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Contextual interference: Contributions of practice
145
Acta Psychologma 73 (1990) 145-157 North-Holland
CONTEXTUAL INTERFERENCE: CONTRIBUTIONS OF PRACTICE Charles
H. SHEA, Robert
KOHL
and Catherine
INDERMILL
*
Texas A&M Unruersrry, College Siarron, USA Accepted May 1989
The present expenment extends the fmdmgs of Shea and Morgan (1979) and Lee and Magi11 (1983) by deternumng the Impact of mampulatmg contextual Interference in 50, 200, and 400 acquisition tnals on retention of a raped force productton task assessed under both random and blocked contexts. Acquisition performance was mfenor for the random acquisitton groups as compared to the blocked groups with httle differences between the 50, 200, and 400 acqmsitton groups’ performance at comparable stages of practtce However, the retentton data indicated that sublects who completed 400 random acquisition trials performed better on both random and blocked retention than subjects who learned under blocked contexts. Increasing the number of blocked acqutsition tnals dtd not tmprove retentton under blocked contexts and had a negattve effect on retentton assessed under random contexts Apparently, the benefits of blocked practtce (low contextual interference) occur early m practice with response production becommg mcreasmgly more rigid and mflexible On the other hand, the benefits of random practtce (high contextual Interference) surface after mmal practice
An increasing number of investigators have contrasted performance under ‘high’ and ‘low’ contextual interference (see Lee and Magi11 (1985) for summary and theoretical perspectives). Typically, a learner is asked to practice a number of related tasks or a number of variations of a single task and the practice schedule is varied to maximize or mimmize contextual interference. Practice schedules that present task variations in a random order (C-A-B, B-C-A, B-A-C) are thought to produce high contextual interference, while schedules that present * The authors would like to thank Richard Magtll, Ttm Lee, Dick Schmidt, Wayne Shebtlske and Craig Wnsberg for readmg an earher draft of thrs paper and contributmg many helpful suggestions for the revmon. In addition, we wash to thank Dana Bernard and Mark Guadagnoh for thetr assistance wtth the data collection. Requests for repnnts should be sent to C H. Shea, Human Performance Laboratones, 276 Read Butldmg, Texas A&M Umverstty, College Statton, TX 77843-4243, USA. OOOl-6918/90/$3 50 0 1990, Elsevier Science Pubhshers B V (North-Holland)
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task variations in a blocked fashion (B-B-B, A-A-A, C-C-C) are thought to produce low contextual interference. In general, it has been found (e.g., Lee and Magi11 1983; Shea and Morgan 1979) that acquisition performance under high contextual interference was inferior to that of low contextual interference. However, measures of retention indicated better recall for subjects who practiced under high contextual interference. Contextual interference has been postulated to increase as the number of task variations interpolated between repetitions on a particular variation increases. More critically, the increase in interpolated task variations is thought to increase the likelihood of more than one variation being held simultaneously in working memory and decrease the availability of the solution to a particular variation. Thus, a random schedule offers the possibility for a large number (relatively) of task variations to be spaced between repetitions of any one variation, and blocked presentation has no variations between repetitions of any one variation. Shea and Morgan (1979) had subjects attempt 54 acquisition trials (6 blocks of 9 trials) on three variations of a task in either a blocked or random sequence. The task variations were similar in that each variation started (picking up a ball) and ended (placing the ball in a slot) the same, but varied in terms of the combinations and/or order that 3 of 6 barriers that were to be ‘knocked down’. The movement time of subjects performing under the random condition decreased more slowly, but eventually (by block 6) approached that of subjects in the blocked condition. After 10 min of rest, retention was measured for one half of the subjects under blocked or random conditions. The other half of the subjects performed the retention tests after 10 days. Performance after both lo-min and lo-day delays, when collapsed across both retention conditions (blocked-blocked, blocked-random and random-blocked, random-random) was significantly higher for subjects who learned the task variations under random conditions. There were no statistical differences between the blocked to blocked, blocked to random and random to random conditions with only the blocked to random condition significantly slower. They infer from the collapsed data, that random acquisition practice leads to superior retention as compared to blocked acquisition practice. However, this was only true when retention was assessed under random conditions. No clear benefit of random acquisition practice was found when retention was under blocked
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conditions for either retention interval. The major finding was that blocked acquisition practice led to very poor retention performance when switched to a random retention schedule. Perhaps the data should be interpreted not in terms of how well the random acquisition group performed on retention under random conditions but rather m terms of the extent to which blocked acquisition practice negatively effected retention under random conditions. A purpose of the present experiment was to extend the retention findings of Shea and Morgan (1979) and subsequent experiments (e.g., Lee and Magill, 1983) to rapid responses, requiring the subject to completely process (preprogram) the response specifications prior to execution. Previous motor behavior research in contextual interference has, for the most part, involved tasks that were regulated on the basis of concurrent feedback or a least offered the opportunity for feedbackbased corrections to intervene in the control of the movement. It should be noted that in all three experiments by Lee and Magi11 (1983) and the experiments by Shea and his colleagues (Shea and Morgan 1979; Shea and Zimny 1983; Morgan 1981) the task involved knocking over a series of barriers or touching a series of buttons either as quickly as possible or in a specific movement time. It is not clear to which extent these findings can be generalized to open-loop tasks. ’ A second purpose was to determine the impact of increasing the number of acquisition trials under blocked and random contexts on subsequent blocked and random retention. For the most part in the motor domain, contextual interference experiments have used relatively few acquisition trials. For example, in all three experiments by Lee and Magi11 (1983) as well as m experiments by Shea and his colleagues (Shea and Morgan 1979; Shea and Zimny 1983) only 54 acquisition trials were used. Contextual interference studies by DelRey et al. (1982) and Goode and Magi11 (1986) also used relatively few (< 65) acquisition trials. Shea and Zimny (1983) express some concern for the amount of practice issue. They suggest that intuitively, ‘random practice should produce optimum effects after subjects had the opportunity to learn the tasks through blocked practice.’ However, they discount ’ In the barner knock down tasks used m the Shea and Morgan (1979) and Lee and Magdl(l983) expenments, feedback-based correctlon may have played an Important role m the control of the movement Thus, the Influence of contextual Interference on the memones subservmg the movement may have been obscured. Perhaps m more rapld tasks where feedback-based correctlons are hmlted, the Influence of contextual Interference may be wewed more clearly.
