ORIGINAL ARTICLE

Sara De Maeght Æ Wolfgang Prinz

Action induction through action observation

Accepted: 11 August 2003 / Published online: 3 February 2004� Springer-Verlag 2004

Abstract Ideomotor movements may arise in individualswhile they watch goal-directed actions or events. In aprevious study we developed a paradigm for investigat-ing ideomotor movements induced through watching theoutcome of one’s own action. In the present study weextended the paradigm to investigate both movementsinduced through watching the outcome of one’s own aswell as somebody else’s action (player mode and ob-server mode respectively). We report three experiments,each with differing conditions for the player mode, butidentical conditions for the observer mode. Resultsindicate that in both modes ideomotor movements aregoverned by two basic principles: Perceptual inductionand intentional induction. In the player mode we repli-cated and extended previous findings, indicating disso-ciation between hand and head movements. In theobserver mode no such dissociation was obtained. Ourfindings suggest that people perform, in their own ac-tions, what they see being performed in other people’sactions. Induction of action through observation canpertain to both the action’s physical surface andunderlying intentions. Furthermore, our results suggestthat perceptual induction is ubiquitous but may be lo-cally suspended for intentional action control. We dis-cuss our results in the framework of theories invoking astrong overlap between representational structures foraction perception and action planning.

Introduction

Ideomotor movements may arise in observers while theywatch others perform certain actions (cf., e.g., Katz,1960; Prinz, 1987; Knuf, Aschersleben, & Prinz, 2001).Typically, such movements tend to occur involuntarily,

sometimes even countervoluntarily. For instance, whilewatching a soccer match on TV, many individuals can-not help but move around in their armchairs—particu-larly when watching dramatic scenes in which theirhome team is facing a serious threat. The same applies tosituations in which individuals watch certain conse-quences of someone else’s actions rather than the actionsthemselves. For example, while sitting in the passengerseat of a car, many people cannot help but push phan-tom brakes when the driver approaches, say, a bend tooquickly.

Ideomotor movements are commonplace; most peo-ple are well familiar with them. Still, it is fair to say thattheir functional basis is as yet poorly understood (forpossible reasons, cf. Prinz, 1987; Knuf et al., 2001). Herewe address the following research question: Given thatideomotor movements occur in an observer in responseto a scene s/he is watching—in what way exactly arethese movements related to the pattern of events in thescene? This article is a follow-up to a previous study inwhich we raised the same basic question (Knuf et al.,2001). In that study we used billiard-like and bowling-like tasks: Participants initially ‘‘kicked’’ a ball toward atarget and then tracked its course. We analyzed invol-untary movements occurring to them while watching theconsequences of their self-performed initial kicks, andtried to find out how these movements were related tothe pattern of events in the scene being watched. Thoughwe were able to identify some basic principles governingideomotor actions in the tasks (see below), the general-izability of these results was clearly limited. So far theseprinciples had been derived from and demonstrated forthe special case in which ideomotor action was inducedby watching the consequences of one’s own precedinginstrumental action. At this point we have no way ofknowing whether these principles also apply to the moregeneral case of watching (the consequences of) otherindividuals’ actions, which is usually considered to bethe prototype absolute of ideomotor action.

In the present study we extend our previous paradigmso that it allowed us to study ideomotor movements not

S. De Maeght Æ W. Prinz (&)Max Planck Institute for Psychological Research,Amalienstrasse 33, 80799 Munich, GermanyE-mail: prinz@psy.mpg.de

Psychological Research (2004) 68: 97–114DOI 10.1007/s00426-003-0148-3

only in active players (i.e., individuals also involved inthe initial instrumental action), but also in passiveobservers (i.e., individuals merely involved in watchingsomebody else’s action). How, then, are ideomotormovements in observers related to what they observe?And how do (passive) observers differ from (active)players in their patterns of induced movements?

Principles of ideomotor action

How can ideomotor action be explained? How is itpossible that observing a certain action performed bysomebody else induces in the observer a tendency toperform an action that is somehow related? This ques-tion can be raised at two levels—empirical generaliza-tions and theoretical mechanisms. Firstly, how exactly isthe performed action related to the induced action?Which aspects or features of the action being observedare actually taken up? And how are they mapped ontoaspects or features of the action being performed? Thisquestion requires empirical generalizations, or principlesof ideomotor action. Secondly, what kinds of processingmechanisms and representational structures can accountfor the involuntary occurrence of ideomotor induction?This question requires functional architectures suited tosupport the operation of those principles. We touchbriefly on principles in the present section and comeback to mechanisms in the General discussion section.

Overviews of the colorful history of major ideas re-lated to ideomotor action have been presented elsewhere(Knuf et al., 2001; Prinz, 1987; Prinz, De Maeght, &Knuf, in press), so that we will only summarize somemajor lines of thought here. Much of the debate on in-duction principles has centered around William James’famous ideomotor principle of voluntary action,according to which ‘‘every representation of a movementawakens in some degree a movement which is its object’’(James, 1890, Vol. II, p. 526). Although this principlewas primarily meant to account for internally induced,voluntary actions, it can likewise be applied to externallyinduced, ideomotor actions. Here, one has to take thenotion of representation in its broad sense, referring notonly to anticipatory representations of one’s own plan-ned action, but also to perceptual representations ofother individuals’ ongoing action. Hence, perceiving acertain action in somebody else tends to stimulate in theperceiver a tendency to perform the movement himselfthat accords with the movement being observed.

How do these movements accord with each other?For classical authorities like Lotze (1852), Carpenter(1874), and James (1890) it was natural to believe thatthe bridge between the perceived action (in the other)and the action being performed (by oneself) is formed bythe principle of similarity. With this view, ideomotoraction is a special form of imitation. Observers tend torepeat in their own actions what they observe othersperforming in theirs. We refer to this view as perceptualinduction (Knuf et al., 2001). According to perceptual

induction, observers tend to perform the same actionsthat they see in the scene. However, in a number ofexamples, anecdotal evidence from introspection seemsto suggest otherwise (e.g., Chevreul, 1833). People oftenfeel they act in an ‘‘as-if’’ mode—as if they could affectthe course of ongoing events in a particular, desireddirection. According to this view, ideomotor actions areidle-running intentional actions. Observers tend to per-form actions that—should they be effective—would leadto the desired results in the observed scene. We refer tothis view as intentional induction (Knuf et al., 2001),according to which observers tend to perform actionsthey would like to see happening in the scene. Evidencefrom our previous study suggests that these principlesare both functional in movements induced by watchingthe outcome of one’s own instrumental action. In thepresent study we wish to explore whether these princi-ples are likewise effective when individuals watch theoutcome of other individuals’ actions.

Paradigms for studying ideomotor action

In our previous study we used a billiard-like task and abowling-like task in order to study ideomotor actioninduced by watching the consequences of one’s owninitial actions (Knuf et al., 2001, Experiments 1 and 2/3,respectively). In the present study we combine thatbowling task with a new tracking task that allows us tostudy ideomotor action induced by watching the con-sequences of somebody else’s actions.

The bowling task was a computer-animated game inwhich participants saw, on each trial, a ball travelingtoward a target. Total traveling time was about 3 s, andball trajectories were preprogrammed so that the ballwould always miss the target if participants did notintervene. However, such corrective interventions couldonly be made in the initial period of the ball’s travel(instrumental period, about 1 s). Corrective interven-tions were horizontal shifts to the left or right that wereeither applied to the traveling ball or to the stationarytarget, depending on the condition (ball vs. target).These corrective shifts were performed by horizontaljoystick movements. Instructions were given so thatparticipants were motivated to maximize their hit rates,a goal they could only achieve by exploiting the possi-bility of early corrections as much as possible.

