Interpolated Activities During the Knowledge-of

Journal of Experimental Psychology: Learning, Memory, and Cognition 1990, Vol. 16, No. 4, 692-705

Copyright 1990 by the American PsychologicalAse~eiation, Inc. 0278-7393/90/$00.75

Interpolated Activities During the Knowledge-of-Results Delay and Post-Knowledge-of-Results Interval: Effects on Performance and Learning Stephan P. Swinnen Institute o f Physical Education, G r o u p Biomedical Sciences, Catholic University o f Leuven, Heverlee, Belgium In the present experiments, the effect of involvement in attention-demanding recognition activities (secondary task) during the KR delay interval on learning/retention of a timing movement (primary task) was investigated. Findings of Experiment 1 and 2 revealed that the interpolated task interfered with the primary task, resulting in inferior retention performance. In a third experiment, the temporal location of the secondary task was manipulated through its interpolation in the KR delay and the post-KR interval across different conditions. In contradiction to earlier predictions, the secondary task tended to generate less interference when administered in the post-KR interval in comparison with the KR delay interval. Current findings raise doubts about the hypothesized minor role of the KR delay interval for learning/retention and indirectly point to important information-processing activities that occur during this interval (Swinnen, Schmidt, Niebolson, & Shapiro, 1990).

reinforcement is recommended because even a slight delay degrades the rate of producing correct responses (Renner, 1964). On the contrary, despite strong research efforts, a number of studies with humans have revealed null effects of K R delay manipulations on motor performance (Bilodeau, 1966; Bilodeau & Bilodeau, 1958; Bilodeau & Ryan, 1960; Lorge & Thorndike, 1935; McGuigan, 1959). In a recent review o f this topic, Salmoni, Schmidt, and Walter (1984) have further underscored these early findings. On the basis o f the well-known learning-performance distinction, they divided studies into two main classes: those that made use o f some type of no-KR retention tesP in order to assess learning effects and those that did not. They concluded that numerous studies failed to show any effects o f K R delay on performance in the presence o f knowledge of results. In the few that did find effects, the differences in performance could often be attributed to concomitant variations in either the post-KR or the intertrial interval, confounded with the K R delay interval. In the limited number o f studies that made use of some type o f no-KR test, the majority did not demonstrate reliable effects o f varying the delay of knowledge of results (Dyal, 1966; Dyal, Wilson, & Berry, 1965; Koch & Dorfman, 1979; McGuigan, 1959; McGuigan, Crockett, & Bolton, 1960; Schmidt, Christenson, & Rogers, 1975; Schmidt & Shea, 1976). The aforementioned general lack of evidence for K R delay effects toned down earlier optimisms, and, as a consequence,

It is generally agreed that knowledge o f results (KR) is an important source of information for skill learning to occur, although it is not always a necessary condition (Swinnen, 1988; Swinnen, Vandenberghe, & Van Assche, 1986). Decades ago, it was thought that knowledge o f results and its various manipulations have the same effect on human learning as positive reinforcement has on animal learning. Thorndike (1931) was a major proponent o f this idea. Although some of his theories are still widely accepted, others have been challenged on the basis o f experimentally confirmed dissonances between animal and human learning (Adams, 1971, 1978). One of them is delay o f reinforcement, commonly known as delay o f knowledge o f results in the area o f motor behavior. The knowledge-of-results delay (KR delay) interval spans the time between completion o f the response and presentation of knowledge of results. It is complementary to the post-knowledge-of-results (post-KR) interval that refers to the time occurring between receipt o f knowledge of results and the next attempt. In animal learning, use o f instantaneous The present experiments were performed at the University of California, Los Angeles (UCLA), while the author was staff research associate in the Motor Control Laboratory, Department of Kinesiology. Research was supported by Contract MDA903-85-K-0225 from the U.S. Army Research Institute (Basic Research) awarded to R. A. Schmidt and D. C. Shapiro. Further support was provided through a Collaborative Research Grant from the NATO Scientific Affairs Division (Contract 86/732) for cooperation between the Motor Control Laboratory, Department of Psychology, UCLA, Los Angeles, California, and the Motor Learning Laboratory of the Institute of Physical Education, Catholic University of Leuven, Leuven, Belgium. The author is indebted to R. A. Schmidt and D. C. Shapiro for their help and support and to Keith Rayner and three anonymous reviewers who provided helpful comments on a previous version of the manuscript. Correspondence concerning this article should be addressed to Stephan P. Swinnen, Labo Motorisch Leren, Instituut voor Lichamelijke Opleiding, Groep Biomedische Wetenschappen, Catholic University of Leuven, Tervuurse Vest 101, 3030 Heverlce, Belgium.

J On the basis of the notion that the presence of knowledge of results often induces temporary effects, Salmoni, Sehmidt, and Walter (1984) have proposed to assess the effects of certain knowledge of results manipulations on learning by evaluating performance on a task after a sufficiently long rest period (following the acquisition phase) and in the absence of knowledge of results, that is, the no-KR retention test. Thus, performance assessed during the acquisition phase with knowledge of results present does not necessarily distinguish between the temporary performance and more permanent learning effects of a variable, whereas the no-KR retention test does. 692

INTERPOLATED ACTIVITIES AND LEARNING this temporal variable was no longer viewed as being critically important for motor learning. For example, in his closed-loop theory and in later work, Adams (1971, 1976,' 1987) argued that delay of knowledge of results has little or no effect on performance (with or without availability of knowledge of results). He further hypothesized that the KR delay interval is an empty waiting period where only minor forgetting might occur (Adams, 1976). Failures in research provide opportunities for new perspecfives, and this also occurred after the decline of KR delay research. On the one hand, because delaying knowledge of results did not appear to profoundly influence success in goal achievement, the hypothesis arose that learners are able to bridge this interval by retaining certain aspects of the movement until presentation of knowledge of results (NeweR, 1981; Salmoni et al., 1984). On the other hand, with the shift from a reinforcement to an informational interpretation of knowledge of results, attention was redirected from the KR delay to the post-KR interval as the important temporal variable (Bilodeau, 1966; Bilodeau & Bilodeau, 1958; Newell, 1976). Questions arose as to what the subject would do with knowledge of results and how much time he or she would need to process the information it contained. The post-KR interval became the locus of information processing, movement evaluation, and planning, in sharp contrast to the supposed minor involvement of the learner in the KR delay interval. The commonly accepted superiority of the post-KR interval was well captured by Schmidt (1988, p. 439), who wrote that the learner is hypothesized to be a holder of information in the KR delay interval, in contrast to being an active and creative movement modifier in the post-KR delay interval. In opposition to this passive attitude, we have recently argued that the learner is actively engaged in processing movement information and in detection of errors during the KR delay interval (Swinnen, 1988; Swinnen, Schmidt, Nicholson, & Shapiro, 1990). The learner does not have to wait until knowledge of results is presented to appreciate the degree of correctness in movement. Instead, after completion of the response--that is, when the response-produced feedback is still vivid--subjects are involved in self-generated error estimarion activities that may benefit future performance. Evidence for this viewpoint was provided through experimental manipulation of the delay of knowledge of results. In contrast to earlier research findings, our experiments revealed that subjects who received knowledge of results instantaneously were less successful in skill learning than were subjects given knowledge of results after a delay, presumably because immediate provision of knowledge of results interfered with spontaneous response evaluation (Swinnen et al., 1989). Two aspects of the experimental setup were different from previous studies. First, the tasks were more complex. Second, assuming that processing response-produced information occurs rapidly in such motor tasks, delay of knowledge of results was reduced to zero (or nearly zero) seconds in the instantaneous knowledge of results condition. Although reducing the length of the KR delay interval is one way to interfere with subjects' information processing activities, an alternative research strategy was developed in the present experiments. Subjects were administered an attention-demanding secondary activity during the KR delay in-

