Eþ ects of manipulating relative and absolute motion information during observational learning of an aiming task

Transcription

1 Journal of Sports Sciences, 2001, 19, 507± 520 Eþ ects of manipulating relative and absolute motion information during observational learning of an aiming task SALEH A. AL-ABOOD, 1 * KEITH DAVIDS, 1 SIMON J. BENNETT, 1 DEREK ASHFORD 1 and MANUEL M ARTINEZ MARIN 2 1 Psychology Research Group, Department of Exercise and Sport Science, The M anchester Metropolitan University, Hassall Road, Alsager ST7 2HL, UK and 2 Facultad de Ciencias del Deporte, University of Granada, Granada, Spain Accepted 22 M arch 2001 In the visual perception perspective of observational learning, the manipulation of relative and absolute motion information in visual demonstrations optimally directs learners search towards appropriate task solutions. We assessed the eþ ect of emphasizing transformational information and removal of structural information using point-light kinematic displays in approximating the model s relative motion patterns. Participants viewed computer-simulated point-light demonstrations or normal video demonstrations before and intermittently throughout 100 acquisition trials with knowledge of results on an underarm modi ed-dart aiming task. On the next day, all participants performed 20 retention trials without demonstrations. The kinematics of spatial and temporal coordination and control variables were examined relative to the model s action, as well as performance scores. The results indicated that approximation of the model s spatial and temporal coordination and control patterns was achieved after observation of either type of demonstrations. No diþ erences were found in movement outcomes. In a second experiment, the eþ ects of manipulating absolute motion information by slowmotion demonstrations were examined relative to real-time demonstrations. Real-time demonstrations led to a closer approximation to the model s spatial and temporal coordination patterns and better outcome scores, contradicting predictions that slow-motion displays convey intact relative motion information. We speculate that the eþ ect of visual demonstration speed on action perception and reproduction is a function of task constraints ± that is, novelty or familiarity of relative motion of demonstrated activities. Keywords: control, coordination, informational constraints, skill acquisition, visual demonstrations. Introduction An important task for sport scientists interested in motor skill acquisition is to evaluate the eþ ectiveness of diþ erent types of visual dem onstrations during sports coaching, training and practice. Scully and Newell (1985) conceptually integrated evidence from research on the perception of biological motion with Newell s (1985) framework of learning stages (i.e. coordination, control, skill) and proposed a visual perception perspective on observational learning as an alternative to traditional theories. Central to this visual perspective is the tenet that visual demonstrations should be used * Author to whom all correspondence should be addressed. s.a.al-abood@mmu.ac.uk primarily to transm it relative motion information essential to the task being learned or performed. From this standpoint, it has been argued that one important role of visual demonstrations during skill acquisition is the manipulation of this source of movement information, so that the search by learners to assemble eþ ective coordination patterns is optimally directed towards appropriate task solutions (Scully and Newell, 1985; Williams et al., 1999). On the basis of this new theoretical rationale for observational learning eþ ects, Scully (1988) suggested some practical implications for the use of visual demonstrations in teaching and coaching motor skills. We address some of the important questions related to the arguments proposed by Scully, in an attempt to examine the empirical support for these suggestions. Journal of Sports Sciences ISSN print/issn X online Ó 2001 Taylor & Francis Ltd

2 508 Al-Abood et al. Speci cally, we assessed whether the use of pointlight displays can facilitate further the acquisition of movement coordination com pared to normal videotaped demonstrations. We also examined whether slow-m otion videotaped demonstrations interfere with the pick-up of relative motion information and the perception of temporal parameters of a modelled action compared to real-time demonstrations, as hypothesized by Scully (1988). In the rst study reported in this paper, we examined the hypothesis that the use of point-light or kinematic displays in visual demonstrations can aid the pick-up of relative motion information, facilitating the acquisition of movement coordination underlying the task being learned (Scully, 1988). This prediction is based on the argument that kinematic displays convey only transformational information essential to perceiving biological motion and minimizing structural information. Previous research on visual perception of human motion has consistently demonstrated that transformational information (i.e. inform ation about the movement of an individual) conveyed by point-light displays is suý cient for observers to discriminate between diþ erent classes of physical activities, such as walking, running, cycling, dancing (Johansson, 1973, 1975), throwing (W illiams, 1985) and throwing and bowling (Scully, 1987). M oreover, the removal of structural infor mation ± information about the shape, colour, size and other characteristics of a performer ± did not hinder observers identi cation and recognition of activities. Research has also shown that kinematic displays aþ ord the perception of dynam ic parameters of observed activities, such as force (Runeson and Frykholm, 1981), speed (Scully, 1987; W illiams, 1989) and even technical execution and aesthetic quality in gymnastics (Scully, 1986). Accordingly, the results of these perception studies are suý ciently compelling to