Ideomotor movements were studied in the inductionperiod of the trial that followed the instrumental periodand lasted for about 2 s. During the induction period thejoystick was no longer effective. Hence, participantscould only watch the consequences of their immediatelypreceding intentional action. Visible action consequencesdiffered in three respects. Firstly, the ball could eithertravel in a northeastern or in a northwestern direction(direction of ball motion). Secondly, the event on thescreen could entail an upcoming hit or an upcoming miss(trial outcome). Thirdly, in the case of a miss, the ballcould pass by the target on its left-hand or right-hand

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side (side of target miss). While participants watched oneof these events that resulted from their preceding inter-vention, we recorded the spontaneous movements madeby their hands, heads, and feet. Hands (which remainedattached to the joystick during the induction period) wereinstrumental effectors (in the sense of having effectuatedthe initial corrections), whereas heads and feet were non-instrumental throughout. This paradigm allowed us toseparate the contributions of perceptual and intentionalinduction to ideomotor action. Perceptual induction isindicated to the extent that the induced movementsmirror the direction of ball motion. Conversely, inten-tional induction is indicated to the extent that themovements reflect participants’ actual goal states (which,in turn, depend on condition—ball vs. target—andshould certainly be different for upcoming hits and mis-ses; for details, see Knuf et al., 2001).

Results indicated that perceptual and intentionalinduction were both effective in this paradigm.Remarkably, however, the pattern of induced action wasdifferent for instrumental and non-instrumental effec-tors. In (instrumental) hands, there was no indication ofperceptual induction, whereas intentional induction waspronounced. Induced hand movements were guided byball-related intentions in the ball condition and by tar-get-related intentions in the target condition. In (non-instrumental) head and foot movements there was evi-dence of both perceptual and intentional induction.Surprisingly, however, for these effectors, intentionalinduction turned out to be independent of task condition(ball vs. target). Instead, hand and foot movements wereguided by ball-related intentions in both conditions.

Clearly, these findings are limited to ideomotormovements that occur in immediate succession to one’sown intentionally guided instrumental action. Partici-pants first make their corrective movements and thenwatch the consequences of their intervention; first, theyplay the game, and then they observe the outcome. In thisregard, the difference between instrumental and non-instrumental effectors may be telling. The fact thatintentional induction was task-dependent in hands (buttask-independent in heads and feet) seems to support astrong after-effect of the initial instrumental act on theensuing induced action. As to hands, instrumental andinduced actions are both carried out by the same effector.Therefore, the observed pattern of results may, at leastpartly, be due to intentional inertia, i.e., some carry-overof initial intentions from the instrumental to the inductionperiod.

The tracking task used in the present study was de-signed for two related purposes. The general purposewas to study the more prototypical case of ideomotoraction, i.e., action induced by watching somebody else’sactions and/or their consequences. The more specificpurpose was to create a situation in which intentionalinertia can play no role—so that induction owing toobservation (of others’ actions) can be dissociated frominduction owing to after-effects of intentions (underlyingone’s own previous actions).

For our new paradigm we created a situation inwhich ideomotor movements can be studied in a pureobserver mode, i.e., while participants observe eventconfigurations generated by somebody else. For theevents to be observed we used the same patterns as in theprevious study, i.e., the various patterns of hits andmisses that can be generated in the bowling game. Thistime, however, while watching these events, participantsperformed a simple tracking task, which served as ameans for recording induced movements. They wererequired to move a joystick up and down along with thevertical position of the traveling ball. Accordingly,instructions for the tracking task had to emphasize thevertical dimension. However, in analyzing its results, wedid not analyze performance on the (relevant) verticaldimension. Instead, we focused on the (irrelevant) hor-izontal dimension. If action induction also occurred inthe pure observer mode, it should show in systematicdrifts of the joystick on the horizontal dimension (fordetails, see the Method section of Experiment 1).

The tracking task differs from the bowling task inthree major respects. Firstly, because the tracking taskrequires the participant to monitor somebody else’sperformance, any ideomotor movement arising in theobserver’s performance should be free from short-termintentional inertia, i.e., from after-effects of a previousself-performed intentional action. Therefore, inducedmovements should exhibit weaker intentional inductionin observers’ tracking than in players’ bowling.1 Sec-ondly, since the tracking task requires the ball to betracked, it requires attention to be focused on the ballthroughout. This differs from the bowling task, whichallows attention to be focused on the ball in the ballcondition, and on the target in the target condition.Therefore, conflicting attentional demands may arise inthe target condition of the tracking task, but not in theball condition. Thirdly, and related to this, we may ex-pect that in this task ball-related movement inductionwill be stronger than target-related induction through-out. This should support both perceptual induction(based on the ball’s manifest motion) and ball-relatedintentional induction (based on the ball’s implied latentgoal).

Overview

We ran three experiments on the combined bowling-and-tracking task, in which participants observed andtracked another individual’s performance in the bowlingtask. In the first two experiments they first played the

1It may also be expected that ideomotor movements would be, ingeneral, weaker and/or less frequent in monitoring the actions ofothers vs. self-performed actions (tracking vs. bowling). However,since the two tasks are quite different in their basic functionalrequirements, we believe that comparisons of absolute magnitudesof induced movements (i.e., main effects) do not make much sense.However, this argument does not affect comparisons of magnitudesof differences (i.e., interactions).

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bowling game themselves (part 1) before tracking an-other (alleged) individual’s performance in that game(part 2). Accordingly, these experiments should allow usto assess ideomotor actions in both tasks. Part 1 ofExperiment 1 was an exact replication of Experiment 3from our previous study (Knuf et al., 2001) and studiedperformance in the bowling task, whereas part 2 studiedtracking performance in the combined task. In Experi-ment 2, participants learned to play the bowling gamewith an ‘‘unnatural’’ (i.e., spatially incompatible) rulefor the mapping of joystick movements onto ball andtarget movements. This allowed the conflict between‘‘natural’’ and ‘‘unnatural’’ mappings in induced move-ments in both the bowling and the tracking task to bestudied. Finally, in Experiment 3, participants startedwith the tracking task right away, i.e., without any priorexperience of playing the game.

Experiment 1

In this experiment we combined the bowling and thetracking task in order to study ideomotor movements inobservers. Participants took part in two experimentalsessions, one devoted to the ball and one to the targetcondition. Within each session, the bowling task (playermode) was performed first, and the tracking task

(observer mode) second. The tracking task was actuallya combination of:

1. Watching somebody else’s performance on thebowling task

2. Tracking the vertical position of the traveling ball

The layout of the screen in the two tasks is outlined inFig. 1.

The goal of the experiment was to study the contri-butions of perceptual and intentional induction to boththe bowling task and the tracking task. With regard tothe bowling task, the present experiment was an exactreplication of Experiment 3 in the Knuf et al. (2001)study. Results from this part should allow us to assessthe reliability of the findings reported there. However,the main focus of the experiment was on ideomotormovements arising in the tracking task. Our central re-search question was to what extent the contributions ofperceptual and intentional induction were the same ordifferent in players’ bowling vs. observers’ tracking.

In order to differentiate between the two inductionprinciples, two factors were manipulated independently,direction of ball motion (left-/rightward) and side oftarget miss (left/right). Perceptual induction predictsthat movements performed during the induction periodshould always point in the same direction as the ballmoves (to the left when the ball travels leftward, to theright when it travels rightward). Intentional inductionpredicts that movements should be induced in cases ofupcoming misses, but not with upcoming hits. This isbecause it is only for misses that corrective intentionsshould be formed. In cases of upcoming misses theseintentions (and the movements they induce) should de-pend on two factors, the object under instrumentalcontrol (ball vs. target) and the side of the expectedtarget miss. In the ball condition, induced movements

Fig. 1 Illustration of a, b the bowling task and c, d the trackingtask. Upper panels (a, c) show the ball condition, lower panels (b, d)the target condition. Effects of joystick movements are indicated byarrows on the screen. In the bowling task joystick movements areonly effective in the instrumental period. In the tracking taskvertical joystick movements control a marker on the right-handmargin on the screen (visible in practice trials, invisible inexperimental trials)

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should reflect the movements required to push the balltoward the target (i.e., rightward in the case of anupcoming miss on the left-hand side, leftward in the caseof an upcoming miss on the right-hand side). Con-versely, in the target condition, induced movementsshould reflect the movements required to push the targettoward the ball (i.e., leftward with left misses andrightward with right misses).