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terval, and its effects on primary task performance were evaluated. In view of the aforementioned research evidence that subjects estimate their responses after performance completion, it was hypothesized that involvement in a secondary estimation task would interfere with learning the primary task. The theoretical rationale underlying this hypothesis was that structural interference results from competition between tasks imposing simultaneous demands on specific perceptual and/or motor mechanisms. Experiment 1 On the basis of the notion that the KR delay interval holds performance evaluation activities, it was hypothesized that interfering with these activities---through administration of a secondary task during this interval--would be detrimental to skill acquisirion. Of crucial importance was the judicious selection of the secondary task, that is, one that would generate maximal structural interference with the presumed primary task activities. Because a major concern for generating structural interference among tasks is their degree of similarity (Kahneman, 1973; Kiusbourne, 1981; Kinsbourne & Hicks, 1978; Navon & Gopher, 1979), the secondary task consisted of estimating the errors made in a timing response performed by the experimenter, that is, a recognition type of activity. There were three groups: (a) a group that estimated errors in a response, performed by the experimenter during the KR delay interval of the primary task (interpolated2); (b) a group with a free KR delay (free); and (c) a group that estimated errors made in the criterion task during the KR delay interval (estimation). The third group was added in order to determine the possible effect of forced estimation of errors on learning a timing skill. Previous studies showed that forced estimation benefits learning, although the evidence was inconclusive (Hogan & Yanowitz, 1978; Shapiro, Schmidt, & Swinnen, 1984). Consequently, the present manipulations can be taken to represent a continuum ranging from forced estimation to prevented estimation, with the free group positioned in between. Indeed, evidence has been reported that subjects estimate errors spontaneously during the KR delay interval even if they are not required to do so (Swinnen, 1988). The following hypotheses were examined: (a) Subjects of the interpolated group will perform with higher error on the criterion task than subjects of the free group, presumably because the secondary recognition task interferes with ongoing information-processing activities during the KR delay interval; (b) subjects requested to estimate their own response 2 The secondary task was interpolated during the KR delay interval of the primary task. It consisted of a timing movement with reversals in direction, similar to the primary movement, although overall movement distance and timing differed. After completion of the primary movement by the subject, the experimenter initiated the secondary timing movement on another apparatus. Subsequently, the subject was to estimate the timing error in the latter movement as accurately as possible, that is, the deviation from a specified target time which was different from the subject's own movement time. Thus, two features characterize the interpolated task: Subjects were to perceive the interpolated movement and to provide an estimation of the timing error.

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errors will perform with lower error on the criterion task than those who are not because it m a y lead to increased involvem e n t in response recognition. Method

Subjects Undergraduates of the University of California, Los Angeles, participated in the study (N ffi 66). Subjects were randomly assigned to conditions, with the criterion that gender be equated between groups. They had not received prior experience with the task.

Task and Apparatus The apparatus consisted of a 2.54-em diameter stainless-steel shaft, approximately 1.3 m in length and mounted horizontally between two vertical supports that were attached to a wooden base (see Figure 1). A slide consisting of a ball-bearing sleeve was fitted to the shaft, allowing the slide to move nearly frictiouless through 130 era. The slide was supplied with a finger grip handle and a pointer. A switch at the fight end (with respect to the subject) ofthe slide's travel closed when the slide was moved away from it, and another switch, mounted 84 em leftward, dosed when the slide passed it, starting and stopping a digital millisoeond timer, respectively. In addition, there were two 3-era-wide target zones located 45 cm and 65 cm from the starting position. The seated' subject was to grasp the handle positioned against the fight end of the shall move the slide leftward to the second target zone (i.e., 65 era), reverse direction to the first zone (45 era), and then reverse direction again to move through the switch at the far left with a follow through. This movement was to be produced in a time as close as possible to 1,000 ms on each trial. The subject was instructed to initiate the movement shortly after illumination of a "go" light, but reaction times were neither stressed nor measured. At the onset of the lighh the computer (DIOIT^L PDP 11/'23) started sweeping for a maximal duration of 2.7 S with a sampling frequency of 500 Hz. The movement time was immediately displayed on the computer terminal. Upon acceptance of the trial, the movement times were automatically filed and saved for further analysis. Scores were in terms of movement time. No measures of accuracy at the intermediate targets were taken, although it was obvious that all subjects typically followed directions to reverse the movements within the target zones. Overemphasis of spatial accuracy would possibly have resulted in a failure to meet the specified target time. A second similar apparatus was put in front of the subject at a distance of 1 m, used by the experimenter to perform the interpolated movements. The characteristics of these movements were similar to the criterion movement (i.e., they contained two reversals in direction), but the overall movement distance from start to finish was shorter (42 era).