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this position by stating that from their ‘theoretical perspective any random practice should result in better retention as measured by recall measures’ (1983: 350). This position predicts that the benefit of random practice will be small early in practice because all subjects receive equal amounts of elaborative processing. With increased random practice experiences the benefit relative to blocked practice will increase because of increased possibility for multiple and variable processing. However, Schmidt in the formulation and development of Schema theory (Schmidt 1975, 1982) suggests that for rapid, discrete responses the purpose of acquisition practice may change as a generalized motor program is developed and subsequently refined. Early in practice, subjects are thought to be extracting basic information from each repetition in order to formulate the basis (invariant features, see Schmidt, 1982) of the motor program to produce subsequent attempts. Thus, blocked practice may be more effective early in practice because it focuses the learner on the essential movement patterning. Since the program has yet to be refined, the contextual variations introduced in random contexts may increase the processing requirements to the point of ‘overload’. Later in practice, when the motor program is more developed, contextual variation may be more manageable and, in fact, may be essential for the performer to become adept at ‘parameterizing’ (variant features, see Schmidt 1982) responses. This notion, predicts that the benefits of random context may not surface until after substantial amounts of acquisition practice and may even be detrimental after very little practice. DelRey et al. (1982) found that the amount of prior experience with tasks (coincident timing) similar to the experimental task influenced the effect of 64 acquisition trials under ‘low’ and ‘high’ contextual interference. Random acquisition schedules (high contextual interference) resulted in enhanced retention over blocked and constant acquisition schedules but only for ‘experienced’ subjects. Retention increased for the ‘novice’ performers as contextual interference during acquisition was decreased. Therefore, the purposes of the present experiment were to extend the findings of Shea and Morgan (1979) to a rapid force production task and to determine the impact of increasing the number of acquisition trials on retention. Retention will be assessed under both random and blocked conditions.
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Method subjects Seventy-two undergraduate students at Texas A&M University participated the purposes of the study.
from the Required Physical Education program m the experiment. All subjects were narve to
Apparatus The apparatus consisted of a dynarruc force measurement system containing a force transducer whtch converted the physical force mto a voltage that represented the instantaneous value of the applied force. The voltage was directed to a rmcrocomputer wluch was programmed to read (1000 Hz) the voltage shtfts upon contact. The peak voltages were then transformed into newtons through a regression equation developed from the cahbratton procedures. The force transducer and microprocessor were also lmked to an oscrlloscope. The display from one channel of the oscilloscope was elevated when an output voltage from the microcomputer increased to present targets, and the display from another channel was elevated when force was apphed to the transducer. Procedure Upon recnntment to the experiment, subjects were randomly assigned to one of twelve groups. The groups dtffered m terms of acquisition practice (50, 200, 400), acqmsitton context (random, blocked), and retention context (random, blocked). The sublect was asked to sit comfortably in a chau next to a table. An oscilloscope was situated so that the screen was m plain view, and the force transducer was posittoned so that it could be comfortably contacted. The subject was informed that the task was to hit the padded arm of the force transducer with the medial surface of their preferred hand m an attempt to elevate a trace dot on the osctlloscope to the current target line (see fig. 1). Ten target lures were generated and displayed across the oscilloscope (see fig. 2) for each acqursttton and retention set pnor to the subject responding. For the blocked context, the same target lme was displayed 10 times across the oscilloscope screen wtthm a set, but vaned across sets. The 10 target lines were randomly presented wtthin sets m the random context. The result of each lut was tmmedtately displayed as a vertical trace displacement of one of the oscilloscope dots from the baseline to a height indicating the magnitude of force at impact. By comparmg the height of the vertical displacement relative to the honzontal target line, subjects could deterrnme whether the hit was ‘too hard’ or ‘too easy’ before attemptmg the next htt. Five target forces were presented 10 times each within a block of trials (50 trials or 5 sets) for both the random and blocked context. The target forces varied from 75 to 225 newtons m 37 newton increments. There were approxtmately 2 set between trials
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Frg. 1 Expenmental
Frg
tnterference
task
2. Sample performance on an acqumtron ‘set’ under blocked (left) and random (r&t) contexts Note that the forces produced were drsplayed after each hrt
w&in
sets and 90 set between
performed tnals)
one block
sets. The retention
of 10 retention
under erther a random
Interval
was 24 hours and all SubJects
trials at each of the five target
or blocked
forces
(total
of 50
context.
Results Acqursttron Acqmsttion of overall
performance
accuracy
(Henry
was evaluated
by total error
1975) that considers
Total
both response
error (E)
bias (constant
IS a measure error) and
Ch H Shea et al / Contextual mterference
,~G---+---a---,
P
o-oRANDOM
10..
a-ABLOCK
0, 0
: 50
: 100
: 150
: 200
ACIIUISITION
Fig
3
151
I 250
I 300
: 350
: 400
TRIAL
Acqmsltlon performance (means f standard errors) for the 50, 200 and 400 trial acqulsltlon groups under blocked and random acqursrtron contexts
response vanabrhty (vanable error). An Acqursttton Context (random, blocked) X Acquisrtton Practtce (SO, 200, or 400) X Block analysts of vanance (ANOVA) with repeated measures on Block was used to analyze acqutsttron performance Acqursttton performance (E) was mferror for the random acqutsttton groups as compared to the blocked acquisrtron groups with little difference between the 50, 200, and 400 acqmsttton groups’ performance at comparable stages of practice (see fig. 3). The ANOVA on the first block faded to indicate differences between Acqmsttton Practice, F(2, 66) = 1.64, p > 0 05, but dtd indicate that the random acqutsttton groups were mfenor (higher E) to blocked acqursrtton, F(1, 66) = 32.46, p < 0.01. Stmtlarly, an Acqutsttton Context (random, blocked) x Acqutsttron Practice (200, 400) X Block (50, 100, 150, 200) ANOVA with repeated measures on Block failed to Indicate an mteractton of any of the independent variables. The analysts dtd indicate mam effects of Acqutsitton Contexts, F(1, 44) = 26.21, p < 0.01, and Block, F(3, 132) = 12.63, p < 0.01, with the blocked acqursrtion supenor to random acqutsttton and the errors decreasing across trials. Retentton
