Journal of Sports Sciences, 2002, 20, 471± 485
Can anticipatory skills be learned through implicit video-based perceptual training? DAMIAN FARROW* and BRUCE ABERNETHY School of Human Movement Studies, The University of Queensland, Brisbane, Queensland 4072, Australia
Accepted 21 December 2001
The aim of this experiment was to determine the eþ ectiveness of two video-based perceptual training approaches designed to improve the anticipatory skills of junior tennis players. Players were assigned equally to an explicit learning group, an implicit learning group, a placebo group or a control group. A progressive temporal occlusion paradigm was used to examine, before and after training, the ability of the players to predict the direction of an opponent’ s service in an in-vivo on-court setting. The players responded either through hitting a return stroke or making a verbal prediction of stroke direction. Results revealed that the implicit learning group, whose training required them to predict serve speed direction while viewing temporally occluded video footage of the returnof-serve scenario, signi® cantly improved their prediction accuracy after the training intervention. However, this training eþ ect dissipated after a 32 day un® lled retention interval. The explicit learning group, who received instructions about the speci® c aspects of the pre-contact service kinematics that are informative with respect to service direction, did not demonstrate any signi® cant performance improvements after the intervention. This, together with the absence of any signi® cant improvements for the placebo and control groups, demonstrated that the improvement observed for the implicit learning group was not a consequence of either expectancy or familiarity eþ ects. Keywords: anticipation, expertise, implicit learning, perceptual training, tennis.
Introduction A logical point to start the search for the most eþ ective means of developing perceptual-motor skill is by seeking a comprehensive understanding of the sources and mechanisms that underpin any expert advantage on the task of interest. As a result of a concerted research eþ ort over the last two decades, an extensive database now exists on the perceptual information sources that contribute to motor expertise (Starkes, 1993). Within sport, research has consistently demonstrated, across a variety of settings, that experts are able to anticipate more eþ ectively than novices, in part due to their ability to pick up useful anticipatory information from early events in their opponent’ s movement pattern to which novices are not attuned. In racquet sports, for example, evidence from the temporal occlusion paradigm has been used to demonstrate that experts are more able * Address all correspondence to Damian Farrow, Australian Institute of Sport, PO Box 176, Belconnen, ACT 2616, Australia. e-mail:
[email protected]
than less skilled players of picking up anticipatory information from the kinematics of events occurring early in the opponent’ s hitting action, such as the motion of the arm holding the racquet (Abernethy and Russell, 1987; Goulet et al., 1989; Abernethy et al., 2001). There is little doubt according to the existing literature that the ability to extract and use the information available from an opponent’ s movement pattern is a limiting factor to successful performance for less skilled performers. However, whether this capacity is amenable to training is something that, while frequently discussed, has not been extensively researched (for a review, see Williams and Grant, 1999). Studies that have tried to determine the eý cacy of training perceptual skill (e.g. Singer et al., 1994; Adolphe et al., 1997; Farrow et al., 1998) have been limited in several ways. Most previous perceptual training research has focused on the use of video-based perceptual training simulations in which an attempt is made to develop the capacity of performers to understand the meaning of advance cues available within their particular sport
Journal of Sports Sciences ISSN 0264-0414 print/ISSN 1466-447X online Ó http://www.tandf.co.uk/journals
2002 Taylor & Francis Ltd
472 display. This training approach typically involves ® lm footage shot from the player’ s perspective (e.g. a tennis server ® lmed from the perspective of the receiver), the footage being selectively edited to provide diþ ering amounts of advance (pre-contact) and ball-¯ ight information. The training task for the learners is to predict the ® nal direction of the ball after occlusion, based on the information available to them. There is evidence of an improvement in the speed and accuracy of the perceptual response after repeated exposure to this type of training (Singer et al., 1994; Farrow et al., 1998; Abernethy et al., 1999; see Williams and Grant, 1999, for a more complete review); however, there is a general lack of clear evidence of transferability to natural performance contexts (for some limited support, see Adolphe et al., 1997; Scott et al., 1998). Although the perceptual training literature oþ ers promise of improvement in the anticipatory performance of less skilled individuals, several methodological issues currently limit application. First, there has been a general failure to adequately design the experimental interventions by including placebo and control groups to remove Hawthorne and test familiarity eþ ects, respectively (for exceptions, see Farrow et al., 1998; Abernethy et al., 1999). Second, the issue of whether perceptual skill can be trained in the absence of extensive physical practice is yet to be suitably examined. Proponents of the ecological approach to perception (Gibson, 1969, 1979; Handford et al., 1997) argue the importance of continuously maintaining the link between perception and action if any perceptual-motor training approach is to be successful. Third, as noted earlier, few studies have examined whether the improved perceptual judgements of participants in perceptual training programmes result in improved sports performance (for exceptions, see Adolphe et al., 1997; Scott et al., 1998). Currently, there are few if any perceptual training studies within the sports domain that have adequately addressed all three of these methodological issues. In practical terms, perceptual training relies on a performer attending to progressively earlier sources of perceptual information in a display and learning the relationship between these display features and the resultant movement requirements. Of particular interest to both coaches and scientists alike is determining the most eþ ective way of conveying this perceptual information to learners. A critical question is whether a person needs to be told what speci® c information sources to look for and attend to, or whether the meaning of the information can be learned equally well, or perhaps even better, without formal verbal instruction. Recently, this practical question has become topical within the motor learning ® eld as interest has arisen in the respective merits of implicit
Farrow and Abernethy and explicit learning approaches, also known variously as unconscious and conscious learning, unselective (U-mode) and selective (S-mode) processing, tacit (implicit) and explicit knowledge (Hayes and Broadbent; 1988; Magill, 1998) and incidental and intentional learning (Neill et al., 1990). Explicit learning involves the use of speci® c instructions about how to develop a particular skill and concomitantly results in the acquisition of a large verbalizable knowledge base about how to perform the skill being acquired. In contrast, implicit motor learning is `the acquisition of a motor skill without the concurrent acquisition of explicit knowledge about the performance of that skill’ (Maxwell et al., 2000, p. 111). Although the de® nitions of the two modes of learning are relatively unambiguous, their contribution to the skill acquisition process is not. Debate exists about the impact of explicit instruction (and subsequent explicit rule formation) on the motor system’ s natural tendency towards implicit learning processes (Beek, 2000). Masters (2000) advocates minimization of explicit rule formation through the use of implicit learning approaches, while Beek (2000) argues that verbalizable knowledge developed because explicit learning is limited and may not necessarily be damaging to motor performance in all contexts. There is evidence to