Procedure There were three performance phases: The acquisition phase consisted of 80 trials with knowledge of results; retention tests were held 10 rain (immediate retention) and 2 days (delayed retention) after acquisition, consisting of 25 trials each without provision of knowledge of results. Although one can conceive of other retention tests (e.g., with knowledge of results), it was decided to withdraw this information because it often induces temporary effects on performance that need to be distinguished from the relatively permanent learning effects (Salmoui et al., 1984; Schmidt, Young, Swinnen, & Shapiro, 1989). Consequently, interpretations in terms of learning effects are based on no-KR retention performance. In addition, because it is uncertain that temporary effects have vanished after some minutes, the main emphasis is on delayed retention performance. There were three groups: Subjects of the interpolated group estimated the timing error (in milliseconds) in the experimenter's movemerit during the KR delay interval of the criterion task. Subjects of the free group rested during the KR delay interval, while subjects of the estimation group guessed their own movement time errors in milliseconds. After completion of the response, the latter were to inform the experimenter about the magnitude of their errors. Temporal intervals were held constant among groups: The KR delay was 16 s, the post-KR interval 5 s. Knowledge of results was presented verbally to all subjects. Subjects were encouraged to meet the spatial requirements of the task although this aspect was not measured. When the reversals were not made within the targets, subjects were asked to correct their movement pattern. This occurred only during the first trial blocks. The interpolated condition had the following course: ARer subjects completed the primary timing task, the experimenter initiated the secondary movement, and subjects were to estimate the deviations of the latter from 1.5 s; then they were informed about accuracy in estimating the experimenter's movement, afar which they received knowledge of results about the primary task; finally, they waited for the warning signal in order to produce the next response. Subjects were asked to perform both activities as well as possible. The interpolated movements were preplanned by the experimenter, that is, they were made independently of the primary task. The possibility of separating the experimenter's from the subject's movement was investigated in a pilot study in which 15 subjects were tested. After some training, this goal was accomplished. To ensure that the interpolated task demanded enough attention, its movement times were varied between 1.2 and 1.8 s. The experimenter ensured that the total range of movement times was provided by shifting regularly from slower to faster movements and vice versa. This gave the interpolated task its unique and unpredictable character. For that reason, it was decided not to analyze performance on the interpolated task. However, inspection of the data of some subjects revealed that improvement was made across acquisition.

Data Analysis The following dependent measures were analyzed: absolute constant error (ICED, variable error (VE), and constant error (CE)? 3 Computation

of the error scores (x, - T) Constant error = ~-t n

E ( x ~ - ~)~

Figure 1. Demonstration of the apparatus.

Variable error =

~

?/

,,

INTERPOLATED ACTIVITIES AND LEARNING Absolute constant error was obtained by taking the absolute value of the constant error. Sehutz (1974, 1979) has recommended the use of absolute constant error as a suitable measure of average bias across a group of individuals, and variable error as a measure of performance consistency. The analysis of constant error was added to obtain an insight into the sign of the errors made. All error scores were expressed in milliseconds. The data were grouped into blocks of five trials. Separate analyses of variance were conducted on acquisition, immediate, and delayed retention data. A 3 x 16 (Group x Trial Block) analysis of variance (^NOVA), with repeated measures on the last factor, was conducted on each of the dependent variables of the acquisition phase. A 3 x 5 (Group x Trial Block) ANOVA, with repeated measures on the last factor, was conducted on the retention data. Results Acquisition Absolute constant error. Figure 2 shows the error scores across 16 five-trial blocks. The free and estimation group performed similarly. The interpolated group performed with higher error than the former two groups across all trial blocks. Error scores decreased in all three groups. The group effect was significant, F(2, 63) = 7.81, p < .01 (MSc = 41,809.09). Overall means for the interpolated, free, and estimation groups were 108.87, 58.49, and 54.02 ms, respectively. Tukey's a posteriori tests revealed significant differences in mean error between the interpolated group and the free group ( p < .01) and between the interpolated and estimation group ( p < .01). The difference between the free and estimation groups was not significant ( p > .05). The trial block effect was significant, F(15, 945) -- 40.76, p < .01 (MS~ = 4,918.31). The G r o u p x Trial Block interaction was not significant ( F .05. Variable error. Although the interpolated group performedwith higher error on most o f the trial blocks, the group effect was not significant, F(2, 63) --- 1.85, p > .05 (MS~ ffi 4,050.94). In order, means for the interpolated, free, and estimation group were 65.67, 57.13, and 57.9 ms. The trial block effect was significant, F(15, 945) = 26.43, p < .01 (MS~

where x ffi score on trial i; ~ = the subject's average response; T ffi target; n -- number of blocked trials. Absolute constant error is determined by taking the absolute value of the constant error as computed per trial block.

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= 1,141.87). The largest decrease in variable error was found between the first and second trial block, followed by a less pronounced decrease toward the end of practice. The Group x Trial Block interaction was not significant ( F < 1). Immediate Retention Absolute constant error. Only small differences among groups were found, as shown in the middle portion of Figure 2. Overall, a small increase in error across trial blocks was evident. The group effect was not significant, F(2, 63) = 1.0, p > .05 (MSe = 8,554.73). Means for the interpolated, free, and estimation groups were 58.62, 75.16, and 61.64 ms, respectively. The trial block effect was significant, F(4, 252) = 2.6, p < .05 (MSe = 1,177.98). The Group x Trial Block interaction was not significant, F(8, 252) = 1.1, p > .05. Constant error. The estimation group and the free group performed with positive constant error on all trial blocks. Althoughthe estimation group remained fairly stable, the free group showed a downward trend toward zero constant error. The interpolated group, performed with positive constant error on the first trial bloCks; then error scores decreased, becoming slightly negative toward the end o f immediate retention. The ANOVA did not reveal a significant group effect ( F < 1). In order, means for the interpolated, free, and estimation groups were 11.58, 17.77, and 31.32 ms. Thearial block effect was significant, F(4, 252) = 4,89, p < .01 (MS~ = 1,763.03), indicative of an overall decrease in constant error across trial blocks. The decrease was probably due to the interpolated and free groups because the estimation group did not display this downward trend. This pattern was supported by a significant Group x Trial Block interaction F(8, 252) = 3.23, p < .01. Variable error. The interpolated group performed with highest variable error ( M = 47.8 ms), followed by the estimation group ( M = 39.78 ms) and the free group ( M = 35.36 ms). The group effect was significant, F(2, 63) = 3.87, p < .05 (MS~ = 1,131.35). Pairwise a posteriori tests revealed that the interpolated group performed with significantly higher variable error than the free group ( p < .05). The mean error scores of the other groups were not significantly different from each other ( p > .05). The trial block effect was not significant ( F < 1). The G r o u p x Trial Block interaction was significant, F(4, 252) = 2.17, p < .05. This was due to an increase in variable error across practice for the interpolated group while the other groups displayed a small decrease.

Delayed Retention Absolute constant error. The following observations were most apparent (Figure 2, far right side): The interpolated group performed with higher error than did the other groups on all trial blocks ( M ffi 120.02 ms); the estimation group performed with lowest error ( M -- 67.13 ms), and the free group was positioned in between the former groups ( M = 89.97 ms). Although the free and estimation groups remained fairly stable across practice, the interpolated group performed with increasing absolute constant error from the first to the

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STEPHAN P. SWINNEN 280

• •aB- interpolation

E

43- estimation

240

,om Z
.05 (MSe = 1,273.93); neither was the Group x Trial Block interaction, F(8, 252) = 1.59, p > .05. Constant error.

