awareness of self-generated actions with awareness of other-generated actions.
Privacy and integrationist accounts of action make different predictions
about these situations. According to the privacy account, one’s awareness
of other people’s actions must be different from one’s awareness of one’s
own actions, because one has special access to only one’s own intentions. According to the integrationist account, one’s awareness of other
people’s actions should be fundamentally similar to one’s awareness of
one’s own actions. A radical integrationism could reject privacy entirely by claiming that one has no better access to one’s own intentions
than to the intentions of others. For example, one might retrospectively
infer intentions from actions in both these cases.
In summary, then, the private view contrasts self-initiated actions
with all other events. The integrationist view contrasts actions (irrespective of the agent) with events not involving an agent. We performed a series of three experiments aimed at exploring the boundaries
between You, Me, and It. We used the method devised by Libet et al.
(1983) because the resulting measure of the perceived onset time of actions provides a common metric for describing diverse events, yet is
also sensitive to the properties of the underlying psychological representations of those events (Haggard, Aschersleben, Gehrke, & Prinz,
2002).
EXPERIMENT 1
In Experiment 1, we compared the perceived onset time of voluntary actions made by the subject with the perceived onset time of actions that the subject observed the experimenter make and of comparable
mechanical events that were not actions and did not involve an agent. In
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each condition, the action that had to be judged was a lever press followed by a pure tone. This sequence was selected for two reasons:
First, making the action operant by including an action effect would
encourage subjects to conceive their and the experimenter’s movements as involving agency, rather than reaction. Second, this common
action effect was designed to emphasize the similarity between the
three actions, notwithstanding that they involved different agents.
We used planned contrasts to distinguish between the private and
the integrationist views. The private view predicts that one’s own intentions play a key role in awareness of action. Therefore, according
to this account, the perceived onset time of subjects’ own action would
differ from the perceived onset times in the other two conditions, which
would be equal. The integrationist view predicts that awareness of
one’s own voluntary action and of observed actions should be comparable, because these activate a common neural system, but that awareness of these actions should be different from awareness of mechanical
events. Therefore, according to the integrationist view, the perceived
onset time for the subject’s own actions would be similar to the perceived onset time for another person’s actions, but both would differ
from the perceived onset time of machine-generated actions.
Method
Twelve subjects, 9 women and 3 men, all right-handed and between
21 and 47 years old, were tested in the apparatus schematically shown in
Figure 1. Subjects viewed a response lever. Depressing the response lever
2 mm closed an electrical contact. An image of a clock was projected onto
the table next to the lever. The clock had a single hand, 1.2 cm long, that
rotated with a period of 2,560 ms within a face marked at conventional
5-“minute” intervals (Libet et al., 1983). The experimenter initiated the
rotation of the clock at the start of each trial. The initial position of the
clock hand was random. The hand then rotated until the response lever
was moved (see the descriptions of the three conditions later in this section), continued for a random period between 1.5 and 2.5 s thereafter, and
then stopped. The subject then verbally reported the position of the clock
hand when the response lever was depressed. In the self-action and machine-action conditions, the experimenter sat at a table about 2 m away
from the experimental apparatus (in the other-action condition, the experiment was closer, as described later). In these conditions, subjects could
see the experimenter only if they raised and turned their head to the left.
Subjects were encouraged to make their verbal reports as precise as possible, and to avoid confining themselves to the intervals marked on the clock
face. The experimenter verified that the subject was looking at the clock
face during each trial.
Subjects performed 40 trials in each of three conditions. The conditions differed according to how the response lever was moved.
In the self-action condition, subjects pressed the lever with the index finger of their right hand at a time of their own choice. They were
instructed to avoid pressing at “obvious” clock positions (e.g., 0, 30)
or in response to the onset of the hand’s rotation. These instructions
were designed to give the subjects’ movements the quality of voluntary actions, rather than reactions. The subjects then judged the perceived onset time of their action using the clock.
In the other-action condition, the subject observed the experimenter (now sitting next to the subject) pressing the response lever
with his right index finger. The experimenter followed the same constraints as those in the self-action condition. He avoided making any
subtle preparatory movements of his hand prior to the lever press itself, as such movements could warn the subjects of his impending
movement. When the clock stopped, the subject judged the perceived
onset time of the experimenter’s lever press.
In the machine-action condition, the response lever was moved by
a solenoid invisibly located at the lever’s fulcrum. Subjects were in-
Fig. 1. Apparatus used in the experiments. Subjects viewed the response lever through a semisilvered mirror (a). The mirror served to project the
image of a clock on the computer screen next to the lever (b).
