- Email: [email protected]
Movement observation affects movement execution in a simple response task
Acta Psychologica 106 (2001) 3±22
www.elsevier.com/locate/actpsy
Movement observation aects movement execution in a simple response task Marcel Brass
a,*
, Harold Bekkering b, Wolfgang Prinz
b
a
b
Department of Neurology, Max Planck Institute for Cognitive Neuroscience, Stephanstr. 1A, D-04103 Leipzig, Germany Department of Cognition and Action, Max Planck Institute for Psychological Research, Amalienstr. 33, D-80799 Munich, Germany Received 28 January 1999; received in revised form 10 April 1999; accepted 10 April 1999
Abstract The present study was designed to examine the hypothesis that stimulus±response arrangements with high ideomotor compatibility lead to substantial compatibility eects even in simple response tasks. In Experiment 1, participants executed pre-instructed ®nger movements in response to compatible and incompatible ®nger movements. A pronounced reaction time advantage was found for compatible as compared to incompatible trials. Experiment 2 revealed a much smaller compatibility eect for less ideomotor-compatible object movements compared to ®nger movements. Experiment 3 presented normal stimuli (hand upright) and ¯ipped stimuli (hand upside-down). Two components were found to contribute to the compatibility eect, a dynamic spatial compatibility component (related to movement directions) and an ideomotor component (related to movement types). The implications of these results for theories about stimulus±response compatibility (SRC) as well as for theories about imitation are discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2340 Keywords: Reaction time; Motion perception; Perceptual motor processes
*
Corresponding author. Tel.: +49-341-9940-213; fax: +49-341-9940-221. E-mail address: [email protected] (M. Brass).
0001-6918/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 1 - 6 9 1 8 ( 0 0 ) 0 0 0 2 4 - X
4
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
1. Introduction The present study can be placed in two dierent theoretical and experimental contexts: research on stimulus±response compatibility (SRC) and on action imitation. Usually these two ®elds are considered largely isolated from each other. This paper, however, argues that there is a close relationship between them. For this purpose, a theoretical framework is introduced that integrates both perspectives. Research on imitation may thus bene®t from theoretical and experimental accounts of the SRC ®eld about the question of how perception and action interact, while SRC research may bene®t from the investigation of more complex stimulus±response (S±R) relations than has typically been done in this ®eld. 1.1. Imitation: the functional problem The study of imitation phenomena has a long history, not only in psychology but in ethology and anthropology as well. Although imitation is well-documented in dierent ®elds of research and species (e.g., Fiorito & Scotto, 1992; Williams & Nottebohm, 1985), it is fair to say that the mechanisms underlying imitation are still poorly understood. From an action perspective, the most fundamental question of imitation may be described as ``How can a motor act be constructed from a perceived act? Since, if we observe somebody executing a movement, we do not perceive the muscle activation underlying the movement. So how can we know which muscle movements will generate a movement that looks like the one we have observed?'' (Prinz, 1987). Several attempts to solve this issue have been made. Some authors tried to reduce the problem to an instrumental learning (Gewirtz, 1969; Gewirtz & Stingle, 1968) or an associative learning mechanism (Pawlby, 1977). One of the most elaborated and in¯uential theories on imitation was proposed by Piaget (1962). His developmental theory of imitation was built around the concept of circular reactions. Infants explore the relation between motor act and the sensory eect of that action by performing speci®c actions repetitively. During this repetition process a link is established between the perceived act and the motor execution. When a child observes an adult while executing a movement similar to the visual component of their own visual±motor circular reactions, the respective motor component is triggered in the infant. While the observer interprets the infantÕs behavior as imitation, the child is just repeating a circular reaction where the impetus to repeat the circle comes from the perception of the visual component of the ensemble (Meltzo & Moore, 1983). This interpretation cannot explain all imitation phenomena, as was pointed out by Meltzo and Moore (1977) who detected very early imitation of facial gestures in neonates. Neonates cannot build a visual representation of their own facial gestures and, hence, the perceived gesture of a model cannot trigger the motor act by replacing this representation. Still the described mechanism underlying circular reactions might provide a ®rst hint to a possible solution of the action problem in imitation. Namely, when we observe a movement we see the expected outcome of a potential self-executed movement. This view of imitation is very close to an early
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
5
theory of motor control, the ideomotor theory, which will be described in the next paragraph. 1.2. Ideomotor theory and the concept of ideomotor compatibility An interesting suggestion concerning the relationship of perceived and executed action was given by James more than hundred years ago with the description of what he called ideomotor action (James, 1890). James formulated this idea as follows: ``Every representation of a movement awakens in some degree the actual movement which is its object'' (James, 1890, p. 1134). Greenwald (1970a) elaborated on this idea in his theory of ideomotor action. He stated (Greenwald, 1970b) that (a) voluntary responses are represented centrally in the form of images of the sensory feedback they produce, and (b) such images play a controlling role in the performance of their corresponding actions. From this perspective it becomes clear how action observation can be used to guide action execution. The visual response image is one of the major parts of sensory action feedback. Due to a similarity relation, observing an action activates the response image of the corresponding response, which in turn controls the observerÕs performance. This ideomotor mechanism can be applied to account for imitation phenomena, but it is not restricted to imitation. Greenwald (1972) extended this idea to compatibility phenomena, forming the concept of ideomotor compatibility, which denotes ``the extent to which a stimulus corresponds to sensory feedback from its required response'' (p. 52). Ideomotor theory implies that stimuli with high ideomotor compatibility lead to an activation of the corresponding response via the activation of the response image. From this perspective, phenomena of SRC and imitation are closely related. Hence, research on imitative behavior and on SRC can be combined under the theoretical framework of ideomotor theory. 1.3. Compatibility eects in a simple response task What assumptions can be drawn from ideomotor theory with regard to the concept of (ideomotor) compatibility? One of the central ideas of the theory is that perceiving the sensory feedback of an action leads to an activation of the response image. This image plays a controlling role in the performance of the action, assuming that no speci®c S±R translation is involved. In contrast, most models of SRC presuppose that S±R compatibility eects arise at the S±R translation stage (e.g., Hasbroucq & Guiard, 1991). Hence, if a response can be selected before the imperative signal appears, the selection process cannot be aected by the stimulus (however, see Hommel, 1997, for an alternative view). In accordance with theories assuming that SRC operates on S±R translation stage, simple response tasks (SRT) yielded no or only very small S±R eects (e.g., Callan, Klisz & Parsons, 1974; Marzi, Bisiacchi & Nicoletti, 1991; Bashore, 1981; Hasbroucq, Kornblum & Osman, 1988). In SRT paradigm, participants always have to execute the same response, that is, a task in which participants react with a predetermined left- or right-hand response, while random left and right stimuli are
6
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
presented. The absence of robust eects in SRT paradigms seems to support the view that SRC is restricted to choice response tasks. However, in most studies the S±R arrangement was not highly ideomotor compatible. This might be the reason why the eects were typically found to be small. According to the ideomotor principle, SRC eects are expected even in an SRT, where the same response is required independent of the stimulus. The ideomotor theory assumes that even in an SRT, a response image is generated. Observing an action, presumably, evokes a response image as well. Therefore, compatibility between the act seen and the act to be performed should facilitate response execution by activating the actual response image, whereas an incompatible action should interfere with response execution. The general aim of the present study was to investigate the implications of the ideomotor theory for views on imitation and on SRC. Particularly, the study addressed the question of whether S±R arrangements with high ideomotor compatibility lead to compatibility eects in a task with minimal response selection requirements. In addition, we wanted to investigate the ideomotor interpretation of imitation, which suggests that observing a movement activates the equivalent movement. In order to do so, participants were required to execute a pre-instructed ®nger movement in response to the onset of a visually presented compatible or incompatible ®nger movement. When the observed movement is compatible to the preinstructed movement this can be interpreted as a Ôquasi-imitativeÕ situation, that is, the subject performs a similar action as the action shown on the screen. Perceiving a compatible movement should activate the pre-instructed movement, which should result in faster reaction times. In addition, if the observed movement is incompatible with the instructed movement, the incompatible response is activated and thus needs to be inhibited, resulting in slower reaction times.
