Exp Brain Res (2006) 175: 353–362
DOI 10.1007/s00221-006-0557-9
RE SE AR CH AR TI C LE
Chia-Chin Tsai · Wen-Jui Kuo · Jung-Tai Jing
Daisy L. Hung · Ovid J.-L. Tzeng
A common coding framework in self–other interaction:
evidence from joint action task
Received: 29 November 2005 / Accepted: 10 May 2006 / Published online: 24 June 2006
© Springer-Verlag 2006
Abstract Many of our actions are inXuenced by the
social context. Traditional approach attributes the inXuence of the social context to arousal state changes in a
socially promotive way. The ideomotor approach, which
postulates common coding between perceived events and
intended actions, uses a conceptual scheme of ideomotor
compatibility to explain self–other interaction. In this
study, we recorded reaction times (RTs) and eventrelated potentials in a Go/NoGo task with stimulus–
response (S–R) compatibility arrangement to examine
how the social context aVects self–other interaction.
Although the social facilitation theory predicted that
RTs would be faster when acting together with audience
rather than acting alone, the ideomotor theory predicted
S–R compatibility eVects only for the joint condition.
The results revealed S–R compatibility on the RTs, lateralized readiness potential of the Go trials, and P3 of the
NoGo trials in the joint condition, which were in line
with the predictions of the ideomotor theory. Owing to
the anticipation of other’s actions, self and other’s
actions are internally and unintentionally coded at the
representational level and their functional equivalency
can be realized through a common coding framework
between perception and action systems. Social facilitation theory was not supported, because we found no signiWcant data diVerences depending on the setting.
C.-C. Tsai · W.-J. Kuo · J.-T. Jing · D. L. Hung · O. J.-L. Tzeng (&)
Laboratory for Cognitive Neuroscience,
National Yang-Ming University, Taipei, Taiwan
E-mail: [email protected]
Tel.: +886-2-28267242
Fax: +886-2-28204903
C.-C. Tsai · J.-T. Jing · D. L. Hung
Institute of Neuroscience,
National Yang-Ming University, Taipei, Taiwan
O. J.-L. Tzeng
Academic Sinica, Taipei, Taiwan
Keywords Self–other interaction · Joint action ·
Ideomotor theory · Social facilitation · Event-related
potentials
Introduction
Many of our actions and decisions are inXuenced by social
contexts, and even caved in social pressure (Asch 1956).
Neural mechanisms of this social interaction have been
related to the mirror neuron system, which suggested that
our action system, which specializes in detecting and
matching other’s goal-directed actions with our own repertoire, can be modulated by observing other’s actions
(Gallese et al. 1996; Rizzolatti et al. 1996; Nishitani and
Hari 2000; Grezes et al. 2003). However, being equipped
with such a system only may not be suYcient to coordinate or cooperate with others (MeltzoV and Decety 2003).
The reason is that evidence previously demonstrated was
derived from experimental environments in isolation,
instead of interactive settings. In this study, we employed a
newly invented motor cognition paradigm, the joint action
paradigm, to investigate self–other interaction with diVerent social contexts. By using this paradigm, processes of
recognition, anticipation, prediction, and interpretation of
others’ action realized by a direct link of perception and
execution can be practically examined at a representational level (Sebanz et al. 2003, 2005, 2006, in press).
Research on self–other interaction, especially for joint
action, leads to diVerent conclusions regarding its underlying mechanisms. One approach emphasizes the social
facilitation eVect (Zajonc 1965), which suggests that coacting with or the presence of others will inXuence task
performance in a general way. Subjects performed better
in simple tasks and worse in diYcult tasks in cooperation
and being observed contexts than in an individual context. This eVect was attributed to the change of arousal
states, social comparison, and cognitive distraction
(Guerin 1993). Apart from this general description, it is
not clear how self and other’s actions interact at the
354
representational level (Sebanz et al. 2003). Another
approach, i.e., ideomotor theory, emphasizes a common
coding or shared representations between the perceived
events and planned actions and the role of internal (volitional) causes of actions (Prinz 1997; Hommel et al.
2001). Its central notion is that “the perception of an
event that shares features with an event that has been
learnt to accompany, or follow from, one’s own action,
will tend to induce that action” (Greenwald 1970a, b,
1972). By this approach, interaction is taking place at a
higher or cognitive level. Given methodological advancements provided by the motor cognition paradigm, characteristics in processing and theory of this interaction
can be further addressed.
As compared to a traditional case of joint action with
language communication format (Clark 1996), the current motor paradigm referred to acting together in a
social context in a more general sense (Sebanz et al.