suggest that people can learn implicitly, yet are unable to verbalize how they perform movements that allow the achievement of goals, such as optimizing force generation (Masters, 1992), timing the application of optimal force (Wulf and Weigelt, 1997) and intercepting objects using knowledge of critical environmental regulatory information (for a review, see Magill, 1998). Furthermore, there is some evidence in the cognitive domain to suggest that complex stimulus information is retained for longer (up to 2 years postintervention) if learned implicitly rather than explicitly (Allen and Reber, 1980). Reber (1989) has argued that implicit learning involves the acquisition of `deep’ information about event sequence structure not typically conveyed by verbal instruction and it is this deeper encoding that enhances retention of the information. In comparison, there is uncertainty about the use of traditional explicit approaches such as coach-directed cueing to key information sources or the provision of detailed instructions about how to perform a motor skill. There is some evidence to suggest that explicit learning may impede preferred implicit processes. Green and Flowers (1991) argued that explicit learning makes participants aware that there is a rule to be discovered and can disrupt implicit learning processes that are otherwise eþ ective in allowing participants to make use of sequential dependencies present in the display. This perspective has also been adopted by Masters (1992, 2000), who has posited that the development
Implicit perceptual training of explicit knowledge causes deautomatization of the motor skill, forcing performers to try to consciously control what are normally subconscious processes. This, in turn, may result in a degradation of performance relative to a skill learned and subsequently controlled implicitly. In contrast, Beek (2000) has suggested that, rather than the development of explicit knowledge being necessarily disruptive to performance, the ability of a performer to override automatic motor processes and develop an alternative strategy on the basis of explicit knowledge may be advantageous when the motor system has sustained an injury or is impaired. Despite the intuitive attractiveness of implicit learning, the limited research conducted on the topic within the motor learning domain has left a number of pertinent issues unresolved. A particular methodological concern has been the use of the verbal rule formation protocol as an indicator of the presence or absence of implicit learning (e.g. Masters, 1992). Postintervention, participants are typically asked to report any rules or strategies they used to control their performance. Discrete pieces of information reported that provide suý cient detail to account for the par ticipants’ performance are considered explicit rules and an indication of explicit knowledge development. However, it is unlikely that there is a linear relationship between the number of rules and total dependency on explicit processing. For example, one `omnibus’ rule used exclusively may potentially generate greater explicit processing than many small rules applied partially. Another issue of concern relates to the pretesting of explicit rule formation. Bennett (2000) has suggested that the omission of such a test has been a weakness of previous implicit learning research. Without a baseline measure of rule formation, the capacity of any particular training approach to develop or restrict explicit knowledge development cannot be determined accurately. To create a truly implicit learning condition, researchers have sought means of drawing attention away from performance of the task at hand. This has typically involved the use of a secondary task paradigm where learners attend to a secondary task while performing the primary task. The basic premise is that attention to the secondary task precludes the opportunity for learners to re¯ ect on the learning of the primary task and hence accumulate explicit knowledge about how it is performed. However, when examining the acquisition of motor skills in natural settings, the dual-task method can be diý cult to administer (Maxwell et al., 2000). Historically, the typical secondary task used has involved non-context-speci® c activities such as random letter generation or auditory tone judgements. These methods, while legitimate techniques when attempting to invoke implicit learning in experimental conditions,
473 have proven impractical in motor skill learning environments. In particular, the secondary task is typically a mentally arduous imposition (particularly for novices), lacks contextual speci® city and typically causes suppressed practice performance (relative to that of explicit learners) that remains even after extensive practice. Collectively, these factors can lead to a reduction in the participant’ s con® dence in their ability to learn the skill (Liao and Masters, 2001). In an attempt to address this problem, Liao and Masters (2001) have recently advanced the concept of analogy learning, in which an analogy or metaphor is used to explain the requirements of a task so that the need for explicit verbal information is minimized. This method is based upon Hayes and Broadbent’ s (1988) conceptualization of S-mode (selective) or U-mode (unselective) learning. S-mode learning is when individuals deliberately seek information about environmental regularities resulting in explicit knowledge. In contrast, U-mode learning is passive or indirect and no conscious strategies are used to develop knowledge about the environmental regularities. Liao and Masters’ (2001) investigation of this concept with participants required to learn a table-tennis topspin forehand revealed that those who learned by analogy demonstrated features associated with an implicit or a U-mode of learning. In particular, fewer explicit rules were formed, the skill was more robust under a secondary task load, and there was a lack of correlation between learner con® dence and performance. Promisingly, the analogy learners demonstrated similar amounts of learning to the explicit learning groups throughout practice without the suppressed performance typically seen when implicit learning modes are generated through the imposition of a secondary task. A similar approach is examined in the current study, where taskrelated but goal-irrelevant instructions are used in an attempt to create an implicit instructional approach based on the principle of U-mode learning. This study was designed to address some of the previously identi® ed issues with perceptual training and approaches to it. A principal aim was to determine whether the information sources used by expert performers, and which form the basis for their superior anticipatory skill, can be used to train less skilled individuals. We did this by comparing the changes in anticipatory skill after training of two groups experiencing video-based perceptual learning and of a placebo group and a control group. Anticipatory skill was measured in vivo using a progressive temporal occlusion paradigm applied to the tennis return of serve so that perceptual skill learning could be examined within the performance environment. An additional response condition, in which only verbal predictions of forthcoming stroke direction were required, was
474 also used to determine whether video-based learning diþ erentially aþ ects perceptual skill dependent on the amount of natural perception± action coupling inherent in the task. A secondary aim was to assess two fundamentally diþ erent instructional methods for conveying the expert information sources to the learners. Speci® cally, explicit and implicit instructional methods of learning were examined within the perceptual training framework with the intention of determining (a) whether an implicit mode of instruction promotes acquisition of performance in a similar way to explicit modes of instruction and (b) whether the resultant perceptualmotor learning from an implicit mode is retained more eþ ectively over an extended retention period. Of related interest was the question of whether an implicit learning approach (based on the use of task-related but goalirrelevant instructions representative of a U-mode of learning) would demonstrate similar learning features as those typically seen in the more traditional implicit learning approaches and in analogy learning.