2. Experiment 1 The ®rst experiment was a simple response task, in which participants executed the same ®nger movement within one block while reacting to the onset of a ®nger movement on a screen. The observed movement, which served as a go-signal for the prede®ned response, was compatible or incompatible with the to-be-executed response. In one block, the required movement was always to lift the index ®nger from a neutral starting position, that is the ®nger was held a few centimeters above the table. In the other block, the task was to tap on the table with the index ®nger from the same neutral starting position. Stimuli were small digitized video sequences of similar ®nger movements (lifting vs. tapping). If ideomotor compatibility between the act seen and the act to be performed in¯uences response execution by activating the actual response image, robust compatibility eects are expected in the present SRT paradigm. For paradigms with spatial S±R arrangements, these eects are about 20±70 ms in choice tasks (see Hommel, 1996). However, if compatibility occurs on the stage of stimulus±response translation, only minor compatibility eects
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
7
are expected in the present SRT paradigm. In SRT experiments with spatial S±R arrangements eects are about 2±6 ms (e.g. Hasbroucq et al., 1988; Hommel, 1996). 2.1. Method 2.1.1. Participants Eight students of the University of Munich (6 female and 2 male) participated in a 30 min session. All were right-handed, had normal or corrected-to-normal vision, and were naive with respect to the purpose of the study. They were paid for participation (10 DM). 2.1.2. Apparatus and material A Compaq Pentium 200 MHz computer controlled stimulus presentation and data acquisition. Short digitized video sequences of an animated hand were presented on a 17-in. screen with a frame frequency of 60 Hz. The video sequences consisted of ®ve frames. The animated hand had a visual angle of about 6.5° vertically and 10° horizontally. Participants were seated about 80 cm in front of the screen. The animated hand was in a mirrored orientation to the participantÕs right hand, which rested on a table. Movements were recorded by an Optotrak system (Norton Digitalä). Six cameras were installed to sample raw position data of an active infrared marker with a sampling rate of 250 Hz which was placed on the ®ngernail of the subject's index ®nger. The system allows for a reconstruction of movement trajectory in three-dimensional space. Movement onsets were computed from these trajectories and o-line analyzed with MATLAB 5.0 routines. For detection of the movement onset, a relative velocity threshold was used (15% of peak velocity). Onset of movement was de®ned as the moment the velocity exceeded this threshold and a minimum displacement (20% of movement amplitude for the lifting movement and 10% of movement amplitude for the tapping movement) took place within a maximum amount of time after this onset (150 ms for the lifting movement and 80 ms for the tapping movement). 2.1.3. Design and procedure The experiment was divided into two blocks. One requiring to lift the index ®nger as soon as a ®nger movement appeared, the other requiring to move the ®nger downwards. The video sequence started with a frame showing the index ®nger in a middle, resting position. This frame, which was identical for both movements, remained visible for 800, 1600, or 2400 ms. Then three movement frames ¯ashed in successive order in which the ®nger moved randomly either up or down. The ®rst two movement frames were presented for two refreshing cycles (about 34 ms) each. The last frame contained a picture of the ®nal ®nger position and remained on the screen for 500 ms. The overall displacement of the ®nger was about 2° for the lifting and for the tapping movement. The three time intervals between the onset of the ®rst stimulus frame and the onset of the stimulus movement served to reduce anticipation errors. Total trial length was kept constant (about 5.5 s). Between the trials the screen turned blue for 1.5±3 s, depending on the length of the start frame in the
8
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
previous trial. Each block contained 120 trials (20 trials for each observed movement * 3 time intervals for stimulus movement onset. Block order was counterbalanced across participants. Participants executed the instructed movement on observing a movement, irrespective of the type of seen movement. Prior to each block, 10 test trials were presented. 2.1.4. Data analysis Errors. Theoretically, two types of errors could occur: First executing the wrong movement (instead of lifting tapping or vice versa). This error only occurred in the ®rst test trials but not during the blocks. Second, a response could be initiated too early (for criterion see below). One participant was replaced for exceeding the 10% error criterion. For the other participants this error type occurred in 2% of the trials. These trials were excluded from analysis. Response times. Response times below 80 ms and above 800 ms were excluded from analysis. An ANOVA with three within-subject factors was computed over the remaining data, the factors being Ôexecuted movementÕ (lifting vs. tapping), Ôobserved movementÕ (lifting vs. tapping), and Ôstimulus movement onsetÕ (800, 1600, 2400 ms). 2.2. Results A main eect was found for `observed movement', F 1; 7 17:2, P < 0:01. Participants reacted faster when they observed a tapping compared to a lifting movement. Also, a main eect for `stimulus movement onset' was found, F 2; 14 44:6, P < 0:01, that is, the longer the interval between the start frame and the ®rst movement frame, the faster the action was initiated. The most interesting eect was the two-way interaction of observed and executed movement (Fig. 1).
Fig. 1. Movement onset (RT) as a function of `observed movement' (tapping vs. lifting) and `executed movement' (tapping vs. lifting).
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
9
Fig. 2. Mean RT quintiles as a function of compatibility of observed and executed movement.
Participants were much faster if they had to execute the compatible movement (278 ms) compared to the incompatible movement (333 ms), F 1; 7 8:2, P < 0:05. This holds for both movements, but there was an asymmetry. That is, the compatibility eect is smaller for the lifting than for the tapping movement t 1; 7 4; 1, P < 0:01. In a distribution analysis (Fig. 2) we divided the response time distribution in quintiles (Ratcli, 1979), indicating that the compatibility eect becomes larger for slower responses, F 4; 28 7:6, P < 0:01. 2.3. Discussion The results of Experiment 1 clearly demonstrate that responses were faster when the observed movement was compatible to the executed movement than if it was not. Interestingly, this eect was pronounced compared to other attempts to demonstrate S±R compatibility in a simple response task, as reviewed in Section 1. Given the block size of 120 trials, the compatibility eect cannot be explained by some kind of inertia eect from one experimental block to the other (Hommel, 1996). Moreover, the distribution analysis shows that the compatibility eect becomes larger for slower responses, indicating that the process underlying this eect becomes more in¯uential over time. From the perspective of ideomotor theory this makes sense, because complex properties of the observed action, which presumably require more timeconsuming processing, correspond to the sensory feedback from the required response, thus being more likely to aect slower responses. Another surprising result of the ®rst experiment was the asymmetry for the tapping and the lifting movement. We can only speculate about the reason for this eect. One explanation might be that the tapping movement is characterized by two distinct movement components separated by tactile feedback, that is, tapping downwards and moving the ®nger back to the
10
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
start position, while for the lifting movement the upward and downward components are not separated by tactile feedback. Taken together, the ®rst experiment supports the prediction that in a condition with high ideomotor compatibility observing a ®nger movement leads to some kind of activation of the corresponding response. This results in a substantial compatibility eect, even in the case in which response selection processes are minimized. However, it is still unresolved whether the eect has to be attributed to high ideomotor compatibility. One can argue that it was due to some kind of dynamic spatial compatibility and not so much to the quasi-imitative character of observed and executed action (cf. St urmer, Aschersleben & Prinz, 1999). To test this alternative explanation, we designed an experiment in which we compared ®nger movements with object movements, assuming that object movements are less ideomotor-compatible to the to-be executed ®nger movements than ®nger movements.
3. Experiment 2 In Experiment 2 participants again were instructed to execute upward and downward ®nger movements in response to the same ®nger movements. However, in addition to this replication of Experiment 1, moving squares were presented. If the large compatibility eect in Experiment 1 was due to the amount of ideomotor compatibility, the compatibility eect should be much smaller when the go-signal is provided by an object movement. If, however, the large compatibility eect of Experiment 1 only owed dynamic spatial compatibility, one might expect to ®nd similar compatibility eects for object and ®nger movement. 3.1. Method 3.1.1. Participants Ten students of the University of Munich (5 female and 5 male) participated in a 30 min session. All were right-handed, had normal or corrected-to-normal vision, and were naive with respect to the purpose of the study. They were paid 10 DM for participation. 3.1.2. Apparatus and material Stimulus presentation and measurement procedure were identical to Experiment 1. The object movement ®lms were produced by a digital version of the point-light technique developed by Johansson (1973). A black square marked the ®ngernail of the index ®nger in each frame of the two movement ®lms (lifting and tapping). The background was turned blue, so that only the squares were visible. Displaying these frames in successive order leads to the impression of square moving up or down. The kinematics and amplitude of the moving squares were identical to those of the ®nger movement.
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
11
3.1.3. Design and procedure The experiment was again divided into two blocks. In each block, ®nger movement (tapping and lifting) and object movement (up and down) were randomly presented. In one block, participants always had to lift their index ®nger on detecting a movement (irrespective of the stimulus type). In the other block, the ®nger was to move downwards. As in Experiment 1 ten test trials were performed prior to each block. The temporal order of the digitized video sequences was identical for ®nger and object movement. As in Experiment 1, there were three intervals between onset of the ®rst stimulus frame and that of the stimulus movement in order to reduce anticipation errors. Each observed ®nger movement (tapping or lifting), each object movement (up and down) and the three stimulus movement onsets (800, 1600, 2400 ms) were presented ten times in random order. Thus each block consisted of 120 trials. Block order was counterbalanced across participants. 3.1.4. Data analysis Errors. In 1.4% of the trials participants made a ®nger movement before the movement on the computer screen started. Response times. An ANOVA was computed with the factors `observed movement' (upward vs. downward), `executed movement' (tapping versus lifting), `stimulus movement onset' and `moving stimulus' (squares vs. ®ngers). 3.2. Results Main eects were found for `observed movement', F 1; 9 6:0, P < 0:05, and for `stimulus movement' onset, F 2; 18 43:9, P < 0:01. Participants reacted faster when they observed a tapping compared to a lifting movement, and the longer the interval between the start frame and the ®rst movement frame, the faster the action was initiated. The main eect for `type of stimuli' (square vs. ®nger), F 1; 9 29:6, P < 0:01, was observed. Participants reacted faster to the ®nger movement (265 ms) than to the object movement (284 ms). Again, a compatibility eect of observed and executed movement was found to be signi®cant, F 1; 9 8:7, P < 0:01, indicating that participants reacted faster to compatible trials (258 ms) than to incompatible trials (291 ms). In addition, the three-way interaction with `stimulus type' reached statistical signi®cance, F 1; 9 5:2, P < 0:05. While the compatibility eect of stimulus ®nger movement was signi®cant, F 1; 9 9:4, P < 0:05, that of stimulus object movement (Fig. 3) showed only a statistical trend, F 1; 9 3:5, P 0:09. An analysis of the RT-distribution (Fig. 4) revealed an interaction between RT-quintiles and compatibility for observed ®nger movement, F 4; 36 7:5, P < 0:01 but not for object movement, F 4; 36 0:1, P 0:59. The three-way interaction (type of stimuli * compatibility * quintile) was also signi®cant, F 4; 36 2:9, P < 0:05. This indicates that the compatibility eect becomes larger for slower responses with observed ®nger movements but not for observed object movements.
12
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Fig. 3. RT as a function of `observed movement' (tapping vs. lifting) and `executed movement' (tapping vs. lifting) and `type of stimuli' (®nger movement vs. object movement). Finger movements were compared with an upward or downward moving square.
Fig. 4. Mean RT quintiles as a function of compatibility of observed and executed movement and type of observed stimuli (®nger movement vs. object movement).