Similar to immediate retention performance, the free (M = 8.45 ms) and estimation groups (M = 14.12 ms) performed with an overall small positive constant error. The interpolated group performed with negative constant error (M = -92.39 ms); that is, a continuation of the downward trend was already manifest during the previous retention test. The ANOVAindicated a significant group effect, F(2, 63) = 7.78, p < .01 (MSc = 64,580.5). The interpolated group performed with significantly higher negative constant error than the other groups (p < .01), which did not differ from each other (p > .05). The trial block effect was significant F(4, 252) = 3.82, p < .01 (MSc = 1,583.24), as well as the Group x Trial Block interaction, F(8, 252) = 2.14, p < .05. Variable error No big difference in error scores between groups were identified, and this effect did not reach significance ( F < 1). Means for the interpolated, free, and estimation groups were 38.99, 42.38, and 40.04 ms, respectively. The slightly lower error scores of the interpolated group may be a result of its lower movement times (see the negative constant error scores reported in the previous paragraph). The trial block effect was significant, F(4, 252) = 7.45, p < .01 (MS, = 264.46), indicative of an overall decrease in variable error across trial blocks. The Group x Trial Block interaction was not significant, F(8, 252) = 1.58, p > .05. Discussion The results of Experiment 1 suggest that involvement in an interpolated recognition activity during the KR delay interval

had a detrimental effect on learning the primary timing task (as indicated by no-KR retention performance). As far as absolute constant error is concerned, the interpolated group performed with significantly higher error than the estimation group; differences between the free and interpolated group, albeit in the expected direction, failed to reach significance. Analysis of constant error scores provides some information about the processes underlying this performance deterioration. During acquisition, subjects of the interpolated condition tended to move too slowly, as indicated by high positive constant error scores. This effect persisted through the end of acquisition. In other words, subjects of the interpolated condition tended to move toward the slower interpolated movement they estimated but never actually performed. Such effects are often called response biasing or assimilation, and they have frequently been observed in short-term motor memory research. This work typically involves the use of interpolated secondary movements that are found to affect recall performance of criterion positioning tasks. More specifically, recall of the criterion movement shifts in the direction of the interpolated movement (Burwitz, 1974; Craft, 1973; Craft & Hinrichs, 1971; Hagman, 1978; Hagman & Williams, 1977; Laabs, 1973, 1974, 1975; Laabs & Simmons, 1981; Patrick, 1971; Pepper & Herman, 1970; Smyth, 1984; Stelmach, 1982; Stelmach & Kelso, 1975; Trumbo, Milone, & Noble, 1972). Although assimilation was dominant during acquisition, a completely different response mode showed up during retention. Here, subjects of the interpolated group speeded up their responses; that is, they performed with negative constant error. This tendency to move faster, reminiscent of a contrast effect, set in during immediate retention but was more pronounced during delayed retention. In comparison with assimilation effects, contrast effects have been identified only rarely in short-term motor memory research (Fishburne, 1985; Laabs & Simmons, 1981; Levin, Norman, & Dolezal, 1973). Performance at retention was different from that during the acquisition phase in two ways. First, the interpolated task was

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no longer administered during retention trials. Second, knowledge of results was withdrawn so that subjects had to rely on their own feedback sources in order to move correctly. Investigation of delayed retention scores reveals a small (nonsignificant) benefit o f error estimation in comparison with no estimation, a result supporting earlier findings (Schmidt & Shapiro, 1986; Swinnen, 1988).

with positive and negative constant error respectively--subjects of the interpolated fast condition will display exactly the opposite behavioral characteristics.

Experiment 2

Subjects were 80 undergraduates from the University of California, Los Angeles. They were randomly assigned to each of four conditions, with the restriction that groups be equated for gender. They were not informed about the purpose of the experiment, and none had received previous experience with the task.

The results of the previous experiment supported the hypothesis that performing a secondary recognition activity during the K R delay interval is detrimental to learning the criterion task. It was further demonstrated that the increased error scores o f the interpolated group were mainly due to a slowing down of the criterion movement during acquisition and a speeding up during retention. Experiment 2 was designed to test the generality o f this effect by adding a group that was to estimate faster interpolated movements. The latter group was also proposed as a possible solution to potential limitations inherent in the interpolated condition o f the previous experiment. As subjects started speeding up the movements at retention, they may have reached their maximal speed limit, preventing a further drift from the target time. This may possibly have resulted in an underestimation o f the detrimental effects caused by the secondary task in Experiment 1. Indeed, because of the spatial constraints o f the task, it was difficult for some o f the subjects to meet the 1-s movement time goal, especially at initiation o f practice. This suggests that subjects already moved at close to maximal speed under normal circumstances. Consequently, there was only a small range for drifting off (i.e., increasing speed) at retention. In an attempt to step aside this presumed physical speed barrier, it was decided to have subjects estimate a secondary task that was to be performed in a shorter movement time than the criterion task in the present experiment. On the basis of the assumption that a process would take place similar to the previous e x p e r i m e n t - - t h a t is, an assimilation effect during acquisition and a contrast effect during retentionmsubjects were no longer assumed to increase movement speed at retention but rather to slow down. Therefore, the use of a faster interpolated task was hypothesized to disrupt criterion task performance at retention more profoundly; this would result in larger performance differences between the interpolated and noninterpolated conditions. There were four groups, three o f which were similar to ones in the previous experiment: two interpolated groups, (a) one of which estimated errors of a slow interpolated response (interpolated slow) and (b) one of which estimated errors o f a fast interpolated response (interpolated fast) during the K R delay interval of the criterion task, (c) a group with a free K R delay intereal (free), and (d) a group in which subjects were requested to estimate their own movement time (estimation) during the K R delay interval. The following hypothesis was examined: Subjects of the interpolated conditions will perform with higher error than subjects o f the free and estimation conditions. More specificaUy, while subjects o f the interpolated slow condition will slow down their movements during the acquisition phase and speed up their movements during retentionmthat is, perform

Method

Subjects

Apparatus and Task The task was identical to that used in the first experiment. The time in which the criterion movement had to be performed was again Is.

Procedure The operationalization of the interpolated slow, free, and estimation conditions was similar to Experiment 1. The interpolated fast condition differed from the interpolated slow condition only in that movement errors in a faster secondary task were to be estimated. More specifically, movement times ranged from 400 to 800 ms. Subjects were to report the deviation of the experimenter's actual movement from 600 ms. In the interpolated slow condition, movement times of the secondary task ranged from 1,200 to 1,600 ms, and subjects estimated the deviation from 1,400 ms. The acquisition phase consisted of 80 trials with knowledge of results. The immediate (6 min later) and delayed retention test (2 days later) consisted of 25 trials each without knowledge of results. Experimental manipulations were applied only during acquisition.