Methods Participants Thirty-two schoolboy tennis players classi® ed as inter mediately skilled junior players based on their local club and school rankings participated voluntarily in this study after parental consent was obtained. The participants were allocated in a quasi-random fashion to form four equal groups. This process ensured that the highest ranked and lowest ranked players within the sample were evenly distributed across the four groups. The participants were aged 12.1± 17.0 years (15.0 ± 1.1; mean ± s) and they had played competitive tennis for an average of 4.7 years. Experimental design The experimental design consisted of a progressive temporal occlusion pre-test used to determine the ability of the participants to predict the direction of an opponent’ s service in situ. This was followed by 4 weeks of training, involving a combination of video-based training and physical practice. Three days after the ® nal training session, a post-test was completed followed by a further retention test 32 days later. The same test of anticipatory skill was administered on the three separate test occasions. In addition to the test of anticipatory skill, the number of explicit rules possessed by participants before and after the training intervention was assessed using a verbal protocol procedure similar to that developed by Masters (1992).
Farrow and Abernethy Apparatus and test procedures All test sessions for the pre- and post-training of anticipatory performance were conducted on a regulation outdoor (plexipave) tennis court. The participants wore a pair of PLATO liquid crystal spectacles (Milgram, 1987) to control visual occlusion and industrial strength earmuþ s to negate any auditory information that may have been used as a source of anticipatory information. The spectacles could be made to either occlude or allow vision of the server within a 3 ms period. The spectacles were controlled by a computer and UHF transmitter interface unit operated manually by the experimenter. This unit was connected to a UHF receiver worn in a carry pack strapped to the player’ s back at waist level. The radio frequency emitted from the transmitter unit concurrently controlled the spectacles (providing either vision or occlusion) and illuminated a light-emitting diode (LED) when occlusion occurred. The illumination of the LED was recorded together with the rest of the return of serve on a `NAC’ HVRB-200 high-speed video camera operating at 200 Hz and positioned behind the receiving player. This procedure captured a synchronized video record (with an accuracy of ± 10 ms) of the following key features of the serve± return scenario: (1) moment of LED illumination (moment of occlusion), (2) service kinematics, (3) moment of racquet± ball contact for the server, (4) direction of serve and (5) receiver movement direction and shot selection. Post-hoc inspection of the video footage enabled the time of visual occlusion relative to racquet± ball contact to be recorded and an objective determination of the receiver’ s prediction accuracy relative to service direction to be made for each trial. All participants were required to respond to tennis serves hit by two diþ erent right-handed male servers, representative of sub-elite tennis skill and unfamiliar to the participants. Importantly, the servers displayed biomechanically competent service actions similar to the servers displayed on the video-based training tapes. After being ® tted with the occlusion spectacles and earmuþ s, the participants were given 10 pre-test practice trials to familiarize them with the task requirements. Although the participants were aware that the task was a measure of their return of serve performance, they did not know that movement direction was the primary variable of interest. This reduced the likelihood of the diþ erent training groups thinking that their training regime was more or less relevant than that experienced by any other training group. To examine the impact of perception± action coupling on the participant’ s ability to predict service direction, both coupled and uncoupled response conditions were used. The participants started each trial from a position corresponding with the typical receiver’ s position in
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Implicit perceptual training tennis. The coupled condition required the players to make a movement response identical to that which they would use in a game ± that is, the participants were speci® cally instructed to try to hit a successful return stroke. In the uncoupled condition, the participants stood in the same position on the court (without a racquet) and were required to make a verbal prediction of whether the service was directed to their forehand or backhand side. This prediction had to be made immediately after occlusion occurred. Through the use of a progressive temporal occlusion approach (see Fig. 1), ® ve diþ erent temporal occlusion conditions were presented for each response condition. The occlusion conditions were selected so as to provide viewing windows into diþ erent features within the server’ s movement patterns. All trials commenced with introductory vision of service preparation that included the server walking up to the baseline, assuming the ser vice stance and bouncing the ball. This introductory phase was simply provided so that participants had an orientation to the general service and were therefore ready to respond. As is evident in Fig. 1, condition T1 occluded the display no later than 900 ms before racquet± ball contact at a point corresponding with the start of the ball toss. In condition T2, occlusion occurred at or before 600 ms prior to racquet± ball contact at the point where the ball toss had nearly reached its zenith. In addition to the information visible at T1, T2 also provided vision of the upward movement of the ball toss and the racquet’ s movement into a `Y’ position with the ball toss hand. Condition T3 was occluded when the racquet was at the top of the backswing and the ball toss hovering at its zenith at a time at or before 300 ms prior to contact. In addition to vision of the information available in T2, this condition presented vision of the server’ s action, including the movement of the racquet head towards the start of the backswing. Condition T4 occluded the display around the point of racquet± ball contact and consequently presented vision, in addition to that included in T3, of the backswing into the `back-scratch’ position, the acceleration or throwing of the racquet head up to the ball, and any ® nal downward movement of the ball toss. Condition T5 occluded the display at a point after contact permitting vision of the server’ s follow-through motion plus, importantly, post-contact movement of the ball until it reached the vicinity of the net. All participants received a total of 150 serves in their test session (75 trials in the coupled condition and 75 in the uncoupled condition), with each of these serves hit at speeds of approximately 80 km ´ h-1. The servers followed an identical predetermined random schedule for each participant, so as to distribute the serves as equally as possible to the left and right sides of the service box. Serves that were hit into the middle of