2006). The motor cognition paradigm is to combine a
target-detection task with stimulus–response (S–R) compatibility under diVerent social contexts, a group, or an
individual social context (Sebanz et al. 2003, in press). In
the detection task, one of the two targets (circles in red or
green) was presented with equal possibility in three spatial positions (left, middle, and right) on a computer
screen. Two response alternatives, a left and a right button press, were designated for red and green stimuli,
respectively. Therefore, two types of experimental trials
can be deWned by the spatial relationship of target and
response button position: compatible trials, in which the
spatial relationship between stimulus and response is
correspondent (i.e., when a red circle appears on the left
side and with a key response on the left side is performed), and incompatible trials, in which the spatial
relationship between stimulus and response is non-correspondent (i.e., when a red circle appears at the right side
and with a key response on the left side is performed). In
traditional cases, only a single subject performs the task
at a time with both their left and right hands for
responding to red and green targets, and typically, reaction times (RTs) on incompatible trials are considerably
slower than RTs on compatible trials. This has been
dubbed spatial S–R compatibility or Simon eVect. With
modiWcation of this S–R compatibility task, designation
of diVerent social contexts was included in a motor cognition paradigm.
In this study, three social contexts were combined
with a motor cognition paradigm to investigate self–
other interaction, especially to test the social facilitation
and the ideomotor accounts. In a joint context, two subjects were paired and instructed to respond to a red and a
green target complementarily. Because the subjects
responded to one target at a time, it became a Go/NoGo
task for each subject. Although the left-seated subject
responded to red circles, another subject on the right seat
responded to green circles. In an individual context, subjects performed the same Go/NoGo task individually. In
a being observed context, two subjects were paired as
well but only one subject performed the task with
another subject served as an observer. In the experiment,
both behavioral and electrophysiological data were
simultaneously recorded for the subjects. Event-related
potential (ERP) was important because it helps to investigate the formation of self–other interaction because it
provides information about the NoGo trials where no
overt response can be recorded at behavioral level.
Two ERP components, the N2 and the P3, are of
direct relevance to the current task. The N2 is a negative
ERP component that occurs approximately 200 ms after
stimulus onset, is maximal at the frontal sites (Falkenstein et al. 1999; Bruin and Wijers 2002), and is typically
larger on the NoGo than on the Go trials (PfeVerbaum
et al. 1985; Falkenstein et al. 1995, 1999; Kopp et al.
1996), reXecting inhibition eVects (Kok 1986; Falkenstein et al. 1999; Lavric et al. 2004). The P3 is generally
reported to have a centro-parietal maximum for the Go
trials and a fronto-centrally maximum for the NoGo trials, with latency ranging from 300 to 500 ms (PfeVerbaum et al. 1985; Falkenstein et al. 1995; Bruin and
Wijers 2002). The P3 is assumed to reXect the processing
of stimulus evaluation (Kok 2001). Previous studies indicated a consensus that the Go P3 occurs after the stimulus has been evaluated, and can occur even on the basis
of partial information. In contrast, the NoGo P3 is suggested to reXect an inhibition mechanism for action control (Falkenstein et al. 2002). Although there are some
controversies regarding the functional attributes of the
NoGo N2 and P3 components in inhibition (Bruin et al.
2001), evidence suggests that these two components are
related to diVerent aspects of inhibitory processes (Falkenstein et al. 1999, 2002). In a task similar to S–R compatibility task, in addition to N2 and P3, there is another
ERP component of interest, i.e., lateralized readiness
potential (LRP). LRP directly relates to the preparation
of motor responses in a real-time scale (Coles 1989) and
its activation changes can provide information of
response selection and preparation for comparison, especially for compatibility diVerence under diVerent social
contexts.
In a study with similar arrangement, Sebanz et al.
exploited a Go/NoGo task with individual and joint contexts. They demonstrated a self–other interaction by
revealing a compatibility eVect in the joint context but
not in the individual context (Sebanz et al. 2003, in press).
The joint compatibility eVect, an interaction of compatibility and social context, suggested that the subjects took
others’ action into consideration, i.e., action representation of self and others’ was considered as functionally
equivalents. As compared to previous studies (Sebanz
et al. 2003, in press), our study has extended issues of current interest in several ways. First, we can directly test
social facilitation against ideomotor account for social
interaction with the introduction of the being observed
condition. Second, electrophysiological indicators, i.e., N2
and P3 of the NoGo trial, help clarify how social context
can inXuence response inhibition. For example, in the
joint setting, the NoGo trials for one of the paired
subjects are the Go trials for another. Subjects might
355
generate two kinds of action representation, i.e., action
inhibition for themselves and action anticipation for others, and which might regulate N2 and P3 components.
Moreover, the third, whether social symbols are necessary
for the joint compatibility eVects can be tested using nonsocial stimuli as the response cues. In contrast to Sebanz
et al. (2003) who used social symbols, i.e., pictures of a
pointing Wnger wearing a color ring, we used color circles
as experimental stimuli to explore this question.