Data Analysis The data were analyzed according to three dependent measures: absolute constant error, constant error, and variable error, computed per 5-trial blocks. Separate ANOVASWereconducted on the acquisition data and the immediate and delayed retention data. A 4 × 16 (Group x Trial Block) ANOVAwith repeated measures on the last factor was used for the acquisition data, and a 4 x 5 (Group x Trial Block) ANOVA with ~ t e d measures on the last factor was used for the retention data. Results

Acquisition Absolute constant error. On most o f the trial blocks, the interpolated groups performed with higher error than the other groups (see Figure 3, left side). Performance error o f the interpolated slow group was highest. All groups displayed a decrease in absolute constant error across trial blocks. The group effect was significant, F(3, 76) = 12.13, p < .01 (MSc = 16,563.63). In order, means for the interpolated slow, free, estimation, and interpolated fast groups were 101.96, 48.28, 51.43, and 57.64 ms. Tukey tests indicated that only the interpolated slow group differed from the other groups ( p
.05). The trial blockeffoct as well as the Group x Trial Block interaction was significant, F(15, 1140) = 10.06, p < .01, and F(45, 1140) = 5 . 9 5 , p < .01 (MSe = 5,874.96). This interaction was mainly a result,0f the interpolated fast group's scores becoming less negative. This contrasted with the remaining groups whose scores became less positive across trial blocks. Variable error. In comparison with the other groups, the interpolated groups tended to perform with slightly higher error on most trial blocks. Nevertheless, the group effect did not reach significance F(3, 76) = 1.05, p > .05 (MS, = 3,481.48). Means for the interpolated slow, free, estimation, and interpolated fast groups were 63.44, 55.59, 57.61, and 60.05 ms, respectively. The trial block effect was significant, F(15, 1140) = 29.33, p < .01 (MSe - 834.85), indicative of a general decrease in error across trial blocks. The Group x

Trial Block interaction also reached significance, F(45, 1140) = 1.83, p < .01.

I m m e d i a t e Retention Absolute constant error. As shown in Figure 3 (right side), the interpolated slow, free, and estimation groups performed alike, and their scores remained less or more stable across trial blocks; their respective means were 58.6, 56.36, and 67.66 ms. These. data ,are comparable~ to those of the corresponding groups in the previous experiment. The interpolated fast group performed with higher error than the other groups, and error scores increased toward the end of immediate retention (M --- 116.77 ms). The group effect was significant, F(3, 76) = 7.72, p < .01 (MSe = 10,427.33). Tukey's a posteriori tests revealed that the mean absolute constant error of the interpolated fast group differed from that of all other groups (p < .01), which were not different among each other (p > .05). The trial block effect was significant, F(4, 304) = 7.27, p < .01 (MSe = 1,356.65). This was mainly due to an increase in error toward the end of no-KR practice for the interpolated fast group, whereas the other groups did not clearly display this trend. Accordingly, the Group × Trial Block interaction also reached significance,F(l 2,.304) -- 2.49, p < .01. Constant error. All groups performed with an overall positive constant error. The interpolated fast group performed with higher positive constant error than the other groups, and errors increased toward the end of immediateretention. The main effect for group was significant, F(3, 76) = 10.41, p < .01 (MSe = 20,684.24). In order, means for the interpolated slow, free, estimation, and interpolated:fast groups were 12.34, 18.05, 45.09, and 113.44 ms. A posteriori tests revealed significant differences between the means of both interpolated groups (p < .01), between the free and interpolated fast groups (p < .01), and between the estimation and interpolated fast

INTERPOLATED ACTIVITIES AND LEARNING groups (p < .01), but no differences between the other groups (p > .05). The trial block effect was significant, F(4, 304) = 2.65, p < .05 (MS~ -- 1,816.27), indicative of an increase in positive constant error across blocks. This effect was not evident for all groups, as underscored by the significant Group x Trial Block interaction, F(12, 304)-- 3.93, p < .01. Variable error. No significant differences among groups were identified, F(3, 76) ffi 1.83, p > .05 (MS~ ffi 1,873.94). Means for the interpolated slow, free, estimation, and interpolated fast groups were 41.42f~39.81, 41.52, and 49.43 ms, respectively. The trial block effect was not significant, F(4, 304) -- 2.06, p > .05 (MS= -- 316.74); neither was the Group x Trial Block interaction, (F < 1).

Delayed Retention Absolute constant error. The differences among groups were more pronounced than at immediate retention (see Figure 3, far right portion). The estimation group performed with lowest error (M = 67.7 ms), followed by the free group (M = 91.11 ms) and the interpolated slow group ( M r 101.64 ms). The interpolated fast group performed with highest absolute constant error across all trial blocks ( M ffi 160,45 ms). The interpolated groups showed an increase in absolute constant error across no-KR practice. A significant group effect ~ was identified, F(3, 76) = 5.30, p < .01 (MSe = 29,347.43). A posteriori tests indicated that only the mean absolute,constant error scores of the free and interpolated fast groups~p < .05) and of the estimation and interpolated fast groups (p < .01) differed significantly. The trial block effect was significant, indicative of an overall increase in error across trial blocks, F(4, 304) = 12.62, p < .01 (MSe ffi 1,599.08). The latter effect was more pronounced for the interpolated groups than for the others, as indicated by a significant Group x Trial Block interaction, F(I 2, 304) ffi 2.05, p < .05. Constant error. The free and estimation groups performed with close to zero constant error. The interpolated fast group performed with large positive constant error, similar to the immediate retention test. The interpolated slow group showed a general decrease toward larger negative constant error. The main effect for group was significant, F(3, 76) = 21.2, p < .01 (MS, = 48,686.71). In order, means for the interpolated slow, free, estimation, and interpolated fast groups w e r e - 8 6 . 7 5 , -2.87, -4.24, and 156.15 ms. Allithe differences between groups were found significant, as indicated by Tukey tests (p < .05, p < .01), except for the comparison of the free and estimation groups (p > .05). The trial block effect was not significant, F(4, 304) = 1.96, p > .05 (MS, = 2,135.5). The Group x Trial Block interaction was significant, F(I 2, 304) = 6.75, p < .01. Variable error. The group effect was significant, F(3, 76) = 4.84, p < .01 (MSe ffi 1,077.20). The interpolated fast group performed with higher variable error than did the other groups? Means for the interpolated slow, free, estimation, and interpolated fast groups were 37.4, 42.6, 39.68, and 53.7 ms, respectively. A posteriori tests indicated that only the performance differences between both interpolated group~(p < .01) and between the estimation and interpolated fast

699

groups (p < .05) were significant. The trial block effect was significant and was indicative of an overall decrease in variable error across trials, F(4, 304) -- 7.39, p < .01 (MSe = 476.22). The Group x Trial Block interaction was significant as well, F(12, 304) = 1.79, p < .05. In comparison with the fairly stable performance pattern of the estimation group, all remaining groups displayed a big decrease in variable error scores across practice.