3.3. Discussion Results demonstrate a clear dierentiation between the eect of both observed ®nger and object movement on response latencies of executed ®nger movements. Importantly, the compatibility eect was much more pronounced for ®nger than for
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
13
object movements. Nevertheless, a statistical trend was also found for the latter. A possible account for this pattern of results is the notion that two separate mechanisms underlie the compatibility eect: One mechanism that is related to the movement direction, and another related to the movement type. What we call movement direction component can either be caused by the direction of the moving stimulus or by the end position of the stimulus relative to the starting point. While the observation of object movements only activated the movement direction component, the observation of ®nger movements activated both the movement direction component and the movement type component. This interpretation is supported by the distribution analysis of the two types of moving stimuli. The compatibility eect for observed object movement was already present for fast responses and remained constant over the whole range of distribution, suggesting that the relatively simple process of identifying movement direction arises early in the processing stream and adds a constant contribution, independent of response times. On the other hand, the compatibility eect for ®nger movement showed a successive increase in the distribution range, thus suggesting that processing the type of movement required more complex processes which needs more time in order to become eective. There is however a strong alternative interpretation to explain the observed differences: Simply the fact that the two stimulus types dier perceptually from each other. That is, the suggestion derived from the results of Experiment 2, that one mechanism is related to direction and one to movement type (tapping or lifting), is confounded with the change in stimulus display. This alternative explanation is supported by the fact that the movement identi®cation for ®nger movement was faster than that for object movement. One might argue that the dierence in the compatibility eect for object movement and ®nger movement was caused by a perceptual dierence only. The main eect for type of moving stimuli also restricts the conclusion that can be drawn from the distribution analysis. The purpose of Experiment 3 was to separate the two components `movement direction' and `movement type' without the perceptual problem of Experiment 2, by using the same stimulus movement throughout. One possibility to achieve such a separation is to oppose the predicted compatibility eects by turning the stimulus display upside down. In addition, this method will also allow us to quantify the contributions of these two processes to the overall compatibility eect. 4. Experiment 3 To distinguish `movement direction' from the `type of movement', Experiment 3 was sub-divided: One part was a pure replication of Experiment 1. In the other part, everything was identical except that the original stimuli were ¯ipped upside down. Thus, the former tapping movement now led to an upward movement, and vice versa (Fig. 5). This design allows for an opposite prediction regarding the compatibility eect for movement direction (up vs. down) and type of movement (tapping vs. lifting). If movement direction was the primary component of the previously observed compatibility eects of Experiments 1 and 2, movement direction should have
14
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Fig. 5. Compatibility for movement type and movement direction with normal and ¯ipped hand orientation. For normal hand orientation, movement direction and movement type are both compatible to a tapping movement. For ¯ipped hand orientation, movement direction is compatible to a lifting movement while movement type is compatible to a tapping movement.
a larger in¯uence on the compatibility eect. Conversely, if movement type was the primary component, compatibility should be more strongly in¯uenced by movement type (tapping or lifting). A comparison between the same ®nger responses on the ¯ipped and the un¯ipped hand-orientation trials will allow us to separate the contributions of movement direction and type of movement to the compatibility eect. 4.1. Method 4.1.1. Participants Six students and two pupils (5 females, 3 males) took part in the experiment. All participants had normal or corrected-to-normal vision, were naive to the purpose of the experiment, and reported to be right-handed. Participants were paid for participation with 20 DM. 4.1.2. Apparatus and material The digitized video images of Experiment 1 were ¯ipped upside down by software (Photoshop 4.0ä). Except this variation the stimulus material and measurement equipment were identical to Experiment 1.
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
15
4.1.3. Procedure The procedure for each session was identical to Experiment 1, consisting of two blocks with 120 trials each. In one session participants reacted to the stimuli of Experiment 1, and had to react to the ¯ipped video images in the second session. The mean interval between both sessions was 2.25 days. The order of sessions (normal hand vs. ¯ipped hand orientation) was counterbalanced across participants. The order of blocks was also balanced. The instruction was to execute the instructed movement as soon as the hand on the screen moved, irrespective of the type of observed movement. 4.1.4. Data analysis Errors. Like in Experiments 1 and 2, only the second type of errors (reacting before the stimulus movement onset) occurred. In the experimental session with normal hand orientation error rate was 1.6%. In the session with ¯ipped hand orientation 1.5% errors occurred. These trials were excluded from analysis. Again one participant was replaced for exceeding the 10% error criterion. Another participant was replaced because he had been assigned to the wrong condition. Response times. An ANOVA with repeated measures was computed with the factors `observed movement' (tapping vs. lifting), `executed movement' (tapping vs. lifting), and `hand orientation' (normal vs. ¯ipped) as well as `onset of stimulus movement'. 4.2. Results The analysis revealed the main eect for `observed movement', F 1; 7 68:6, P < 0:01, that is, participants had shorter response latencies when observing a tapping rather than a lifting movement. The time of stimulus movement onset also showed the main eect, F 2; 14 33:2, P < 0:01: the longer the interval between start frame and ®rst movement frame, the faster the response was initiated. `Stimulus movement onset' interacted with `observed movement', F 1; 7 9:1, P < 0:01, indicating that the decrease of reaction time for stimulus movements that started late was steeper when observing a lifting rather than a tapping movement. The interaction of observed and executed movement was found to be again signi®cant, F 1; 7 27:8, P < 0:01, indicating that when the observed and executed movement were compatible, movement initiation was faster compared to incompatible trials (237 vs. 268 ms). Importantly, this interaction changed with hand orientation, indicated by a three-way interaction, `observed movement' ´ `executed movement' ´ `hand orientation', F 1; 7 28:9, P < 0:01, revealing larger compatibility eects for normal than for ¯ipped hand orientation. Nevertheless, the compatibility eect was signi®cant for normal hand orientation, F 1; 7 31:2, P < 0:01 as well as for ¯ipped hand orientation, F 1; 7 9:7, P < 0:05 (Fig. 6). In order to disentangle the factors `movement type' and `movement direction', we rearranged the data. We added the two conditions of the un¯ipped session, in which `movement direction' and `movement type' were both compatible. We also added the two conditions where they were both incompatible, and compared these two means with
16
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Fig. 6. RT as a function of `observed movement' (tapping vs. lifting), `executed movement' (tapping vs. lifting) and `hand orientation' (¯ipped vs. un¯ipped). The hand was presented in a regular or upside-down (¯ipped) orientation.
the situations where only one of them was compatible (Table 1). This analysis showed that both `movement type', F 1; 7 27:8, P < 0:01 and `movement direction', F 1; 7 28:9, P < 0:01 in¯uenced the compatibility eect (Fig. 7). However, the in¯uence of `movement type' was larger than that of `movement direction' (31 vs. 20 ms, t 8 3:11, P < 0:05). There was no interaction between these two factors, F 1; 7 0:8, P 0:39. The distribution analysis (Fig. 8) showed an interaction between quintile and compatibility for Ômovement typeÕ, F 4; 28 24:1, P < 0:01 as well as `movement direction', F 4; 28 2:8, P < 0:05. This indicates that the compatibility eect grew for slower responses for both components. The three-way interaction (component * compatibility * quintile) was also signi®cant, F 4; 28 8:1, P < 0:01. The increase of the compatibility eect was larger for `movement type' than for `movement direction'. 4.3. Discussion The within-subject comparison of hand orientation uncovered some interesting details about the previously observed compatibility ®ndings. In Experiment 3, a compatibility eect for movement type was observed even if movement direction and type of movement were displayed in opposite directions. This ®nding demonstrates that observed movement type has a stronger in¯uence than the observed movement direction in controlling response execution. Nevertheless, by separating the factors movement direction and movement type, it became clear that both factors independently in¯uence the compatibility eect. Interestingly, the size of the ®nger movement direction component was comparable to the compatibility eect we observed in Experiment 2 for the object movement condition. We speculate that the object movement condition in Experiment 2 and the movement direction component
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
17
Table 1 Congruency of `movement type' and `movement direction' components for normal (upright) and ¯ipped (upside down) orientation of the displayed hand Congruency (normal hand orientation)
Movement type
Movement direction
Executed movement
Type Type Type Type
Tapping Lifting Tapping Lifting
Downward Upward Downward Upward
Lifting Lifting Tapping Tapping
Congruency (¯ipped hand orientation)
Movement type
Observed direction
Executed movement
Direction congruent/type incongruent Type congruent/direction incongruent Type congruent/direction incongruent Direction congruent/type incongruent
Tapping Lifting Tapping Lifting
Upward Downward Upward Downward
Lifting Lifting Upward Upward
and and and and
direction direction direction direction
incongruent congruent congruent incongruent
Fig. 7. RT as a function of compatibility and the components `movement type' (tapping vs. lifting) and Ômovement directionÕ (upward vs. downward).
in Experiment 3 represent the same mechanism, namely, dynamic spatial compatibility. The distribution analysis of movement type and direction demonstrated that the increase of the compatibility eect for slow as compared to fast responses is larger for the movement type component than that for movement direction. This ®nding again supports the assumption that two mechanisms were eective in this task: a fast mechanism that showed only a small increase over the entire distribution
18
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Fig. 8. Mean RT quintiles as a function of compatibility of observed and executed movement and the components `movement type' (tapping vs. lifting) and `movement direction' (upward vs. downward).
and a slow mechanism that increases in time. While the fast mechanism seems to be related to some kind of dynamic spatial compatibility component, the slow mechanism appears to be related to the movement type component, that is, lifting or tapping, which requires processing of complex properties of the action.
5. General discussion The primary aim of the present study was to investigate whether a stimulus± response arrangement with high ideomotor compatibility leads to a compatibility eect in a task with minimal response-selection requirements as was predicted by the ideomotor theory. The result of Experiment 1 clearly supports this prediction showing a pronounced compatibility eect. In Experiment 2 we tested whether the eect observed in Experiment 1 was primarily related to some kind of general dynamic spatial compatibility and not to ideomotor compatibility. Even if the results of Experiment 2 have to be handled with care, as outlined above, the comparison of object movements and ®nger movements indicates that the compatibility eect decreases if the amount of ideomotor compatibility is reduced. Comparing the distribution analysis for ®nger and object movements brought up the suggestion that the in¯uence of movement observation on movement execution in this task can be split into a movement direction component (up vs. down) and a movement type component (lifting vs. tapping). Changing the hand orientation in Experiment 3 allowed for the separation of the two mechanisms, without the perceptual problem of Experiment 2. The data, particularly the distribution analysis, again supported a distinction within the compatibility eect.