consider whether kinematic point-light displays can aid perception in the modelling process (W illiams et al., 1999). To date, few studies have examined whether pointlight displays can be at least as eþ ective as normal videotaped demonstrations in supporting the performance and learning of motor skills. The lim ited ndings suggest that further research is required before practical implications can be advised. For example, Williams (1985) examined the eþ ect of presenting point-light displays and norm al videotaped demonstrations on observers perform ance of a throwing action consisting of a sequence of 2± 8 sub-m ovements. The results indicated no diþ erences in the production of limb displacement and timing (phasing) between observers of point-light displays and normal videotapes. However, as there was no retention or transfer test, Williams did not compare the eþ ects on lear ning of using diþ erent types of display. There were also several inherent methodological limitations, the m ost important of which was that the point-light displays demonstrated only the throwing arm without the torso, which may have mediated observers perception of movement. W hen point-light displays are used in perception research, researchers typically use points representing the key joints of the whole body rather than just those directly involved in the movement. Furthermore, W illiams (1985) did not consider how kinematic displays in uence movement outcome scores. Consequently, in the rst study reported here, we tried to establish the eþ ect of point-light displays, compared to normal videotaped demonstrations, on the learning of an aiming task by including more practice trials and a retention test in the design. M oreover, we wished to determine the eþ ect of these displays on movement outcomes as well as movement kinematics, which would allow us to investigate the acquisition of movement coordination and control, as operationalized by Scully and Newell (1985). In a second study, we examined whether the observation of slow-m otion videotaped demonstrations has facilitative or detrimental eþ ects on the acquisition of movement coordination and control, as well as task outcomes, compared to videotaped demonstrations presented in real-time. This is an issue of some signi cance to sport scientists, since it has practical as well as theoretical implications for those involved in sports coaching and training. Previous research on this issue has been equivocal. For example, Nelson (1958) found no signi cant diþ erences in outcome scores when observing a golf swing between a slow-m otion group and a real-time videotape group. H owever, the slowmotion group did show a greater gain in scores later in practice, while the real-time modelling group showed a greater gain earlier. M cguire (1961) studied the eþ ect of demonstrating some steps of performance on a pursuit-rotor tracking task in slow motion while keeping other steps at real-time speed. He found enhanced performance for the items presented in slow motion, whereas performance on other items was impaired. W illiams (1985, 1989) assessed the eþ ects of slowing down or speeding up the presentation of a throwing action in normal videotaped demonstrations or point-light displays and found that slow-m otion dem onstrations embedded correct reproduction of timing param eters of an action but had little eþ ect on limb displacement. M ore recently, and using a more complex action ± a ballet dance sequence ± Scully and Carnegie (1998) found that the observation of slowmotion demonstrations, compared to normal-speed dem onstrations, slightly facilitated the pick-up and replication of the model s coordination function (e.g. relative timing). H owever, slow motion was found to

3 Visual dem onstrations and skill acquisition 509 have impaired the pick-up and replication of movement control parameters such as movement time and force. Visual perception research has also revealed mixed ndings on the relative eý cacy of slow-m otion and realtime video demonstrations. For example, Barclay et al. (1978) found that slow motion hindered action recognition, while Scully (1987) observed little in uence on action identi cation but an eþ ect for speed discrimination. Although Cutting and Proý tt (1982) indicated that real-time demonstrations are essential to the perception of a frame of reference for each activity, Scully (1988) theorized that slow-m otion displays provide the same relative motion demonstrated by normal videotapes but exaggerate the temporal parameters of a modelled action. In other words, slow-m otion displays oþ er intact relative motion information of an action but convey distorted `unreal-time features of the action, thus destroying the perception of absolute motion. Relating the visual perception perspective on observational learning to N ewell s (1985) model of motor learning results in two major predictions on the use of slow-m otion demonstrations. First, it is expected that slow-m otion will be as eþ ective as real-time demonstrations in supporting the reproduction of an observed coordination pattern. Secondly, slow motion is predicted to impair the approximation of control parameters of the dem onstrated action. In short, although few perception and action studies have examined the eþ ectiveness of viewing slow-m otion as opposed to real-time dem onstrations, the evidence is still inconclusive and further research is required before practical implications can be proposed. Accordingly, the aim of the second experiment was to address this issue from a visual perception perspective. Experim ent 1 The main aim of Experiment 1 was to establish whether demonstrating an action with a point-light kinematic display, compared to a normal videotaped display, would aid further the pick-up of relative motion information, facilitating the acquisition of a movement coordination pattern. We hypothesized that, if pointlight displays do support the pick-up of relative motion information, observers