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structed that the lever would move automatically. In fact, the experimenter invisibly activated the solenoid at a different latency on each
trial using a switch hidden under his table, out of subjects’ view.
The order of conditions was completely counterbalanced across subjects. In all conditions, closure of the lever was followed after 250 ms by
a short auditory stimulus (100-ms tone of 1 kHz, delivered over loudspeakers).
The position of the clock hand at the time of the lever’s contact was
recorded on a computer for off-line analysis. The temporal resolution of
response times was 1 ms (as determined by the software package LabVIEW). We calculated a judgment error for each trial by comparing the
position of the clock hand at lever closure with the subject’s judgment of
when the lever moved. Negative judgment errors indicate anticipatory
awareness of action; positive judgment errors indicate delayed awareness. A few trials were excluded from the analysis because the subject
reported not paying attention to the clock or response lever during the
trial. Mean judgment errors were used for statistical analysis.
Results and Discussion
Mean judgment errors of the 12 subjects are shown in the top panel
of Figure 2. Subjects had slightly delayed awareness of their own actions (5 ms) and of the experimenter’s actions (11 ms), but an anticipatory awareness of the machine’s displacement of the response lever
(77 ms).
We tested the integrationist hypothesis with a planned contrast in
which we assigned contrast weights of 1 to the self-action and otheraction conditions and 2 to the machine-action condition. The integrationist hypothesis was strongly supported, t(11) 9.45, p .000001. A second planned contrast tested the privacy hypothesis by
assigning contrast weights of 2 to the self-action condition and 1
to the other conditions. This hypothesis was also supported, t(11) 3.98, p .002. Because these two contrasts are nonorthogonal (r .5), they are not mutually exclusive. However, comparison of the t statistics and inspection of the mean values (Fig. 2) shows that our data
are more consistent with the integrationist hypothesis than with the
privacy hypothesis. The same conclusion is supported by the coefficients of determination, r2 .89 for the integrationist hypothesis and
r2 .59 for the privacy hypothesis.
The results clearly show that the perceived onset time of one’s own
actions is comparable to the perceived onset time of other people’s actions. Both are substantially later than the perceived onset time of a
physically comparable machine event. The similarity between the voluntary-action and observed-action conditions deserves particular comment. The perceived onset times of self-generated and other-generated
actions are quite similar, despite the very different sources of perceptual information about them. Information about one’s own voluntary
actions includes at least two private sources of information, namely,
proprioceptive feedback and efference copy. Of course, no such private information is available about the actions of other people.
Experiment 1 provides clearer support for the integrationist view
than for the privacy view. We suggest that conscious representations of
action (a) may derive from the common neural system for action generation and action understanding (Gallese et al., 1996), (b) are independent of the agent who makes the action, and (c) are distinct from
physical events that are not actions and are not associated with any
agent.
Experiment 1 showed a substantial difference between the machine-action condition and the other two conditions. Judgments were
588
Fig. 2. Mean error in judgment of onset time in the three conditions of
Experiments 1, 2, and 3. Vertical lines indicate standard deviations of the
judgment error. Negative judgment errors indicate anticipatory awareness of action; positive judgment errors indicate delayed awareness.
most inaccurate in the machine-action condition (mean judgment error 89 ms). The direction of this difference was not predicted, and we
return to this point in the General Discussion. Viviani and Stucchi
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(1992) reported large differences in the spatiotemporal perception of
more extended, continuous trajectories according to whether the
movements were made by human agents or were mechanical in origin;
perception of biological motion was more accurate. In our study, the
kinematic displacement was minimal and without significant spatial
pattern, yet a comparable difference was observed. Such differences
may reflect either an “intentional stance” (Dennett, 1987) or a specialized module for the perception of biological actions (Stucchi & Viviani, 1993).
The agency effect in Experiment 1 was confounded with a visual
difference between conditions. In the self-action and other-action conditions, subjects saw a hand pressing the lever, whereas no hand was
present in the machine-action condition. Experiment 2 removed this
confound.
EXPERIMENT 2
Method
The methods and conditions for Experiment 2 exactly replicated
those of Experiment 1 except for the subject’s vision of the response
lever. Subjects always saw a gloved hand on the response lever, so that
visual inputs across conditions were balanced. In the self-action condition, they saw their own hand. In the other-action condition, they
saw the experimenter’s hand, wearing an identical glove. In the machine-action condition, the subjects saw the experimenter placing a
rubber hand, wearing an identical glove, on the response lever. The lever was inserted into a slit at the tip of the index finger of the glove, so
that the index finger of the rubber hand would always move with the
lever. The posture of the hand was made comparable across conditions, with the index finger extended to contact the response lever, and
the other fingers and thumb held against the palm. The subjects’ view
of the hand extended to a point just distal to the edge of the glove, and
subjects never saw more proximal body parts. Twelve new subjects
(10 women and 2 men, all right-handed, ages between 21 and 27) participated in Experiment 2.