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
19
5.1. Ideomotor compatibility and compatibility eects in SRT The demonstration of a pronounced compatibility eect in an SRT when using an S±R arrangement with high ideomotor compatibility has interesting implications for current views on SRC. Theories arguing that S±R compatibility is only related to the response-selection stage would probably have serious problems to explain a compatibility eect in a task, in which response selection requirements are minimal. Hence, this ®nding is more in agreement with models that argue for additional mechanisms involved in SRC (Kornblum, Hasbroucq & Osman, 1990; Hommel, 1997). The model of dimensional overlap (Kornblum et al., 1990; Kornblum & Lee, 1995) proposes two mechanisms: a con®rmation mechanism that is rule-based and is involved in identifying the correct response, and a mechanism that leads to an automatic response activation in cases of dimensional overlap between stimulus and response (Kornblum et al., 1990). Because this model was developed to account for results of choice-reaction tasks, it does not include any predictions about compatibility eects in SRT. Unlike most models of SRC, the action-concept model of Hommel (1997) predicts compatibility eects in SRT (Hommel, 1996). It holds that stimulus and response codes are represented in the same representational domain. With feature overlap between stimulus and the response code, prede®ning a response does not prevent the response from being in¯uenced by the irrelevant stimulus (Hommel, 1997). Although this model is closely related to the ideomotor principle, the concept of ideomotor compatibility stresses the importance of the similarity between observed movement and response image. So far, empirical evidence for compatibility eects in SRT has been weak. Since the eects observed with lateralized stimuli and blocked responses were typically small, there has been an extensive discussion whether this eect re¯ects a cognitive function at all. It has been argued that the advantage of ipsilateral responses may be due to anatomical factors only (Berlucchi, Crea, Di Stefano & Tassinari, 1977; Bashore, 1981; Marzi et al., 1991; Berlucchi, Heron, Hyman, Rizzolatti & Umilta, 1971). Berlucchi et al. (1977) argued that at least with spatial compatibility, compatibility eects only occur if there are two response alternatives, so that the subject has to choose between the responses. In agreement with this notion is the fact that studies which did ®nd substantial compatibility eects in SRT had to introduce a second response alternative (e.g., Hommel, 1996). In this context, this refers to the fact that the alternative response actually needs to be executed. Our results, however, clearly demonstrate that for an ideomotor kind of stimulus±response arrangement, it is possible to establish the second response alternative purely on the stimulus side. The large compatibility eect in our experiments point to the major dierence between paradigms used in previous compatibility studies and the present study, that is, we used a quasi-imitative S±R arrangement. Consequently, we think that the ®nding of a robust eect in our paradigm is best captured with the concept of ideomotor compatibility. To speak with Greenwald (1972): ``The ideomotor compatibility dimension overlaps with, but is not identical to, S±R compatibility'' (p. 52). The results of Experiment 2 are in agreement with this dierentiation; a decrease of the compatibility eect was observed when the observed ®nger movement was
20
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
changed into an object movement, a change probably reducing ideomotor compatibility. Additional support for the assumption that ideomotor compatibility differs from other dynamical spatial S±R compatibility phenomena came from the separation of two distinct mechanisms underlying the compatibility eect in the present study. One mechanism seems to be related to some kind of spatial compatibility, the other to more complex properties of the action. This second mechanism may be identi®ed as GreenwaldÕs notion of ideomotor compatibility. This component can be best described as a match between the event, perceived and the representation of what one intends to do, that is, an anticipation of the sensory consequences of the planned action. 5.2. Ideomotor theory and its implication for an action theory of imitation Our results are not only interesting from the perspective of SRC, but also with regard to a functional theory of imitation. The term ``imitation'' is typically used for those cases where a ``you do what I do'' model-imitator situation is present. In our experiments participants were not instructed to imitate the observed movements but to respond to an observed movement on a screen with a pre-instructed movement. In a sense, it seems to be dicult to draw any conclusions from these experiments with regard to mechanisms underlying imitation proper. On the other hand, it may not be necessary to study intentional imitation directly in order to shed light on the mechanisms underlying action control in imitation. Many conclusions about how human beings imitate have been drawn from research in early infancy (Meltzo & Moore, 1977; Piaget, 1962), much of which re¯ects unintentional imitation (Prinz, 1987). The functioning of the mechanism underlying imitation is certainly not restricted to situations in which the intention to imitate is involved. Rather, imitation might involve a process of automatic response priming in correspondence to the observed movement. The present study demonstrates that movement observation in¯uences movement execution even in a task in which the response is prede®ned, which strongly supports the assumption of automaticity in the sense that this priming process occurs involuntarily and is not under the actorÕs control (Bargh, 1992). What conditions are necessary to produce such an automatic response activation process is open to further investigation. Variables like the intention to act and the possibility to execute the response alternative may play an important role. The assumption of an automatic in¯uence of movement observation was also supported by a study using transcranial magnetic stimulation (Fadiga, Fogassi, Pavesi & Rizzolatti, 1995). They demonstrated that observing a movement leads to speci®c activation of the corresponding response measured by electromyographic muscle activity, when the cortical activation was increased by transcranial magnetic stimulation of the motor cortex. Such an activation process may lead to overt behavior in early infancy while in adults it may be only observable in a reaction time experiment. Nevertheless, from the ideomotor point of view, it can be argued that imitation of action is a special case of S±R compatibility such that the stimulus prespeci®es certain aspects of the perceivable consequences of the action.
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
21
Acknowledgements We thank Dirk Kerzel and Bas Neggers for technical support and comments on an earlier version of this paper, Heidi John for improving the English, Susanne Schorb for data collection and Frank Miedreich for assistance in programming.
References Bargh, J. A. (1992). The ecology of automaticity: toward establishing the conditions needed to produce automatic processing eects. American Journal of Psychology, 105(2), 181±199. Bashore, T. R. (1981). Vocal and manual reaction time estimates of interhemispheric transmission time. Psychological Bulletin, 89(3), 352±368. Berlucchi, G., Crea, F., Stefano, M., & Tassinari, G. (1977). Di In¯uence of spatial stimulus±response compatibility on reaction time of ipsilateral and contralateral hand to lateralized light stimuli. Journal of Experimental Psychology, 3(3), 505±517. Berlucchi, G., Heron, W., Hyman, R., Rizzolatti, G., & Umilt a, C. (1971). Simple reaction times of ipsilateral and contralateral hand to a lateralized visual stimuli. Brain, 94(3), 419±430. Callan, J., Klisz, D., & Parsons, O. A. (1974). Strength of auditory stimulus±response compatibility as a function of task complexity. Journal of Experimental Psychology, 102(6), 1039±1045. Fadiga, L., Fogassi, L., Pavesi, G., & Rizzolatti, G. (1995). Motor facilitation during action observation: a magnetic stimulation study. Journal of Neurophysiology, 73(6), 2608±2611. Fiorito, G., & Scotto, P. (1992). Observational learning in octopus vulgaris. Science, 256, 545±547. Gewirtz, J. L. (1969). Mechanisms of social learning: some roles of stimulation and behavior in early human development. In D. A. Goslin, Handbook of socialization theory and research. Chicago, IL: Rand McNally. Gewirtz, J. L., & Stingle, K. G. (1968). Learning of generalized imitation as the basis for identi®cation. Psychological Review, 75(5), 374±397. Greenwald, A. G. (1970b). A choice reaction time test of ideomotor theory. Journal of Experimental Psychology, 80(1), 20±25. Greenwald, A. G. (1970a). Sensory feedback mechanisms in performance control: with special reference to the ideomotor mechanism. Psychological Review, 77, 73±99. Greenwald, A. G. (1972). On doing two things at once: time sharing as a function of ideomotor compatibility. Journal of Experimental Psychology, 94(1), 52±57. Hasbroucq, T., & Guiard, Y. (1991). Stimulus±response compatibility and the Simon eect: toward a conceptual clari®cation. Journal of Experimental Psychology: Human Perception and Performance, 17(1), 246±266. Hasbroucq, T., Kornblum, S., & Osman, A. (1988). A new look at reaction time estimates of interhemispheric transmission time. European Bulletin of Cognitive Psychology, 8(3), 207±221. Hommel, B. (1996). S±R compatibility eects without response uncertainty. Quarterly Journal of Experimental Psychology, 49(3), 546±571. Hommel, B. (1997). Toward an action-concept model of stimulus±response compatibility. In B. Hommel, W. Prinz, Theoretical issues in stimulus±response compatibility (pp. 281±320). Amsterdam: Elsevier. James, W. (1890). The principles of psychology. New York: MacMillan. Johansson, G. (1973). Visual perception of biological motion and a model for its analysis. Perception and Psychophysics, 14, 201±211. Kornblum, S., Hasbroucq, T., & Osman, A. (1990). Dimensional overlap: cognitive basis for stimulus± response compatibility: a model and taxonomy. Psychological Review, 97(2), 253±270. Kornblum, S., & Lee, J.-W. (1995). Stimulus±response compatibility with relevant and irrelevant stimulus dimensions that do and do not overlap with the response. Journal of Experimental Psychology: Human Perception and Performance, 21(4), 855±875.