Action induction in the observer mode should becritically dependent on the extent to which participantsunderstand the game they observe and share the player’sintentions. Observers’ understanding was ensured byhaving them first play the game themselves. Sharingintentions was ensured by paying them on a schedulethat depended on both their own performance attracking and the (alleged) player’s performance atbowling.

As discussed in the Introduction section, we expectedtwo major differences in the pattern of ideomotormovements induced in the player vs. observer mode.Firstly, since short-term intentional inertia was lacking,we anticipated weaker contributions of intentionalinduction in observers’ tracking than in players’ bowling.Secondly, since the tracking task required attention to befocused on the ball, we anticipated that in this task theball would act as a more potent movement-inducer thanthe target. This should show in both perceptual induction(based on the ball’s manifest motion) and ball-relatedintentional induction (based on the ball’s implied goal).

Method

Participants

Twelve students (6 male and 6 female) of the University of Munichwith a mean age of 23.3 years were paid to participate. Elevenparticipants had a right-hand preference, one had no hand pref-erence. All were requested to hold the joystick with their righthands. All were blind to the aim of the experiment.

Apparatus and stimuli

The experiment was run on an IBM-compatible (586) laboratorycomputer with a 17’’ monitor (EIZO 9080i-M). Movements wereinduced by shifting ball-like patterns with every retrace of themonitor (refresh rate 70 Hz). The graphic resolution was 640 · 350pixels, and the display dimensions were adjusted such that a 20 · 15pixel map constituted a circular presentation of the ball-like ob-jects. Viewing distance was 160 cm, resulting in a display size of10.27� · 7.48� (degrees of visual angle) and a diameter of 0.0044�(for ball and target). All further settings were the same as in Knufet al. (2001). A special joystick was constructed on the basis of amodified computer tracking ball (Qtronix Libra 90 Pro). A com-pact metal lever with a spherical knob as a handle (overall length65 mm) was mounted on the tracking ball. The resulting (ratherdelicate) ball-joint construction had one advantage in that it al-lowed nearly friction-free lever movement.

Head and hand movements were registered with an opticalmotion-measurement system (Optotrak 3D, Northern Digital Inc.,Waterloo, ON, Canada). This noncontact motion-measurementsystem tracked small infra-red markers attached to the participant.Marker position data were collected by six position sensors(distributed over three cameras) arranged at a distance of

approximately 3–4 m from the participant. The precision of a po-sition measure was about 0.15 mm, the sampling rate 250 Hz.Experimental presentation and marker position were synchronizedwith a hardware interface between the computer and the Optotraksystem. Every marker position could thus be precisely assigned tothe respective events on the screen.

Design

Task conditions were manipulated through instructions and de-pended on which of the two objects (ball vs. target) could bemanipulated or whose manipulation could be observed during theinstrumental period of each trial (player vs. observer moderespectively). Trials were presented in blocks in two sessions per-formed on 2 days. The order of the conditions was balanced acrossparticipants. Each session began with the player mode of one taskcondition (bowling task), followed by the observer mode of thesame condition (tracking task). In the second session, the other taskcondition was performed. The factor ‘‘direction of ball motion’’was randomized within blocks, as was the factor ‘‘side of targetmiss,’’ which depended on the outcome of the bowling game. Inshort, three factors were manipulated in both player and observermode: Task condition (ball vs. target), direction of ball motion(leftward vs. rightward), and outcome of trial (hit vs. left miss vs.right miss). This allowed a 2 (task condition) · 2 (direction of ballmotion) · 2 (side of target miss) design to be applied to trials inwhich the outcome was a miss.

A block of 240 trials was assigned to each of the four taskcombinations (i.e., two tasks, bowling/tracking, and two taskconditions, ball/target). Hence, there was a total of 960 trials perparticipant. Within each block the first 48 trials were consideredpractice trials. The remaining 192 experimental trials consisted of arandomized sequence of 4 (replications) · 6 (pairs of starting andtarget positions) · 8 (trajectories per pair of positions; see below).

Task and instructions

Participants were seated in front of a computer screen. In each trial,a green screen with two balls was presented. There was a blue ballat the bottom of the screen and a red ball at the top (‘‘ball’’ and‘‘target’’ respectively). The trial started when the ball began to rollin a certain direction. Figure 2 shows the possible starting positionsof ball and target: Two possible starting positions for the ball at thebottom and three positions for the target at the top. For each pairof starting position and target position, the ball could travel on 1 of8 computer-controlled trajectories. As illustrated in Fig. 2, thesetrajectories were programmed so that the ball would never hit thetarget if no corrections were performed.

Each trial was divided into an instrumental phase and aninduction phase. During the instrumental phase, which lasted 1 sand terminated when the ball crossed an invisible horizontal lineindicated by two markers on the margin of the screen (cf. Fig. 1),participants either performed correction movements using theirjoystick (in the player mode) or observed somebody else’s correc-tion movements on the screen (in the observer mode). In the ballcondition, the joystick acted to shift the ball to the left or the right(while its traveling direction remained unchanged). In the targetcondition, the joystick acted to shift the target to the left or theright. Each session started with the bowling task and ended up withthe tracking task (240 trials each).

In each trial of the bowling task, participants first held the leverof the joystick toward their body. The ball started to move as soonas they moved the lever away from that position. Then, during theinstrumental period, they observed the event configurationemerging on the screen and could perform corrective interventionson either the ball or the target.

In each trial of the tracking task participants first held the lever ofthe joystick toward their body, too, and the ball started to move assoon as they moved the lever away from that position. Then, duringthe instrumental period, they observed the event configuration

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emerging on the screen, including corrective interventions per-formed by the (alleged) co-player.2 At the same time, for the rest ofthe trial, participants were required to track the upward travel of theball with joystick movements pointing away from the body. Duringinitial practice trials a blue marker at the right-hand margin of thescreen indicated the joystick position relative to the ball, and par-ticipants were instructed to keep the marker on the ball as closelyand smoothly as possible. This marker was no longer shown in thesucceeding experimental trials.

At the beginning of the experiment, participants were told acover story. They were made believe that another person wouldparticipate. According to the cover story each participant per-formed one of the two tasks (i.e., bowling or tracking) in a separatelab and after some time participants would take turns at per-forming the tasks. Because the tasks were performed simulta-neously participants could, in certain situations (i.e., observermode), watch the movements of the other participant. Performancein both the bowling and the tracking task was scored independentlyby a point system. Scoring was based on trial outcome (hits/misses)for the bowling task and on precision of tracking (mean verticalerror) for the tracking task. Feedback about the points earned inboth tasks was always provided at the end of a trial. The pointsgathered by the two participants were summed up as if both were ateam. Furthermore, in order to support team solidarity, partici-pants’ points were multiplied when they both achieved a criticalnumber of points during the same trial. Moreover, the team thatattained the highest score out of 12 experimental teams received abonus of Euro 25.

Procedure

Participants were seated in front of a screen in a dimly lit room. Themonitor was mounted on a slanted platform close to the floor (at aheight of about 50 cm), making sure that participants sat com-fortably, with a slightly downward gaze. The screen was at a dis-tance of about 160 cm to the participant’s body. The joystick wasnot fixed, and participants were instructed to hold it themselves.Most of them placed the box on their knees, holding it steady withtheir left hand while operating the lever with their right hand.