Retention performance was evaluated by total error (E) An Acqutsttton Context (random, blocked) X Acqutsttton Trials (50, 200, 400) X Retention Context (random, blocked) ANOVA was performed. A stnkmgly different pattern of results as found when retention was assessed under blocked (low contextual interference) and random (high contextual interference) retention contexts (see fig 4). The ANOVA indicated mam effects of Acqutsrtton Context, F(1, 229) = 6.32, p < 0.01 and Retention Context, F(1, 229) = 63 65, p < 0.01. The analysis also mdrcated Acqursttton Practice X Acqursttton Context, F(2, 229) = 6 91, p < 0.01, and Acquisttton Context X Retention Context, F(1, 229) = 5.45, p < 0.05, Interactions as well as an Acquisltlon Practice X Acqulsltton Context X Retentton Context, F(2, 229) = 4.85, p < 0.01, mteractton Simple mam effects analysts and subsequent Duncan’s New Multiple Range Tests were performed on the proftle illustrated m fig 4 When retention was measured under
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RETENTION
.-. o--o
a--aElLOCK-BLOCK .-. RANDOM-BLOCK
RANDOM-RANDOM BLOCK-RANDOM
I 50
100
150
200
ACQUISITION
Fig. 4 Retention
performance groups
250
300
350
400
TRIAL
(means k standard errors) for the blocked and random under blocked and random retention contexts
acqusltlon
blocked contexts, there were no differences between the performance of SubJects who learned under blocked or random contexts, except for subjects who completed 400 acqutsitlon trials. After 400 acquisition tnals, the sublects who learned under random contexts performed better on blocked retention than subjects who learned under blocked contexts. Even though blocked retentton was generally supenor to random retention, mcreasmg acqmsrtron practice from 50 to 200 trials dtd not Improve retentton under blocked contexts Only after 400 acqutsttton tnals was retention performance srgniftcantly improved, but only for the subjects who learned under random contexts. The analysts of retention measured under random contexts mdtcated differences between random and blocked acqutsitton for subjects performing 50, 200, and 400 acqursrtton tnals However, retention for subjects under blocked acqutsttton was supenor to random after only 50 tnals of acqursttron, but mcreasmgly mfenor when 200 and 400 tnals of acqutsrtron were permitted. Indeed, increasing the number of acquisrtton trials under blocked contexts had a negatrve effect on retention under a random context, whtle mcreasmg the number of acqutsttron trials under random contexts had a postttve effect on retention under a random context In addrtron, retention under random contexts for the random acqmsrtron group and 400 acqursrtton trials was srmrlar to that of the blocked retention for blocked acqutsttton subjects when 50, 200, and 400 acqursrtron trials were completed, and the blocked retention for random acqutsrtron subJects when either 50 or 200 acquisttion trials were performed.
Discussion Acquisition performance under random contexts was less accurate, particularly early in practice, than performance under blocked conditions. However, retention accuracy depended not only on the number of trials of acquisition but the context (random or blocked) under
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which the acquisition was conducted and retention measured. 2 When retention was measured under blocked contexts, the random acquisition groups performed similarly to those of the blocked acquisition groups. After 400 acquisition trials, the random acquisition group performed with less error than the blocked acquisition group even though retention was measured under blocked contexts. When retention was measured under random contexts, the random acquisition group was inferior early in practice (after 50 trials) but increasingly superior to the blocked acquisition groups as the number of acquisition trials increased. In fact, with 400 acquisition trials, the random acquisition group under random retention performed similarly to the blocked acquisition groups under blocked retention. The practical implications of the retention data are somewhat paradoxical (as noted by Battig 1979). Random practice schedules will best (or at least equally well) promote the learning of a rapid motor task relative to blocked practice. In fact, increasing the amount (number of trials) of blocked acquisition practice had no affect on retention accuracy when retention was assessed under blocked contexts and a negative effect when measured under random contexts. Apparently, the benefits of blocked practice occur very early in practice with the response production strategy becoming increasingly more rigid and inflexible. On the other hand, the benefits of random practice surface after initial practice, presumably as a result of a more flexible response/control strategy (perhaps a generahzable response schema as proposed by Schmidt (1975)). One notable exception was found where the random acquisition group (50 trials) performed less accurately on random retention than did the blocked acquisition group (50 trials). This suggests, that very early m practice, it is difficult for subjects to determine the appropriate strategies when faced with random contexts. Fitts (1964) and Fitts and Posner (1967) have termed this the cognitive phase of learning, and Adams (1971) labeled this initial phase the verbal-motor stage. They suggest that the subjects’ primary concern early in practice is to understand what is to be done and how perfor* It should be noted that acquwtlon and retention performance 1s plotted m 50 tnal blocks so that 10 tnals at each target force IS Included. This restricts attempts to compare performance at the end of acquwtlon wth performance on retention tnals, particularly for the groups recewmg only 50 acquwtlon trials Dlstnbutlons of errors for all groups on the first 50 tnals were posmvely skewed wth extreme scores occurnng early m the block. Thus, the average error score for the first block may not reflect the true performance level of the subjects at the end of that block
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mance is to be evaluated, rather than determining the most efficient way of meeting the task demands. The increased interference, and perhaps increased task complexity mduced by a random trials schedule, may retard this process. Consistent with the findings of DelRey et al. (1982), the data suggest that a blocked, then random trials schedule, may benefit the retention of the task particularly if retention is to be assessed under random contexts. The results of the 400 trial acquisition groups are similar to the retention results of Shea and Morgan (1979) with one major exception. In the present study, random acquisition groups were superior on retention to blocked acquisition groups regardless of the retention condition. Shea and Morgan (1979) found random acquisition to be superior to blocked acquisition but only on retention assessed under random conditions. As a result of this finding and subsequent experiments by Shea and his colleagues (Shea and Zimny 1983; Morgan 1981) and Magi11 and Lee (1983) retention was only assessed under random conditions. Perhaps with additional acquisition practice differences would also be detected on barrier knock down tasks. Shea and his colleagues (Shea and Morgan 1979; Shea and Zimny 1983) utilizing Battig’s (1979) conceptualization suggested the dramatic reversal of acquisition group effects from acquisition to retention arises as a result of the multiple and variable processing strategies adopted by subjects when faced with increased contextual interference and thus increased intertask interference. That is, under blocked contexts the subject is more likely to invoke similar strategies from attempt to attempt and the state of the memory system (i.e., Stimulus Sampling theories (Estes 1955) or Multistate Markov models (Bjork, 1979)) under which the memory interactions occur is more likely to remain stable than under random contexts. Retention is facilitated under random acquisition conditions because the trial schedule induces the subject to engage in more complete processmg under more varied coding/decoding contexts relative to practice under blocked contexts. Lee and Magi11 suggest that on each trial under random conditions, subjects tend to reconstruct each response from memory because the varied responses interfere with the short-term memory for a particular response. Thus, subjects store (code in memory) information about the present response and subsequently access (decode) varied retrieval channels/strategies to prepare for the demands of the next response. However, under blocked contexts the memory representation(s)