Taken together, current interest and arrangements led
to several predictions. First, RTs of three social contexts
would diVer. If the social facilitation account holds, RTs
in the joint and being observed conditions would be similar and faster than that of the individual condition. In
contrast, if the ideomotor theory holds, we should
observe compatibility eVects of RT only in the joint condition. Second, electrophysiological components of N2
and P3 in the NoGo trials would behave diVerently in
diVerent social contexts. If the social facilitation account
holds, because it predicts that in a simple task people
would perform better when there is audience, behaving
proWle of N2 which relates to monitoring or inhibition
process might diVer among the three social contexts.
However, if the ideomotor account holds, NoGo P3 in
joint action condition might diVer from the other two
conditions because action anticipation of other’s is engendered only in the joint context. Third, as onset of P3 component in the Go trials has been regarded as emergence of
evaluation processing, the Go P3 latency in the joint and
being observed conditions should be faster than that in
the individual condition if the social facilitation account
holds. Moreover, as LRP component is sensitive to brief
preparation for manual response transformation (Kutas
and Donchin, 1980), for ideomotor approach, which suggests that conXict might occur after the stage of stimulus
evaluation, the inXuence of social contexts would aVect
LRP rather than Go P3. Social contexts would be possible to modulate the response activation, especially
aVected the manual transformation on the non-corresponding side (incompatible trials) in the joint condition
if the ideomotor account holds.
Fig. 1 Illustration of the formation of three social contexts in
our experimental setting: left
joint Go/NoGo condition; middle being observed Go/NoGo
condition; right individual Go/
NoGo condition
Materials and methods
Subjects
Twenty-six right-handed volunteers, all of whom were students from National Yang-Ming University, participated
in this experiment (19–24 years old). Each subject was paid
for $14.28 for participation. Four data sets had to be discarded because of serious eye blink artifacts. Twenty-two
subjects’ data (ten males and 12 females) were analyzed.
Experimental setting and stimuli
Figure 1 displays the three social contexts and task settings. Subjects performed a Go/NoGo task. (1) In the joint
condition, paired subjects sat side by side and performed
the task in a complementary way. Although the left-seated
subject responded to red circles, another subject on the
right seat responded to green circles. (2) In the individual
condition, subjects performed the task alone. An empty
chair remained beside for each participant. (3) In the being
observed condition, two subjects were paired but only one
subject performed the task with another subject serving as
an observer. The two paired subjects were instructed
together and that the observer was told to observe and
learn the procedure and would covertly count the errors
committed. By doing this, we expected to make an audience eVect for the subject performing the task. The order
of conditions was counterbalanced. A rectangle
(9 £ 3.5 cm2 in width and height) with three discs horizontally arranged inside (1 cm in radius and 0.5 cm between
the discs) was presented centrally on a PC monitor. Targets were either red or green dots presented on one of the
three discs at a time. The Wxation and target extended
approximately 1.17° and 3° in height and width.
Procedure and design
Each trial started with a cue for 1,500 ms for eye blinking.
In the following, a Wxation was presented for 500 ms, and
356
then a target was presented either on the right, central, or
left disc for 100 ms. There was a time period of about
1,500 ms for responding before the next trial started. Subjects were instructed to respond to red or green target by
pressing the “shift” keys on the left or right side of the keyboard with equal emphasis on the response speed and accuracy. In consideration of S–R compatibility, one-third of
the trials were compatible arrangement, and incompatible
for another one-third of the trials. In all conditions, subjects
completed four blocks of 48 trials (including 24 Go and 24
NoGo trials) and switched seats before the third block.
ERP recording
EEG data were recorded from 64 scalp electrodes, and
vertical and horizontal EOGs were recorded for eye
movements. All channels were referenced to the linked
mastoids. EEG and EOG were recorded with a sampling
rate of 500 Hz. EEG epochs set at the range of ¡100 to
800 ms. Data were digitally Wltered with a band-pass Wlter
(1–30 Hz, 12 dB/oct). Only artifact-free trials were averaged to create ERP. Trials containing eye movement artifact, A/D saturation, or with a baseline drift exceeding
60 V in any channel, were excluded. Stimulus-locked
ERPs of electrodes C3 and C4 were used for LRP calculation. For response on the left side, ERP of C3 was subtracted from that of C4; for response on the right side,
ERP of C4 was subtracted from that of C3. After subtraction, the results were averaged and digitally Wltered (lowpass cut-oV frequency of 12 Hz). LRPs of compatible and
incompatible conditions were calculated separately.
being observed, and individual) and compatibility (compatible and incompatible) factors was conducted. SigniWcance was found for compatibility (F(1,21) = 22.99,
p < .05) and interaction (F(2,42) = 15.52, p < .05), but
not for social context (F(2,42) = 0.391, p > .05). As our
prediction, this compatibility was only signiWcant in the
joint condition. Subjects responded slower to incompatible than to compatible trials (joint: F(1,63) = 53.538,
p < .05; being observed: F(1,63) = 1.274, p > .05; individual: F(1,63) = 2.89, p > .05).