At the beginning of the first session, the participant was intro-duced to a second participant, actually the experimenter’s confed-erate. After this brief meeting, the participant entered the lab, satdown in front of the screen and was instructed about the bowlingtask of the respective task condition. Participants then practicedthe bowling task. Before the experimental trials started, they weretold that two computers were connected, and from then on the twoparticipants would play together. The experimenter left the lab andstarted the experiment. During experimental trials on the bowlingtask, feedback about the alleged co-player’s tracking performancewas the sole indication of the other’s presence. In the second part ofthe experiment participants received instructions for the trackingtask (same task condition). In each trial the correction movementsperformed by the alleged other participant were shown during theinstrumental period. Participants watched the event configurationemerging on the screen while tracking the ball’s vertical position.During experimental trials in the tracking task, the co-player’spresence was indicated by the bowling patterns he/she was gener-ating and the feedback earned on those patterns.

In the second session performed on another day within a week,participants did not meet the alleged participant, but were told thats/he was already sitting in the other lab. The procedure was thenrepeated with the other (complementary) task condition. Finally,the experimenter explained the actual goals of the experiment, andparticipants had a chance to compare their scores with those of theothers.

Data processing

The data reported below are based on position measures providedby the Optotrak system. As supplementary measures, ball andtarget positions on the screen were recorded and used for classi-fying a given trial according to its outcome (hit, left miss/right miss)and for identifying the pertinent temporal markers in the Optotrakposition data (i.e., trial onset, trial offset, and point of transitionbetween instrumental and induction period).

As we were interested in horizontal displacements of hands andheads, the results we report below are exclusively based on thehorizontal component extracted from Optotrak recordings. Fig-ure 3 gives an example of hand movements (of one trial). Theordinate shows the relative vertical ball position on the screen usingtwo different scales, each with the same range but different scalingfactors. The transformation into relative coordinates was necessaryfor comparing trajectories of different lengths (due to the more orless tilted paths), offering the advantage that the same value reflectsnot only the relative ball position within a period but also its rel-ative point in time. The ordinate may therefore be interpreted bothspatially and temporally.

The plot in Fig. 3 should be read from bottom to top: Thelower 0% value depicts the hand’s position at the beginning of atrial (i.e., when the ball starts rolling). The lower 100% value showsthe hand’s position at the end of the instrumental period (i.e., whenthe ball crosses the invisible dividing line), and the upper 100%value the hand’s position at the end of the trial. On the x-axis, thevertical zero line represents the hand’s position at the beginning ofthe trial. Horizontal coordinates depict hand displacements to theleft or the right.

Below we report the information contained in these trajectoriesin terms of two summary statistics. First of all, we computedaverage trajectories per condition based on the relative displace-ment data as shown in Fig. 3. This measure will mainly be used forthe sake of illustration. It characterizes average movement trendsacross samples of movements and/or participants. Secondly, wecomputed ‘‘net induced shift (NIS) values’’. This measure will beentered into the ANOVAs. The NIS measure served as an indexsummarizing lateral hand and/or head movements during theinduction period. The term ‘‘net’’ indicates that the measure reflectsthe result of the full history of induced movements during thatperiod, which may consist of a variable number of shifts to the leftand/or right. NIS values were computed as the difference betweenthe effector’s spatial position at the beginning and the end of the

Fig. 2 Starting positions for balls (bottom), target positions (top),and examples of preselected trajectories for two combinations ofstarting and target positions

2Randomized sequences of 192 event configurations from thebowling task were prepared as stimuli for each of the two condi-tions of the tracking task. These bowling patterns had actually beenrecorded from players’ performance in previous experiments.Depending on the task condition they showed initial correctiveinterventions either on the ball or the target. As in the bowling task,the sequences of 192 bowling patterns consisted of four replicationsof the 48 possible trajectories. Furthermore, these patterns exhib-ited an equal number of hits, left misses, and right misses (one-thirdeach).

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induction period.3 NIS-values can thus be considered vectorscoding both length and direction of the effective induced shiftoccurring during the induction period.

In the player mode it was often observed that a particularmovement began in the instrumental period and was continued inthe induction period (as illustrated in Fig. 3). If a pattern like this isobserved in the player mode, it is questionable whether that part ofthe movement that falls into the induction period should be con-sidered instrumental vs. induced. Therefore, in order to differenti-ate between instrumental and induced movements in the playermode, we applied the standstill criterion suggested by Knuf et al.(2001): Only those movements during the induction period thatwere preceded by a standstill of at least 200 ms within that periodwere considered induced.4 This criterion was based on the consid-eration that any movement following the standstill must have beenprogrammed and initiated during the induction period, i.e., afterthe joystick had become ineffective (for further details, see Knufet al., 2001).

Results

Player mode

Data were missing for technical reasons in 18% of alltrials. The application of the 200-ms standstill criterion

led to a further loss of 29% of trials. In the remainingdata set, 481 left misses, 431 right misses, and 199 hitswere observed for the ball condition, and 312 left misses,333 right misses, and 369 hits for the target condition(implying hit rates of 21.8% and 57.2% respectively).

The following results are based on the remaining dataset. Hand and head movements were categorized fordirection of ball motion (left/right), outcome of trials(hit/left miss/right miss), and task condition (ball/tar-get). Average trajectories are shown in Fig. 4. NIS val-ues for misses were entered into a 2 (task condition) · 2(side of target miss) · 2 (direction of ball motion) re-peated measurement ANOVA.

Hand movements

Strongly induced hand movements were observed withupcoming misses, but not with upcoming hits (cf.Fig. 4). In the ANOVA for misses, the main effects wereinsignificant. There was, however, a significant interac-tion between task condition and side of target miss,F(1,11) = 9.82, MSE = 32.33, p = .01. This interactionwas in line with the pattern predicted for intentionalinduction. In the ball condition leftward movementswere obtained for right target misses and rightwardmovements for left target misses. In the target conditionthe pattern was the other way round.

Head movements

Head movements showed a significant main effect ofdirection of ball motion, F(1,11) = 7.37, MSE = 10.96,p < .05, indicating a tendency to move in accordancewith the direction of ball motion. Concerning side oftarget miss, a nonsignificant trend was observed in-dicating that the head tended to move more to the rightin cases of left misses and more to the left in cases ofright misses (F(1,11) = 3.59, MSE = 24.61, p = .085).This difference was reliable in the ball condition but notin the target condition. Interactions were not significant.

Summary

Results for the player mode provided a full replication ofthe pattern of findings reported by Knuf et al. (2001) intheir Experiment 3. Hand movements showed task-dependent intentional induction. In the ball condition,hands acted as if to push the ball toward the target.Conversely, in the target condition, hands acted as if topush the target toward the ball. In hand movementsthere was no indication of perceptual induction at all.The pattern was different for head movements. Headstended to follow the direction of ball motions. Headswere also influenced by side of target miss. However, inthis case, intentional induction was not dependent ontask condition. Instead, heads moved in accordance withball-related intentions throughout.

Fig. 3 Trajectory of horizontal effector movements in a single trial,covering both the instrumental and induction period (bottom scaleon the right vs. top scale on the left). From these trajectories, netinduced shift values (NIS) are obtained as indicated, i.e., asdifferences between horizontal positions at the beginning (0%) andthe end (80%) of the induction period. Note: In the player modethe application of the 200-ms standstill criterion will often lead tosubstantial reductions in effective NIS-values (not shown here; forillustration, cf. Knuf et al., 2001, Fig. 3)

3As is illustrated in Fig. 3 the beginning was set at the true onset ofthat period (0%), whereas the end was set at 80%. This latter pointwas chosen because pilot studies and earlier experiments had shownthat in the very final phase of the induction period, inducedmovements may often be confounded with reset movements, i.e.,movements that go back to the effector’s original default position(cf. Knuf et al., 2001, p. 787).4The application of the 200-ms standstill criterion could have oneof two consequences, depending on whether or not a sufficientlylong standstill was observed during the induction period of a trial.If there was no standstill of sufficient length, the trial was dis-carded. If there was a standstill of sufficient length, effective NISvalues were computed from the net shift arising between the end ofthe standstill period and the 80% line. These effective NIS valueswere entered into the ANOVAs to be performed.