of point-light displays should show a greater, or at least an equal, approximation to the tem poral and spatial relative motion patterns demonstrated by a model than observers of normal videotapes of the same action. Furthermore, in relation to Newell s (1985) model of motor learning, we also wished to determine whether point-light displays have the potential to support the pick-up of movement control variables such as movement time and velocity. Methods Participants. Ten male participants (mean age = 24.2 years) volunteered for the experiment. They were all right-handed and had normal or corrected-to-normal vision. All participants were naive to the experimental task, provided informed consent before participation and were told that they were free to withdraw at any time. Apparatus and task. The task involved an underarm throw of a modi ed dart originally used by Al-Abood et al. (in press). The aim of the task was to score as many points as possible by aiming the modi ed dart with the dominant arm towards a target dartboard. The target was a standard dartboard (Unicorn) modi ed for the outcome scoring system of the experiment. It contained 10 concentric circles. The bullseye had a diameter of 2.25 cm, with each other circle increasing by 2.25 cm in radius. To provide outcome scores as a dependent variable, the bullseye was awarded 10 points, with each concentric circle radiating out from the bullseye decreasing by one point so that the outerm ost circle was worth only one point. The target was placed on the oor 3 m away from a throwing line. A regular (Unicorn) dart was also modi ed by attaching an additional shaft to the free end of the ight. This allowed the participants to hold the dart from its end, permitting an underhand aiming movement. The modi ed dart had a mass of 40 g and was 20 cm long (see Fig. 1). The task was unfamiliar to all participants and involved multiple biom echanical degrees of freedom, permitting the examination of modelling eþ ects on movement coordination and control (M cdonald et al., 1989). Data collection and dem onstration preparation. An ELITE on-line motion analysis system (see Pedotti and Ferrigno, 1995) was used to collect and analyse movement kinematics. Three markers were positioned on three joints of the dominant arm (i.e. the right upper limb) of the model and all participants: the acromion process of the shoulder and the lateral condyles of the elbow and wrist. A fourth marker was attached to the modi ed dart at the additional shaft to determine the release time of the dart. The two-dimensional coordinate data were recorded on-line at a sampling frequency of 100 Hz. After testing, the three-dimensional coordinates were reconstructed from the transformed two-dimensional coordinate data of the re ective markers recorded from the two cameras. Then, the raw displacement data were ltered with a recursive second-order Butterworth lter with a cut-oþ frequency of 5 Hz, which was applied twice to negate the phase shift (Wood, 1982). The ltered displacement data were then diþ erentiated oþ -line to derive velocity data.

4 510 Al-Abood et al. Fig. 1. The underhand modi ed-dart aiming task. Finally, the resultant displacement and velocity data were calculated. A video cam era (Panasonic, F-15) and recorder (Samsung, SV 821K) were used to videotape the model for later demonstrations. Before being videotaped, the model practised 3000 trials on the experimental task over 30 days at a rate of 100 trials per day. The model was videotaped during the nal 20 trials on the last day of practice. Then, a videotaped colour recording was edited by selecting six representative trials to be presented as visual demonstrations to participants. The mean outcome score for these selected trials was 8.8 out of a maximum possible score of 10 points. The viewing time of the videotape recording was approximately 3 min. The view of the demonstrations in this videotape recording contained the model s whole body movement, the target and the trajectory of the dart. The videotape recording also provided auditory knowledge of results after every trial demonstrated by the model. To present the videotape demonstrations to the participants, the videotape recorder was connected to a colour television (Hantarex, screen dim ensions = cm). While the m odel was being videotaped, a record of his movement kinematics was also taken via the ELITE system following the aforementioned procedures. The model s kinematic data were collected for later comparative analyses. Speci cally, for all trials demonstrated by the model, angular displacement and velocity of the aiming arm were calculated from the instant of movement initiation to the release of the dart. A computer-simulated point-light demonstration was created based on the digitized kinematic data of the six trials selected for presentation in the normal videotaped recording for the model. Therefore, the point-light display provided the same transformational movement information demonstrated by the normal videotaped presentation. H owever, no structural information was available in the point-light display. In the point-light dem onstration, 21 points represented the key joints of the model s body and two points represented the dart. Furthermore, one segm ent-light and one pointlight represented the target and its centre, respectively. A computer-based, rather than videotape, display was used in the study to eliminate the detrimental eþ ect of `blooming that may occur as a result of using bright light sources against a dark background (Kozlowski and Cutting, 1977). Procedure. The participants were assigned at random to one of two experimental treatment groups (n = 5): normal-videotape group and the point-light group. All participants followed the same experimental procedures but diþ ered with respect to whether they observed normal videotaped demonstrations or pointlight demonstrations. All participants observed their respective demonstrations before practice and intermittently every 10 consecutive practice trials throughout the acquisition session. Before and throughout the retention trials, all participants perform ed without observing any demonstrations. All participants were instructed to use the information available in the dem onstrations to help them to perform the task and to improve their perform ance. They also received preliminary instructions from the experimenter on how to hold the dart. Each participant was tested individually in the presence of the experimenter only. The study consisted of two sessions, acquisition and retention, held on two