In order to diVerentiate between the facilitation and
inhibition of S–R compatibility, one procedure was further exploited. In the procedure, RT performance of the
trials with targets appearing on the central disc was
taken as a baseline to be subtracted from the RTs of
compatible and incompatible trials. The results were
divided by the baseline afterward for indication of facilitation and inhibition (Spieler et al. 1996). For the convenience of calculation, we modiWed this index by
multiplying it with 104. Table 1 also reports the indication of facilitation and inhibition in the three social contexts. A repeated-measures two-way ANOVA with the
factors social context and the facilitation/inhibition
showed that there was no signiWcant main eVect for the
facilitation/inhibition (F(1,21) = 2.69, p > .05), but there
was a signiWcant eVect for social context
(F(2,42) = 15.512, p < .05). The interaction was signiWcant (F(2,42) = 10.897, p < .05). Post hoc analysis
revealed a signiWcant diVerence in inhibition and showed
a maximum in the joint condition rather than in the
other two conditions.
Electrophysiological data
Results
Repeated-measures ANOVA was used to analyze the
behavioral and ERP data. The Greenhouse–Geisser
adjusted p values were reported when necessary, but
original degrees of freedom are given.
Behavioral results
Table 1 presents RT data of the Go trial in three social
contexts. A two-way ANOVA with social context (joint,
Averaged ERPs for compatibility on the NoGo and Go
trials in three social contexts at three electrodes are
shown in Figs. 2, 3, and 4. The P3 component was quantiWed by measuring the mean amplitude and peak latency
in the range 310–360 ms for the Go trial and 350–400 ms
for the NoGo trial (see Tables 3 and 4). The NoGo N2
was quantiWed by measuring mean amplitudes from 220
to 270 ms for the analysis of social context and compatibility (see Table 2). Time windows for compatibility
eVect on the Go and NoGo trials and the electrodes of
interest, i.e., Fz, Cz, and Pz, were selected in line with the
Table 1 Mean RT (Go trials) on compatible and incompatible trials in three social contexts
RTs (ms)
Joint
COM
Mean RT1
Mean RT2
Compatibility
EVect size
434.4 (31.9)
Being observed
IMC
449.6 (33.4)
442.0 (33.5)
15.2
Facilitation
Interference
97.785
248.453
COM
437.5 (36.1)
Individual
IMC
441.0 (33.0)
439.3 (34.6)
3.5
Facilitation
Interference
66.995
17.892
COM
IMC
437.5 (43.7)
439.9 (41.0)
438.7 (42.4)
2.4
Facilitation
Interference
37.602
20.046
COM compatible trial; IMC incompatible trial. In parentheses are the standard deviations of the mean RT on Go trials. The compatibility
eVect = RTincompatible trial ¡ RTcompatible trial. EVect size (corrected) for facilitation = [(RTcompatible ¡ RT neutral)/RTneutral] £ 104. EVect size
(corrected) for inhibition = [(RTincompatible ¡ RTneutral)/RTneutral] £ 104. The RTneutral: 438.7 ms (joint); 439.1 ms (individual); 440.4 ms (being observed)
357
previous studies with typical Go/NoGo paradigms
(PfeVerbaum et al. 1985; Falkenstein et al. 1995, 1999).
For LRP analysis, it was quantiWed by measuring mean
amplitudes from 200 to 300 ms for the Go trials and 100
to 200 ms for the NoGo trials, locked by the onset time
of stimulus (see Table 5).
NoGo N2 component
Table 2 shows the NoGo N2 amplitude (see also Fig. 2),
and a 3 £ 2 £ 3 ANOVA of amplitude with factors of
social context, compatibility, and electrode was conducted. For the electrode factor, it had a signiWcant eVect
(F(2,42) = 8.328, p < .05), and showed an anterior minimum.
Compatibility
eVect
was
signiWcant
(F(1,21) = 7.124, p < .05). No interaction was observed.
NoGo P3 component
Figure 2 also displays the grand averages for the NoGo
P3 (see also Table 3). A 3 £ 2 £ 3 ANOVA of amplitude
with factors of social context, compatibility, and electrode was conducted. First, for the electrode factor, it
had a signiWcant eVect (F(2,42) = 31.973, p < .05) and
showed an anterior-central maximum (Fz: 8.281 V, Cz:
8.811 V, Pz: 5.075 V). This was in line with the wellestablished Wnding of an anteriorization of topography
of the scalp sites on the NoGo trials (PfeVerbaum et al.