22
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Marzi, C. A., Bisiacchi, P., & Nicoletti, R. (1991). Is interhemispheric transfer of visuomotor information asymmetric? Evidence from a meta-analysis. Neuropsychologia, 29(12), 1163±1177. Meltzo, A. N., & Moore, M. K. (1977). Imitation of facial and manual gestures by human neonates. Science, 198, 75±78. Meltzo, A. N., & Moore, M. K. (1983). The origins of imitation in infancy: paradigm phenomena, and theories. Advances in Infancy Research, 2, 265±301. Pawlby, S. (1977). Imitative interaction. In H. Schaer, Studies in mother±infant interaction. New York: Academic Press. Piaget, J. (1962). Play, dreams and imitation in childhood. New York: Norton. Prinz, W. (1987). Ideomotor action. In H. Heuer, A. F. Sanders, Perspectives on perception and action (pp. 47±76). Hillsdale, NJ: Lawrence Erlbaum. Ratcli, R. (1979). Group reaction time distributions and an analysis of distribution statistics. Psychological Bulletin, 86, 446±461. St urmer, B., Aschersleben, G., & Prinz, W. (1999). Correspondence eects with manual gestures and postures: a study on imitation. Journal of Experimental Psychology: Human Perception and Performance, in press. Williams, H., & Nottebohm, F. (1985). Auditory responses in avian vocal motor neurons: a motor theory for song perception in birds. Science, 229, 279±282.
www.elsevier.com/locate/actpsy
Movement observation aects movement execution in a simple response task Marcel Brass
a,*
, Harold Bekkering b, Wolfgang Prinz
b
a
b
Department of Neurology, Max Planck Institute for Cognitive Neuroscience, Stephanstr. 1A, D-04103 Leipzig, Germany Department of Cognition and Action, Max Planck Institute for Psychological Research, Amalienstr. 33, D-80799 Munich, Germany Received 28 January 1999; received in revised form 10 April 1999; accepted 10 April 1999
Abstract The present study was designed to examine the hypothesis that stimulus±response arrangements with high ideomotor compatibility lead to substantial compatibility eects even in simple response tasks. In Experiment 1, participants executed pre-instructed ®nger movements in response to compatible and incompatible ®nger movements. A pronounced reaction time advantage was found for compatible as compared to incompatible trials. Experiment 2 revealed a much smaller compatibility eect for less ideomotor-compatible object movements compared to ®nger movements. Experiment 3 presented normal stimuli (hand upright) and ¯ipped stimuli (hand upside-down). Two components were found to contribute to the compatibility eect, a dynamic spatial compatibility component (related to movement directions) and an ideomotor component (related to movement types). The implications of these results for theories about stimulus±response compatibility (SRC) as well as for theories about imitation are discussed. Ó 2001 Elsevier Science B.V. All rights reserved. PsycINFO classi®cation: 2340 Keywords: Reaction time; Motion perception; Perceptual motor processes
*
Corresponding author. Tel.: +49-341-9940-213; fax: +49-341-9940-221. E-mail address: [email protected] (M. Brass).
0001-6918/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 1 - 6 9 1 8 ( 0 0 ) 0 0 0 2 4 - X
4
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
1. Introduction The present study can be placed in two dierent theoretical and experimental contexts: research on stimulus±response compatibility (SRC) and on action imitation. Usually these two ®elds are considered largely isolated from each other. This paper, however, argues that there is a close relationship between them. For this purpose, a theoretical framework is introduced that integrates both perspectives. Research on imitation may thus bene®t from theoretical and experimental accounts of the SRC ®eld about the question of how perception and action interact, while SRC research may bene®t from the investigation of more complex stimulus±response (S±R) relations than has typically been done in this ®eld. 1.1. Imitation: the functional problem The study of imitation phenomena has a long history, not only in psychology but in ethology and anthropology as well. Although imitation is well-documented in dierent ®elds of research and species (e.g., Fiorito & Scotto, 1992; Williams & Nottebohm, 1985), it is fair to say that the mechanisms underlying imitation are still poorly understood. From an action perspective, the most fundamental question of imitation may be described as ``How can a motor act be constructed from a perceived act? Since, if we observe somebody executing a movement, we do not perceive the muscle activation underlying the movement. So how can we know which muscle movements will generate a movement that looks like the one we have observed?'' (Prinz, 1987). Several attempts to solve this issue have been made. Some authors tried to reduce the problem to an instrumental learning (Gewirtz, 1969; Gewirtz & Stingle, 1968) or an associative learning mechanism (Pawlby, 1977). One of the most elaborated and in¯uential theories on imitation was proposed by Piaget (1962). His developmental theory of imitation was built around the concept of circular reactions. Infants explore the relation between motor act and the sensory eect of that action by performing speci®c actions repetitively. During this repetition process a link is established between the perceived act and the motor execution. When a child observes an adult while executing a movement similar to the visual component of their own visual±motor circular reactions, the respective motor component is triggered in the infant. While the observer interprets the infantÕs behavior as imitation, the child is just repeating a circular reaction where the impetus to repeat the circle comes from the perception of the visual component of the ensemble (Meltzo & Moore, 1983). This interpretation cannot explain all imitation phenomena, as was pointed out by Meltzo and Moore (1977) who detected very early imitation of facial gestures in neonates. Neonates cannot build a visual representation of their own facial gestures and, hence, the perceived gesture of a model cannot trigger the motor act by replacing this representation. Still the described mechanism underlying circular reactions might provide a ®rst hint to a possible solution of the action problem in imitation. Namely, when we observe a movement we see the expected outcome of a potential self-executed movement. This view of imitation is very close to an early
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
5
theory of motor control, the ideomotor theory, which will be described in the next paragraph. 1.2. Ideomotor theory and the concept of ideomotor compatibility An interesting suggestion concerning the relationship of perceived and executed action was given by James more than hundred years ago with the description of what he called ideomotor action (James, 1890). James formulated this idea as follows: ``Every representation of a movement awakens in some degree the actual movement which is its object'' (James, 1890, p. 1134). Greenwald (1970a) elaborated on this idea in his theory of ideomotor action. He stated (Greenwald, 1970b) that (a) voluntary responses are represented centrally in the form of images of the sensory feedback they produce, and (b) such images play a controlling role in the performance of their corresponding actions. From this perspective it becomes clear how action observation can be used to guide action execution. The visual response image is one of the major parts of sensory action feedback. Due to a similarity relation, observing an action activates the response image of the corresponding response, which in turn controls the observerÕs performance. This ideomotor mechanism can be applied to account for imitation phenomena, but it is not restricted to imitation. Greenwald (1972) extended this idea to compatibility phenomena, forming the concept of ideomotor compatibility, which denotes ``the extent to which a stimulus corresponds to sensory feedback from its required response'' (p. 52). Ideomotor theory implies that stimuli with high ideomotor compatibility lead to an activation of the corresponding response via the activation of the response image. From this perspective, phenomena of SRC and imitation are closely related. Hence, research on imitative behavior and on SRC can be combined under the theoretical framework of ideomotor theory. 1.3. Compatibility eects in a simple response task What assumptions can be drawn from ideomotor theory with regard to the concept of (ideomotor) compatibility? One of the central ideas of the theory is that perceiving the sensory feedback of an action leads to an activation of the response image. This image plays a controlling role in the performance of the action, assuming that no speci®c S±R translation is involved. In contrast, most models of SRC presuppose that S±R compatibility eects arise at the S±R translation stage (e.g., Hasbroucq & Guiard, 1991). Hence, if a response can be selected before the imperative signal appears, the selection process cannot be aected by the stimulus (however, see Hommel, 1997, for an alternative view). In accordance with theories assuming that SRC operates on S±R translation stage, simple response tasks (SRT) yielded no or only very small S±R eects (e.g., Callan, Klisz & Parsons, 1974; Marzi, Bisiacchi & Nicoletti, 1991; Bashore, 1981; Hasbroucq, Kornblum & Osman, 1988). In SRT paradigm, participants always have to execute the same response, that is, a task in which participants react with a predetermined left- or right-hand response, while random left and right stimuli are
6
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
presented. The absence of robust eects in SRT paradigms seems to support the view that SRC is restricted to choice response tasks. However, in most studies the S±R arrangement was not highly ideomotor compatible. This might be the reason why the eects were typically found to be small. According to the ideomotor principle, SRC eects are expected even in an SRT, where the same response is required independent of the stimulus. The ideomotor theory assumes that even in an SRT, a response image is generated. Observing an action, presumably, evokes a response image as well. Therefore, compatibility between the act seen and the act to be performed should facilitate response execution by activating the actual response image, whereas an incompatible action should interfere with response execution. The general aim of the present study was to investigate the implications of the ideomotor theory for views on imitation and on SRC. Particularly, the study addressed the question of whether S±R arrangements with high ideomotor compatibility lead to compatibility eects in a task with minimal response selection requirements. In addition, we wanted to investigate the ideomotor interpretation of imitation, which suggests that observing a movement activates the equivalent movement. In order to do so, participants were required to execute a pre-instructed ®nger movement in response to the onset of a visually presented compatible or incompatible ®nger movement. When the observed movement is compatible to the preinstructed movement this can be interpreted as a Ôquasi-imitativeÕ situation, that is, the subject performs a similar action as the action shown on the screen. Perceiving a compatible movement should activate the pre-instructed movement, which should result in faster reaction times. In addition, if the observed movement is incompatible with the instructed movement, the incompatible response is activated and thus needs to be inhibited, resulting in slower reaction times.