5 Visual dem onstrations and skill acquisition 511 consecutive days; these sessions consisted of 100 and 20 trials, respectively. The participants were allowed 2 min rest after every 20 acquisition trials. The time between each set of 10 consecutive acquisition trials was the same for participants in both groups. Each trial was initiated by a `ready command given by the experimenter approximately 2 s before the `go command to start the movem ent. All participants received knowledge of results based on their perform ance on each trial. Knowledge of results was provided for two reasons: rst, to determine the potency of modelling in motor skill acquisition when such knowledge is available; secondly, to generalize the ndings of the present experiment to learning and performance settings in which such knowledge is provided. Dependent measures. Data on movement outcomes and kinematics were collected for all acquisition and retention trials following the data collection procedures described above. However, for movement kinematics only, the rst six and last six acquisition trials as well as the rst six retention trials were analysed. One movement outcome measure and several movement kinematic measures were selected as appropriate variables to examine the predictions advanced in this study about the eþ ects of visual demonstrations on movement outcom es, coordination and control. Movement outcomes. To determine the eþ ects of the type of demonstrations on movement outcomes, means and standard deviations (s) of performance scores were calculated to establish the aiming accuracy of each participant for each block of 10 trials. This procedure resulted in 10 acquisition and two retention blocks. The resultant data were subm itted to separate (2 groups 12 trial blocks) two-factor analyses of variance (ANOVA) with repeated measures on the trial blocks. Movement coordination (relative motion). To examine the eþ ects of the type of demonstrations on movement coordination, we used the relative motion of upperand lower-arm segments of the aiming arm. Speci cally, we computed angular velocities and displacements of arm segm ents to the vertical axis from the start signal to the instant of dart release. The positions of these two segm ents to the vertical represent the most coherent shape in terms of the topological properties of a throwing arm (Scully, 1987). To determine precisely the approximation of participants relative motion patterns to those of the model, a cross-correlation of recognition coeý cient (R ) (Sparrow et al., 1987) was calculated between the median relative motion values of the model and that of every trial performed by a participant and included in the analysis. The cross-correlation of recognition coeý cient is a measure of the extent to which the angles between adjacent data points from one angle± angle or velocity± velocity plot are sim ilar to angles from another plot. It is a measure of similarity between two coordination patterns. The value of cross-correlation (R) ranges from -1.0 to +1.0 according to the similarity. As R approaches zero, the velocity± velocity or angle± angle relative motions of two plots become increasingly dissimilar in shape. In the present study, therefore, the higher the positive R-value, the greater the approximation to the model s relative motion. To calculate cross-correlations, we followed three steps. First, one movement pattern was selected as representative of the six trials demonstrated by the model. To determine the model s median pattern, we plotted all relative motions of the model s six trials in a single angle± angle diagram and velocity± velocity diagram. From these motions, we selected the median relative motions of the model (see Fig. 2). Secondly, after selecting the model s median pattern, all participants trials were time-normalized to the same number of data points as the model s pattern using a cubic spline interpolation technique. Finally, a crosscorrelation with a zero-time lag was calculated between each participant s trial and the model s median trials. These procedures were rst followed to calculate crosscorrelations for spatial coordination (i.e. angle± angle relations) and were then repeated to compute temporal coordination (i.e. velocity± velocity relations). After calculating cross-correlations, means and standard deviations of cross-correlations were computed for each participant for the rst six acquisition trials, the last six acquisition trials and the rst six retention trials. Standard deviations were computed as an index of within-participant variability in coordination patterns around the model s median pattern. The cross-correlations were averaged by using a Fischer Z- transformation procedure. The calculated means and standard deviations of cross-correlations were submitted to separate (2 groups 3 trial blocks) two-factor multivariate analyses of variance (M ANOVA) with repeated measures on the trial block factor. Movement control. Although there were various kinematic variables available to examine the eþ ects of demonstration type on movement control, we decided to focus on those variables re ecting the control of elbow angle. Changes in this angle represent the scaling or parameterization of the coordination function of upper- and lower-arm segments (Scully, 1987). The selected variables were: elbow angle at release, elbow velocity at release and movement time (i.e. the time taken from the start signal to release of the dart). For each variable, means and standard deviations for each