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responsible for the present response is not forgotten and can be utilized (perhaps updated) to accomplish the subsequent responses. Thus, permanent memory stores may be updated and/or accessed less frequently under blocked acquisition. Retention is factlitated because the process of ‘forgetting’ and ‘reconstruction’ is thought to promote better qualitative and quantitative analysis of the response and thus more varied interactions with permanent memory stores. Two additional findings are of particular interest for theoretical and practical reasons. First, the experiments found that practice per se (number of practice trials) was not the sole determiner of retention. Increasing the number of blocked acquisition trials had no influence on retention assessed under blocked conditions and a negative effect on retention measured under random conditions. Secondly, specificity did not appear to play a vital role in determining retention performance. The specificity of learning (Barnett et al. 1973) hypothesis predicts that conditions in acqutsition which most closely match the criterion conditions will be most effective for learning that criterion. While the findings of a number of contextual interference studies were consistent with the specificity notions, the results of the present experiments contradict this hypothesis. The findings indicated that, after sufficient practice, random acquisition schedules resulted in enhanced retention even under blocked retention contexts. Further, after relatively little practice blocked acquisition practice leads to better retention under random retention conditions than random acquisition. The specificity of learning hypothesis does not appear to be supported in this and other motor (e.g., Schmidt 1987) and cognitive/verbal studies (e.g., Bransford et al. 1979). For example, practice on a set of variations of a given task, as opposed to constant practice on one variation, is more effective for performance on a novel variation of the same task, regardless of whether the retention was under variable or constant conditions (e.g., Schmidt 1982). Clearly, specificity notions need to be examined more carefully in order to determine whether they are being ‘overridden’ by other more powerful factor(s) or the specificity hypothesis is simply in error. Finally, it is interesting to note that a wide variety of experimental manipulattons have resulted in detrimental effects during acquisition but comparable or even enhanced retention. Experiments concerning knowledge of results (Schmidt et al. 1987) contextual interference (Shea and Morgan 1979), massed/distributed practice (Stelmach 1969)
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and lag/spaced conditions (Kohl et al. 1987) have all produced similar profiles. Clearly the similarity of the acquisition-retention profile is not sufficient cause to propose similar mechanisms. However, the similarities m the performance profiles (and the explanations for them) may have some common threads that should not be discounted. It may be that a host of conditions can invoke comparable changes in the amount/type of processing that sacrifices acquisition performance m lieu of increased retention.
Summary The results suggest that the introduction of contextual interference mto the practice schedule influences the retention of a simple motor task. Retention, regardless of the condition under which retention is assessed, can be enhanced by altermg the practice schedule so as to increase contextual interference provided sufficient acquisition practice is provided. Further, increases in blocked acquisition practice do not lead to increased retention under blocked and results in decrements m retention under random retention conditions. Blocked acquisition appears to be effective only when relatively few acquisition trials are permitted and retention is assessed under random conditions.
References Adams, J A 7971 A closed loop theory of motor learnmg. Journal of Motor Behawor 3, 111-149 Barnett, M.L , D Ross, R A. Schmidt and B Todd, 1973 Motor skdls learnmg and the speclflclty of trammg prmclple Research Quarterly 44, 440-447 Battlg, W F , 1966 ‘Faclhtatlon and Interference’ In A A Bdodeau (ed ), Acquwtlon of skdl New York Acadenuc Press. Battlg, W F , 1979 ‘The flexlbdlty of human memory’ In L S. Cermak and F.1 M Craig (eds ), Levels of processmg In human memory. HI&dale, NJ Erlbaum BJork, R A, 1979. ‘Repetltton and rehearsal mechamsms Models of short-term memory’ In D A Norman (ed ), Models of memory New York Academic Press BJork, R.A. and T W Allen, 1970 The spacmg effect Consohdatlon or dlfferentlal encodmg? Journal of Verbal Learnmg and Verbal Behavior 9, 567-572 Bransford, J D, J J. Franks, C D Morris and B.S Stem, 1979 ‘Some general constramts on learnmg and memory research’. In L S Cermak and F.1 M Cralk (eds ), Levels of processmg m human memory Hdlsdale, NJ Erlbaum DelRey, P , E H Wughalter and M WhItehurst, 1982 The effects of contextual Interference on females wth vaned expenence m open sports skdls Research Quarterly for Exercise and Sport 53, 108-115
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Estes, W.K , 1955 StatIstIcal theory of spontaneous recovery and regresston. Psychologtcal Review 62,145-154 Fltts, P M , 1964. ‘Perceptual-motor skdls leammg’ In A W Melton (ed ), Categones of leammg New York: Acadenuc Press. Fltts, P M and M.I. Posner, 1967 Human performance Belmont, CA Brooks/Cole. Goode, S and RA Magdl, 1986 Contextual mterference effects m learmng three badmmton serves Research Quarterly for Exercise and Sport 57, 308-314 Henry, F.M , 1975 Absolute error versus ‘e’ m target accuracy Journal of Motor Behavior 7, 227-228. IOrk, R E., 1968 Expenmental design procedures for behavior sciences Belmont, CA Wadsworth. Kohl, R M., C H. Shea and M. Guadagnoh, 1987 Contextual Interference Contnbutlons of lag and spacing Paper presented at the annual meetmg of the Amencan Alhance of Health, Physlcal Educatton, Recreation and Dance, Las Vegas, NY, April Lee, T D. and R A Magdl, 1983 The locus of contextual Interference m motor-Sk111 acqmsltlon. Journal of Expenmental Psychology Leammg, Memory and Cogmtlon 9, 730-746 Lee, T.D. and RA Magdl, 1985 ‘Can forgettmg faclhtate skdl acqutsltlon?’ In D Goodman, R B Wdberg and Franks (eds ), Dlffenng perspectives m motor learmng and control Amsterdam North-Holland Morgan, R L., 1981. An exammatlon of the memory processes underlying contextual Interference m motor skdls. An unpubhshed doctoral dlssertatlon, University of Colorado, Boulder. Schmidt, R A , 1975 A schema theory of discrete motor skdl learnmg Psycholo@cal Review 82, 225-260. Schnudt, R A, 1982 Motor learning and control, Champaign, IL Human Kmettcs Schnudt, R A, D.C Shaptro, C J Wmstem, D E. Young and Swmnen, 1987 Feedback and motor skill tralmng Relative frequency of KR and summary KR (Techmcal Report l/87). Alexandria, VA US Army Research lnstttute Shea, J B and R L. Morgan, 1979 Contextual Interference effects on the acqmsltlon. retention and transfer of a motor skdl. Journal of Expenmental Psychology Human Learmng and Memory 5, 179-187 Shea, J.B and S.T. Zlmny 1983 ‘Context effects m memory and learmng movement mformatlon’ In R A Magdl (ed.), Memory and control of actlon New York Elsevler Stelmach, G.E., 1969 Efflclency of motor learning as a function of mtertnal rest. Research Quarterly 40, 198-202.