Results and Discussion
The mean judgment errors were 1 ms for the self-action condition,
5 ms for the other-action condition, and 19 ms for the machine-action condition (see Fig. 2, middle panel). The same planned contrasts
were performed as for Experiment 1. The integrationist hypothesis was
supported, t(11) 3.14, p .0095. The privacy hypothesis was not supported, t(11) 0.73. In addition, we performed a post hoc comparison
between the machine-action conditions of Experiments 1 and 2. This
comparison showed a significant difference, t(22) 5.39, p .0002.
The pattern of effects found in Experiment 1 were clearly replicated,
again confirming the integrationist position. Moreover, the privacy hypothesis was clearly not supported. Although Experiment 2 made the
other- and machine-action conditions more similar, which should have
favored the privacy hypothesis, it clearly could be rejected.
Interestingly, awareness in the machine-action condition was less
anticipatory in Experiment 2 than in Experiment 1. This reduced anticipation for machine displacement could reflect better balancing of
visual inputs. Alternatively, the presence of a more realistic hand in
the machine-action condition in Experiment 2 may have activated to
some extent a system for understanding biological actions. Pavani, Spence,
and Driver (2000) have suggested that subjects can attribute rubber hands
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to their own body as long as the postural arrangement of the hands is plausible. Moreover, the actions of anthropomorphic, but not nonanthropomorphic, machines can prime imitation by humans (Castiello, Lusher,
Mari, Edwards, & Humphreys, 2002). We speculate that the realistic
hand in this experiment partially evoked attributions of intentions. Other
studies have shown that viewing another person’s body parts can activate schema-level representations of the same parts of one’s own body
(Parsons, 1987, 1994; Reed & Farah, 1995). Similarly, one’s own hand
position can influence judgments about visual hand stimuli (Sirigu &
Duhamel, 2001). Future research might vary the anthropomorphic
qualities of an artificial hand parametrically and investigate corresponding changes in awareness of action.
EXPERIMENT 3
Experiments 1 and 2 supported the hypothesis of a similar time
course for the awareness of the actions of self and others. However, in
both experiments, actions were followed by an auditory effect. In principle, differences in the perceived onset time of action between conditions could arise because of differential inference of agency. Indeed,
Haggard et al. (2002) found that an action that produces an effect is
perceived to occur later than an action that does not produce an effect.
The results of Experiments 1 and 2 might be predicted if self- and othergenerated actions bind with the ensuing beep, but machine actions do
not. Experiment 3 repeated the design of Experiment 1 without any beep
after the lever press.
Method
Except for the absence of the beep, the methods were identical to
those in Experiment 1. Twelve new right-handed subjects (3 men and
9 women, ages 21–28) participated in Experiment 3.
Results and Discussion
The mean judgment errors were 26 ms for the self-action condition, 9 ms for the other-action condition, and 57 ms for the machine-action condition (see Fig. 2, bottom panel). The same planned
contrasts were performed as for Experiment 1. The integrationist hypothesis was supported, t(11) 4.12, p .0017. The privacy hypothesis was not supported, t(11) 0.70.
Hence, again, even without an auditory action effect that might have
caused a selective temporal shift of awareness in the self-action and the
other-action condition, the privacy hypothesis was rejected and the integrationist view was supported. We conclude that our results reflect a
genuine difference in the time course of action awareness rather than
differences between conditions in the strength of associations between
actions and effects.
GENERAL DISCUSSION
Our results point toward the surprising conclusion that the time
course of people’s awareness of their own actions resembles the time
course of their awareness of the actions of others. In three experiments,
the integrationist hypothesis was strongly supported, whereas the privacy hypothesis received little or no support. This suggests that people’s conscious awareness of intentions and related mental states does
not arise from a private source within their own minds. Traditionally,
the prototype of private access to mental states has been somatic proprioception (Melzack & Wall, 1982; Wittgenstein, 1953). In this view,
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conscious experience is necessarily restricted to the subject because
subjects receive somatic information only from their own body and
not from the bodies of others. Nevertheless, our results do not reflect
this differential access. We conclude that private information plays little or no part in the aspects of action awareness we studied.
Instead, our results suggest awareness of action distributes successfully across different agents. In this respect, action awareness resembles the mirror-neuron system reported in monkeys (Gallese et al.,
1996) and its apparent analogue in humans (Fadiga et al., 1995; Rizzolatti, Fadiga, Matelli, et al., 1996). Our data suggest that the neural
circuit underlying action generation and action understanding may
also participate in constructing conscious experience. Interestingly, the
same neural network has been implicated in other high-level functions,
which typically require consciousness, such as communication (Rizzolatti & Arbib, 1998).