1985; Falkenstein et al. 1995, 1999; Fallgatter and Strik
1999; Bokura et al. 2001). Second, the main eVect of
social context (F(2,42) = 7.373, p < .05) was signiWcant.
Post hoc analysis showed that NoGo P3 amplitude was
larger in the joint condition rather than other two conditions. Third, the three-way interaction (F(4,84) = 3.663,
p < .05) and two-way interaction between social context
and compatibility at Fz electrode (F(2,126) = 4.023,
p < .05) were also signiWcant. At Fz electrode, a signiWcant compatibility eVect was found only in the joint condition rather than other two conditions (joint:
F(1,189) = 14.934,
p < .05;
being
observed:
F(1,189) = 0.482, p > .05; individual: F(1,189) = 0.283,
p > .05). In the joint condition, the NoGo P3 amplitude
was larger on incompatible than on compatible trials.
For the analysis of peak latency, there was only a main
eVect for electrode (F(2,42) = 6.203, p < .05). Post hoc
analysis showed that the latency of NoGo P3 is faster at
anterior-central electrodes (Fz: 361.2 ms, Cz: 360.3 ms,
Pz: 366.4 ms).
Table 2 Electrophysiological results of the compatibility eVect (compatible vs. incompatible trial) of NoGo N2 on mean amplitude (V)
and peak latencies (ms) at three electrode sites (Fz, Cz, Pz) in three social contexts
Compatibility eVect of mean amplitudes of NoGo N2 (time window: 220–270 ms)
Joint
Fz
Cz
Pz
Being observed
Individual
Compatible
Incompatible
Compatible
Incompatible
Compatible
Incompatible
1.546 (2.94)
3.081 (3.22)
2.042 (3.16)
3.009 (2.64)
4.672 (3.10)
3.064 (3.13)
2.496 (2.77)
4.307 (3.74)
3.139 (4.05)
2.801 (3.17)
4.492 (3.79)
3.126 (3.75)
2.117 (2.40)
3.670 (3.20)
2.574 (3.36)
2.367 (2.90)
4.130 (3.61)
3.186 (3.62)
In parentheses are the standard deviations of the mean on NoGo N2
Fig. 2 ERP waveforms associated with compatibility eVect in
three diVerent social contexts
for NoGo stimulus at three electrodes: left joint NoGo, middle
being observed NoGo, right
individual NoGo; upper Fz, middle Cz, lower Pz; black line compatible trial, grey line
incompatible trial
358
Table 3 Electrophysiological results of the compatibility eVect (compatible vs. incompatible trial) of NoGo P3 on mean amplitude (V) and
peak latencies (ms) at three electrode sites (Fz, Cz, Pz) in three social contexts
Joint
Compatible
Being observed
Incompatible
Compatible
Individual
Incompatible
Compatible
Incompatible
7.770 (4.18)
8.188 (3.96)
4.306 (2.95)
7.991 (4.78)
8.478 (4.43)
4.603 (2.83)
Compatibility eVect of mean amplitudes of NoGo P3 (time window: 350–400 ms)
Fz
Cz
Pz
8.336 (3.48)
9.051 (3.41)
5.820 (2.69)
9.934 (4.38)
10.335 (3.87)
5.967 (2.80)
7.686 (3.63)
8.296 (3.78)
4.945 (2.70)
7.973 (3.55)
8.517 (3.35)
4.812 (2.33)
Compatibility eVect of peak latencies of NoGo P3 (time window: 350–400 ms)
Fz
Cz
Pz
360.2 (14.3)
359.2 (12.1)
364.6 (13.8)
361.5 (9.7)
360.6 (10.0)
366.8 (14.5)
360.9 (13.0)
358.4 (12.6)
362.1 (15.6)
363.1 (14.2)
358.1 (12.2)
364.6 (17.1)
361.5 (13.6)
363.5 (14.2)
372.5 (17.1)
359.9 (13.1)
362.3 (15.2)
368.0 (16.0)
In parentheses are the standard deviations of the mean on NoGo P3
Go P3 component
Figure 3 presents the Go P3 proWles at three representative
electrodes (Fz, Cz, Pz) in the three social contexts (see also
Table 4). A 3 £ 2 £ 3 ANOVA of amplitude with factors of
social context, compatibility, and electrode was conducted.
P3 amplitudes diVered across electrodes (F(2,42) = 20.609,
p < .05) and showed an anterior minimum. Interaction
between social context and compatibility was signiWcant
(F(2, 42) = 3.472, p < .05). For the analysis of peak latencies,
the main eVect for compatibility (F(1,21) = 8.434, p < .05)
and electrode (F(2,42) = 9.837, p < .05) was signiWcant. Post
hoc analysis showed that Go P3 peak latency was faster at
Pz than at Fz (Fz: 334.7 ms, Cz: 330.1 ms, Pz: 326.1 ms). No
signiWcance was found further.