6 512 Al-Abood et al. Fig. 2. Angle± angle (a) and velocity± velocity (b) relative motion patterns for the six trials selected for the model. The abbreviations Int, Rev and Rel indicate the instants of movement initiation, arm reversal and dart release respectively. block of six trials were calculated for each participant. Separate multivariate analyses of variance (2 groups 3 trial blocks: Acquisition 1, Acquisition 2 and Retention), with repeated measures on the last factor, were computed on the control variables. Signi cance for all statistical analyses conducted in

7 Visual dem onstrations and skill acquisition 513 this study was set at a = For repeated-m easure analyses, the Huynh-Feldt correction factor was used to adjust the degrees of freedom of the univariate F because of the potential violation of the sphericity assumption owing to the many levels of the repeated measures (see Schutz and G essaroli, 1987). Furthermore, signi cant multivariate analyses of variance were followed by separate univariate analyses of variance and discriminant analysis (i.e. standardized coeý cients) to determine which of the dependent variables contributed most to signi cant diþ erences within each factor (Bray and M axwell, 1985; Schutz and Gessaroli, 1987). The values reported for the multivariate analyses of variance are W ilks lambda ratios. Signi cant analyses of variance were followed by Tukey HSD tests when required. Results Movement outcome. The AN OVA on mean outcome scores showed no signi cant eþ ect for groups (F 1,8 = 1.28, P = 0.29) or groups trial blocks interactions (F 11,88 = 1.21, P = 0.29). However, there was a signi - cant main eþ ect for trial blocks (F 11,88 = 2.37, P = 0.01). Univariate analysis, including a H uynh-feldt procedure to adjust the degrees of freedom because of a violation of the sphericity assum ption, maintained the trial block eþ ect (F 11,88 = 2.37, P = 0.013). Follow-up Tukey HSD tests on the block factor revealed an improvement in movement outcome with practice. The performance of both groups on the second retention block was signi - cantly better than that on the rst acquisition block (P = 0.00). No other signi cant diþ erences were found, although the diþ erences between the rst acquisition block and the seventh and eighth acquisition blocks approached conventionally accepted levels of statistical signi cance (P = 0.06; see Table 1). For standard deviations of outcome scores, there was no main eþ ect for groups (F 1,8 = 0.291, P = 0.60) or trial blocks (F 11,88 = 1.14, P = 0.34), or a groups trial blocks interaction (F 11,88 = 1.04, P = 0.42). Movement coordination (relative motion). The multivariate analyses of variance computed on the means of velocity± velocity cross-correlations and angle± angle cross-correlations showed no main eþ ect for groups (Wilks l 2,7 = 0.865, P = 0.60) or trial blocks (W ilks l 4,5 = 0.296, P = 0.13), or a groups trial blocks interaction (W ilks l 4,5 = 0.655, P = 0.65) (see Table 2). For standard deviations of cross-correlations, as an index of intra-individual variability of movement coordination, no signi cant diþ erence was observed between groups (W ilks l 2,7 = 0.861, P = 0.59) or trial blocks (W ilks l 4,5 = 0.538, P = 0.46), and there was no groups trial blocks interaction (Wilks l 4,5 = 0.839, P = 0.90) (see Table 2). Table 1. Movement outcomes as a function of groups and trial blocks (mean ± s) Trial block Acquisition Retention 1 2 Normal-videotape group 3.20 ± ± ± ± ± ± ± ± ± ± ± ± 3.00 Point-light group 3.20 ± ± ± ± ± ± ± ± ± ± ± ± 3.09 Table 2. Cross-correlations of recognition coeý and trial blocks (mean ± s) cient (R) as a function of groups Trial block Acquisition 1 Acquisition 2 Retention Angle± angle (R) Normal-videotape group Point-light group Slow-motion group 0.46 ± ± ± ± ± ± ± ± ± 0.05 Velocity± velocity (R ) Normal-videotape group Point-light group Slow-motion group 0.43 ± ± ± ± ± ± ± ± ± 0.03

8 514 Al-Abood et al. Table 3. Movement control variables as a function of groups and trial blocks (mean ± s) Trial block Acquisition 1 Acquisition 2 Retention Release angle (8 ) Normal-videotape group Point-light group Slow-motion group 142 ± ± ± ± ± ± ± ± ± 4.63 Release velocity (degrees per second) Normal-videotape group 61.8 ± 20.6 Point-light group 104 ± 40.6 Slow-motion group 74.2 ± ± ± ± ± ± ± 35.5 Movem ent tim e (m s) Normal-videotape group Point-light group Slow-motion group 1469 ± ± ± ± ± ± ± ± ± 123 M ovement control. The multivariate analyses of variance on the means of elbow release angle, elbow release velocity and movement time showed no signi cant diþ erences between groups (W ilks l 3,6 = 0.921, P = 0.91) or trial blocks (W ilks l 6,3 = 0.152, P = 0.21), and there was no groups trial blocks interaction (W ilks l 6,3 = 0.266, P = 0.43). Similarly, there were no signi cant eþ ects for groups (W ilks l 3,6 = 0.700, P = 0.51) or trial blocks (Wilks l 6,3 = 0.281, P = 0.45), or a groups trial blocks interaction (W ilks l 6,3 = 0.368, P = 0.55), when a M ANOVA was computed on the standard deviations of the same dependent variables (see Table 3). Discussion The aim of Experiment 1 was to determine whether point-light kinematic displays, compared to normal videotaped displays, support further the pick-up of relative motion information, thus facilitating the acquisition of a movement coordination pattern. We hypothesized that if point-light displays do facilitate the pick-up of relative motion information, compared to normal videotapes, participants observing point-light displays should show a closer, or at least an equal, approximation to the temporal and spatial relative motion patterns dem onstrated by the model. This prediction was supported in part, since no diþ erences in approximating the m odel s relative m otion patterns were observed between