the service box were eliminated from the analysis, as it was deemed too diý cult to classify objectively a player’ s response to such serves. Furthermore, on those occasions where participants did not move either to the left or right, an incorrect response was recorded. Each server completed all required serves for one response condition before the second server began his trial block of the same response condition. The same procedure was then followed for the other response condition. Response conditions (coupled or uncoupled) were therefore blocked for each participant but the order of presentation of the response conditions and servers was counterbalanced across participants. Check on the post-hoc sorting of data Because manual triggering of the liquid crystal spectacles does not permit precise trial-to-trial control of occlusion onset, post-hoc video inspection of the trials was necessary to sort the data into the ® ve 300 ms occlusion conditions relative to racquet± ball contact. It was then necessary to determine whether as a result of this process the post-hoc determined means within each 300 ms occlusion condition were evenly distributed across the groups. For example, a group’ s superior performance in a particular occlusion condition may simply be the result of receiving more visual information (a later occlusion) than the other groups. To check that this possible confound was not present, the exact occlusion times for each group’ s data within each of the ® ve 300 ms occlusion conditions were subjected to a 4 ´ 5 ´ 3 ´ 2 (group ´ occlusion ´ time of testing ´ response) analysis of variance (ANOVA) with repeated measures on the last three factors. Results revealed a signi® cant main eþ ect for the time of occlusion (F4,112 = 9703, P < 0.05) and group (F3,28 = 4.60, P < 0.05). Importantly, there was no signi® cant group ´ occlusion interaction or any other higher-order interaction. The group main eþ ect revealed that the control group received less visual information than the placebo group when the means where collapsed across all occlusion conditions. Therefore, we concluded that the resultant means of each individual 300 ms occlusion condition experienced by each experimental group after the posthoc sorting procedure were essentially equal between the groups. This indicates that any changes in performance by the training groups post-intervention can be attributed to factors other than methodological nuances inherent in the post-hoc sorting of temporal occlusion conditions. The mean number of trials included for analysis within each of the ® ve occlusion conditions is outlined in Table 1. These data were also subjected to a 4 ´ 5 ´ 3 ´ 2 (group ´ occlusion ´ time of testing ´ response) ANOVA with repeated measures on the last
Fig. 1. Schematic representation of the ® ve 300 ms occlusion conditions imposed during the progressive temporal occlusion test.
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Implicit perceptual training Table 1. Number of trials experienced per participant across each occlusion condition and test phase in the motor and verbal response conditions (mean ± s) T1
T2
T3
T4
Motor response Pre-test Post-test Retention test
13.3 ± 3.4 10.8 ± 3.0 7.3 ± 3.7
10.7 ± 3.3 12.6 ± 3.7 15.9 ± 3.6
18.1 ± 2.9 20.0 ± 3.4 19.4 ± 3.5
18.9 ± 5.3 16.5 ± 5.8 16.1 ± 5.6
11.4 ± 4.7 12.6 ± 5.3 13.6 ± 5.5
Verbal response Pre-test Post-test Retention test
14.6 ± 3.6 11.3 ± 3.7 8.6 ± 3.8
9.9 ± 3.2 13.3 ± 4.3 15.2 ± 4.2
20.1 ± 3.4 20.3 ± 3.2 21.2 ± 2.8
18.8 ± 4.2 14.8 ± 5.4 14.6 ± 5.6
9.9 ± 5.2 13.4 ± 6.3 13.1 ± 5.2
three factors. Results revealed a signi® cant three-way interaction between group, occlusion condition and time of testing (F24,224 = 1.89, P < 0.05). Within-participants contrasts revealed that the control group had fewer trials included for analysis at the T1 post-test condition relative to the other experimental groups; however, this eþ ect was reversed at the T2 post-test condition (F3,28 = 4.30, P < 0.05). These diþ erences are trivial and do not preclude valid comparisons of the training groups from being undertaken as the central analysis within the study. Training procedures During training, the two experimental groups (the implicit and explicit learning groups) watched identical video-based temporal occlusion footage of tennis servers. Three video-based training sessions were completed each week. Each session consisted of 50 practice trials and took approximately 20 min to complete. Altogether, 600 practice trials were completed over the 4 weeks of training. A placebo group watched video footage of elite tennis matches for an equivalent period of time. Both the placebo and the experimental groups were given introductory statements about the expected positive bene® ts the training sessions would have on their return of serve performance. This was done to provide comparable expectancies of success between the three training groups. Finally, a control group simply completed one physical practice training session per week that consisted of each participant returning 25 serves hit by another member of their training group. They undertook no video-based training and were given no introductory statements as to the expected bene® ts from participation in the practice they were under taking. The physical practice session completed by the control group each week was also completed by the other three training groups. The purpose of interspersing physical training with perceptual training was to provide
T5
the players with the opportunity of continuously recalibrating any changes in visual-perceptual capabilities with the motor system requirements of the task. This may be critical given the apparent importance of maintaining strong functional links between perception and action (Gibson, 1979). During each of the 4 training weeks, the three video-based groups typically completed two video-based sessions on one day and then returned 2 days later to complete a further video-based session and the physical practice session. Diary training records were kept by the participants throughout the study. Importantly, these indicated that the participants played comparable amounts of tennis during the experiment and were unaware about the speci® c nature or content of the practice sessions undertaken by the other training groups. Explicit learning group. The participants in the explicit learning group viewed temporally occluded video-based footage of various professional tennis players hitting serves. The footage was ® lmed from the receiver’ s perspective and was then edited so that the serves were occluded at various stages before and after racquet± ball contact and provided diþ ering amounts of kinematic service information. The participants watched the edited footage on a 480 mm television monitor in a quiet room. Their task, for each service they viewed, was to manually record whether they thought the ball was being hit to the left or right side of the service box. This response had to be made immediately after occlusion occurred. After recording their response, the participants watched an unoccluded replay to assess the accuracy of their prediction. Participants in this group were explicitly instructed about the relationship between speci® c advance information sources in the opponent’ s service action and the resultant service direction. The anticipatory information sources highlighted during the training were selected according to previously identi® ed kinematic service characteristics known to diþ erentiate between the return of serve of expert and novice tennis