LRP component for the Go and NoGo trials
Figure 4 presents the LRP data in the three social contexts separately on the Go and NoGo, compatible and
incompatible trials (see also Table 5). For the Go
Fig. 3 ERP waveforms associated with compatibility eVect in
three diVerent social contexts
for Go stimulus at three electrode sites: left joint Go, middle
being observed Go, right Individual Go; upper Fz, middle Cz,
lower Pz; black line compatible
trial, grey line incompatible trial
response of the incompatible trials, LRPs had a positive
deXection Wrst and a delayed negative deXection later
due to a conXict between spatial and response dimensions (Gratton et al. 1988). Thus, a one-way ANOVA
with factor of social context was conducted separately
for compatible and incompatible trials. For compatible
trials, the eVect of social context (F(2,42) = 0.037, p > .05)
was not signiWcant during the negative deXection (200–
300 ms). For incompatible trials, the negative deXection
(200–300 ms) was modulated by social context
(F(2,42) = 5.425, p < .05). Post hoc analysis showed a
more negative potential in the joint condition rather than
other two conditions. Positive deXection (125–175 ms)
was not aVected by social context (F(2,42) = 0.024,
p > .05). For the NoGo response, incompatible trials
also demonstrated a similar positive deXection. A oneway ANOVA with factor of social context was conducted separately for compatible and incompatible trials
but only the positive deXection was tested on incompatible trials. For compatible trials, the negative deXection
(100–200 ms) was modulated by social context
359
Table 4 Electrophysiological results of the compatibility eVect (compatible vs. incompatible trial) of Go P3 on mean amplitude (V) and
peak latencies (ms) at three electrode sites (Fz, Cz, Pz) in three social contexts
Joint
Compatible
Being observed
Incompatible
Compatible
Individual
Incompatible
Compatible
Incompatible
4.582 (2.83)
8.342 (3.32)
6.863 (2.69)
5.432 (3.23)
9.115 (3.88)
6.828 (2.71)
5.618 (3.65)
9.441 (4.03)
6.978 (2.73)
Compatibility eVect of mean amplitudes of Go P3 (time window: 310–360 ms)
Fz
Cz
Pz
5.720 (2.83)
8.574 (3.37)
6.964 (2.80)
6.024 (2.64)
9.133 (3.12)
7.750 (2.58)
5.471 (3.01)
8.875 (3.92)
6.932 (3.11)
Compatibility eVect of peak latencies of Go P3 (time window: 310–360 ms)
Fz
Cz
Pz
328.6 (19.2)
326.7 (17.0)
323.3 (15.4)
334.2 (14.2)
331.6 (15.8)
328.9 (15.2)
333.7 (17.2)
330.0 (15.6)
325.0 (13.7)
337.0 (17.8)
333.6 (16.4)
328.9 (15.2)
336.2 (17.9)
327.8 (17.1)
324.6 (15.2)
338.7 (17.4)
330.9 (18.5)
326.1 (15.6)
In parentheses are the standard deviations of the mean on Go-P3
Behavioral evidence
Fig. 4 LRP waveforms associated with compatibility eVect in three
diVerent social contexts for Go and NoGo stimulus: left Go trial,
right NoGo trial; upper compatible trial, lower incompatible trial
(F(2,42) = 5.411, p < .05). Post hoc analysis showed a
more negative potential in the joint condition rather than
other two conditions. On incompatible trial, there is no
social context diVerence (F(2,42) = 0.697, p > .05) for the
positive reXection (100–200 ms).
Discussion
By using a motor cognition paradigm with diVerent
social contexts, while behavioral S–R compatibility eVect
was shown only in the joint context, electrophysiological
changes in response to the S–R correspondence was also
found only in the joint context. Both behavioral and
electrophysiological evidence beneWt the idea that the
individual will represent and anticipate others’ actions
when acting together. In general, the ideomotor theory
but not social facilitation theory gains support from our
behavioral and electrophysiological Wndings.
RT compatibility eVect was shown in the joint condition
rather than in the individual or being observed condition, which indicated that acting together or alone
caused diVerence in action planning, i.e., stimulus evaluation or response selection of the task. This observation
lent little support to the social facilitation account
because the subjects behaved similarly regardless of the
being observed or individual conditions. However, one
might concerns, when taking a close look at the being
observed condition, there seemed to be a facilitation
eVect in the being observed condition, and which might
favor the social facilitation account. Even so, the social
facilitation eVect is very limited as compared to the joint
compatibility eVect.