Our results also raise the issue of whether people represent the actions of others by analogy with their own. According to this interpretation, one has private access to one’s own intentions, and these intentions
generate conscious awareness of action (Haggard & Eimer, 1999). One
can then infer the intentions of others from their actions, by analogy
with one’s own private case. This account rescues the privacy view, but
at the same time softens the concept of privacy, because other people
can infer one’s “private” intentions from one’s behavior. Moreover, if
awareness of action could arise from mental states inferred in other
people, rather than directly experienced, then one’s awareness of one’s
own action might likewise be inferential, and therefore not especially
privileged.
This analogical interpretation recalls Dennett’s (1987) concept of
intentional stance. When one adopts an intentional stance, one necessarily attributes agency to another individual. Previous philosophical
work has focused on quite high-level effects of intentional stance, such
as the way people verbally describe the behavior of others. Our experiments provide quantitative data suggesting that much more fundamental aspects of perceptual awareness are affected. Gallese has similarly
placed the ability to perceive the sensorimotor actions of others at the
foundation of social understanding (Gallese & Goldman, 1998) and social empathy (Gallese, 2001).
Work on the theory of mind (Baron-Cohen, 1995) has focused on
how people represent the beliefs of others. Our data suggest that representing the intentional actions of others may follow similar principles.
Indeed, in development, it seems likely that representation of others’
actions should precede representation of their beliefs.
Finally, our experiments repeatedly showed that the “actions” of a
nonbiological machine were perceived differently from the actions of
true biological agents, even when the actions were visually identical
(see also Shiffrar & Freyd, 1993; Stevens, Fonlupt, Shiffrar, & Decety
2000). In particular, the other-action and machine-action conditions
differed only in the subject’s concept of how the action was generated.
We suggest that subjects took the intentional stance in the other-action
condition, but not in the machine-action condition. This clearly influenced their perception of the subsequent physical event. Castiello et
al. (2002) found that observing biological actions influences with motor
performance, whereas observing mechanical actions does not. Moreover, a study by Meltzoff (1995) confirmed that such effects depend
on representing the mental states of others. Children correctly inferred
the intention of an actor to separate two parts of an object even though
he failed to achieve the intended goal: When the children imitated the
actor, they immediately separated the two parts of the object. Conversely, when a machine demonstrated kinematically similar fumbling
590
movements that likewise failed to separate the two parts of the object,
the children did not produce the target action. These data suggest that
the concept of an agent influences motor performance. We suggest that
the concept of agency also influences the construction of conscious
experience (see Knoblich, 2002, for the role of action cues in self-recognition).
One final feature of our data deserves additional comment. In three
experiments, we found that the perceived onset time of a machine action differed significantly from the perceived onset time of biological
actions. Moreover, we found that this difference consistently involved
anticipatory awareness of machine actions, with subjects claiming that
the machine actions occurred before they actually did. Although a difference between the machine-action and other-action conditions was
predicted, the negative sign of the judgment error in the machine-action
condition was not predicted, and may seem surprising. Given that subjects cannot have advance information about the movement of a machine, and appear not to attribute intentions to a machine, one might
have expected a delayed awareness in the machine-action condition.
Binding between actions and effects (Haggard et al., 2002) could explain the anticipation of machine actions in Experiments 1 and 2. Selfand other-generated actions are perceived shifted toward effects
that they cause. Our results suggest machine actions are not perceived
in this way. In Experiment 3, no beep occurred. Nevertheless, using
the response lever inevitably produced a click. The click could
have become the “effect” of the action, again allowing some intentional binding for the self- and other-action conditions, but not for the
machine-action condition. However, this remains an ad hoc explanation: In future research, we plan to vary the presence of an effect more
systematically. This may further clarify how and why the actions of
biological agents are perceived differently from the actions of nonagents. We speculate that machine actions may be perceived differently from self- and other-generated actions because machine actions
are not perceived with reference to the effects they cause.
In the present experiments, we studied the perceived onset times of
actions. Awareness of action clearly involves, in addition to subjective
time, dimensions such as force, effort, and pain. However, the proprioceptive system ensures that one has privileged access to these other dimensions. Time, in contrast, is a common metric dimension that can
be compared across all events. Our results show that differential activation of the proprioceptive system in self-generated and observed actions may mask the operation of a common mental process that integrates
these two classes of action. Moreover, this common integrating process
operates at levels of the human mind high enough to enter conscious
awareness.
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