Discussion The results of Experiment 2 were conloxtent with those of Experiment 1 and strengthen the contention that secondary recognition activities, interpolated during the K R delay interval, interfere with learning the criterion task. As predicted, the analysis of constant error has shown that subjects tend to move toward the faster o r slower times of the interpolated movements they estimate, during acquisition (assimilation effect), whereas respon~ characteristics are r e r e a d during retention (contrast effect). In other words, subjects who estimated slower interpolated movements speed up the criterion movements, but those who estimated faster movements slow down considerably. Apparently, both the assimilation and contrast effects are robust findings. In the previous discussion, attention was already drawn to similar assimilation effects in short-term motor memory research. However, the contrast effect at retention fails to comply with this anaiogy and needs further explanation. One reason why it may have occurred is that subjects no longer experienced the influence of the interpolated task at retention because it was withheld, whereas they did remember the kind of knowledge of results they were predominantly receiving during the acquisition trials. Moreover, it may also be a result of the subject's persevering attempt to cope with or resist assimilation. Although assimilation and contrast effects have been investigated extensively in perceptual judgment studies (McKenna, 1984), the particular occurrence of these effects in the present experiments does not meet a clearcut analogy with this work. In line with the findings of Experiment 1, estimation of performance errors made in the criterion task tended to benefit performance. However, the effect again failed to reach significance. Therefore, forced estimation appears to be a variable of only marginal importance. This may be due to the fact that learners estimate errors spontaneously (Swinnen, 1988). 4 Some caution is required for interpretation of the variable error data. It is possible ithat subjects of the interpolated fast condition ~rformed with higher variable error than did the other groups, partly because they moved much slower, and higher movement times give rise to higher variable error scores. In order to test this assumption, the variable error scores for each block were divided by their respective mean movement time, providing a measure of relative variable error. In opposition to the statistical analysis of variable error, the ANOVA on relative variable error scores did not reveal a mgnificant group effect, F(3, 76) ffi 1.07, p > .05. The mean scores of the interpolated slow, free, estimation;and interpolated fast groups were 0.040, 0.043, 0.039~ and 0.046, respectively. Again, the interpolated fast group perfoi~aed, with higher error than the other groups, but differences were no longer significant.

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STEPHAN P. SWINNEN Experiment 3

The previous experiments consistently demonstrated that interpolation of secondary recognition activities during the K R delay interval interferes with learning a timing task. The question remains, however, if the observed interference effects are strictly tied to the K R delay interval or if they occur independently of the location o f the interpolated task. This problem was addressed in Experiment 3 through comparison of the degree of interference induced by interpolation o f the secondary task in the K R delay and post-KR interval. To my knowledge, such an experiment has not been conducted so far, although it appears most suitable when assessing the relative importance o f these intervals for motor performance and learning. As it became evident from Experiment 2 that the slow interpolated task was subject to experimental artifacts, it was decided to preserve the fast interpolated task in the present experiment. There were three groups: (a) One group performed the fast interpolated task during the K R delay; (b) another group, during the post-KR interval; and (c) subjects o f the free group were not assigned any secondary activities during either of these intervals. On the basis of the general line of reasoning that subjects are spontaneously involved in performance evaluation right after response completion (i.e., during the K R delay interval), it was predicted that the secondary task would interfere more with learning the criterion task when applied in the K R delay interval then in the post-KR interval. The assumption underlying this hypothesis is that interference results predominantly from competition between tasks imposing simultaneous demands on specific perceptual mechanisms. In other words, the interference is of a "structural" nature (Kahneman, 1973). Method

Subjects Undergraduates from the University of California, Los Angeles were tested and received credit for participation (N = 66). An equal number of male and female subjects was assigned to each condition. Participants had not received prior experience with the task.

Apparatus and Task The task was identical to that used in the previous experiments.

Procedure There were three groups: Group 1 performed the interpolated task during the KR delay interval (interpolated KR delay), Group 2 during the post-KR interval (interpolated post-KR); performance of these groups was compared with Group 3, which was not assigned any secondary tasks during either the Kit delay or the post-KR interval (free). With respect to the interpolated KR delay group, the secondary task was presented 5 s after completion of the primary task--rather than immediately after, as in the first two experiments--in order to investigate the degree of perseverance of secondary task interference after this time lag. The interpolated activity consisted of movements ranging from 400 to 800 ms. Subjects reported the deviation of the experimenters' movement time from the target time of 600 ms.

The intertrial interval (26 s) was held constant among groups. In both the interpolated Kit delay group and the free group, knowledge of results was delayed by 21 s while the post-KR interval duration was 5 s. In the interpolated post-KR group, the KR delay interval was 5 s, the post-KR interval 21 s. Consequently, the most important interval to affect learning, namely the intertrial interval (Salmoni et al., 1984), was held constant among groups. The acquisition phase consisted of 80 trials with knowledge of results. Retention tests were held 5 min and 2 days later, consisting of 25 trials each without knowledge of results.

Data Analysis The statistical analysis of the data was similar to that in Experiment 2. Results

Acquisition Absolute constant error. Although the interpolated groups initially performed with lower error than the free group, error scores were higher on most o f the remaining trial blocks (see Figure 4, left portion). This initial performance benefit might be due to the former groups' tendency to speed up the criterion movement. This is in contrast to no-interpolation circumstances under which subjects usually perform the task too slow at the start of practice. All groups showed a progressive decrease in error across trial blocks, an effect that was most pronounced for the free group. The group effect was not significant, F(2, 63) = 1.26, p > .05 (MSe = 5,279.25). Means were 37.46, 39.45, and 45.79 ms for the interpolated K R delay group, the free group, and the interpolated post-KR group, respectively. The trial block effect was significant, F(15, 945) = 19.07, p < .01 (MSe = 1,467.9), as well as the Group x Trial Block interaction, F(30, 945) = 2.25, p < .01. Constant error. The free group performed with an overall positive constant error on all trial blocks, but scores became less positive across practice. The interpolated K R delay group performed with negative constant error on most of the trial blocks except the last ones. The interpolated post-KR group performed with only a small negative constant error on some of the trial blocks. The statistical analysis of constant error revealed a significant group effect, F(2, 63) = 8.42, p < .01 (MS~ = 12,484.21). In order, means for the interpolated K R delay, free, and interpolated post-KR groups were -9.09, 25.25, and 4.66 ms. A posteriori tests on the group means revealed that both interpolated groups differed from the free group ( p < .01, p < .05), but they were not significantly different from each other ( p > .05). The trial block effect was not significant ( F < 1). The G r o u p x Trial Block interaction was significant, F(30, 945) = 2.86, p < .01. Variable error. Although both interpolated groups performed with higher variable error on the majority of the trial blocks, the group effect was not significant, F(2, 63) = 1.38, p > .05 (MSo = 3,478.31). Means for the interpolated K R delay, the free, and the interpolated post-KR groups were 59.52, 52.13, and 56.17 ms, respectively. The trial block effect