478 players (Goulet et al., 1989; Hernandez and Sicilia, 1998). In particular, the signi® cance of the location of the ball toss and the movement and angle of the racquet head in the last 150 ms before contact were explained. Secondary features such as the server’ s grip, stance and shoulder rotation relative to resultant ball direction were also discussed to encourage conscious attention to these features. This information was presented to the explicit learning participants through a variety of media. First, they watched an instructional video that outlined the similarities and diþ erences in the biomechanics of the various service types (i.e. slice, ¯ at and topspin). This was complemented with verbal and written instruction and the use of diagrams to reinforce the key information sources likely to provide anticipatory information about the service direction. Furthermore, all participants in this group received a summary sheet that reviewed the key information± service direction relationships. Each training session was supervised and the participants were provided with further feedback throughout each training session, reinforcing the important kinematic service features. Implicit learning group. This training group watched exactly the same training tapes and completed the same number of practice trials as the explicit learning group but, importantly, did not receive any explicit instruction regarding which speci® c information sources they should attend to so as to facilitate anticipatory prediction performance. Instead, the task of the players in this group was to estimate the speed of each occluded serve. When the serve was replayed unoccluded, the experimenter read out the serve’ s speed and the participants recorded the correctness of their approximation. The logic behind the design of this training approach was two-fold: ® rst, the approach was designed to minimize conscious (explicit) processing of possible anticipatory information sources for prediction of service direction; second, the approach forced the participants to closely monitor service mechanics in an attempt to predict the serve’ s speed. We reasoned that attention to this information might indirectly facilitate the players’ service prediction performance, as they would implicitly establish relationships between the service kinematics and resultant service outcome, without necessarily consciously processing such infor mation and establishing verbalizable rules for the task. Placebo group. This group watched competitive matchplay footage of professional tennis matches (e.g. 1998 US Open ® nal, 1992 Rio Masters matches) for a length of time (20 min) equivalent to the duration of the perceptual training sessions undertaken by the two experimental groups. To maintain their attention, the
Farrow and Abernethy participants in this group were asked questions about the match they were viewing at various times during the training session. Typical questions included: Why is a particular player winning? What is the point score in a game? What are the favourite shots of a player in a given set of circumstances? Additionally, this group was given a statement about the expected positive eþ ects of increased match-play knowledge on return of serve performance. Control group. A fourth group acted as a control and was required only to complete the weekly physical practice session and the three test sessions. The main aims of this group were to provide baseline measures for improvement on the physical practice task and to ascertain the extent of any training improvements in return of serve performance that may have arisen as a result of test familiarity. Data analysis Both before and after the 4 week training period, the participants were required to write down all the rules, coaching tips and strategies that they felt were impor tant in returning serve. The pre-intervention question permitted their responses to relate to any coaching tips that they had received previously, while their post-intervention responses (completed after the ® nal training session) were based upon both prior knowledge and the training they had completed during the experiment. An explicit rule was operationally de® ned as any rule drawn from written instructions received or speci® cally relating to the biomechanical or anticipatory cues used in returning a tennis serve. Examples of explicit rules reported included: `when the ball toss is out to the left, the serve is going to go right’ and `if there is a big back arch, it usually means a kick serve down the left’ . Statements not referring to the biomechanical or anticipatory aspects of returning serve were excluded, for example `I watched for racquet speed’ and `whether the serve is ¯ at or topspin’ . The credibility of the explicit rules elicited from the written protocols was then corroborated by checking one list against the other (Masters, 1992). The pre-intervention questioning was used to determine whether there were any diþ erences between participants in the amount of explicit knowledge they possessed before starting training. The post-intervention questioning was used to determine whether the instructional sets given to the diþ erent training groups worked as expected and diþ erentially aþ ected the development of explicit rule formation. Response accuracy was the critical dependent variable extracted from the test of anticipatory skill. Responses in the coupled condition were de® ned as the
479
Implicit perceptual training ® nal direction (left or right) that the participants moved their body in an attempt to intercept the oncoming serve. The uncoupled responses were recorded as left (verbalized as backhand by a right-hand player) or right (verbalized as forehand by a right-hand player). The percentage of successful responses under each response condition for each time window was then calculated. The accuracy data of the participants were analysed using a 4 ´ 5 ´ 2 ´ 2 (group ´ temporal occlusion condition ´ time of testing ´ response condition) factorial ANOVA with repeated measures on the last three factors. Relevant signi® cant interactions were followed up by a series of fully repeated-measures 5 ´ 2 ´ 2 (temporal occlusion condition ´ time of testing ´ response condition) analyses of variance conducted separately for each training group. The GreenhouseGeisser correction was applied to any violations of sphericity. Signi® cant interactions as a result of these analyses were examined using within-participant contrasts and/or t-tests with Bonferroni correction where appropriate. In keeping with the main aim of the study, key points of comparison were signi® cant betweengroup changes in prediction accuracy as a result of the training intervention at each of the occlusion points. Any mean data points that demonstrated a signi® cant change after the intervention were then compared against 50% to determine if the prediction performance diþ ered reliably from that which would have arisen simply by guessing. (In a two-choice prediction task, a mean prediction of approximately 50% would be expected if a participant was simply guessing.) The same process was also applied to within-group prediction accuracy changes across adjacent temporal occlusion conditions to provide a measure of predictive information pick-up across the enclosed viewing period. The validity of the ® ndings was assessed further by examining individual participant data to determine how many participants within each group signi® cantly improved performance at the time points or time window found to be signi® cantly above chance for the group as a whole. A further analysis was also undertaken to allow a direct comparison of the prediction accuracy of the participants in the two diþ erent response conditions (coupled and uncoupled). The purpose of this was to determine if the training interventions diþ erentially aþ ected verbal versus motor response accuracy.