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Observer mode

Data were missing for technical reasons in 6.4% of alltrials. In the remaining data set, there were 703 leftmisses, 702 right misses, and 737 hits in the ball condi-tion, and 723 left misses, 709 right misses, and 741 hits inthe target condition (implying hit rates of 34.4% and34.1%, respectively). In this task there was no reason toapply the 200-ms standstill criterion, since participants’movements were never instrumental with respect toevent configurations on the screen. Hand and headmovements were analyzed as before.

As is shown in Fig. 5, patterns of induced move-ments were obtained in the observer mode, too. Itshould be noted, however, that the amplitudes of thosemovements were, on average, much smaller than in theplayer mode. (Note the different scaling of effector po-sition in Figs. 4, 5.)

Hand movements

Hands tended to move in accordance with the direc-tion of ball motion throughout, i.e., with bothupcoming hits and misses (cf. Fig. 5). In the ANOVAfor misses, the main effect of direction of ball motion

was significant, F(1,11) = 10.02, MSE = 15.61, p <.01. Furthermore, the interaction between task condi-tion and side of target miss turned out to be sig-nificant, F(1,11) = 7.45, MSE = 7.77, p < .05. In theball condition, there was a main effect of side of targetmiss, F(1,11) = 11.51, MSE = 7.32, p < .01, in-dicating a slight tendency to move as if to push theball toward the target. In the target condition, the sideof target miss had no influence on hand movements atall, F < .01.

Head movements

For both hits and misses the pattern of head movementslargely corresponded to that of hand movements (cf.Fig. 5). In the ANOVA for misses two main effects weresignificant: Direction of ball motion, F(1,10) = 6.08,MSE= 17.42, p< .05, and side of target miss, F(1,10)=11.29, MSE = 1.41, p < .01. Separate analyses for thetwo task conditions showed that side of target miss was asignificant factor in the ball condition, F(1,10) = 6.62,MSE = 3.34, p < .05, but not in the target condition,F(1,10) = 2.07, MSE = .42, p > .05. The impact ofdirection of ball motion was equally reliable in bothconditions.

Fig. 4 Results fromExperiment 1/player mode.Average trajectories for handsand heads are shown for twotask conditions (ball/target),two directions of ball motion(left/rightward), and three trialoutcomes (hits, left misses/rightmisses). As in Fig. 3 trajectoriesunfold from bottom to top. Thehorizontal line indicates thetransition between instrumentaland induction period

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Summary

In the observer mode the same pattern of induced move-ments was observed for hands and heads. In both effec-tors, perceptual inductionwas strong throughout. In bothtask conditions and with upcoming hits as well as misses,hands and heads moved in accordance with the directionof ball motion. Furthermore, intentional induction wasobserved as well. However, in this part of the experiment,intentional induction was reliable only in the ball condi-tion, not in the target condition (in both effectors again).

Discussion

Results for the player mode provided a full replication ofthe findings by Knuf et al. (2001, for their Experiment 3;see also Prinz, De Maeght, & Knuf, in press). Instru-mental effectors (i.e., hands) exhibited strong intentionalinduction, but no perceptual induction at all. In theseeffectors, intentional induction was task-dependent, i.e.,it was governed by ball- and target-related intentions inthe respective conditions. Non-instrumental effectors(i.e., heads) exhibited both perceptual and intentionalinduction. However, in these effectors, intentional

induction was always ball-related, reliable in the ballcondition but not in the target condition. These findingssuggest that different patterns of movement inductionmay apply to instrumental and non-instrumental effec-tors (cf., Knuf et al., 2001). Instrumental effectormovements may be governed by the goal under volun-tary pursuit, i.e., by ball-related intentions in the ballcondition, and by target-related intentions in the targetcondition. At the same time, while following the induc-tions emerging from internal goals, instrumental effec-tors may shield themselves from any perceptualinduction arising from the external environment. In away, instrumental effectors behave like rational agents,strongly committed to internal sources of action controland more or less decoupled from external sources.

More importantly, rich patterns of induced move-ments were also obtained in the observer mode, and bothinduction principles turned out to be effective in thismodetoo. Basically, our findings were in line with expectations.Firstly, there was no longer any difference between handsand heads. This had to be expected because both effectorswere now non-instrumental. Secondly, pronounced per-ceptual induction was obtained throughout, i.e., in allconditions and including hands. This had to be expected

Fig. 5 Results fromExperiment 1/observer mode.Average trajectories as inFig. 4. (Note, however, thedifferent scaling of the effectorposition.)

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under the assumption that active suppression of percep-tual induction is only obtained as long as the hands areinvolved in instrumental action. Thirdly, the ball nowplayed a clearly stronger attentional and intentional rolethan the target. This bias had in fact been anticipated as aresult of the asymmetrical attentional demands imposedby the tracking task. Because this task required attentionto be focused on the ball throughout, conflicting demandswere expected to arise in the target condition (wherebowling and tracking are in conflict), whereas no suchconflict is entailed in the ball condition (where intentionand attention always focus on the ball).

The main goal of the present experiment was tostudy patterns of movement induction under conditionsin which individuals observe the outcome of otherindividuals’ actions, i.e., under conditions whereintentional inertia arising from their own instrumentalactions can play no role. Our results suggest two majorconclusions. Firstly, perceptual induction appears to beubiquitous under this condition: People tend to movein accordance with the ball throughout. Secondly, inthe same situation, intentional induction is obtained aswell: People move in accordance with ball-relatedintentions. Given these results we submit that inten-tional induction is more than an after-effect of one’sown preceding instrumental action. It arises not only inthe player mode but also in the observer mode, i.e.,under conditions in which participants are not at allinvolved in instrumental action.

Experiment 2

The second experiment was an exact replication ofExperiment 1—with one crucial difference. We now dis-sociated instrumental joystick movements from theirvisible effects on the screen. Participants had to learn anon-natural, spatially incompatible relationship betweenmovements and their effects on the screen (or betweenintended effects on the screen and joystick movementssuited to realize them). In this experiment the ball or thetarget could be shifted in a particular direction by pushingthe joystick in the opposite direction.

We reasoned that this reversal of the ‘‘natural’’relationship between movements and their effectsshould help us to pin down what the intentions that areeffective in intentional induction may precisely refer to.In Experiment 1 movement-related intentions were al-ways confounded with effect-related intentions. Hence,there was no way of distinguishing induction arisingfrom intended (distal) effects on the screen andinstrumental (proximal) movements to realize them.This should become possible, however, with an inverserelationship between effectuating movements andeffectuated motions.

Given the results of Experiment 1 we anticipateddifferent consequences for the two tasks. Players inthe bowling task have, in principle, equal access to bothmovements and their effects. However, if it is true that in

this task intentional induction relies on short-termintentional inertia (partly, at least) we should expect tosee a strong role for movement-based intentions andonly a marginal role (if any) for effect-based intentions.At least, this must be expected if short-term intentionalinertia is indeed based on after-effects of immediatelypreceding instrumental movements.