2. Experiment 1 The ®rst experiment was a simple response task, in which participants executed the same ®nger movement within one block while reacting to the onset of a ®nger movement on a screen. The observed movement, which served as a go-signal for the prede®ned response, was compatible or incompatible with the to-be-executed response. In one block, the required movement was always to lift the index ®nger from a neutral starting position, that is the ®nger was held a few centimeters above the table. In the other block, the task was to tap on the table with the index ®nger from the same neutral starting position. Stimuli were small digitized video sequences of similar ®nger movements (lifting vs. tapping). If ideomotor compatibility between the act seen and the act to be performed in¯uences response execution by activating the actual response image, robust compatibility eects are expected in the present SRT paradigm. For paradigms with spatial S±R arrangements, these eects are about 20±70 ms in choice tasks (see Hommel, 1996). However, if compatibility occurs on the stage of stimulus±response translation, only minor compatibility eects
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
7
are expected in the present SRT paradigm. In SRT experiments with spatial S±R arrangements eects are about 2±6 ms (e.g. Hasbroucq et al., 1988; Hommel, 1996). 2.1. Method 2.1.1. Participants Eight students of the University of Munich (6 female and 2 male) participated in a 30 min session. All were right-handed, had normal or corrected-to-normal vision, and were naive with respect to the purpose of the study. They were paid for participation (10 DM). 2.1.2. Apparatus and material A Compaq Pentium 200 MHz computer controlled stimulus presentation and data acquisition. Short digitized video sequences of an animated hand were presented on a 17-in. screen with a frame frequency of 60 Hz. The video sequences consisted of ®ve frames. The animated hand had a visual angle of about 6.5° vertically and 10° horizontally. Participants were seated about 80 cm in front of the screen. The animated hand was in a mirrored orientation to the participantÕs right hand, which rested on a table. Movements were recorded by an Optotrak system (Norton Digitalä). Six cameras were installed to sample raw position data of an active infrared marker with a sampling rate of 250 Hz which was placed on the ®ngernail of the subject's index ®nger. The system allows for a reconstruction of movement trajectory in three-dimensional space. Movement onsets were computed from these trajectories and o-line analyzed with MATLAB 5.0 routines. For detection of the movement onset, a relative velocity threshold was used (15% of peak velocity). Onset of movement was de®ned as the moment the velocity exceeded this threshold and a minimum displacement (20% of movement amplitude for the lifting movement and 10% of movement amplitude for the tapping movement) took place within a maximum amount of time after this onset (150 ms for the lifting movement and 80 ms for the tapping movement). 2.1.3. Design and procedure The experiment was divided into two blocks. One requiring to lift the index ®nger as soon as a ®nger movement appeared, the other requiring to move the ®nger downwards. The video sequence started with a frame showing the index ®nger in a middle, resting position. This frame, which was identical for both movements, remained visible for 800, 1600, or 2400 ms. Then three movement frames ¯ashed in successive order in which the ®nger moved randomly either up or down. The ®rst two movement frames were presented for two refreshing cycles (about 34 ms) each. The last frame contained a picture of the ®nal ®nger position and remained on the screen for 500 ms. The overall displacement of the ®nger was about 2° for the lifting and for the tapping movement. The three time intervals between the onset of the ®rst stimulus frame and the onset of the stimulus movement served to reduce anticipation errors. Total trial length was kept constant (about 5.5 s). Between the trials the screen turned blue for 1.5±3 s, depending on the length of the start frame in the
8
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
previous trial. Each block contained 120 trials (20 trials for each observed movement * 3 time intervals for stimulus movement onset. Block order was counterbalanced across participants. Participants executed the instructed movement on observing a movement, irrespective of the type of seen movement. Prior to each block, 10 test trials were presented. 2.1.4. Data analysis Errors. Theoretically, two types of errors could occur: First executing the wrong movement (instead of lifting tapping or vice versa). This error only occurred in the ®rst test trials but not during the blocks. Second, a response could be initiated too early (for criterion see below). One participant was replaced for exceeding the 10% error criterion. For the other participants this error type occurred in 2% of the trials. These trials were excluded from analysis. Response times. Response times below 80 ms and above 800 ms were excluded from analysis. An ANOVA with three within-subject factors was computed over the remaining data, the factors being Ôexecuted movementÕ (lifting vs. tapping), Ôobserved movementÕ (lifting vs. tapping), and Ôstimulus movement onsetÕ (800, 1600, 2400 ms). 2.2. Results A main eect was found for `observed movement', F 1; 7 17:2, P < 0:01. Participants reacted faster when they observed a tapping compared to a lifting movement. Also, a main eect for `stimulus movement onset' was found, F 2; 14 44:6, P < 0:01, that is, the longer the interval between the start frame and the ®rst movement frame, the faster the action was initiated. The most interesting eect was the two-way interaction of observed and executed movement (Fig. 1).
Fig. 1. Movement onset (RT) as a function of `observed movement' (tapping vs. lifting) and `executed movement' (tapping vs. lifting).
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
9
Fig. 2. Mean RT quintiles as a function of compatibility of observed and executed movement.
Participants were much faster if they had to execute the compatible movement (278 ms) compared to the incompatible movement (333 ms), F 1; 7 8:2, P < 0:05. This holds for both movements, but there was an asymmetry. That is, the compatibility eect is smaller for the lifting than for the tapping movement t 1; 7 4; 1, P < 0:01. In a distribution analysis (Fig. 2) we divided the response time distribution in quintiles (Ratcli, 1979), indicating that the compatibility eect becomes larger for slower responses, F 4; 28 7:6, P < 0:01. 2.3. Discussion The results of Experiment 1 clearly demonstrate that responses were faster when the observed movement was compatible to the executed movement than if it was not. Interestingly, this eect was pronounced compared to other attempts to demonstrate S±R compatibility in a simple response task, as reviewed in Section 1. Given the block size of 120 trials, the compatibility eect cannot be explained by some kind of inertia eect from one experimental block to the other (Hommel, 1996). Moreover, the distribution analysis shows that the compatibility eect becomes larger for slower responses, indicating that the process underlying this eect becomes more in¯uential over time. From the perspective of ideomotor theory this makes sense, because complex properties of the observed action, which presumably require more timeconsuming processing, correspond to the sensory feedback from the required response, thus being more likely to aect slower responses. Another surprising result of the ®rst experiment was the asymmetry for the tapping and the lifting movement. We can only speculate about the reason for this eect. One explanation might be that the tapping movement is characterized by two distinct movement components separated by tactile feedback, that is, tapping downwards and moving the ®nger back to the
10
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
start position, while for the lifting movement the upward and downward components are not separated by tactile feedback. Taken together, the ®rst experiment supports the prediction that in a condition with high ideomotor compatibility observing a ®nger movement leads to some kind of activation of the corresponding response. This results in a substantial compatibility eect, even in the case in which response selection processes are minimized. However, it is still unresolved whether the eect has to be attributed to high ideomotor compatibility. One can argue that it was due to some kind of dynamic spatial compatibility and not so much to the quasi-imitative character of observed and executed action (cf. St urmer, Aschersleben & Prinz, 1999). To test this alternative explanation, we designed an experiment in which we compared ®nger movements with object movements, assuming that object movements are less ideomotor-compatible to the to-be executed ®nger movements than ®nger movements.
3. Experiment 2 In Experiment 2 participants again were instructed to execute upward and downward ®nger movements in response to the same ®nger movements. However, in addition to this replication of Experiment 1, moving squares were presented. If the large compatibility eect in Experiment 1 was due to the amount of ideomotor compatibility, the compatibility eect should be much smaller when the go-signal is provided by an object movement. If, however, the large compatibility eect of Experiment 1 only owed dynamic spatial compatibility, one might expect to ®nd similar compatibility eects for object and ®nger movement. 3.1. Method 3.1.1. Participants Ten students of the University of Munich (5 female and 5 male) participated in a 30 min session. All were right-handed, had normal or corrected-to-normal vision, and were naive with respect to the purpose of the study. They were paid 10 DM for participation. 3.1.2. Apparatus and material Stimulus presentation and measurement procedure were identical to Experiment 1. The object movement ®lms were produced by a digital version of the point-light technique developed by Johansson (1973). A black square marked the ®ngernail of the index ®nger in each frame of the two movement ®lms (lifting and tapping). The background was turned blue, so that only the squares were visible. Displaying these frames in successive order leads to the impression of square moving up or down. The kinematics and amplitude of the moving squares were identical to those of the ®nger movement.
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
11
3.1.3. Design and procedure The experiment was again divided into two blocks. In each block, ®nger movement (tapping and lifting) and object movement (up and down) were randomly presented. In one block, participants always had to lift their index ®nger on detecting a movement (irrespective of the stimulus type). In the other block, the ®nger was to move downwards. As in Experiment 1 ten test trials were performed prior to each block. The temporal order of the digitized video sequences was identical for ®nger and object movement. As in Experiment 1, there were three intervals between onset of the ®rst stimulus frame and that of the stimulus movement in order to reduce anticipation errors. Each observed ®nger movement (tapping or lifting), each object movement (up and down) and the three stimulus movement onsets (800, 1600, 2400 ms) were presented ten times in random order. Thus each block consisted of 120 trials. Block order was counterbalanced across participants. 3.1.4. Data analysis Errors. In 1.4% of the trials participants made a ®nger movement before the movement on the computer screen started. Response times. An ANOVA was computed with the factors `observed movement' (upward vs. downward), `executed movement' (tapping versus lifting), `stimulus movement onset' and `moving stimulus' (squares vs. ®ngers). 3.2. Results Main eects were found for `observed movement', F 1; 9 6:0, P < 0:05, and for `stimulus movement' onset, F 2; 18 43:9, P < 0:01. Participants reacted faster when they observed a tapping compared to a lifting movement, and the longer the interval between the start frame and the ®rst movement frame, the faster the action was initiated. The main eect for `type of stimuli' (square vs. ®nger), F 1; 9 29:6, P < 0:01, was observed. Participants reacted faster to the ®nger movement (265 ms) than to the object movement (284 ms). Again, a compatibility eect of observed and executed movement was found to be signi®cant, F 1; 9 8:7, P < 0:01, indicating that participants reacted faster to compatible trials (258 ms) than to incompatible trials (291 ms). In addition, the three-way interaction with `stimulus type' reached statistical signi®cance, F 1; 9 5:2, P < 0:05. While the compatibility eect of stimulus ®nger movement was signi®cant, F 1; 9 9:4, P < 0:05, that of stimulus object movement (Fig. 3) showed only a statistical trend, F 1; 9 3:5, P 0:09. An analysis of the RT-distribution (Fig. 4) revealed an interaction between RT-quintiles and compatibility for observed ®nger movement, F 4; 36 7:5, P < 0:01 but not for object movement, F 4; 36 0:1, P 0:59. The three-way interaction (type of stimuli * compatibility * quintile) was also signi®cant, F 4; 36 2:9, P < 0:05. This indicates that the compatibility eect becomes larger for slower responses with observed ®nger movements but not for observed object movements.
12
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Fig. 3. RT as a function of `observed movement' (tapping vs. lifting) and `executed movement' (tapping vs. lifting) and `type of stimuli' (®nger movement vs. object movement). Finger movements were compared with an upward or downward moving square.
Fig. 4. Mean RT quintiles as a function of compatibility of observed and executed movement and type of observed stimuli (®nger movement vs. object movement).