protocols. Computation of a Pearson product± moment correlation coeý cient showed suitable inter-rater reliability (r = 0.91, P < 0.01). The scores of the two raters were then averaged to provide a ® nal verbal protocol score. Surprisingly, none of the participants possessed any explicit rules pertaining speci® cally to prediction or anticipation of service direction before the start of training. The results of a one-way ANOVA revealed that there was a signi® cant diþ erence in post-intervention rule formation between the explicit, implicit, placebo and control groups (F2,21 = 20.5, P < 0.05). Explicit rule formation post-intervention con® rmed that the explicit group acquired signi® cantly more explicit rules (2.5 ± 0.89) than any of the other training groups. Importantly, the learning condition based on the notion of task-related but irrelevant instructions (the implicit learning group) demonstrated, as expected, much less explicit rule formation (0.43 ± 0.51). Similarly, the placebo group (0.93 ± 0.68) and the control group (0.25 ± 0.44) also demonstrated minimal explicit rule development (see Fig. 2). Prediction accuracy: acquisition performance One of the aims of this study was to examine changes in the anticipatory skill of the two groups experiencing video-based perceptual training and of the placebo and control groups. Furthermore, we were interested in whether a more implicit mode of learning promoted acquisition performance in a similar way to that of an explicit mode of instruction. We predicted that success in predicting service direction by moving in the correct direction to hit a successful return (i.e. the coupled response condition) and by verbalizing whether a serve was hit to the participant’ s forehand or backhand side (i.e. the uncoupled response condition) would be
Results Explicit rule formation The ® rst author and an independent rater summed the number of explicit rules written down by each par ticipant and then independently scored these written
Fig. 2. Mean number of explicit rules reported by each group after the post-test. Error bars represent the standard error.
480 signi® cantly in¯ uenced both by the time of testing and the extent of temporal occlusion applied. Figure 3 presents pre- and post-intervention prediction accuracy, collapsed across response modes, as a function of occlusion for the explicit, implicit, placebo and control groups. Of importance was the demonstration of betweengroup similarity before the start of the intervention. The absence of a signi® cant main eþ ect for group (F3,28 = 0.554, P > 0.05) and the lack of a signi® cant interaction between group and time of testing (F3,28 = 0.381, P > 0.05) support the conclusion that the four training groups were not signi® cantly diþ erent at the start of the intervention. In relation to post-intervention performance, a signi® cant main eþ ect for time of testing (F1,28 = 14.6, P < 0.05) indicated that in general posttest performance was better than pre-test performance. Although there was no signi® cant interaction between
Farrow and Abernethy group and time of testing (F3,28 = 1.06, P > 0.05), this was overshadowed by a three-way interaction between group, occlusion and time of testing (GreenhouseGeisser F12,112 = 2.44, P < 0.05), indicating that the groups responded diþ erently to the training intervention. Follow-up analyses revealed signi® cant occlusion ´ time interactions for both the implicit learning group (F4,28 = 3.93, P < 0.05) and the placebo group (F4,28 = 3.47, P < 0.05). Within-participant contrasts revealed that the post-test prediction performance of the implicit learning participants was signi® cantly better than their pre-test performance at the T3± T4 occlusion condition (F1,7 = 10.6, P < 0.05). This ® nding points to this group’ s use of pre-contact information sources for predictive purposes (see Fig. 3). Additionally, both the implicit learning group (F1,7 = 11.6, P < 0.05) and placebo group (F1,7 = 3.47, P < 0.05) demonstrated signi® cantly worse post-test scores than pre-test
Fig. 3. Pre- and post -test training prediction accuracy (percentage of correct responses) for the explicit, implicit, placebo and control groups. Standard error measures are displayed in the negative direction for the pre-test (r ) and in the positive direction for the post-test ( ). Fifty percent constitutes guessing on this task.
481
Implicit perceptual training scores across the T4± T5 occlusion condition. Closer inspection of the implicit learning group’ s means at T5 (pre-test 88.43, post-test 83.38) indicated that this interaction was attributable to the contrast approach where the magnitude of the change is sensitive to the value of the ® rst condition. In relation to the placebo group, the most likely explanation for this result was the presence of a statistical outlier in the post-test data causing this group’ s mean to be unusually suppressed relative to the pre-test mean. This was con® rmed by further analysis in which the statistical diþ erence disappeared when the outlying data point was excluded from the analysis. Surprisingly, the explicit learning group, unlike the implicit learning group, did not demonstrate any speci® c improvement in prediction performance from before to after the intervention (F4,28 = 0.193, P > 0.05). Similarly, the control group’ s performance did not change between before and after the intervention (F4,28 = 1.20, P > 0.05) (see Fig. 3). Inspection of the means that were signi® cantly different from chance revealed that the implicit learning group’ s post-training improvement occurred selectively at the T4 occlusion condition (see Fig. 3). Although the implicit learning group’ s mean prediction score at T4 was not signi® cantly diþ erent from the other experimental groups, examination of individual participants’ data at T4 revealed that seven of the eight participants within the implicit learning condition improved their mean prediction performance after the intervention. Comparative improvements of this type were observed in only three of the eight explicit learners, while the prediction performance of two members of this group decreased post-training. Prediction accuracy: retention performance