The functional situation is different for observers inthe tracking task. In the observer mode participants havedirect access to the movement effects they observe on thescreen, but no direct access to the movements generatingthose effects. In the present paradigm, in which observerswere players before, observers may be able to retrievesome knowledge about those movements, be it proce-dural and/or declarative. At any rate, however, effect-related information picked up from the screen should bemuch stronger than movement-related information re-trieved from long-term memory. As a consequence weshould expect to see a strong role for effect-based inten-tions and only a marginal role (if any) for movement-based intentions.

In summary, then, we expected that in the playermode movements would preponderate over movementeffects whereas in the observer mode effects would pre-ponderate over movements.

Method

Participants

Twelve students (10 female, 2 male) of the University of Munichwith a mean age of 24.0 years were paid to participate. All hadright-hand preference and were requested to hold the joystick withtheir right hands. All were blind to the aim of the experiment.

Apparatus, stimuli, design, and procedure

These were the same as in Experiment 1, the only exception beingthat at the beginning of the first session participants were explicitlyinformed about the reverse relationship between joystick move-ments and object movements on the screen.

Results

Player mode

Data were missing for technical reasons in 16.8% of alltrials. The application of the 200-ms standstill criterionled to a further loss of 29.9% and 29.7% in the ball andthe target condition respectively. In the remaining dataset, there were 460 left misses, 467 right misses, and 164hits for the ball condition, and 443 left misses, 435 rightmisses, and 251 hits for the target condition (hit rates of15.0 and 22.2% respectively).

Hand movements

Pronounced induced hand movements were observedwith upcoming misses but not hits (cf. Fig. 6). In the

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ANOVA for misses the main effects were insignificant.Therewas, however, a significant interaction between taskcondition and side of target miss, F(1,11)= 6.88,MSE=22.32, p<.05.Thepattern underlying this interactionwasin line with what has to be expected if intentions refer tomovements rather than movement effects. In the ballcondition handsmoved leftward in cases of leftmisses andrightward in cases of right misses, whereas the reversepattern was obtained in the target condition.

Head movements

Head movements exhibited a reliable main effect ofdirection of ball motion, F(1,11) = 34.29, p < .01, in-dicating a tendency to move in accordance with the di-rection of ball motion. Furthermore, the main effect ofside of target miss was significant, F(1,11) = 6.27, indi-cating that the head tended tomove to the right in cases ofleft misses and to the left in the cases of right misses.Separate analyses were performed for the two task con-ditions. In the ball condition side of target miss was sig-nificant, whereas direction of ball motion was not.Conversely, in the target condition, direction of ball mo-tion was significant, whereas side of target miss was not.

Summary

Results were in line with expectations. Hand movementsshowed task-dependent intentional induction. Themovement pattern was completely reversed, however. Inthe ball condition hands seemed to act as if to pull theball away from the target, and in the target conditionthey seemed to act as if to pull the target away from theball—movements that actually resulted in the reverseeffects on the screen. The pattern was different again forhead movements. As in Experiment 1 heads were influ-enced by both direction of ball motion and side of targetmiss, indicating that heads tended to move in accor-dance with the ball on the screen in terms of both theball’s traveling direction and ball-related intentions. Insummary, then, hands were guided by movement-relatedafter-effects, whereas heads were guided by the patternof movement effects on the screen.

Observer mode

Data were missing for technical reasons in 6.5% of alltrials. In the remaining data set, there were 720 left mis-ses, 695 right misses, and 728 hits in the ball condition,

Fig. 6 Results fromExperiment 2/player mode.Average trajectories as in Fig. 4

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and 711 left misses, 709 right misses, and 748 hits in thetarget condition (implying hit rates of 34.9% and 34.5%respectively). In this task there was again no reason toapply the 200-ms standstill criterion.

Hand movements

As can be seen in Fig. 7, hands tended to move inaccordance with the direction of ball motion through-out, i.e., with both upcoming hits and misses. However,in the ANOVA for misses, the main effect of direction ofball motion turned out to be insignificant, F(1,11) =2.79, MSE = 16.37, p > .05. Side of target miss was asignificant source of variance in the ball condition,F(1,11) = 7.52, MSE = 4.19, p < .05, but not in thetarget condition, F < 1. In the ball condition handsmoved in accordance with ball-related intentions, i.e.,rightward in cases of upcoming left misses and leftwardin cases of upcoming right misses.

Head movements

Head movements also exhibited a tendency to move inaccordance with the traveling ball (with both hits and

misses). However, the main effect of direction of ballmotion again failed to reach significance in the ANO-VA for misses, F(1,11) = 4.33, MSE = 2.57, p = .062.Side of target miss was a highly significant source ofvariance, F(1,11) = 7.95, MSE = .86, p < .05, in-dicating a tendency to move in accordance with ball-related intentions. Separate analyses for the two taskconditions showed that these effects were both sig-nificant in the ball condition, but not in the targetcondition.

Summary

As in Experiment 1, the pattern of induced movementsin the observer mode was basically identical for handsand heads. Perceptual induction was obtainedthroughout. Hands and heads tended to move inaccordance with the traveling ball, and this was ob-served with hits and misses in both task conditions.Furthermore, intentional induction was obtained in theball condition, but not in the target condition. Inten-tions referred to events on the screen—and not tojoystick movements previously performed to controlthose events.

Fig. 7 Results fromExperiment 2/observer mode.Average trajectories as inFig. 4. (Note, however, thedifferent scaling of the effectorposition.)

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Discussion

In Experiment 2 we dissociated instrumental movementsfrom their associated motions on the screen. The maingoal of the experiment was to study the extent to whichintentional induction refers to proximal movements vs.distal movement effects. We expected to see movement-based intentions in the player mode (at least for instru-mental hands to which intentional inertia applies) andeffect-based intentions in the observer mode, where short-term intentional inertia can play no role. Basically, theresults confirmed these expectations. In the player mode,we observed a clear dissociation between the pattern ofinduced movements for instrumental hands and non-instrumental heads. Hands were exclusively guided bymovement-related intentions, whereas heads were cap-tured by the events on the screen, moving in accordancewith the ball’s traveling direction as well as ball-relatedintentions. Overall, head movements were basicallyidentical to those inExperiment 1, whereas the pattern forhand movements was completely reversed.

In the observer mode no such dissociation betweenhands and heads was obtained. Instead, both effectorsshowed the same pattern of induced movements, exhib-iting both perceptual induction and ball-related inten-tional induction. Perceptual induction was onlymarginally reliable. Intentional induction was reliable inthe ball condition only. Overall, both effectors in theobserver mode exhibited the same basic pattern of in-duced movements as in Experiment 1. The sole differenceseemed to be that perceptual induction was less reliablein Experiment 2 than in Experiment 1. Possibly, this re-flects some participants’ uncertainty due to prior expe-rience with conflicting rules for playing the game.

In summary, these findings corroborate the viewthat intentional inertia plays an important role inthe player mode but not in the observer mode. Short-term intentional inertia in the player mode is basedon after-effects of immediately preceding instru-mental movements. These after-effects are strongdeterminants of induced movements initiated duringthe induction period. Later on, when the same taskis applied in the observer mode, these movement-related after-effects are no longer effective (except,perhaps, for their possible role in weakening perceptualinduction). Instead, intentional induction is now guidedby intentions referring to movement effects on the screen.

Experiment 3

So far participants had played the bowling game them-selves before being transferred to a situation in whichthey watched another (alleged) individual’s performanceof that task. In the present experiment we wanted tostudy performance in what could be called a pure ob-server mode, i.e., without any experience of prior activeplay. At least partly, the pattern of induced movementsobtained in the observer mode of Experiments 1 and 2

could be due to the fact that participants were initiallyengaged in playing the game actively building upintentions referring to movements and/or movementeffects on the screen. In other words, although perfor-mance on the tracking task cannot be affected by short-term intentional inertia (resulting from after-effects ofimmediately preceding instrumental movements), it isstill possible that long-term intentional inertia plays arole (resulting from after-effects of the first block in theexperimental session, where the same task was run in theactive player mode). Would induced movements still beobtained when participants are from the outset studiedin the observer mode, i.e., without a prior build-up ofintentions in active playing?