3.3. Discussion Results demonstrate a clear dierentiation between the eect of both observed ®nger and object movement on response latencies of executed ®nger movements. Importantly, the compatibility eect was much more pronounced for ®nger than for
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
13
object movements. Nevertheless, a statistical trend was also found for the latter. A possible account for this pattern of results is the notion that two separate mechanisms underlie the compatibility eect: One mechanism that is related to the movement direction, and another related to the movement type. What we call movement direction component can either be caused by the direction of the moving stimulus or by the end position of the stimulus relative to the starting point. While the observation of object movements only activated the movement direction component, the observation of ®nger movements activated both the movement direction component and the movement type component. This interpretation is supported by the distribution analysis of the two types of moving stimuli. The compatibility eect for observed object movement was already present for fast responses and remained constant over the whole range of distribution, suggesting that the relatively simple process of identifying movement direction arises early in the processing stream and adds a constant contribution, independent of response times. On the other hand, the compatibility eect for ®nger movement showed a successive increase in the distribution range, thus suggesting that processing the type of movement required more complex processes which needs more time in order to become eective. There is however a strong alternative interpretation to explain the observed differences: Simply the fact that the two stimulus types dier perceptually from each other. That is, the suggestion derived from the results of Experiment 2, that one mechanism is related to direction and one to movement type (tapping or lifting), is confounded with the change in stimulus display. This alternative explanation is supported by the fact that the movement identi®cation for ®nger movement was faster than that for object movement. One might argue that the dierence in the compatibility eect for object movement and ®nger movement was caused by a perceptual dierence only. The main eect for type of moving stimuli also restricts the conclusion that can be drawn from the distribution analysis. The purpose of Experiment 3 was to separate the two components `movement direction' and `movement type' without the perceptual problem of Experiment 2, by using the same stimulus movement throughout. One possibility to achieve such a separation is to oppose the predicted compatibility eects by turning the stimulus display upside down. In addition, this method will also allow us to quantify the contributions of these two processes to the overall compatibility eect. 4. Experiment 3 To distinguish `movement direction' from the `type of movement', Experiment 3 was sub-divided: One part was a pure replication of Experiment 1. In the other part, everything was identical except that the original stimuli were ¯ipped upside down. Thus, the former tapping movement now led to an upward movement, and vice versa (Fig. 5). This design allows for an opposite prediction regarding the compatibility eect for movement direction (up vs. down) and type of movement (tapping vs. lifting). If movement direction was the primary component of the previously observed compatibility eects of Experiments 1 and 2, movement direction should have
14
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Fig. 5. Compatibility for movement type and movement direction with normal and ¯ipped hand orientation. For normal hand orientation, movement direction and movement type are both compatible to a tapping movement. For ¯ipped hand orientation, movement direction is compatible to a lifting movement while movement type is compatible to a tapping movement.
a larger in¯uence on the compatibility eect. Conversely, if movement type was the primary component, compatibility should be more strongly in¯uenced by movement type (tapping or lifting). A comparison between the same ®nger responses on the ¯ipped and the un¯ipped hand-orientation trials will allow us to separate the contributions of movement direction and type of movement to the compatibility eect. 4.1. Method 4.1.1. Participants Six students and two pupils (5 females, 3 males) took part in the experiment. All participants had normal or corrected-to-normal vision, were naive to the purpose of the experiment, and reported to be right-handed. Participants were paid for participation with 20 DM. 4.1.2. Apparatus and material The digitized video images of Experiment 1 were ¯ipped upside down by software (Photoshop 4.0ä). Except this variation the stimulus material and measurement equipment were identical to Experiment 1.
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
15
4.1.3. Procedure The procedure for each session was identical to Experiment 1, consisting of two blocks with 120 trials each. In one session participants reacted to the stimuli of Experiment 1, and had to react to the ¯ipped video images in the second session. The mean interval between both sessions was 2.25 days. The order of sessions (normal hand vs. ¯ipped hand orientation) was counterbalanced across participants. The order of blocks was also balanced. The instruction was to execute the instructed movement as soon as the hand on the screen moved, irrespective of the type of observed movement. 4.1.4. Data analysis Errors. Like in Experiments 1 and 2, only the second type of errors (reacting before the stimulus movement onset) occurred. In the experimental session with normal hand orientation error rate was 1.6%. In the session with ¯ipped hand orientation 1.5% errors occurred. These trials were excluded from analysis. Again one participant was replaced for exceeding the 10% error criterion. Another participant was replaced because he had been assigned to the wrong condition. Response times. An ANOVA with repeated measures was computed with the factors `observed movement' (tapping vs. lifting), `executed movement' (tapping vs. lifting), and `hand orientation' (normal vs. ¯ipped) as well as `onset of stimulus movement'. 4.2. Results The analysis revealed the main eect for `observed movement', F 1; 7 68:6, P < 0:01, that is, participants had shorter response latencies when observing a tapping rather than a lifting movement. The time of stimulus movement onset also showed the main eect, F 2; 14 33:2, P < 0:01: the longer the interval between start frame and ®rst movement frame, the faster the response was initiated. `Stimulus movement onset' interacted with `observed movement', F 1; 7 9:1, P < 0:01, indicating that the decrease of reaction time for stimulus movements that started late was steeper when observing a lifting rather than a tapping movement. The interaction of observed and executed movement was found to be again signi®cant, F 1; 7 27:8, P < 0:01, indicating that when the observed and executed movement were compatible, movement initiation was faster compared to incompatible trials (237 vs. 268 ms). Importantly, this interaction changed with hand orientation, indicated by a three-way interaction, `observed movement' ´ `executed movement' ´ `hand orientation', F 1; 7 28:9, P < 0:01, revealing larger compatibility eects for normal than for ¯ipped hand orientation. Nevertheless, the compatibility eect was signi®cant for normal hand orientation, F 1; 7 31:2, P < 0:01 as well as for ¯ipped hand orientation, F 1; 7 9:7, P < 0:05 (Fig. 6). In order to disentangle the factors `movement type' and `movement direction', we rearranged the data. We added the two conditions of the un¯ipped session, in which `movement direction' and `movement type' were both compatible. We also added the two conditions where they were both incompatible, and compared these two means with
16
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Fig. 6. RT as a function of `observed movement' (tapping vs. lifting), `executed movement' (tapping vs. lifting) and `hand orientation' (¯ipped vs. un¯ipped). The hand was presented in a regular or upside-down (¯ipped) orientation.
the situations where only one of them was compatible (Table 1). This analysis showed that both `movement type', F 1; 7 27:8, P < 0:01 and `movement direction', F 1; 7 28:9, P < 0:01 in¯uenced the compatibility eect (Fig. 7). However, the in¯uence of `movement type' was larger than that of `movement direction' (31 vs. 20 ms, t 8 3:11, P < 0:05). There was no interaction between these two factors, F 1; 7 0:8, P 0:39. The distribution analysis (Fig. 8) showed an interaction between quintile and compatibility for Ômovement typeÕ, F 4; 28 24:1, P < 0:01 as well as `movement direction', F 4; 28 2:8, P < 0:05. This indicates that the compatibility eect grew for slower responses for both components. The three-way interaction (component * compatibility * quintile) was also signi®cant, F 4; 28 8:1, P < 0:01. The increase of the compatibility eect was larger for `movement type' than for `movement direction'. 4.3. Discussion The within-subject comparison of hand orientation uncovered some interesting details about the previously observed compatibility ®ndings. In Experiment 3, a compatibility eect for movement type was observed even if movement direction and type of movement were displayed in opposite directions. This ®nding demonstrates that observed movement type has a stronger in¯uence than the observed movement direction in controlling response execution. Nevertheless, by separating the factors movement direction and movement type, it became clear that both factors independently in¯uence the compatibility eect. Interestingly, the size of the ®nger movement direction component was comparable to the compatibility eect we observed in Experiment 2 for the object movement condition. We speculate that the object movement condition in Experiment 2 and the movement direction component
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
17
Table 1 Congruency of `movement type' and `movement direction' components for normal (upright) and ¯ipped (upside down) orientation of the displayed hand Congruency (normal hand orientation)
Movement type
Movement direction
Executed movement
Type Type Type Type
Tapping Lifting Tapping Lifting
Downward Upward Downward Upward
Lifting Lifting Tapping Tapping
Congruency (¯ipped hand orientation)
Movement type
Observed direction
Executed movement
Direction congruent/type incongruent Type congruent/direction incongruent Type congruent/direction incongruent Direction congruent/type incongruent
Tapping Lifting Tapping Lifting
Upward Downward Upward Downward
Lifting Lifting Upward Upward
and and and and
direction direction direction direction
incongruent congruent congruent incongruent
Fig. 7. RT as a function of compatibility and the components `movement type' (tapping vs. lifting) and Ômovement directionÕ (upward vs. downward).
in Experiment 3 represent the same mechanism, namely, dynamic spatial compatibility. The distribution analysis of movement type and direction demonstrated that the increase of the compatibility eect for slow as compared to fast responses is larger for the movement type component than that for movement direction. This ®nding again supports the assumption that two mechanisms were eective in this task: a fast mechanism that showed only a small increase over the entire distribution
18
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Fig. 8. Mean RT quintiles as a function of compatibility of observed and executed movement and the components `movement type' (tapping vs. lifting) and `movement direction' (upward vs. downward).
and a slow mechanism that increases in time. While the fast mechanism seems to be related to some kind of dynamic spatial compatibility component, the slow mechanism appears to be related to the movement type component, that is, lifting or tapping, which requires processing of complex properties of the action.
5. General discussion The primary aim of the present study was to investigate whether a stimulus± response arrangement with high ideomotor compatibility leads to a compatibility eect in a task with minimal response-selection requirements as was predicted by the ideomotor theory. The result of Experiment 1 clearly supports this prediction showing a pronounced compatibility eect. In Experiment 2 we tested whether the eect observed in Experiment 1 was primarily related to some kind of general dynamic spatial compatibility and not to ideomotor compatibility. Even if the results of Experiment 2 have to be handled with care, as outlined above, the comparison of object movements and ®nger movements indicates that the compatibility eect decreases if the amount of ideomotor compatibility is reduced. Comparing the distribution analysis for ®nger and object movements brought up the suggestion that the in¯uence of movement observation on movement execution in this task can be split into a movement direction component (up vs. down) and a movement type component (lifting vs. tapping). Changing the hand orientation in Experiment 3 allowed for the separation of the two mechanisms, without the perceptual problem of Experiment 2. The data, particularly the distribution analysis, again supported a distinction within the compatibility eect.