Acta Psychologma 73 (1990) 145-157 North-Holland
CONTEXTUAL INTERFERENCE: CONTRIBUTIONS OF PRACTICE Charles
H. SHEA, Robert
KOHL
and Catherine
INDERMILL
*
Texas A&M Unruersrry, College Siarron, USA Accepted May 1989
The present expenment extends the fmdmgs of Shea and Morgan (1979) and Lee and Magi11 (1983) by deternumng the Impact of mampulatmg contextual Interference in 50, 200, and 400 acquisition tnals on retention of a raped force productton task assessed under both random and blocked contexts. Acquisition performance was mfenor for the random acquisitton groups as compared to the blocked groups with httle differences between the 50, 200, and 400 acqmsitton groups’ performance at comparable stages of practtce However, the retentton data indicated that sublects who completed 400 random acquisition trials performed better on both random and blocked retention than subjects who learned under blocked contexts. Increasing the number of blocked acqutsition tnals dtd not tmprove retentton under blocked contexts and had a negattve effect on retentton assessed under random contexts Apparently, the benefits of blocked practtce (low contextual interference) occur early m practice with response production becommg mcreasmgly more rigid and mflexible On the other hand, the benefits of random practtce (high contextual Interference) surface after mmal practice
An increasing number of investigators have contrasted performance under ‘high’ and ‘low’ contextual interference (see Lee and Magi11 (1985) for summary and theoretical perspectives). Typically, a learner is asked to practice a number of related tasks or a number of variations of a single task and the practice schedule is varied to maximize or mimmize contextual interference. Practice schedules that present task variations in a random order (C-A-B, B-C-A, B-A-C) are thought to produce high contextual interference, while schedules that present * The authors would like to thank Richard Magtll, Ttm Lee, Dick Schmidt, Wayne Shebtlske and Craig Wnsberg for readmg an earher draft of thrs paper and contributmg many helpful suggestions for the revmon. In addition, we wash to thank Dana Bernard and Mark Guadagnoh for thetr assistance wtth the data collection. Requests for repnnts should be sent to C H. Shea, Human Performance Laboratones, 276 Read Butldmg, Texas A&M Umverstty, College Statton, TX 77843-4243, USA. OOOl-6918/90/$3 50 0 1990, Elsevier Science Pubhshers B V (North-Holland)
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task variations in a blocked fashion (B-B-B, A-A-A, C-C-C) are thought to produce low contextual interference. In general, it has been found (e.g., Lee and Magi11 1983; Shea and Morgan 1979) that acquisition performance under high contextual interference was inferior to that of low contextual interference. However, measures of retention indicated better recall for subjects who practiced under high contextual interference. Contextual interference has been postulated to increase as the number of task variations interpolated between repetitions on a particular variation increases. More critically, the increase in interpolated task variations is thought to increase the likelihood of more than one variation being held simultaneously in working memory and decrease the availability of the solution to a particular variation. Thus, a random schedule offers the possibility for a large number (relatively) of task variations to be spaced between repetitions of any one variation, and blocked presentation has no variations between repetitions of any one variation. Shea and Morgan (1979) had subjects attempt 54 acquisition trials (6 blocks of 9 trials) on three variations of a task in either a blocked or random sequence. The task variations were similar in that each variation started (picking up a ball) and ended (placing the ball in a slot) the same, but varied in terms of the combinations and/or order that 3 of 6 barriers that were to be ‘knocked down’. The movement time of subjects performing under the random condition decreased more slowly, but eventually (by block 6) approached that of subjects in the blocked condition. After 10 min of rest, retention was measured for one half of the subjects under blocked or random conditions. The other half of the subjects performed the retention tests after 10 days. Performance after both lo-min and lo-day delays, when collapsed across both retention conditions (blocked-blocked, blocked-random and random-blocked, random-random) was significantly higher for subjects who learned the task variations under random conditions. There were no statistical differences between the blocked to blocked, blocked to random and random to random conditions with only the blocked to random condition significantly slower. They infer from the collapsed data, that random acquisition practice leads to superior retention as compared to blocked acquisition practice. However, this was only true when retention was assessed under random conditions. No clear benefit of random acquisition practice was found when retention was under blocked
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conditions for either retention interval. The major finding was that blocked acquisition practice led to very poor retention performance when switched to a random retention schedule. Perhaps the data should be interpreted not in terms of how well the random acquisition group performed on retention under random conditions but rather m terms of the extent to which blocked acquisition practice negatively effected retention under random conditions. A purpose of the present experiment was to extend the retention findings of Shea and Morgan (1979) and subsequent experiments (e.g., Lee and Magill, 1983) to rapid responses, requiring the subject to completely process (preprogram) the response specifications prior to execution. Previous motor behavior research in contextual interference has, for the most part, involved tasks that were regulated on the basis of concurrent feedback or a least offered the opportunity for feedbackbased corrections to intervene in the control of the movement. It should be noted that in all three experiments by Lee and Magi11 (1983) and the experiments by Shea and his colleagues (Shea and Morgan 1979; Shea and Zimny 1983; Morgan 1981) the task involved knocking over a series of barriers or touching a series of buttons either as quickly as possible or in a specific movement time. It is not clear to which extent these findings can be generalized to open-loop tasks. ’ A second purpose was to determine the impact of increasing the number of acquisition trials under blocked and random contexts on subsequent blocked and random retention. For the most part in the motor domain, contextual interference experiments have used relatively few acquisition trials. For example, in all three experiments by Lee and Magi11 (1983) as well as m experiments by Shea and his colleagues (Shea and Morgan 1979; Shea and Zimny 1983) only 54 acquisition trials were used. Contextual interference studies by DelRey et al. (1982) and Goode and Magi11 (1986) also used relatively few (< 65) acquisition trials. Shea and Zimny (1983) express some concern for the amount of practice issue. They suggest that intuitively, ‘random practice should produce optimum effects after subjects had the opportunity to learn the tasks through blocked practice.’ However, they discount ’ In the barner knock down tasks used m the Shea and Morgan (1979) and Lee and Magdl(l983) expenments, feedback-based correctlon may have played an Important role m the control of the movement Thus, the Influence of contextual Interference on the memones subservmg the movement may have been obscured. Perhaps m more rapld tasks where feedback-based correctlons are hmlted, the Influence of contextual Interference may be wewed more clearly.