Method

Participants

Twelve students (4 male, 8 female) of the University of Munichwith a mean age of 23.6 years were paid to participate. All hadright-hand preference and were requested to hold the joystick withtheir right hands. Participants were blind to the purpose of theexperiment.

Apparatus, stimuli, and design

Basically, these were the same as in Experiment 1, except for thefact that participants were now studied in the observer mode only.Again, there were two sessions with two blocks each. The two taskconditions (ball vs. target) were assigned to the two sessions (withorder of task conditions balanced across subjects). Accordingly, ineach session, 2 blocks of 48 practice trials and 192 experimentaltrials were applied in succession (both in the observer mode).

Procedure

Basically, the same procedure was used as in Experiment 1. Initialinstructions were slightly different. When participants arrived at thelab, they were introduced to the other (alleged) player and wereinformed that the two participants had to perform two differenttasks. The two tasks were then explained. First of all, participantswere familiarized with the rules of the bowling task, and they weretold that their co-player would be performing this task during theexperiment. Secondly, the tracking task was explained, requiringthem to watch the co-player’s performance and track the verticalposition of the ball. Participants’ performance scores were based onthe same point system as before.

Results

Data were missing for technical reasons in 5.1% of alltrials. In the remaining data set, 717 left misses, 722 rightmisses, and 724 hits were observed in the ball condition,and 729 left misses, 722 right misses, and 761 hits in thetarget condition (implying hit rates of 33.5% and 34.4%respectively).

Hand movements

As shown in Fig. 8, hands moved in accordance with thedirection of ball motion. This effect was observed withboth upcoming hits and misses. Numerically, it was even

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larger for hits than for misses. In the ANOVA for mis-ses, a nonsignificant trend was observed for direction ofball motion, F(1,11) = 4.32, MSE = 24.14, p = .062.Furthermore, the interaction between task conditionand side of target miss was significant. In the ballcondition, side of target miss was a significant source ofvariance, F(1,11) = 7.30, MSE = 2.19, p < .05, in-dicating the usual pattern of ball-related intentional in-duction. No such effect was observed in the targetcondition (F < 1).

Head movements

Head movements also showed an impact of direction ofball motion with both upcoming hits and misses. In theANOVA for misses, this effect turned out to be signifi-cant, F(1,11) = 5.01, MSE = 2.12, p < .05, indicatingthat the head tended to follow the direction of the tra-veling ball. All other effects failed to reach significance.

Summary

Hands and heads both exhibited perceptual induction.In hands perceptual induction was numerically

larger but statistically less reliable than in heads. Fur-thermore, intentional induction was obtained in the ballcondition but not in the target condition. Intentionalinduction was restricted to hands and did not show inheads.

Discussion

In this experiment we studied induced movements whenparticipants acted in the observer mode from the outset,i.e., without prior experience of playing the bowlinggame. Remarkably, the pattern of results did not differgreatly from that obtained for the observer mode in thetwo preceding experiments. Perceptual induction wasobtained throughout, suggesting that it may be more orless ubiquitous in the observer mode, capturing handsand heads. More importantly, intentional induction wasobtained too, suggesting that intentions can be effectivein passive observation, even if they have not beenestablished before in active performance.

The scope of intentional induction was limited in tworespects, however. Firstly, like in Experiments 1 and 2,intentional induction was effective in the ball condition,but not in the target condition. This is in line with the

Fig. 8 Results fromExperiment 3/observer mode.Average trajectories as inFig. 4. (Note, however, thedifferent scaling of the effectorposition.)

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difference in the attentional requirements of these twoconditions, as discussed above. Secondly, intentionalinduction was observed in hands, but not in heads. Thisdifference is surprising in view of the fact that in thisexperiment participants had never used their hands toperform correction movements on the objects on thescreen. Still, it may be argued that hands and heads dif-fered in three important respects. Firstly, participantsused their own hands for tracking the ball. Secondly, theybelieved that their co-players used their hands for shiftingthe ball or the target. Last but not least, they were, ofcourse, aware from life-long experience that hands areprototypical tools for realizing instrumental intentions(much more so than heads, the movements of which playa major role in exploration and communication). Takentogether these factors may account for the intentionallead of hands over heads in a situation in which this leadcannot be derived from prior active experience of thesame task.

General discussion

The main goal of the present experiments was to studypatterns of induced movements arising in observerswhile they watch another (alleged) individual’s actionsand their consequences. In three experiments we studiedaction induction through action observation. In Exper-iment 1, observers first played the game before watchinganother individual’s performance in that game. InExperiment 2, participants first played a reversed ver-sion of the game and then watched another individual’sperformance in that reversed game. Finally, in Experi-ment 3, participants watched from the outset, withoutany prior experience of active play.

Principles

Results for the observer mode showed that the pattern ofinduced movements was virtually identical across thethree experiments:

1. Watching the ball’s travel induced participants tomove in accordance with the direction of that travel.This occurred with both upcoming hits and misses

2. Intentional induction was less ubiquitous, clearlyeffective in the ball condition but not in the targetcondition. A result along these lines had been antic-ipated, given the conflicting attentional demandsarising in the target condition of the combinedbowling and tracking task

3. Basically, the pattern of induced movements was thesame for hands and heads, except that in Experi-ment 3 intentional induction was restricted to handsand did not occur in heads

These findings suggest that action induction throughobservation does not depend much on observers’ prior

experience of the experimental situation. Instead, whenpeople watch other people’s actions and/or their out-comes, they tend to move in accordance with both thoseactions’ physical structure and underlying intentions.For the time being, these conclusions can only be de-rived from the ball condition, not from the target con-dition. However, we believe that this does not weakenthe conclusions themselves. As we discussed in theintroductory section, results for the target condition aredifficult to interpret because they reflect conflictingattentional and intentional task demands.

Results for the player mode replicated previous find-ings and corroborated the explanatory scheme offeredby Knuf et al. (2001):

1. Hands showed strong intentional induction, but noperceptual induction at all. Intentional induction inhands was task-dependent, i.e., hand movementswere ball-related in the ball condition and target-related in the target condition. Moreover, Experi-ment 2 revealed that effective intentions did not reallyrefer to the distal objects on the screen (ball vs. tar-get), but to proximal effectors performing thosemovements (hands)

2. In both experiments head movements exhibited per-ceptual induction as well as ball-related intentionalinduction. Head movements followed the ball’s traveland moved in accordance with ball-related intentions

In the player mode we encountered the same disso-ciation between instrumental and non-instrumental ef-fectors that had been discussed by Knuf et al. (2001).Instrumental effectors (hands) acted as if they were stillcommitted to the previously active goal. At the sametime they were not susceptible to perceptual induction,as if to shield themselves from any other inductions thanthose relevant to the actual task. Conversely, non-instrumental effectors (i.e., heads) seemed to be entirelycaptured by the dynamic object (i.e., the travelingball)—captured in the sense of moving in accordancewith both the ball’s motion (perceptual induction) andball-related intentions (intentional induction). As re-gards the player mode, this difference between ball andtarget in their power to capture attention and intentioncannot be accounted for by conflicting intentional de-mands between two subtasks. Instead, as was pointedout by Knuf et al. (2001), we must trace it back to thedifference in perceptual salience of the two objects.Unlike the target, which is more or less stationary, theball is dynamic in the sense that it moves throughout atrial. It seems that non-instrumental effectors that arenot committed to task-specific intentions are captured inorder to move in accordance with the most salient objecton the display.