Another aim of this study was to determine if any improvements in prediction accuracy of service direction as a result of the intervention period would be maintained after a 32 day un® lled retention interval. In particular, we reasoned that the implicit learning group should demonstrate improved retention performance relative to the other learning conditions. The results of a 4 ´ 5 ´ 2 ´ 2 (group ´ temporal occlusion condition ´ time of testing ´ response condition) factorial ANOVA with repeated measures on the last three factors revealed a signi® cant time of testing ´ occlusion interaction (F4,112 = 3.84, P < 0.05). Follow-up within-participant contrasts demonstrated that the retention test performance of the groups was better than their post-test anticipatory performance between the T4 and T5 occlusion conditions (F1,28 = 9.95, P < 0.05). However, the absence of a signi® cant main eþ ect for group (F1,28 = 14.8, P > 0.05) or any signi® cant two -way group interactions (group ´ time:
F3,28 = 0.980, P > 0.05; group ´ occlusion: F12,112 = 1.04, P > 0.05) or higher-order interactions indicates that any improvements due to training were not maintained at the time of the retention test (see Fig. 4). In particular, the improvement in prediction performance seen at the T3± T4 occlusion condition for the implicit learning group 3 days after completing the training intervention (i.e. at the post-intervention test) was not retained after a 32 day un® lled retention interval. Response mode comparisons The prediction accuracy within the coupled (movement) and uncoupled (verbal) modes of response was also compared directly within the results of the analyses of variance for the acquisition and retention phases. Similar ® ndings were found for the two experimental phases. Although there was no main eþ ect for response mode (acquisition: F1,28 = 0.233, P > 0.05; retention: F1,28 = 0.142, P > 0.05), the extent of perception± action coupling did interact signi® cantly with the time of occlusion (acquisition: F4,112 = 10.2, P < 0.05; retention: F4,112 = 6.82, P < 0.05). The absence of any other higher-order interactions with the extent of perception± action coupling employed indicates that similar conclusions about the relative eþ ect of implicit and explicit training would be observed regardless of the response mode used. However, as Fig. 5 shows, the relative prediction accuracy in the coupled and uncoupled conditions varied according to the information sources available to the players. Speci® cally, while there was no diþ erence in prediction accuracy between the response conditions for occlusions before racquet± ball contact, prediction accuracy was better for the coupled response mode in the post-contact occlusion condition (where ball ¯ ight information was available). Therefore, more accurate responses were evident when ball ¯ ight information was available for movement-based responses rather than verbal responses.
Discussion In this study, we examined the eþ ects of two diþ erent video-based perceptual training interventions designed to improve the ability of tennis players to anticipate the direction of an opponent’ s serve. In particular, the use of a more traditional explicit method of instruction was compared to an implicit mode of learning designed to minimize recourse to conscious processing of anticipatory information sources. Changes in anticipatory performance for the groups given perceptual training were assessed in a natural setting relative to appropriate placebo and control groups.
482
Farrow and Abernethy
Fig. 4. Post-test and retention test prediction accuracy (percentage correct responses) for the explicit, implicit, placebo and control groups. Standard error measures are displayed in the negative direction for the post-test (r ) and in the positive direction for the retention-test ( ). Fifty percent constitutes guessing on this task.
The key point of interest in this study was to ascertain if any of the diþ erent training requirements would be successful in improving anticipatory performance. Analyses revealed some critical improvements in prediction accuracy after training for the implicit learning group that were not replicated by any of the other groups. Speci® cally, prediction accuracy at the T4 (contact) occlusion point was signi® cantly better for the implicit group immediately after the 4 week training period than before, with the capacity to improve signi® cantly prediction accuracy in the T3± T4 time window evident after, but not before, the implicit training intervention (Fig. 3). This improvement in prediction accuracy and the acquired capacity to pick up information in the T3± T4 time window (to take prediction accuracy from chance to well above) was evident for seven of the eight participants in this
learning group. Importantly, these improvements for the implicit group occurred despite an absence of explicit rules about how to complete the task, suggesting a U-mode of learning (Hayes and Broadbent, 1988). The signi® cance of the time period at which the training eþ ects were evident for the implicit group is that the kinematic information available at T4 and during the T3± T4 time window has been previously demonstrated (Goulet et al., 1989; D. Farrow and B. Abernethy, unpublished) to be one of the most infor mative phases of the tennis serve for the extraction of anticipatory information relevant for service prediction. Furthermore, the kinematic information within the T3± T4 time window is also likely to be the information that would be learned implicitly by this group, as this window provides visibility to the throwing action of the racquet head up to the point of contact. In attempting to
Implicit perceptual training
Fig. 5. Pre- and post-test intervention prediction accuracy as a function of the time of occlusion and response condition. The coupled condition involves a movement response to hit the ball; the uncoupled response involves a verbal prediction of service direction. Standard errors are shown in the negative direction for the coupled data (r ) and in the positive direction for the uncoupled data ( ). The data from the acquisition phase are used as an exemplar for it and the retention phase.