INTERPOLATED ACTIVITIES AND LEARNING

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delay groups of Experiment 3 during acquisition, immediate retention, and delayed retention. was significant, F(15, 945) = 11.05, p < .01 (MS~ = 793.94), indicative of big decreases in error across practice. The Group × Trial Block interaction was not significant, F(30, 945) = 1, p > .05. Immediate Retention Absolute constant error. Both interpolated groups performed with much higher error than the free group (Figure 4, right portion). All groups showed an increase in absolute constant error across trial blocks, but it was more pronounced for the interpolated groups. The group effect was significant, F(2, 63) - 7.86, p < .01 (MS~ -- 11,403.82). In order, means for the interpolated K R delay, free, and interpolated POst-KR groups were 93.48, 48.11, and 100.83 ms. Paitwise a posteriori tests indicated that mean performance of both interpolated groups differed significantly from that of the free group (p < .01). The interpolated groups were not significantly different from each other (p > .05). As expected, the trial block effect was significant, F(4, 252) = 14.81, p < .01 (MSe = 1,403.56). The Group × Trial Block interaction was not significant (F < 1). Constant error. All groups performed with positive constant error, but scores were much higher for both interpolated groups. The main effect for group was significant, F(2, 63) = 9.88, p < .01 (MS= = 16,422.14). Means for the interpolated KR delay, free, and interpolated post-KR groups were 90.07, 25.78, and 94.33 ms, respectively. Again, mean constant error scores of both interpolated groups differed from that of the free group (p < .01), but they did not differ from each other (p > .05). The trial block effect was significant,/7(4, 252) = 16.05, p < .01 (MSo = 1,577.95). The Group × Trial Block interaction was not significant,/7(8, 252) = 1.59, p > .05. Variable error. The interpolated groups performed with higher error than the free group, resulting in a significant group effect, F(2, 63) = 8.71, p < .01 (MSo = 633.36). In

order, mean scores for the interpolated K R delay, free, and interpolated post-KR groups were 50.66, 36.71, and 45.83 ms. The means of both interpolated groups differed significantly from the free group (p < .01, p < .05), but they were not different from each other (p > .05). No apparent changes across no-KR practice were evident. Accordingly, the trial block effect was not significant, (F < 1); neither was the Group × Trial Block interaction, F(8, 252) = 1.13, p > .05. Delayed Retention Absolute constant error. As shown in Figure 4 (far fight portion), the free group performed with lowest error on all trial blocks (M = 84.85 ms); the interpolated KR delay group with highest error (M = 162.36 ms) and the interpolated postK R group were positioned in between the former two groups (M = 124.98 ms). Significant differences in mean absolute constant error among groups were identified, F(2, 63) = 3.38, p < .05 (MS~ = 48,892.57). Tukey tests indicated that only the difference in mean performance between the interpolated K R delay and free group was significant (p < .05). Both interpolated groups performed with increasing error across trial blocks, but this was not as clearly evident for the free group. Accordingly, the trial block effect was significant, F(4, 252) = 10.23, p < .01 (MSe = 2,079.66), as well as the Group × Trial Block interaction, F(8, 252) = 2.23, p < .05. Constant error. All groups performed with an overall positive error, but scores of both interpolated groups were much higher than those of the free group. In accordance with absolute constant error, all groups performed with increasing constant error across trial blocks, but this effect was more prevalent for the interpolated groups. The main effect for group was significant, F(2, 63) = 5.97, p < .01 (MS= -77,009.44). In order, means for the interpolated KR delay, free, and interpolated post-KR groups were 146, 18.9, and 103.21 ms. Only the difference between the interpolated K R

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STEPHAN P. SWINNEN

delay and free group reached significance (p < .05). The trial block effect was significant, F(4, 252) -- 21.94, p < .01 (MSe = 2,039.15), as well as the Group x Trial Block interaction, F(8, 252) = 2.2 l, p < .05. Variable error. Similar to immediate retention, the free group performed with lower variable error than the other groups. Nevertheless, the group effect was not significant, F(2, 63) = 2.90, p > .05 (MS, = 1,271.25). Means for the interpolated KR delay, free, and interpolated post-KR groups were 50.3, 41.02, and 51.66 ms, respectively. The trial block effect was significant, F(4, 252) = 7.99, p < .01 (MSe = 395.25), indicative of an overall decrease in error. The Group x Trial Block interaction was not significant, F(8, 252) = 1.37, p > .05. Discussion In keeping with Experiment I and 2, Experiment 3 indicated that interpolating a secondary task during the KR delay interval is detrimental to learning the primary task. Taking delayed retention data as a major guideline for assessing learning effects, it appears that the secondary task also tended to affect learning when applied in the post-KR interval, although to a smaller extent. Performance of the interpolated post-KR group was not significantly different from the free group, but it was not significantly different from the interpolated KR delay group either. Accordingly, a midway point is indicated for interpretation of these findings. It appears that the interference produced by the secondary task is only partly bounded to the KR delay interval. Although this is the only study of its kind in which the temporal location of a secondary task has been manipulated within a knowledge-of-results paradigm, some short-term motor memory studies have addressed the effects of the temporal position of interpolated tasks on recall performance of positioning movements (Patrick, 1971; Stelmach & Walsh, 1973). These studies revealed that recall performance is more subject to interference when the interpolated movement is presented at the end of the retention interval instead of at the beginning. Insofar as generalizations from the short-term memory paradigm to the learning paradigm are plausible, these findings suggest that interpolations in the post-KR interval are more detrimental to performance than are interpolations in the KR delay interval. Only a small and nonsignificant trend in this direction was found with respect to the acquisition and immediate retention data. However, rather than being higher, performance error at delayed retention was lower for the postKR group. Consequently, delayed retention performance does not meet the analogy with short-term motor memory research. It should be noted that current findings are not likely to be due to differences in the duration of the KR delay and postKR interval that accompanied the secondary task manipulations. In their extensive review, Salmoni et al. (1984) coneluded that of all three intervals, the intertrial interval was most influential, and it was held constant in the present experiment. Because amount of practice was not manipulated across the three experiments, it is unclear whether level of initial learning has any interaction with the effects so obtained.