Taken together our results suggest that the contri-butions of the two principles to the guidance of ideo-motor action are different for players and observers. Inplayers there is a strong dissociation between instru-mental and non-instrumental effectors. Instrumentaleffectors show the after-effects of previous instrumental

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actions and shield themselves against inductions arisingfrom the actual stimulus configuration. Non-instru-mental effectors are susceptible to both perceptual andintentional induction arising from salient objects on thedisplay. In observers no such dissociation is obtained.Instead, both perceptual and intentional induction areeffective throughout, suggesting that, when people watchother people’s actions, they cannot help but move inaccordance with those actions and their implied inten-tions.

Mechanisms

How can we account for action induction through themere observation of other individuals’ action? As wehave pointed out elsewhere (Prinz, 1987, 1990; Hommelet al., 2001; Knuf et al., 2001) much of the debate aboutfunctional architectures for ideomotor action has fo-cused on two basic theoretical premises, both of whichalso play a major role in theories of imitation (Prinz,2002): strong overlap (between representations for per-ception and action) and intention-based representation(in action perception and performance).

The notion of strong overlap between perceptionand action has always been commonplace in explan-atory approaches to ideomotor action and imitation.For instance, James’ ideomotor principle was, at thattime, part of the then popular motor theories ofcognition (James, 1890; cf. Scheerer, 1984). Thosetheories believed in close functional links betweenperception and action—to the effect that perceptualrepresentations were regarded as relying or dependingon the actions they imply or lead to. Strong overlapbetween perception and action is also inherent in morerecent elaborations on the logic of the ideomotorprinciple, such as the mechanism of ideomotor com-patibility (Greenwald, 1970, 1972), the common-codingprinciple (Prinz, 1990, 1997, 2002), or the theory ofevent coding (TEC: Hommel et al., 2001)—approachessharing assumption that the planning of future actionengages the same representational resources as theperception of present events. According to these ap-proaches, the representational structures subservingperception and action planning are co-extensional—tothe effect that perceiving certain actions may oftenprime, or induce, the execution of corresponding ac-tions, or vice versa.

The notion of intention-based representation refers tothe nature of action representation in the alleged com-mon representational domain for perceived and plannedaction. One of the commonplace notions in the literatureon action perception is that the perception of humanaction tends to go beyond the information given, i.e.,beyond the visual information about the spatio-temporalpatterns of limb movements it is based on (for overview,see Stranger & Hommel, 1995). For instance, whenwatching a point-light walker lifting an object, observerssee more than just the pattern of the walker’s body

movements. As has been shown in the seminal work byRuneson and Frykholm (1981, 1983), observers see theweight of the lifted object in the same way as they see thebody movements—although, functionally speaking, thevisual stimulus pattern does not specify dynamic prop-erties (e.g., weight) in the same direct way as it specifieskinematic properties (e.g., movements). Observers seemto see the dynamics ‘‘through’’ the kinematics (whichmay even imply that for them perceived dynamics aremore salient than perceived kinematics).

More importantly, in the same way as they see dy-namic patterns ‘‘through’’ or ‘‘beyond’’ the kinematicsgiven, observers often spontaneously see intentionalpatterns ‘‘behind’’ the kinematics and dynamics. Forinstance, if an object is too heavy to lift, observers seestraight away that the point-light walker tries to lift theweight. In other words, they see goal-directed actionthrough the spatio-temporal pattern of movementprovided by the stimulus. Furthermore, as was shownin the pioneering work of Heider (1926/1959, 1930/1959), such spontaneous attribution of internal states(like intentions or desires) to visual movements is by nomeans restricted to body movements of intentionalagents, such as people or animals. Heider and Simmel(1944) were the first to show that such attribution islikewise ubiquitous in the perception of animated car-toons, where perceivers take motion patterns of artifi-cial objects for action patterns of intentional agents(e.g., two triangles moving around each other as toagents fighting each other).

Intention-based representation is also inherent indemonstrations of automaticity in social perception andcognition (e.g., Bargh, 1997; Bargh & Barndollar, 1996).These studies show that the perception of social stimulilike people, their actions, or underlying intentions tendto induce related traits, attitudes, or actions in ob-servers—and often do so more or less automatically, i.e.,without conscious awareness. Importantly, Bargh (1997)has shown that this automaticity can be conditional orunconditional, depending on whether the representationformed of a given social stimulus, say, a goal-directedaction, is dependent on or independent of the observer’sown actual goal state.

Recent evidence suggests that intentional interpreta-tions of kinematic patterns may emerge very early in life.For instance, habituation studies by Csibra and Gergelyhave demonstrated that 9-month-olds show evidence ofintentional interpretation of object motion (e.g., twodots moving around on a screen so that one seems tochase the other or an object moving so that it appears toavoid an obstacle, etc.; Csibra & Gergely, 1998; Csibra,Gergely, Biro, Koos, & Brockbank, 1999). Evidence ofan intentional understanding of human arm and handmovements has been demonstrated even in 6-month-olds(Woodward, 1998, 1999; Jovanovic et al., 2003). Thesefindings suggest that intentional interpretation of actionsand events may be deeply rooted in the architecturesubserving their representation, since even very younginfants see intentional action in physical movement.

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Strong overlap and intention-based representationare also supported by recent evidence on brain struc-tures subserving action perception. Over the past dec-ade, Rizzolatti and coworkers have analyzed theproperties of ‘‘mirror neurons’’ in the premotor cortexof monkeys (Gallese, Fadiga, Fogassi, & Rizzolatti,1996; Rizzolatti, Fadiga, Fogassi, & Gallese, 1996; foroverview, see Rizzolatti, Fogassi, & Gallese, 2001).Remarkably, these neurons are active not only whilethe monkey performs a certain action (as their locationin premotor cortex suggests), but also when the mon-key observes the same action being performed by theexperimenter. Parallel to this line of research, Perrettand coworkers have documented no less remarkableproperties of neurons in the temporal cortex of mon-keys (Jellema, Baker, Perrett, & Wicker, 2000; Perrettet al., 1989, 1990; for an overview, see Jellema & Per-rett, 2002). These neurons are particularly active whenthe animal observes some other animal acting, andmany of them code selectively for meaningful classes ofactions—meaningful in the sense of sharing commongoals or intentions. For instance, a given cell mightcode for a particular direction of the observed animal’sgaze (e.g., downward) irrespective of whether the headis seen from the front or in profile. Further evidencefrom TMS studies and brain-imaging studies with hu-mans suggests that structures with similar propertiesmay be operating in the human brain too (e.g., Decetyet al., 1997; Fadiga, Fogassi, Pavesi, & Rizzolatti, 1995;Grafton, Arbib, Fadiga, & Rizzolatti, 1996; Iacoboniet al., 1999; Nishitani & Hari, 2000). Obviously, then,intention-based representation and strong overlap areboth incorporated in mirror systems. These systemsseem to be capable of classifying perceived actions ac-cording to intentional content (intention-based re-presentation) and they draw on the very same neuronpopulation for action control and action perception(strong overlap).

Based on new findings on the interconnections be-tween the cortical areas harboring these two cell popu-lations (Gallese, Fadiga, Fogassi, & Rizzolatti, 2002),the Parma group has recently come up with an in-tegrated model for the combined activity of thesetwo populations in what they call a mirror system.According to the model, this system is built for actionunderstanding. Action is understood by the system whenaction observation causes the motor system of theobserver to ‘‘resonate.’’ We ‘‘understand others throughan ‘internal act’ that recaptures the sense of their acting’’(Rizzolatti et al., 2001, p. 661). If this is true the ubiq-uitous occurrence of induced movements suggests thatthose internal acts may often occur alongside externalactions.

Acknowledgements We wish to thank Lothar Knuf for extensivesupport in planning, running, and analyzing the experiments andfor critical comments on a earlier version of the paper; Heide Johnfor handling and support in shaping the manuscript; and MaxSchreder for preparing the figures.

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