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this position by stating that from their ‘theoretical perspective any random practice should result in better retention as measured by recall measures’ (1983: 350). This position predicts that the benefit of random practice will be small early in practice because all subjects receive equal amounts of elaborative processing. With increased random practice experiences the benefit relative to blocked practice will increase because of increased possibility for multiple and variable processing. However, Schmidt in the formulation and development of Schema theory (Schmidt 1975, 1982) suggests that for rapid, discrete responses the purpose of acquisition practice may change as a generalized motor program is developed and subsequently refined. Early in practice, subjects are thought to be extracting basic information from each repetition in order to formulate the basis (invariant features, see Schmidt, 1982) of the motor program to produce subsequent attempts. Thus, blocked practice may be more effective early in practice because it focuses the learner on the essential movement patterning. Since the program has yet to be refined, the contextual variations introduced in random contexts may increase the processing requirements to the point of ‘overload’. Later in practice, when the motor program is more developed, contextual variation may be more manageable and, in fact, may be essential for the performer to become adept at ‘parameterizing’ (variant features, see Schmidt 1982) responses. This notion, predicts that the benefits of random context may not surface until after substantial amounts of acquisition practice and may even be detrimental after very little practice. DelRey et al. (1982) found that the amount of prior experience with tasks (coincident timing) similar to the experimental task influenced the effect of 64 acquisition trials under ‘low’ and ‘high’ contextual interference. Random acquisition schedules (high contextual interference) resulted in enhanced retention over blocked and constant acquisition schedules but only for ‘experienced’ subjects. Retention increased for the ‘novice’ performers as contextual interference during acquisition was decreased. Therefore, the purposes of the present experiment were to extend the findings of Shea and Morgan (1979) to a rapid force production task and to determine the impact of increasing the number of acquisition trials on retention. Retention will be assessed under both random and blocked conditions.
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Method subjects Seventy-two undergraduate students at Texas A&M University participated the purposes of the study.
from the Required Physical Education program m the experiment. All subjects were narve to
Apparatus The apparatus consisted of a dynarruc force measurement system containing a force transducer whtch converted the physical force mto a voltage that represented the instantaneous value of the applied force. The voltage was directed to a rmcrocomputer wluch was programmed to read (1000 Hz) the voltage shtfts upon contact. The peak voltages were then transformed into newtons through a regression equation developed from the cahbratton procedures. The force transducer and microprocessor were also lmked to an oscrlloscope. The display from one channel of the oscilloscope was elevated when an output voltage from the microcomputer increased to present targets, and the display from another channel was elevated when force was apphed to the transducer. Procedure Upon recnntment to the experiment, subjects were randomly assigned to one of twelve groups. The groups dtffered m terms of acquisition practice (50, 200, 400), acqmsitton context (random, blocked), and retention context (random, blocked). The sublect was asked to sit comfortably in a chau next to a table. An oscilloscope was situated so that the screen was m plain view, and the force transducer was posittoned so that it could be comfortably contacted. The subject was informed that the task was to hit the padded arm of the force transducer with the medial surface of their preferred hand m an attempt to elevate a trace dot on the osctlloscope to the current target line (see fig. 1). Ten target lures were generated and displayed across the oscilloscope (see fig. 2) for each acqursttton and retention set pnor to the subject responding. For the blocked context, the same target lme was displayed 10 times across the oscilloscope screen wtthm a set, but vaned across sets. The 10 target lines were randomly presented wtthin sets m the random context. The result of each lut was tmmedtately displayed as a vertical trace displacement of one of the oscilloscope dots from the baseline to a height indicating the magnitude of force at impact. By comparmg the height of the vertical displacement relative to the honzontal target line, subjects could deterrnme whether the hit was ‘too hard’ or ‘too easy’ before attemptmg the next htt. Five target forces were presented 10 times each within a block of trials (50 trials or 5 sets) for both the random and blocked context. The target forces varied from 75 to 225 newtons m 37 newton increments. There were approxtmately 2 set between trials
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Frg. 1 Expenmental
Frg
tnterference
task
2. Sample performance on an acqumtron ‘set’ under blocked (left) and random (r&t) contexts Note that the forces produced were drsplayed after each hrt
w&in
sets and 90 set between
performed tnals)
one block
sets. The retention
of 10 retention
under erther a random
Interval
was 24 hours and all SubJects
trials at each of the five target
or blocked
forces
(total
of 50
context.
Results Acqursttron Acqmsttion of overall
performance
accuracy
(Henry
was evaluated
by total error
1975) that considers
Total
both response
error (E)
bias (constant
IS a measure error) and
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,~G---+---a---,
P
o-oRANDOM
10..
a-ABLOCK
0, 0
: 50
: 100
: 150
: 200
ACIIUISITION
Fig
3
151
I 250
I 300
: 350
: 400
TRIAL
Acqmsltlon performance (means f standard errors) for the 50, 200 and 400 trial acqulsltlon groups under blocked and random acqursrtron contexts
response vanabrhty (vanable error). An Acqursttton Context (random, blocked) X Acquisrtton Practtce (SO, 200, or 400) X Block analysts of vanance (ANOVA) with repeated measures on Block was used to analyze acqutsttron performance Acqursttton performance (E) was mferror for the random acqutsttton groups as compared to the blocked acquisrtron groups with little difference between the 50, 200, and 400 acqmsttton groups’ performance at comparable stages of practice (see fig. 3). The ANOVA on the first block faded to indicate differences between Acqmsttton Practice, F(2, 66) = 1.64, p > 0 05, but dtd indicate that the random acqutsttton groups were mfenor (higher E) to blocked acqursrtton, F(1, 66) = 32.46, p < 0.01. Stmtlarly, an Acqutsttton Context (random, blocked) x Acqutsttron Practice (200, 400) X Block (50, 100, 150, 200) ANOVA with repeated measures on Block failed to Indicate an mteractton of any of the independent variables. The analysts dtd indicate mam effects of Acqutsitton Contexts, F(1, 44) = 26.21, p < 0.01, and Block, F(3, 132) = 12.63, p < 0.01, with the blocked acqursrtion supenor to random acqutsttton and the errors decreasing across trials. Retentton