participants in the two groups. This suggests that the transformational information available in pointlight kinematic displays was suý cient to convey relative motion information to observers learning an underarm throw. This is consistent with ndings of visual perception research for the potency of such displays in transmitting accurate information about physical activity identi cation and recognition (Johansson, 1973, 1975; W illiams, 1985; Scully, 1987). The analysis of movement control variables also showed no group diþ erences caused by type of visual dem onstrations. Again, the results suggest that pointlight displays were as eþ ective as normal videotape dem onstrations in conveying information about movement control. This is also consistent with ndings in visual perception research (Scully, 1987; W illiams, 1989), indicating that the removal of structural information has no detrimental eþ ect on the perception of control-relevant information. The results are also consistent with the work of Williams (1985), who found no diþ erences in production of movement timing between observers of point-light displays and normal demonstrations. M oreover, the results of the present study extend W illiam s ndings to learning contexts. Finally, for achieving the goal of the movement (i.e. outcome scores), the observation of point-light displays was also found to be as eþ ective as the observation of normal videotaped demonstrations. Experim ent 2 The main aim of Experiment 2 was to assess the eþ ect of slow-m otion demonstrations, compared to real-tim e dem onstrations, on the acquisition of movement coordination and control of an aiming task. Visual perception research has revealed mixed results on how slow m otion demonstrations in uence perception of movement characteristics. Furthermore, the eþ ect of

9 Visual dem onstrations and skill acquisition 515 slow-m otion demonstrations on movement production has typically received little attention in the modelling literature, although this method of instruction is commonly used by instructors of motor skills. This lack of consideration may have been due in part to a lack of understanding of the nature of movement information conveyed by visual demonstrations in traditional perspectives on observational learning. However, Scully (1988) suggested that slow-m otion displays provide the same relative m otion demonstrated by normal videotapes, but distort the temporal parameters of a modelled action. Slow-m otion displays oþ er intact relative motion information of an action but convey `unreal time features of that action, perturbing the perception of absolute motion. Accordingly, a key prediction on the use of slow-m otion demonstrations from a visual perception perspective is that slow-m otion will be as eþ ective as normal real-time demonstrations for the reproduction of an observed coordination pattern, although it may im pair the approximation of control aspects of the demonstrated action. Methods Participants. Five additional male participants (mean age = 23.2 years), none of whom participated in Experiment 1, volunteered for this experiment. All participants were right-handed and had normal or corrected-tonormal vision. They were naive to the experimental task, provided informed consent before participation and they were told that they were free to withdraw at any time. Apparatus and task. The task and apparatus were the same as those used in Experiment 1. Data collection and demonstration preparation. The procedures used to collect kinematic data in Experiment 1 were followed in Experiment 2. Furthermore, the videotaped demonstrations by the skilled model prepared in Experiment 1 were presented in slow motion for participants in the slow-m otion group in Experiment 2. The viewing time of the videotape recording presented in slow motion was approximately 4.5 min. Procedure. The ve additional participants who volunteered for this experiment were assigned to the slow-m otion group. To examine the predictions advanced for this experiment, this group was compared to the normal-videotape group who viewed normal videotape demonstrations presented in real time in Experiment 1. All participants in the slow-m otion group followed the same experimental procedures as the normal-videotape group in Experiment 1 for the number of acquisition and retention trials, knowledge of results and inter-block rest intervals. H owever, the groups diþ ered with respect to the nature of the visual demonstrations they observed; the participants in the normal-videotape group viewed normal real-time videotaped demonstrations (a normal lm rate of 25 H z), whereas those in the slow-m otion group observed slowmotion demonstrations (15 H z) of the same movement. Participants in both groups viewed their respective demonstrations before practice and intermittently every 10 consecutive practice trials throughout the acquisition session. All participants were instructed to use the information available in the dem onstrations to help them to learn the task and to improve their performance. Data analysis. In this experim ent, the dependent variables representing movement coordination, control and outcome scores, as well as the statistical analyses, were similar to those used in Experiment 1. Results Movement outcomes. The ANOVA computed on mean outcome scores showed a signi cant eþ ect for groups (F 1,8 = 9.32, P = 0.02) and trial blocks (F 11,88 = 2.31, P = 0.01); however, there was no signi cant eþ ect for the interaction between groups and trial blocks (F 11,88 = 1.42, P = 0.18). Inspection of the means of both groups indicated that the normal-videotape group (4.69) performed signi cantly better than the slowmotion group (3.10) (see Fig. 3). A univariate analysis, including a H uynh-feldt procedure to adjust the degrees of freedom because of a violation of the sphericity assumption, maintained the trial block eþ ect (F 11,88 = 2.37, P = 0.015). Follow-up Tukey HSD tests on the block factor revealed improvement in movement outcome with practice. The performance of both groups on the eighth acquisition block (mean = 4.44) and second retention block (mean = 4.35) was signi cantly better than that for the rst acquisition block (mean = 2.83) (P = 0.01 and P = 0.02, respectively). For standard deviations of outcome scores, there was no signi cant eþ ect for groups (F 1,8 = 0.507, P = 0.50) or trial blocks (F 11,88 = 1.22, P = 0.29), and there was no groups trial blocks interaction (F 11,88 = 0.808, P = 0.63). Movement coordination (relative motion). The multivariate analyses of variance computed on the means of velocity± velocity cross-correlations (temporal coordination) and angle± angle cross-correlations (spatial coordination) showed signi cant main eþ ects for groups (Wilks l 2,7 = 0.227, P = 0.01); however, no signi cant eþ ect was found for trial blocks (Wilks l 4,5 = 0.393,