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
19
5.1. Ideomotor compatibility and compatibility eects in SRT The demonstration of a pronounced compatibility eect in an SRT when using an S±R arrangement with high ideomotor compatibility has interesting implications for current views on SRC. Theories arguing that S±R compatibility is only related to the response-selection stage would probably have serious problems to explain a compatibility eect in a task, in which response selection requirements are minimal. Hence, this ®nding is more in agreement with models that argue for additional mechanisms involved in SRC (Kornblum, Hasbroucq & Osman, 1990; Hommel, 1997). The model of dimensional overlap (Kornblum et al., 1990; Kornblum & Lee, 1995) proposes two mechanisms: a con®rmation mechanism that is rule-based and is involved in identifying the correct response, and a mechanism that leads to an automatic response activation in cases of dimensional overlap between stimulus and response (Kornblum et al., 1990). Because this model was developed to account for results of choice-reaction tasks, it does not include any predictions about compatibility eects in SRT. Unlike most models of SRC, the action-concept model of Hommel (1997) predicts compatibility eects in SRT (Hommel, 1996). It holds that stimulus and response codes are represented in the same representational domain. With feature overlap between stimulus and the response code, prede®ning a response does not prevent the response from being in¯uenced by the irrelevant stimulus (Hommel, 1997). Although this model is closely related to the ideomotor principle, the concept of ideomotor compatibility stresses the importance of the similarity between observed movement and response image. So far, empirical evidence for compatibility eects in SRT has been weak. Since the eects observed with lateralized stimuli and blocked responses were typically small, there has been an extensive discussion whether this eect re¯ects a cognitive function at all. It has been argued that the advantage of ipsilateral responses may be due to anatomical factors only (Berlucchi, Crea, Di Stefano & Tassinari, 1977; Bashore, 1981; Marzi et al., 1991; Berlucchi, Heron, Hyman, Rizzolatti & Umilta, 1971). Berlucchi et al. (1977) argued that at least with spatial compatibility, compatibility eects only occur if there are two response alternatives, so that the subject has to choose between the responses. In agreement with this notion is the fact that studies which did ®nd substantial compatibility eects in SRT had to introduce a second response alternative (e.g., Hommel, 1996). In this context, this refers to the fact that the alternative response actually needs to be executed. Our results, however, clearly demonstrate that for an ideomotor kind of stimulus±response arrangement, it is possible to establish the second response alternative purely on the stimulus side. The large compatibility eect in our experiments point to the major dierence between paradigms used in previous compatibility studies and the present study, that is, we used a quasi-imitative S±R arrangement. Consequently, we think that the ®nding of a robust eect in our paradigm is best captured with the concept of ideomotor compatibility. To speak with Greenwald (1972): ``The ideomotor compatibility dimension overlaps with, but is not identical to, S±R compatibility'' (p. 52). The results of Experiment 2 are in agreement with this dierentiation; a decrease of the compatibility eect was observed when the observed ®nger movement was
20
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
changed into an object movement, a change probably reducing ideomotor compatibility. Additional support for the assumption that ideomotor compatibility differs from other dynamical spatial S±R compatibility phenomena came from the separation of two distinct mechanisms underlying the compatibility eect in the present study. One mechanism seems to be related to some kind of spatial compatibility, the other to more complex properties of the action. This second mechanism may be identi®ed as GreenwaldÕs notion of ideomotor compatibility. This component can be best described as a match between the event, perceived and the representation of what one intends to do, that is, an anticipation of the sensory consequences of the planned action. 5.2. Ideomotor theory and its implication for an action theory of imitation Our results are not only interesting from the perspective of SRC, but also with regard to a functional theory of imitation. The term ``imitation'' is typically used for those cases where a ``you do what I do'' model-imitator situation is present. In our experiments participants were not instructed to imitate the observed movements but to respond to an observed movement on a screen with a pre-instructed movement. In a sense, it seems to be dicult to draw any conclusions from these experiments with regard to mechanisms underlying imitation proper. On the other hand, it may not be necessary to study intentional imitation directly in order to shed light on the mechanisms underlying action control in imitation. Many conclusions about how human beings imitate have been drawn from research in early infancy (Meltzo & Moore, 1977; Piaget, 1962), much of which re¯ects unintentional imitation (Prinz, 1987). The functioning of the mechanism underlying imitation is certainly not restricted to situations in which the intention to imitate is involved. Rather, imitation might involve a process of automatic response priming in correspondence to the observed movement. The present study demonstrates that movement observation in¯uences movement execution even in a task in which the response is prede®ned, which strongly supports the assumption of automaticity in the sense that this priming process occurs involuntarily and is not under the actorÕs control (Bargh, 1992). What conditions are necessary to produce such an automatic response activation process is open to further investigation. Variables like the intention to act and the possibility to execute the response alternative may play an important role. The assumption of an automatic in¯uence of movement observation was also supported by a study using transcranial magnetic stimulation (Fadiga, Fogassi, Pavesi & Rizzolatti, 1995). They demonstrated that observing a movement leads to speci®c activation of the corresponding response measured by electromyographic muscle activity, when the cortical activation was increased by transcranial magnetic stimulation of the motor cortex. Such an activation process may lead to overt behavior in early infancy while in adults it may be only observable in a reaction time experiment. Nevertheless, from the ideomotor point of view, it can be argued that imitation of action is a special case of S±R compatibility such that the stimulus prespeci®es certain aspects of the perceivable consequences of the action.
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
21
Acknowledgements We thank Dirk Kerzel and Bas Neggers for technical support and comments on an earlier version of this paper, Heidi John for improving the English, Susanne Schorb for data collection and Frank Miedreich for assistance in programming.
References Bargh, J. A. (1992). The ecology of automaticity: toward establishing the conditions needed to produce automatic processing eects. American Journal of Psychology, 105(2), 181±199. Bashore, T. R. (1981). Vocal and manual reaction time estimates of interhemispheric transmission time. Psychological Bulletin, 89(3), 352±368. Berlucchi, G., Crea, F., Stefano, M., & Tassinari, G. (1977). Di In¯uence of spatial stimulus±response compatibility on reaction time of ipsilateral and contralateral hand to lateralized light stimuli. Journal of Experimental Psychology, 3(3), 505±517. Berlucchi, G., Heron, W., Hyman, R., Rizzolatti, G., & Umilt a, C. (1971). Simple reaction times of ipsilateral and contralateral hand to a lateralized visual stimuli. Brain, 94(3), 419±430. Callan, J., Klisz, D., & Parsons, O. A. (1974). Strength of auditory stimulus±response compatibility as a function of task complexity. Journal of Experimental Psychology, 102(6), 1039±1045. Fadiga, L., Fogassi, L., Pavesi, G., & Rizzolatti, G. (1995). Motor facilitation during action observation: a magnetic stimulation study. Journal of Neurophysiology, 73(6), 2608±2611. Fiorito, G., & Scotto, P. (1992). Observational learning in octopus vulgaris. Science, 256, 545±547. Gewirtz, J. L. (1969). Mechanisms of social learning: some roles of stimulation and behavior in early human development. In D. A. Goslin, Handbook of socialization theory and research. Chicago, IL: Rand McNally. Gewirtz, J. L., & Stingle, K. G. (1968). Learning of generalized imitation as the basis for identi®cation. Psychological Review, 75(5), 374±397. Greenwald, A. G. (1970b). A choice reaction time test of ideomotor theory. Journal of Experimental Psychology, 80(1), 20±25. Greenwald, A. G. (1970a). Sensory feedback mechanisms in performance control: with special reference to the ideomotor mechanism. Psychological Review, 77, 73±99. Greenwald, A. G. (1972). On doing two things at once: time sharing as a function of ideomotor compatibility. Journal of Experimental Psychology, 94(1), 52±57. Hasbroucq, T., & Guiard, Y. (1991). Stimulus±response compatibility and the Simon eect: toward a conceptual clari®cation. Journal of Experimental Psychology: Human Perception and Performance, 17(1), 246±266. Hasbroucq, T., Kornblum, S., & Osman, A. (1988). A new look at reaction time estimates of interhemispheric transmission time. European Bulletin of Cognitive Psychology, 8(3), 207±221. Hommel, B. (1996). S±R compatibility eects without response uncertainty. Quarterly Journal of Experimental Psychology, 49(3), 546±571. Hommel, B. (1997). Toward an action-concept model of stimulus±response compatibility. In B. Hommel, W. Prinz, Theoretical issues in stimulus±response compatibility (pp. 281±320). Amsterdam: Elsevier. James, W. (1890). The principles of psychology. New York: MacMillan. Johansson, G. (1973). Visual perception of biological motion and a model for its analysis. Perception and Psychophysics, 14, 201±211. Kornblum, S., Hasbroucq, T., & Osman, A. (1990). Dimensional overlap: cognitive basis for stimulus± response compatibility: a model and taxonomy. Psychological Review, 97(2), 253±270. Kornblum, S., & Lee, J.-W. (1995). Stimulus±response compatibility with relevant and irrelevant stimulus dimensions that do and do not overlap with the response. Journal of Experimental Psychology: Human Perception and Performance, 21(4), 855±875.
22
M. Brass et al. / Acta Psychologica 106 (2001) 3±22
Marzi, C. A., Bisiacchi, P., & Nicoletti, R. (1991). Is interhemispheric transfer of visuomotor information asymmetric? Evidence from a meta-analysis. Neuropsychologia, 29(12), 1163±1177. Meltzo, A. N., & Moore, M. K. (1977). Imitation of facial and manual gestures by human neonates. Science, 198, 75±78. Meltzo, A. N., & Moore, M. K. (1983). The origins of imitation in infancy: paradigm phenomena, and theories. Advances in Infancy Research, 2, 265±301. Pawlby, S. (1977). Imitative interaction. In H. Schaer, Studies in mother±infant interaction. New York: Academic Press. Piaget, J. (1962). Play, dreams and imitation in childhood. New York: Norton. Prinz, W. (1987). Ideomotor action. In H. Heuer, A. F. Sanders, Perspectives on perception and action (pp. 47±76). Hillsdale, NJ: Lawrence Erlbaum. Ratcli, R. (1979). Group reaction time distributions and an analysis of distribution statistics. Psychological Bulletin, 86, 446±461. St urmer, B., Aschersleben, G., & Prinz, W. (1999). Correspondence eects with manual gestures and postures: a study on imitation. Journal of Experimental Psychology: Human Perception and Performance, in press. Williams, H., & Nottebohm, F. (1985). Auditory responses in avian vocal motor neurons: a motor theory for song perception in birds. Science, 229, 279±282.