Retention performance was evaluated by total error (E) An Acqutsttton Context (random, blocked) X Acqutsttton Trials (50, 200, 400) X Retention Context (random, blocked) ANOVA was performed. A stnkmgly different pattern of results as found when retention was assessed under blocked (low contextual interference) and random (high contextual interference) retention contexts (see fig 4). The ANOVA indicated mam effects of Acqutsrtton Context, F(1, 229) = 6.32, p < 0.01 and Retention Context, F(1, 229) = 63 65, p < 0.01. The analysis also mdrcated Acqursttton Practice X Acqursttton Context, F(2, 229) = 6 91, p < 0.01, and Acquisttton Context X Retention Context, F(1, 229) = 5.45, p < 0.05, Interactions as well as an Acquisltlon Practice X Acqulsltton Context X Retentton Context, F(2, 229) = 4.85, p < 0.01, mteractton Simple mam effects analysts and subsequent Duncan’s New Multiple Range Tests were performed on the proftle illustrated m fig 4 When retention was measured under
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RETENTION
.-. o--o
a--aElLOCK-BLOCK .-. RANDOM-BLOCK
RANDOM-RANDOM BLOCK-RANDOM
I 50
100
150
200
ACQUISITION
Fig. 4 Retention
performance groups
250
300
350
400
TRIAL
(means k standard errors) for the blocked and random under blocked and random retention contexts
acqusltlon
blocked contexts, there were no differences between the performance of SubJects who learned under blocked or random contexts, except for subjects who completed 400 acqutsitlon trials. After 400 acquisition tnals, the sublects who learned under random contexts performed better on blocked retention than subjects who learned under blocked contexts. Even though blocked retentton was generally supenor to random retention, mcreasmg acqmsrtron practice from 50 to 200 trials dtd not Improve retentton under blocked contexts Only after 400 acqutsttton tnals was retention performance srgniftcantly improved, but only for the subjects who learned under random contexts. The analysts of retention measured under random contexts mdtcated differences between random and blocked acqutsitton for subjects performing 50, 200, and 400 acqursrtton tnals However, retention for subjects under blocked acqutsttton was supenor to random after only 50 tnals of acqursttron, but mcreasmgly mfenor when 200 and 400 tnals of acqutsrtron were permitted. Indeed, increasing the number of acquisrtton trials under blocked contexts had a negatrve effect on retention under a random context, whtle mcreasmg the number of acqutsttron trials under random contexts had a postttve effect on retention under a random context In addrtron, retention under random contexts for the random acqmsrtron group and 400 acqursrtton trials was srmrlar to that of the blocked retention for blocked acqutsttton subjects when 50, 200, and 400 acqursrtron trials were completed, and the blocked retention for random acqutsrtron subJects when either 50 or 200 acquisttion trials were performed.
Discussion Acquisition performance under random contexts was less accurate, particularly early in practice, than performance under blocked conditions. However, retention accuracy depended not only on the number of trials of acquisition but the context (random or blocked) under
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which the acquisition was conducted and retention measured. 2 When retention was measured under blocked contexts, the random acquisition groups performed similarly to those of the blocked acquisition groups. After 400 acquisition trials, the random acquisition group performed with less error than the blocked acquisition group even though retention was measured under blocked contexts. When retention was measured under random contexts, the random acquisition group was inferior early in practice (after 50 trials) but increasingly superior to the blocked acquisition groups as the number of acquisition trials increased. In fact, with 400 acquisition trials, the random acquisition group under random retention performed similarly to the blocked acquisition groups under blocked retention. The practical implications of the retention data are somewhat paradoxical (as noted by Battig 1979). Random practice schedules will best (or at least equally well) promote the learning of a rapid motor task relative to blocked practice. In fact, increasing the amount (number of trials) of blocked acquisition practice had no affect on retention accuracy when retention was assessed under blocked contexts and a negative effect when measured under random contexts. Apparently, the benefits of blocked practice occur very early in practice with the response production strategy becoming increasingly more rigid and inflexible. On the other hand, the benefits of random practice surface after initial practice, presumably as a result of a more flexible response/control strategy (perhaps a generahzable response schema as proposed by Schmidt (1975)). One notable exception was found where the random acquisition group (50 trials) performed less accurately on random retention than did the blocked acquisition group (50 trials). This suggests, that very early m practice, it is difficult for subjects to determine the appropriate strategies when faced with random contexts. Fitts (1964) and Fitts and Posner (1967) have termed this the cognitive phase of learning, and Adams (1971) labeled this initial phase the verbal-motor stage. They suggest that the subjects’ primary concern early in practice is to understand what is to be done and how perfor* It should be noted that acquwtlon and retention performance 1s plotted m 50 tnal blocks so that 10 tnals at each target force IS Included. This restricts attempts to compare performance at the end of acquwtlon wth performance on retention tnals, particularly for the groups recewmg only 50 acquwtlon trials Dlstnbutlons of errors for all groups on the first 50 tnals were posmvely skewed wth extreme scores occurnng early m the block. Thus, the average error score for the first block may not reflect the true performance level of the subjects at the end of that block
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mance is to be evaluated, rather than determining the most efficient way of meeting the task demands. The increased interference, and perhaps increased task complexity mduced by a random trials schedule, may retard this process. Consistent with the findings of DelRey et al. (1982), the data suggest that a blocked, then random trials schedule, may benefit the retention of the task particularly if retention is to be assessed under random contexts. The results of the 400 trial acquisition groups are similar to the retention results of Shea and Morgan (1979) with one major exception. In the present study, random acquisition groups were superior on retention to blocked acquisition groups regardless of the retention condition. Shea and Morgan (1979) found random acquisition to be superior to blocked acquisition but only on retention assessed under random conditions. As a result of this finding and subsequent experiments by Shea and his colleagues (Shea and Zimny 1983; Morgan 1981) and Magi11 and Lee (1983) retention was only assessed under random conditions. Perhaps with additional acquisition practice differences would also be detected on barrier knock down tasks. Shea and his colleagues (Shea and Morgan 1979; Shea and Zimny 1983) utilizing Battig’s (1979) conceptualization suggested the dramatic reversal of acquisition group effects from acquisition to retention arises as a result of the multiple and variable processing strategies adopted by subjects when faced with increased contextual interference and thus increased intertask interference. That is, under blocked contexts the subject is more likely to invoke similar strategies from attempt to attempt and the state of the memory system (i.e., Stimulus Sampling theories (Estes 1955) or Multistate Markov models (Bjork, 1979)) under which the memory interactions occur is more likely to remain stable than under random contexts. Retention is facilitated under random acquisition conditions because the trial schedule induces the subject to engage in more complete processmg under more varied coding/decoding contexts relative to practice under blocked contexts. Lee and Magi11 suggest that on each trial under random conditions, subjects tend to reconstruct each response from memory because the varied responses interfere with the short-term memory for a particular response. Thus, subjects store (code in memory) information about the present response and subsequently access (decode) varied retrieval channels/strategies to prepare for the demands of the next response. However, under blocked contexts the memory representation(s)
