Response preparation and control of movement sequences

Response preparation and control of movement sequences

Ian M Franks

Response Preparation and Control of Movement Sequencesl

Abstract Two experiments investigated the response complexity effect using elbow extension/flexion movements. In the first experiment, RT for an extension movement was significantly less than RT for an extension/flexion movement. However, this difference in RT was not evident when participants were asked to pause at the reversal of the extension/flexion for approximately 260 ms. The second experiment manipulated the duration of the pause between these movements and also measured the electromyographical (EMG) activity of the triceps and biceps muscles. When the pause was reduced to 75 ms participants were not able to program the flexion portion of the movement at the reversal, forcing them to preprogram this movement; hence, increasing their premotor reaction time.

It has been suggested that a sequence of actions can be programmed in advance of movement initiation (Lashley, 1951). One prediction from this suggestion is that a simple movement will take a shorter amount of time to plan than a more complex movement. This prediction has been tested empirically by measuring the reaction time (RT) required to prepare and initiate movements which vary in complexity. RT is defined as the time between stimulus onset and movement initiation. Typically, it has been found that more complex movements require a greater amount of time to prepare and initiate than simpler movements. The variation in RT was assumed to be due to response programming activity, because the tasks required stimulus identification and response selection to be held constant

Freeman (1907) was possibly the first to measure the relationship between RT and movement complexity. He found that when participants drew geometric figures such as a straight line, a circle, or a pentagon, the RT became longer as the figure increased in complexity. These variations in latency were said to have occurred because of antagonistic muscular tensions originating from anticipation of the necessary movement reversals which occurred in the more complex sequences. However, the most influential researcher to work within the RT/movement complexity paradigm was Franklin Henry. In a series of papers in the 1950s and 1960s, Henry and his co-workers laid the groundwork for his “Memory Drum Theory” (Henry & Rogers, 1960). In this theory, the basis of movement control was likened to the memory drum hardware in the computers of the day. It was suggested that the more complex the movement, the more complicated the memory drum program representing it. This more complicated program takes longer to initiate because: a larger amount of stored information will be needed, and thus the neural impulses will require more time for co-ordination and direction into the eventual motor neurons and muscles. (Henry & Rogers, 1960, p. 450) Henry and Rogers tested this theory by having participants produce either a single key lift response or a key lift response concatenated with movements to targets. They found that the simple movement required less time to prepare and initiate than the more complicated sequence of movements. This phenomenon has since been studied extensively. Various researchers have shown that RT increases with increased movement complexity using such tasks as keystrokes and tapping (Fischman, 1984; Klapp & Rodriguez, 1982; Rosenbaum & Patashnik, 1980), speech (Erickson, Pollack, & Montague, 1970; Sternberg, Monsell, Knoll, & Wright, 1978), and handwriting (Hulstijn & van Galen, 1983; Teulings, Mullins, & Stelmach, 1986). The parameter which appears to be most strongly related to changes in RT in these studies is the number of response elements that comprise the movement. Thus, it is the number of taps or keystrokes, number of stress groups in a sequence of speech, and number of strokes taken in writing a letter which has the greatest impact on the time required to prepare and initiate these responses.