In contrast, the ideomotor theory gains support from
current Wndings. Compatibility eVects revealed in the
joint condition indicated that other’ action representation was taken into account. That is, in the incompatible
trials of joint condition, subjects responded slower due to
a conXict between the two competing action codes from
relevant response and irrelevant stimulus dimensions.
This conXict did not exist in the other two conditions
(Fig. 5). Especially, the audience in the being observed
condition was not enough to activate the common coding system of self–other interaction. Our data were in line
with the Wndings of Sebanz et al. (2003), and which suggested that when acting together, the self–other interaction can be realized through a common coding
framework between perception and action systems. The
joint compatibility eVect in a Go/NoGo task suggested
that self and others’ actions can be represented as the
functional equivalents (Sebanz et al. 2003).
From the ideomotor point of view, the compatibility
eVect can be explained in terms of dimensional overlap
between irrelevant stimulus and response dimensions
(Kornblum et al. 1990; Kornblum and Lee 1995; Prinz
1997). In this study, although the relevant stimulus (i.e.,
color of the stimulus) is processed via a controlled route
to activate the correct response, the irrelevant stimulus
360
Table 5 Electrophysiological results of Go/NoGo LRP on compatible and incompatible trials in three social contexts
Go LRP (200–300 ms)
Compatible
Incompatible
NoGo LRP (100–200 ms)
Joint
Being observed
Individual
Joint
Being observed
Individual
¡2.510 (2.21)
¡3.220 (2.07)
¡2.104 (2.26)
¡2.186 (2.10)
¡2.071 (2.16)
¡2.105 (2.17)
¡2.097 (1.70)
1.192 (1.57)
¡1.352 (1.29)
0.890 (1.67)
¡1.280 (1.11)
0.863 (1.64)
In parentheses are the standard deviations of the LRPs on Go and NoGo trials
Fig. 5 Illustration of direct link and task demand associated with
three social settings and compatibility situations: upper joint Go/
NoGo condition; middle being observed Go/NoGo condition; lower
individual Go/NoGo condition
dimension (i.e., the location of the stimulus) can be activated via an automatic route. Thus, a slower RT on incompatible trials is due to a conXict between the irrelevant
stimulus dimension and the response dimension. Therefore,
this theory also proposed that the locus of interference
might occur after identiWcation of relevant dimension, and
possibly up to response selection stage (Kornblum et al.
1990; Masaki et al. 2000; Valle-Inclan 1996).
Electrophysiological evidence in the NoGo trials
In a NoGo trial, the Wnal action plan of a subject is to
inhibit his motor response. However, when a NoGo trial
in a joint context, it will serve as a Go trial for another
subject as well, which will lead one to anticipate other’s
action. Therefore, in a joint NoGo trial, a control for
inhibiting self and anticipating other’s actions will be
necessitated. As there is no behavioral response in the
NoGo trials, our electrophysiological data help to clarify
these processing proWles. First, we found that NoGo P3
amplitude in the joint condition varied from the other
two conditions, whereas the NoGo N2 did not change
among the three social contexts. As suggested by the
physiological meaning of NoGo N2 and P3 discussed
aforementioned (Falkenstein et al. 1999, 2002), these
results together indicated that action inhibition (NoGo
N2) occurred in the three social contexts equally, but
action anticipation (NoGo P3) revealed only in the joint
situation and caused more control processing for action
monitoring. Bokura et al. (2005) also suggested that
NoGo P3 rather than NoGo N2 might reXect executive
function including expectation and planning by observing decreased NoGo P3 amplitude in patients with Parkinson’s disease.
Additional action control in the joint condition was
perfectly in justiWcation of the ideomotor theory. When
perceiving a stimulus requiring an action from the coactor, it would also awaken related action representation
in the self. NoGo trials, therefore, not only induced
response inhibition but also action anticipation. In order
to suppress the increased activation following anticipation
of the other’s action, additional action control in joint
condition was engendered. We might anticipate the coactor’s action representation or plan in terms of the taskspeciWc relationship he faced (Sebanz et al. 2005). Accordingly, S–R compatibility on NoGo trials was reXected in
the electrophysiological response, i.e., P3 amplitude. For
example, when a green stimulus appeared on the right side
(incompatible NoGo trial), not only irrelevant stimulus
dimension (RIGHT) and related task demand (GREEN is
the NoGo response for self) were processed, but also
anticipation toward other’s actions activated automatically (now she/he has to respond to GREEN stimulus with
a RIGHT key). Thus, larger NoGo P3 amplitude on
incompatible trials might result from the reconciliation
between action anticipation and response inhibition.
As noted, in a joint action the individuals need to
devote more eVort than acting alone. Given that the mirror system is ego-centered and does not imply another
agent in one’s own action plan (Knoblich and Jordan
2002, 2003), one needs another group-centered coordination system to represent joint action and to modulate
one’s action plan.