10 516 Al-Abood et al. Fig. 3. Mean movement outcomes as a function of the normal videotape group (d ) and slow-motion group (s ) across blocks of 10 practice trials. Fig. 4. Angle± angle relative motion patterns for a representative participant from the normal videotape group (NVp) and slowmotion group (SMp) on the last acquisition trial, compared to the median pattern of the model. The abbreviations Int, Rev and Rel indicate the instants of movement initiation, arm reversal and dart release respectively. P = 0.24) and there was no groups trial blocks interaction (Wilks l 4,5 = 0.319, P = 0.16). Follow-up univariate analyses of variance and discriminant analysis on the group eþ ect revealed signi - cant diþ erences between groups for both dependent variables: spatial coordination (F 1,8 = 22.1, P = 0.00) and temporal coordination (F 1,8 = 16.3, P = 0.00). The discriminant functions (i.e. standardized coeý cients) were and for spatial and temporal coordination, respectively. Inspection of group means for both dependent variables during acquisition and retention revealed that participants in the normalvideotape group had higher cross-correlations than those in the slow-m otion group, indicating closer approxim ation to the skilled model s relative motion (see Table 2 and Figs 4 and 5). For standard deviations of cross-correlations, as an estimate of intra-individual variability of movement coordination, no signi cant diþ erences were observed between groups (W ilks l 2,7 = 0.719, P = 0.32) or trial

11 Visual dem onstrations and skill acquisition 517 Fig. 5. Velocity± velocity relative motion patterns for a representative participant from the normal videotape group (NVp) and slow-motion group (SMp) on the last acquisition trial, compared to the median pattern of the model. The abbreviations Int, Rev and Rel indicate the instants of movement initiation, arm reversal and dart release respectively. blocks (W ilks l 4,5 = 0.571, P = 0.51), and there was no groups trial blocks interaction (Wilks l 4,5 = 0.638, P = 0.62) (see Table 2). Movement control. The multivariate analyses of variance on the means of elbow release angle, elbow release velocity and movement time showed no signi - cant diþ erences between groups (W ilks l 3,6 = 0.861, P = 0.81) or trial blocks (W ilks l 6,3 = 0.423, P = 0.68), and there was no groups trial blocks interaction (Wilks l 6,3 = 0.327, P = 0.53). Similarly, there was no signi cant eþ ect for groups (Wilks l 3,6 = 0.487, P = 0.20), or an interaction between groups and trial blocks (Wilks l 6,3 = 0.211, P = 0.32), when a M ANOVA was computed on the standard deviations of the same dependent variables. H owever, a signi cant eþ ect was found for trial blocks (W ilks l 6,3 = 0.021, P = 0.01). Follow-up univariate analyses of variance and discriminant analysis on the trial block eþ ect revealed signi cant diþ erences between trial blocks for movement tim e only (F 2,16 = 7.73, P = 0.00). The discriminant function (i.e. standardized coeý cient) was Follow-up Tukey H SD tests revealed a signi cant decrease in variability of movement time with practice. The performance of both groups on the second acquisition block and retention block was more consistent than that in the rst acquisition block (see Table 3). Discussion The aim of Experiment 2 was to determine the eþ ect of observing slow-m otion demonstrations, compared to real-time visual demonstrations, on motor skill acquisition from a visual perception perspective (Scully and Newell, 1985; Scully, 1988). According to this perspective, slow-m otion displays may convey intact perception of relative m otion information of a demonstrated activity but may impede the recognition of absolute motion parameters such as speed and duration of an activity. Following this hypothesis, and given that perception constrains action, we predicted that there would be no diþ erences in relative motion patterns between the slow-m otion and normalvideotape groups. However, diþ erences due to speed of demonstration were expected in movement control variables such as velocity at release and movement time. The results of this experiment showed that, throughout the acquisition and retention sessions, participants in the normal-videotape group approximated the model s temporal and spatial relative motions more closely than those in the slow-m otion group. It was apparent that observers of slow-m otion demonstrations were unable to perceive intact relative motion information from these demonstrations, resulting in a less accurate approximation of the model s relative motion patterns. This nding does not concur with the predictions of the visual perception perspective. Rather, the results are more consistent with the suggestion of Cutting and Proý tt (1982) that demonstrating an action in real-time is an indispensable procedure for establishing accurate perception of a coordinate frame of reference unique to each activity. This is also in line with the work of Barclay et al. (1978), who reported poor recognition of actions after the observation of slow-m otion presentations.