Studies that have confirmed the relationship between RT and movement complexity have typically used maximal speeds of response. Specifically, participants were not only required to initiate their movements as quickly as possible but also to complete them in as short a time as possible. More recently, a number of investigators, using variable rates of response, have uncovered some interesting results (Canic & Franks, 1989; Franks & van Donkelaar, 1990; Garcia-Colera & Semjen, 1987, 1988; van Donkelaar & Franks, 1991a, 1991b). They found that RT increased linearly in relation to the number of movement elements at the quickest rates, but either failed to do so, or did so in a non-linear fashion at slower rates. Thus, when a movement sequence is completed at a less than maximal speed, changing the complexity of it by adding extra elements does not reliably cause an increase in the RT required to prepare and initiate it. This finding has been explained by suggesting that some form of on-line or parallel system of movement preparation is taking place (Rosenbaum, Hindorff, & Munro, 1986). The logic is as follows: If participants preprogram a movement sequence, then increasing the number of elements within that sequence should lead to an increased latency during response initiation. However, if increasing the number of elements does not cause an increase in RT, then the participants must not have to plan the entire sequence beforehand; rather, some aspect of this process can carry on into the period of movement execution. If the planning process does not occur during the period prior to actual movement, then some evidence of on-line control should be present within the movement itself. Because, by definition, the planning process takes time, several researchers have looked to the inter-response intervals (IRIs) for such evidence. Ostry (1980, 1983) has been perhaps the most successful in this regard. He has found that, in typing a series of keystrokes, participants tend to lengthen the IRIs in the middle of the sequence (Sternberg et al.,1978, found a similar result but did not discuss it at any length). Ostry suggested that this midsequence slowing was used by the participants to prepare, on-line, the terminal elements in the sequence. Such a form of preparation is required because, according to Ostry, we have a limited motor output span. In a similar vein, another line of IRI evidence has been used to demonstrate the differences between preprogrammed movements and those prepared on-line. Specifically, Canic and Franks (1989; see also Franks & Canic, 1991 and 1992) found the RT required to prepare and initiate isochronous sequences of tapping movements increased as the number of taps increased at fast rates (200 and 300 ms/tap), but failed to do so at slow rates (400, 600 and 800 ms/tap). In addition, the IRIs produced in such sequences displayed lengthened beginning and terminating elements at the fast rates but not the slow rates. Povel and Essens (1985) have suggested that such temporal accenting (termed “agogic” accenting) reflects a tendency to perceive the sequence under question as a unitary whole. Thus, in Canic’s studies, it appears that the individual taps in the sequences completed at the faster rates were both perceived and prepared (as evidenced by the IRI and RT data, respectively) as a group. On the other hand, the taps produced in the sequences completed at the slower rates appeared to have been perceived and prepared individually. In such a situation, it is likely that each tap was prepared during the execution of those preceding it. Evidence from serial pattern learning studies has also demonstrated the viability of analysing the variations within IRI’s. In studies by Povel and Collard (1982) and Rosenbaum, Kenny, and Derr (1983), the IRIs between movement sequence subunits in a discrete movement pattern were lengthened in relation to those within the subunits. This was used as evidence to suggest that the patterns were prepared in parts as the movement was being executed. Similarly, in a study by Franks, Wilberg, and Fishburne (1985), probe RT to a secondary task was measured at points within and between subunits of a movement tracking pattern. If the patterns were planned entirely before their execution, then probe RT should remain similar at each point in time. However, it was found that probe RT was greater during the intervals between subunits, suggesting that more processing was required during this part of the pattern. This increase in probe RT also indicated that parts of the pattern were prepared on-line as opposed to entirely beforehand. Unfortunately, other than the studies by Canic and Franks (1989) and Ostry (1980,1983), litle else has been done within the RT/movement complexity paradigm to uncover how and when the on-line control processes occur during the preparation and production of a movement sequence. The following two experiments were designed to examine this issue in detail using simple forearm extension and extension/flexion movements. Reaction times and movement times of extension movements were compared to those of extension/flexion movements that varied in the amount of time allowed at the reversal of the movement sequence. Following Henry and Roger’s (1960) original hypothesis and more recently Klapp’s (1996) reaction time analysis of central motor control, a simple forearm extension requires less programming (hence lower reaction time) than a more complex extension/flexion movement. However, if the duration of time between the extension and flexion movement segments is extended to permit on-line control of the flexion movement, then the reaction time was not expected to be significantly longer than the reaction time for an extension movement. Furthermore, if an increase in pause duration does encourage on-line control at the reversal, then it would be of interest to know what the temporal limitation of this on-line control is? That is, how much time is required to prepare the flexion portion of the extension/flexion movement sequence? Experiment 1 METHOD

Participants. A total of 30 male and female university students volunteered to serve as participants in this experiment. This was a randomized group design with 15 participants per group. All were naive as to the hypothesis being tested and inexperienced at the experimental task. The participants were paid 10 dollars for their participation in the study. The experiment was carried out in accord with the ethical guidelines of the University of British Columbia. Apparatus. All participants performed a repetitive arm extension/flexion movement in the horizontal plane through an extension range of 22.5 degrees ( from 55 degrees to 77.5 degrees — where 180 degrees was defined as full extension) and a flexion range of 11.25 degrees. The right forearm was positioned on a forearm manipulandum, which consisted of a padded horizontal lever attached to a bearing mounted vertical shaft. The elbow was coaxial with the axis of rotation, and the right hand was supinated to grasp a vertical handle at the free end of the lever. The position of the handle was adjusted to accommodate for varying forearm lengths. The height at which the participants were seated was also adjusted so that the shoulder angle remained constant in the frontal plane and the elbow was raised to the height of the shoulder, thus ensuring a true flexion and extension movement. All participants were required to wear a pair of translucent PLATO spectacles (Translucent Technologies Inc., Model P1). These spectacles use liquid crystal lenses that turn translucent through an electrical signal, allowing the complete occlusion of vision within 2 ms. The glasses were connected to the parallel printer port of the computer, thus allowing the lens to be cleared (open state) or occluded (closed state) under software control. The reason for using these spectacles was to reduce the visual cues that were available to the participant following the onset of the imperative stimulus. In their open state the spectacles did not affect the perception (brightness, sharpness, spectral composition) of the stimulus and response cursors. Moreover, in their closed state, the spectacles not only blocked out all pertinent visual cues, but the participant’s eye remained illuminated and close to the level of illumination in the open state. Hence the amount of adjustment the eye makes when switching takes place was minimal.