361
Electrophysiological evidence in the Go trials
In general, no signiWcant compatibility and social context
eVect were found in the Go-P3 amplitude, although the
compatibility eVect in joint condition seemed to be larger
than other two conditions (Table 4). In a further analysis
suggested by interaction between social context and compatibility, we only found nearly signiWcant compatibility
eVect in the joint condition (Joint: F(1,63) = 3.378,
p = 0.07). Changes of Go P3 amplitude have been related
to the loading of stimulus evaluation, and which would
occur prior to action selection and preparation (Kok
2001). No diVerence was found in this study suggesting
that the subjects were equally loaded with stimulus evaluation for the Go trials under diVerent social contexts. This
Wts with the hypothesis that the interference eVect in an
S–R compatibility task occurs at the response selection
stage instead of evaluation (Kornblum et al. 1990;
Masaki et al. 2000). Our results were partly not consistent
with the Wndings of Sebanz et al. (in press). In their Wndings, they found reduced P3 amplitude on incompatible
trials at the posterior electrodes in both the group and
individual conditions, and a smaller positivity for incompatible Go trials at anterior electrode in the group condition than in the individual condition. This diVerence
might result from the cuing stimuli of diVerent domains,
i.e., social vs. non-social symbols.
Information derived from P3 latency of the Go trial is
more controversial. Some studies reported that P3
latency correlates with stimulus evaluation rather than
response selection and is insensitive to S–R compatibility
(McCarthy and Donchin 1981; Magliero et al. 1984;
Smulders et al. 1995). Some studies reported longer
latency for P3 component and slower RT to the incompatible stimuli, suggesting a locus of interference at
response selection (Ragot and Renault 1981; Ragot
1984). Our results were consistent with the later cases.
However, the present ERP data were not enough for
claiming that the locus of interference on a joint action
only occurs at the stage of response selection due to the
lack of rule or other manipulation in the experiment.
More research is necessitated, and one of our unpublished studies does support this possibility.
Electrophysiological evidence of LRP
LRP is an index for motor preparation and it provides
another way to investigate self–other interaction. As
ideomotor theory suggests that perceiving other’s action
will evoke a corresponding motor activation at representation level, modulation of social context on LRP should
be possible to reXect response priming and transformation. In this study, LRP from compatible trials of NoGo
response and incompatible trials of Go responses were
signiWcantly modulated by the social context. In a joint
context, it behaved distinctly to other contexts, and it
might indicate a priming eVect of cortical response in
response to other’s action. The Wndings supported the
ideomotor approach. When taking P3, LRP, and RTs for
the Go trials, compatibility eVects on RTs and LRP (sensitive to the stage of response selection) but not on P3
(sensitive to the stage of stimulus evaluation) suggested
that conXict in a joint action in this case might occur at
the stage after the stimulus evaluation, probably at the
stage of response selection.
A common coding framework for action perception and
execution in a joint context
Our Wndings support the idea that self–other interaction
can be realized through a common coding framework
between perception and action systems (Hommel et al.
2001), in which one’s own and others’ actions are represented in a functional equivalent way. Three current Wndings support this claim. First, behavioral compatibility
eVects in the joint condition suggested that action perception and action execution share a common coding at
the representation level and thus are commensurate
(Hommel et al. 2001). Second, the NoGo P3 Wndings
demonstrate that action anticipation causes a speciWc
demand and play a crucial role in joint action. Finally,
subjects might be forced to form a relevant response set
for self and others’ actions. When there is a conXict
between responses for relevant and irrelevant stimulus
dimensions, subjects have to inhibit responses for irrelevant dimension and select responses for relevant information dimension. Accordingly, the Simon-like
interference takes place at a response selection stage, or
at least after the stage of identifying stimuli of relevance.
It might be the reason that no compatibility eVect in the
Go P3 was found in our study.
As a concluding remark, we would like to emphasize
that the ideomotor approach not only provides a new
and appropriate framework to understand joint action
and social perception theoretically, but also allows us to
examine these phenomena empirically. In this study, evidence from behavioral and electrophysiological data
suggested a common representation framework for self–
other interaction. Many relevant issues are in need of
being explored, e.g., the role of agency (doing joint action
with an agent or with a computer program). Moreover, it
is also useful to explore issues of joint action with neuroimaging tools, e.g., fMRI, to clarify what the neural networks underpinning this common framework.
Acknowledgments This research was conducted in the Laboratory
for Cognitive Neuroscience and was partly supported by Academic
Sinica, National Science Council (NSC 94-2572-H-010-002-PAE),
and the Tzong Jwo Jang Educational Foundation of Taiwan. We
also thank Shin-Mai Sun for her help in collecting the ERP data.
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