12 518 Al-Abood et al. One plausible explanation for the discrepancies between the ndings of the present study and the predictions of the visual perception perspective is the novelty of relative motion patterns available in the underarm-throwing task of this study. M ore generally, it is possible that whether observers can perceive intact information about relative motion from slow-m otion displays depends, to a large extent, on the observers familiarity with the experimental task. It is plausible that altering the speed of demonstrations (i.e. a scaling up or down of the optimal relative motion) will not impede movement perception if observers have previously viewed the task. Accordingly, it is possible that the participants in the studies of Scully (1987) and Williams (1985, 1989) were able to pick up and use appropriate relative motion patterns because the overarm throwing and bowling actions demonstrated in those experiments were fam iliar to them. In contrast, if a slow-m otion manipulation of relative motion occurs with a novel experimental task, then the perceptual pick-up of the optimal relative motion underlying the successful perform ance m ay be perturbed, thus hindering learning as in this study. Further research is required to verify this explanation by examining the eþ ects of learners perceptual experience on action perception and reproduction of novel and familiar tasks to be learned through visual demonstrations presented in slow motion. The analysis of movement control variables (i.e. elbow angle and velocity at release plus movement time) indicated no diþ erences between participants in the normal-videotape and slow-m otion groups in the reproduction of these variables during acquisition and retention sessions. This is also inconsistent with the predictions of the visual perception perspective. We predicted that slow-m otion presentations may hinder the perception of movement control variables relevant to the speed of movement and thus may lead to inaccurate scaling of movement parameters. The present results are also not in line with those of W illiams (1985, 1989) and Scully and Carnegie (1998), who showed incorrect reproduction of timing variables (e.g. absolute timing, force) after the observation of slow-m otion dem onstrations. One possible explanation for these contradictory ndings about movement control is that participants in the slow-m otion group were able to discover appropriate timings of the aiming action through physical practice. That is, because participants in the present study had more physical practice (i.e. 100 trials) than those in the studies of W illiams (1989) and Scully and Carnegie (1998) ± 6 and 10 trials, respectively ± they were able to nd the appropriate control parameters. Slow-m otion demonstrations may have hindered observers perception of movement timing characteristics, but had little eþ ect on movement production because learning was m ediated by discovery of key parameters through physical practice. However, because perception was not assessed in the present study, it is not possible to con rm this. Nevertheless, subsequent additional analyses of movement production of these control variables on the rst practice trial suggested that this explanation may be plausible. We ran separate t-tests on the data of the rst practice trial for each control variable in an attempt to dissociate the eþ ect of observation of dem onstrations from that of physical practice. The results showed no signi cant diþ erences between the slow-m otion and normal-videotape groups in elbow angle at release (t 8 = 0.627, P = 0.55) and movement time (t 8 = , P = 0.37). However, a signi cant diþ erence between groups was found for elbow velocity at release (t 8 = -2.66, P = 0.03). The normal-videotape group (mean = 49.5 per second) approximated the model s velocity at release (mean = 48.1 per second) much more closely than the slow-m otion group (mean = 131 per second). Furthermore, although there were no group differences in movement time, closer inspection of individual data for each participant in both groups, and of within-group standard deviations, revealed an interesting nding. The movement times on the rst practice trial of the participants in the normal-videotape group ranged from 1200 to 1790 ms (s = 230), while those of the participants in the slow-m otion group ranged from 1040 to 2680 ms (s = 667). This implies that the search by participants for the appropriate movement time of the task was more constrained or directed by the model s (mean = 1820 ms) movement time in real-time demonstrations compared to slow-m otion presentations. Taken collectively and prompted by initial ndings, further analyses of movement control variables for the rst practice trial suggested that the observation of slow-m otion demonstrations could have hindered the pick-up of absolute m otion information. H owever, through physical practice, the participants in the slow-m otion group were able to discover and approxim ate the appropriate movement speed and tim e essential for solving the task problem. Future research should assess both perception and action to better understand how manipulating absolute motion information in visual demonstrations mediates action perception and reproduction. Finally, analysis of movement outcome scores indicated that the normal-videotape group signi cantly outperformed the slow-m otion group during acquisition and retention sessions. This suggests that the closer approximation of the model s relative motion by participants in the former group resulted in better outcome scores.

13 Visual dem onstrations and skill acquisition 519 General discussion In the experiments reported here, we examined the eþ ect of manipulating relative and absolute motion information available in visual demonstrations on the acquisition of an aiming task from a visual perception perspective of observational learning (Scully and Newell, 1985; Scully, 1988). As predicted, Experiment 1 showed that the inclusion of structural information, found in normal demonstrations, had little additional eþ ect on the acquisition of movement coordination and control or on maximizing outcome scores. The use of point-light demonstrations, compared to normal demonstrations, was of equal value in facilitating the acquisition of movement coordination patterns by emphasizing the relative motion information. The implication is that learners can pick up relative motion information from both normal videotaped demonstrations of a skill and in point-light formats to direct learners attention towards transformational information. Because of evolutionary in uences on the human visual system, learners can pick up as much transformational information from normal demonstrations as from point-light displays to eþ ectively use this source of information in subsequent movement reproductions. In Experiment 2, some evidence indicated that the observation of slow-m otion demonstrations, compared to presentations at regular speed, was detrimental to the approximation of the model s spatial and temporal relative motions as well as to performance scores. This nding contradicted the predictions of the visual perception perspective that slow-m otion displays convey intact relative motion information. H owever, further work is required to determine whether the eþ ect of the speed of visual demonstrations on action perception and reproduction is a function of task constraints, such as the novelty of relative motion corresponding to the demonstrated activity. The more unfam iliar the relative motion patterns used in task performance, the more likely that the perception of optimal relative motion will be hindered by slow-m otion presentation. H owever, this im plication awaits further research before precise suggestions for the use of slow-m otion videotapes in motor skill acquisition can be advanced. It could be the case that slow-m otion demonstrations are more eþ ective at later stages of learning after the movement percept and coordination functions underlying the task have been established. 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