Additional practice might have resulted in a reduction of these performance decrements. However, 80 trials was considered to provide ample opportunity for acquiring this timing skill. General Discussion The present experiments have convincingly demonstrated that performing an interpolated recognition activity during the KR delay interval is detrimental to learning the criterion task. Learning was assessed through evaluation of performance in the absence of knowledge of results and with the interpolated task removed. The effects were clearly not of a temporary nature but persisted at least until 2 days following the acquisition phase. Subjects who were administered a secondary task during the KR delay interval performed with higher absolute constant error on subsequent retention trials than did subjects not involved in any attention-demanding secondary activities. It is noteworthy here that interference in recall emerged even though subjects did not actually perform the interpolated movement. Trying to recognize the experimenter's performance errors was sufficient to induce large interference effects on the primary task. These findings complement earlier evidence in which interpolation of activities in the KR delay interval affected learning a variety of tasks. Boulter (1964) investigated the role of interpolated verbal and motor activities during the KR delay on learning a blind lever positioning movement and found that motor and verbal-motor activity hampered immediate retention performance ("trials to extinction," as he called it). Shea and Upton (1976) had subjects perform a motor shortterm memory task during the KR delay interval, and this affected acquisition of a double linear positioning movement, as well as retention performance. More recently, Marteniuk (1986) conducted a series of experiments in which subjects learned an arm movement consisting of reversals. In a f ~ t experiment, he, unlike Shea and Upton, failed to identify interference with an interpolated short-term memory task. In a second experiment, Marteniuk (1986) demonstrated interference with an interpolated movement to be learned. The present experiments differ from this earlier work in that a recognition type of secondary task was used which (at least initially) confines the locus of the resulting interference to the sensory moralities. Moreover, performance detriments persisted at least across 2 days following the acquisition phase. This implies that the long-term memory representation of the primary task was corroded by these interpolations. Although it is beyond doubt that interference emerged among the tasks, it is more difficult to identify the specific causes of this effect. In order to address this matter, at least two aspects of the interpolated task warrant further consideration. Subjects only "perceived" the interpolated movement without actually producing it, and they "estimated" errors made by the experimenter in the secondary task. Estimating timing errors, made in the interpolated movement, may have led to the development of a recognition memory schema for that movement (see Schmidt, 1975). This schema may have interfered with the development of a recognition schema for the criterion task, resulting in a performance deterioration.

INTERPOLATED ACTIVITIES AND LEARNING Stated differently, performing the interpolated recognition activity prevented subjects from building a reliable reference of correctness for the primary task, indispensable for successful error detection in producing future responses. Theories of motor learning have underscored the importance of such references against which learners compare and evaluate the response-produced feedback information in order to detect and correct errors (Adams, 1971; Schmidt, 1975 ). These selfinitiated response evaluation activities are also referred to as "subjective reinforcement," and they are especially critical for performance in the absence of knowledge of results (Adams, 1971). When subjects fail to build a reliable reference of correctness, performance is most likely to deteriorate. This can be observed when looking at Figures 2 to 4. Although initial retention performance in the interpolated activity groups was only slightly worse than in the other groups, it deteriorated progressively across trial blocks, as if subjects failed to detect and correct their errors. Thus, deterioration in recall, as shown by this progressive drift away from the target, may have evolved as a result of a corroded recognition memory representation for the primary task. The particular expression of this detriment at retention as a "contrast" effect can preferablybe explained by the performer's recollectionof the sign of knowledge of resultsgiven predominantly during acquisition and his or her continuing strategyto resistassimilation. Put into a more general perspective, the present findings lead to the conclusion that m e m o r y for timing movements is a dynamic process, undergoing changes duc to contextual phenomena, even days after acquisition has taken place. It results from a complex interaction between the phenomena of perception (recognition) and production (recall).Because subjects could watch as well as hear the sound of the interpolated movement, vision and, perhaps more important, audition, arc the mediatory sensory systems that indirectlyaffected the cognitive representation of the primary timing movement. Indeed, previous research has shown that audition m a y be best suited for dealing with temporal information (Doody, Bird, & Ross, 1985; O'Conner & Hcrmelin, 1978). In the present experiments, moving the slide over the stainlesssteel shaft generated a distinct sound. In view of the recently provided evidence that subjects are actively involved in estimating their timing errors before presentation of knowledge ofresults (Swinnen, 1988; Swinnen et al., 1990), it is conceivable that the secondary task interfered more specificallywith estimation of errors made in the primary task. In that ease, itshould not have generated interference when interpolated in the post-KR interval.This prediction was tested in Experiment 3 and was only partly confirmed. Delayed retention performance of the group that was administered the interpolated task during the post-KR interval was no longer affected significantly,but some degree of interference was stillmanifest. This leads to the suggestion that the interferencewas not strictlybounded to the K R delay interval.One reason this effectmay have occurred is that the post-KR interval also embeds activitiesrelated to development of the reference of correctness. Indeed, availabilityof response-produced feedback and self-generatederror information, in addition to objectiveinformation about the degree

703

of success in goal achievement (through knowledge of results), may provide a new impetus to building and strengthening this mechanism. Therefore, mental activities that occur during both the KR delay and post-KR interval may play a role in developing the recognition memory representation for movement; conversely, interpolation of secondary tasks during each of these intervals may interfere with this process. In summary, the present findings are at issue with the longstanding viewpoint that, in comparison with the post-KR interval, the KR delay interval plays only an inferior role in motor skill learning. The evidence put forward here converges with and complements our work on manipulation of the delay of knowledge of results in skill learning (Swinnen et al., 1990). Removing or severely shortening the K R delay interval, on the one hand, or interpolating a secondary task, on the other hand, is detrimental to learning, presumably because it prevents subjects from building a reliable reference of correctness. Furthermore, although it has been argued that secondary tasks more easily produce interference when applied in the post-KR interval instead of in the KR delay interval (Adams, 1971)--presumably because active processing of information takes place after knowledge of results has been presented--the present studies demonstrate that interpolated recognition activities tend to generate more interference with learning the primary task when administered in the KR delay interval. In light of these findings, it is not embarrassing that motor behavior researchers have repeatedly failed to demonstrate interference with interpolation of secondary tasks during the post-KR interval (Lee & Magill, 1983, 1987; Magill, 1988; but see McGuigan, 1959, and McGuigan, Hutchens, Eason, & Reynolds, 1964, for an exception) in contrast to repeated successes with interpolations during the KR delay interval. Magill (1988) and Lee and Magill (1987) have gone even further by pointing out that attention-demanding activity during the post-KR interval can benefit motor skill learning. In order to resolve these issues, it will be necessary to further assess the potential role of interpolated tasks in motor performance and to identify the boundary conditions wherein such tasks induce benefits versus detriments on learning and retention. Experiments in which the effects of interpolation of the same task in the KR delay and post-KR interval are compared across different conditions may be one fruitful line for future research in this area.

References Adams, J. A. (1971). A closed-looptheory of motor learning. Journal of Motor Behavior, 3, I 11-149. Adams, J. A. (1976). Learning and memory: An introduction. Homewood, IL: Dorsey Press.

Adams, J. A. (1978). Theoretical ~ for knowledge of results. In G. E. Stvlmach (EEL),Information processing in motor control and learning (pp. 229-240). New York: Academic Press. Adams, J. A. (1987). Historical review and appraisal of research on the learning, retention, and transfer of human motor skills Psychoiogicul Bulletin, 101, 41-74. Bflodeau, I. M. (1966). Information feedback. In E. A. Bilodeau (Ed.), Acquisition of skill (pp. 255-296). New York: Academic Press. Bilodeau, E. A., & Bilodeau, I. M. (1958). Variation of temporal intervals among critical events in five studies of knowledge of

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