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responsible for the present response is not forgotten and can be utilized (perhaps updated) to accomplish the subsequent responses. Thus, permanent memory stores may be updated and/or accessed less frequently under blocked acquisition. Retention is factlitated because the process of ‘forgetting’ and ‘reconstruction’ is thought to promote better qualitative and quantitative analysis of the response and thus more varied interactions with permanent memory stores. Two additional findings are of particular interest for theoretical and practical reasons. First, the experiments found that practice per se (number of practice trials) was not the sole determiner of retention. Increasing the number of blocked acquisition trials had no influence on retention assessed under blocked conditions and a negative effect on retention measured under random conditions. Secondly, specificity did not appear to play a vital role in determining retention performance. The specificity of learning (Barnett et al. 1973) hypothesis predicts that conditions in acqutsition which most closely match the criterion conditions will be most effective for learning that criterion. While the findings of a number of contextual interference studies were consistent with the specificity notions, the results of the present experiments contradict this hypothesis. The findings indicated that, after sufficient practice, random acquisition schedules resulted in enhanced retention even under blocked retention contexts. Further, after relatively little practice blocked acquisition practice leads to better retention under random retention conditions than random acquisition. The specificity of learning hypothesis does not appear to be supported in this and other motor (e.g., Schmidt 1987) and cognitive/verbal studies (e.g., Bransford et al. 1979). For example, practice on a set of variations of a given task, as opposed to constant practice on one variation, is more effective for performance on a novel variation of the same task, regardless of whether the retention was under variable or constant conditions (e.g., Schmidt 1982). Clearly, specificity notions need to be examined more carefully in order to determine whether they are being ‘overridden’ by other more powerful factor(s) or the specificity hypothesis is simply in error. Finally, it is interesting to note that a wide variety of experimental manipulattons have resulted in detrimental effects during acquisition but comparable or even enhanced retention. Experiments concerning knowledge of results (Schmidt et al. 1987) contextual interference (Shea and Morgan 1979), massed/distributed practice (Stelmach 1969)
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and lag/spaced conditions (Kohl et al. 1987) have all produced similar profiles. Clearly the similarity of the acquisition-retention profile is not sufficient cause to propose similar mechanisms. However, the similarities m the performance profiles (and the explanations for them) may have some common threads that should not be discounted. It may be that a host of conditions can invoke comparable changes in the amount/type of processing that sacrifices acquisition performance m lieu of increased retention.
Summary The results suggest that the introduction of contextual interference mto the practice schedule influences the retention of a simple motor task. Retention, regardless of the condition under which retention is assessed, can be enhanced by altermg the practice schedule so as to increase contextual interference provided sufficient acquisition practice is provided. Further, increases in blocked acquisition practice do not lead to increased retention under blocked and results in decrements m retention under random retention conditions. Blocked acquisition appears to be effective only when relatively few acquisition trials are permitted and retention is assessed under random conditions.
References Adams, J A 7971 A closed loop theory of motor learnmg. Journal of Motor Behawor 3, 111-149 Barnett, M.L , D Ross, R A. Schmidt and B Todd, 1973 Motor skdls learnmg and the speclflclty of trammg prmclple Research Quarterly 44, 440-447 Battlg, W F , 1966 ‘Faclhtatlon and Interference’ In A A Bdodeau (ed ), Acquwtlon of skdl New York Acadenuc Press. Battlg, W F , 1979 ‘The flexlbdlty of human memory’ In L S. Cermak and F.1 M Craig (eds ), Levels of processmg In human memory. HI&dale, NJ Erlbaum BJork, R A, 1979. ‘Repetltton and rehearsal mechamsms Models of short-term memory’ In D A Norman (ed ), Models of memory New York Academic Press BJork, R.A. and T W Allen, 1970 The spacmg effect Consohdatlon or dlfferentlal encodmg? Journal of Verbal Learnmg and Verbal Behavior 9, 567-572 Bransford, J D, J J. Franks, C D Morris and B.S Stem, 1979 ‘Some general constramts on learnmg and memory research’. In L S Cermak and F.1 M Cralk (eds ), Levels of processmg m human memory Hdlsdale, NJ Erlbaum DelRey, P , E H Wughalter and M WhItehurst, 1982 The effects of contextual Interference on females wth vaned expenence m open sports skdls Research Quarterly for Exercise and Sport 53, 108-115
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Estes, W.K , 1955 StatIstIcal theory of spontaneous recovery and regresston. Psychologtcal Review 62,145-154 Fltts, P M , 1964. ‘Perceptual-motor skdls leammg’ In A W Melton (ed ), Categones of leammg New York: Acadenuc Press. Fltts, P M and M.I. Posner, 1967 Human performance Belmont, CA Brooks/Cole. Goode, S and RA Magdl, 1986 Contextual mterference effects m learmng three badmmton serves Research Quarterly for Exercise and Sport 57, 308-314 Henry, F.M , 1975 Absolute error versus ‘e’ m target accuracy Journal of Motor Behavior 7, 227-228. IOrk, R E., 1968 Expenmental design procedures for behavior sciences Belmont, CA Wadsworth. Kohl, R M., C H. Shea and M. Guadagnoh, 1987 Contextual Interference Contnbutlons of lag and spacing Paper presented at the annual meetmg of the Amencan Alhance of Health, Physlcal Educatton, Recreation and Dance, Las Vegas, NY, April Lee, T D. and R A Magdl, 1983 The locus of contextual Interference m motor-Sk111 acqmsltlon. Journal of Expenmental Psychology Leammg, Memory and Cogmtlon 9, 730-746 Lee, T.D. and RA Magdl, 1985 ‘Can forgettmg faclhtate skdl acqutsltlon?’ In D Goodman, R B Wdberg and Franks (eds ), Dlffenng perspectives m motor learmng and control Amsterdam North-Holland Morgan, R L., 1981. An exammatlon of the memory processes underlying contextual Interference m motor skdls. An unpubhshed doctoral dlssertatlon, University of Colorado, Boulder. Schmidt, R A , 1975 A schema theory of discrete motor skdl learnmg Psycholo@cal Review 82, 225-260. Schnudt, R A, 1982 Motor learning and control, Champaign, IL Human Kmettcs Schnudt, R A, D.C Shaptro, C J Wmstem, D E. Young and Swmnen, 1987 Feedback and motor skill tralmng Relative frequency of KR and summary KR (Techmcal Report l/87). Alexandria, VA US Army Research lnstttute Shea, J B and R L. Morgan, 1979 Contextual Interference effects on the acqmsltlon. retention and transfer of a motor skdl. Journal of Expenmental Psychology Human Learmng and Memory 5, 179-187 Shea, J.B and S.T. Zlmny 1983 ‘Context effects m memory and learmng movement mformatlon’ In R A Magdl (ed.), Memory and control of actlon New York Elsevler Stelmach, G.E., 1969 Efflclency of motor learning as a function of mtertnal rest. Research Quarterly 40, 198-202.