predict service speed, the participants in the implicit learning group needed to pay close attention to the motion of the racquet head (including racquet arm speed and movement) to determine the amount of spin imparted on the ball. A large amount of imparted spin would indicate to the players that the ball speed would be reduced for that serve. It was apparent that, in addition to acquiring some (implicit) understanding of the association between racquet head motion, spin and service speed, this group also implicitly acquired an understanding of the relationship between racquet head movement and resultant ball direction, thereby facilitating their anticipatory performance. The contention that the implicit learning group acquired useful anticipatory knowledge is strengthened further by the absence of any improvements in performance across any of the other pre-contact time conditions. As the training for both the explicit and implicit learning groups predominantly focused upon information sources that occurred during the middle to late stages of the service action (in particular conditions T3 and T4), it was to be expected that if any signi® cant improvements in performance were to emerge as a result of the training interventions, they would do so during these key periods. In contrast to the implicit learning group, the group
483 trained explicitly somewhat surprisingly showed no systematic improvements in prediction accuracy from pre- to post-training (Fig. 3). It should be noted, however, that the explicit learning group still had a mean predictive accuracy similar to the implicit learning group post-training. One possible explanation for the lack of post-intervention improvement is that because participants in this group were exposed to several strategies suggested to improve anticipatory performance, they did not consistently attend to the most valuable anticipatory strategy but rather sampled a number of diþ erent information sources with varying success. Although the motion of the racquet approaching contact was emphasized as the most reliable cue for service direction prediction, the availability of less conclusive but more consciously perceptible information arising from sources such as the ball toss may have inadvertently captured the attention of most members of the explicit learning group. Ball toss information appears earlier in the service action and at slow speed relative to racquet head information. In contrast, the members of the implicit learning group, by the nature of the instruction provided to them, were likely to only attend to the one information source (racquet head motion), which also proved to be the most pertinent for anticipating service direction. The improvements with training for the implicit learning group occurred against a background of no measurable pre- to post-training improvements for either the placebo group or the control group (Fig. 3). Therefore, the on-court anticipatory performance changes observed after the intervention for the implicit learning group can be considered to be a direct consequence of the nature of the training completed and not an artefact of either expectancy or test familiarity. The number of explicit rules acquired by the par ticipants in each group was used in this study to provide a rough estimate of the extent of explicit knowledge development. This measure, although not perfect, indicated two key things. First, the pre-test measure revealed that the participants had not experienced any formal explicit training regarding the use of perceptual information for anticipatory purposes before entering this study. The only `rules’ provided by the participants were in terms of reference to opponent probability information such as the direction of their favourite serve. Most of the verbal sets provided pre-training were technical catch phrases like `move in and take the ball early’ and `shorten your backswing when returning ® rst serves’ . These responses not only demonstrated the participants’ lack of explicit knowledge before the start of the training intervention but also highlighted the typical bias displayed in many tennis coaching environments where technical skill development, rather than perceptual skill development, is emphasized. Second,
484 the post-training examination of explicit rule formation (Fig. 2) con® rmed that the explicit learning group acquired most explicit rules while the implicit learning, placebo and control groups acquired very few explicit rules of relevance to the task. Although the explicit learning group reported signi® cantly more explicit rules than the other training groups, it is noteworthy that they reported fewer rules than reported for other tasks (e.g. Masters, 1992), where ® ve or more rules is the norm. This diþ erence can be readily explained in that there were essentially only ® ve rules provided to the explicit learners in this study. Rules related to ball toss, racquet trajectory, back arch, knee bend and, to a lesser extent, grip were the predominant ones emphasized during the training intervention undertaken by the explicit learning group. Previous research, particularly that investigating cognitive tasks, has suggested that one of the key advantages of the implicit learning approach is improved retention of task-relevant information over an extended period (Allen and Reber, 1980). This eþ ect has not been conclusively demonstrated within the motor domain, as retention tests are typically conducted as soon as 3 days after practice ends (e.g. Maxwell et al., 2000). The current study administered a post-test 3 days after the ® nal training session and a delayed retention test 32 days after the post-test. The results revealed that the implicit learning group’ s improvement in anticipatory performance at the critical T3± T4 time window that was apparent 3 days after training was not retained after a 32 day un® lled retention interval. This regression in performance during retention may be explained by a lack of suý cient ongoing practice to sustain the implicitly acquired anticipatory skill. Although the 4 weeks of experimental training was suý cient to produce learning over a short period (at least for 3 days after training ceased), further practice or training may be necessary if players expect to maintain the improvement in performance over an extended duration. This observation further highlights the paucity of evidence on the robustness of implicit motor learning and highlights the importance of including more extended, practically relevant, retention intervals in studies of implicit learning of movement skills. Direct comparison of the prediction accuracy generated under the coupled motor response condition and the uncoupled verbal response mode (Fig. 5) demonstrates an important interaction between the relative accuracies of the two response modes and the nature of the information available to guide anticipatory judgements. The availability of ball ¯ ight information produced signi® cantly better anticipatory performance within the coupled response condition, whereas no diþ erences were evident when only advance postural information was available to guide the prediction. In the
Farrow and Abernethy coupled response mode, participants were required to produce the task-speci® c motor response of attempting to hit the ball, as opposed to the static uncoupled verbal response condition typical of most traditional anticipation research. The results of this study mirror those from other recent work both from our own laboratory, which demonstrated that expert tennis players also perform more eþ ectively in a coupled rather than in an uncoupled response condition (D. Farrow and B. Abernethy, unpublished), and from other researchers investigating perceptual-motor skill using `real-world’ tasks and settings (Beilock et al., in press). Such observations are consistent with, but certainly not proof of, Milner and Goodale’ s (1995) conceptualization of two diþ erent visual processing streams for perception and action processes. Furthermore, the results highlight the importance of providing a test setting that replicates the natural task requirements and environment, in this case for anticipatory skill, as closely as possible. Failure to do so may impede the capacity to observe the full extent of any perceptual learning changes that occur (Abernethy et al., 1993). Practically, the results of this study highlight several issues that warrant the consideration of both coaches interested in using perceptual training and scientists interested in further re® ning knowledge about perceptual-motor skill acquisition. First, learning approaches such as video-based perceptual training, where perception and action are separated during some of the training process, do appear to have the capacity to improve an individual’ s perceptual performance and, importantly, transfer this enhanced ability into the performance environment. However, a related issue that is not yet fully understood is whether the volume of physical practice engaged in by performers may in some way mediate the extent of perceptual training transfer. Second, learning can be achieved by individuals who receive no explicit instruction about information sources that facilitate anticipatory performance; indeed, such approaches can induce improvements in anticipatory performance that are not apparent for players given explicit instruction. The use of an indirect instructional approach, such as the use of instructions that are task -related but goal-irrelevant, like those given to the implicit learning group in the present study, can generate performance improvements. This provides both researchers and coaches with another viable method to minimize explicit rule formation during skill acquisition yet not overload the processing capacities of the learner during this process. The search for other, more eþ ective instructional approaches based upon implicit learning principles is also encouraged to provide a more comprehensive picture of the impact of implicit learning in motor skill learning domains. The capacity of such implicit learning approaches to bring
Implicit perceptual training about enduring changes in anticipatory skill, which last months rather than days after training has ceased, still needs to be demonstrated.
Acknowledgements Appreciation is expressed to the Australian Sports Commission, in particular the Australian Institute of Sport Tennis programme, for funding this project. The authors are most grateful to Rob Bryant for his technical expertise and assistance throughout the project and Wayne Mason (Head Tennis Coach, Brisbane Boys College) for the organization and provision of the players during testing and training.
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