The participants were seated in front of a 10 x 12 cm oscilloscope screen (Tektronix, Model 620) that was positioned at eye level, directly in front of them, at a distance of oscilloscope screen at a distance of 0.50 cm. Two cursors, the computer generated stimulus, and below the participant controlled response, were displayed on the oscilloscope screen. The range of movement of these cursors on the 12 cm wide screen at a distance of 0.5 cm above and below was 5 cm from the horizontal centre line of the oscilloscope screen. This screen was calibrated to give of movement of these cursors on the 12 cm wide screen was 5 cursor, and the centre of the screen. This screen was specified at less than 1%. The calibrated to give 0.1 mm poscope screen was controlled by dual accuracy for the 0.25 mm analogue (D/A) converters (4096 digital values) and the linearity was specified at less than 1%. The oscilloscope screen was controlled by dual 12 bit digital output for brightness control (Tecmar Labmaster Data Acquisition System). The linearity of this D/A) converters (4096 digital values) and a single measured at 0.5 bits of the 12 bit output. The start for brightness control (Tecmar Labmaster Data targets were defined by vertion System). The lines drawn of the output was measured at 0.5 bits of the 12 bit output. These lines were start accurate to within 0.005 cm and the target width was 2 cm. These targets were defined by vertical lines drawn on the oscillotantly positioned on the oscilloscope screen. These lines were drawn accurate to within 0.005 cm and the target width was 2 cm. These targets were equidistantly positioned on the oscilloscope.

The angular displacement values of the forearm manipulandum were mechanically transmitted to an angular potentiometer (Helipot 6187, One Turn, 10K Ohm), and the resulting linear voltage was sampled by an A/D converter (Labmaster) at 500 Hz with an accuracy of 12 bits (4096 values). The A/D converter was linearly accurate to one bit, while the potentiometer was 1% linear over its entire range. The forearm manipulandum was calibrated at 90 degrees and measured to approximately 0.06 degrees per bit accuracy.

An accelerometer (Kistler, type 8638B50, + 50 G) was positioned at the end of the forearm manipulandum, 42 cm from the centre of rotation. The accelerometer signal was filtered with an active lowpass filter (Krone-Hite, # 3750) set at 50 Hz and then sampled. Setting the filter lower would have resulted in a significant signal delay in the filter (5 ms measured at 20 Hz lowpass). There was no signal delay at 50 Hz. These accelerometer signals were measured using the same A/D converter that was detailed earlier, and this signal, measured in volts, was converted to an angular acceleration value (deg/sec/sec) using the accelerometer’s calibration conversion factor.

The acceleration data for a single extension movement was double integrated and the derived displacement curve very closely matched the collected displacement data (maximum error at any point in time

Procedure. Initially, each participant was given five practice trials at both movement conditions. The order of presentation for practice was extension movement followed by extension/flexion movement. During these practice trials the participant was given feedback after each trial and after successful completion they would move on to the test phase of the experiment to complete 15 acceptable trials for each movement condition. An acceptable trial was one in which the RT measured between 100 and 500 ms to offset the possibilities of anticipation and inattentiveness. The test phase of this experiment used a reaction time protocol in which the participants had to initiate their movement as soon as possible after the imperative stimulus, which was the occlusion of vision via the PLATO spectacles (the spectacles remained occluded until the entire movement was finished). Participants were then to move as fast and as accurately as possible to contact the centre of the targets. Although the movements were made to remembered positions and therefore did not involve visually based corrections, participants were asked not to make corrective submovements that resulted from any proprioceptive feedback from the initial movement impulse. In addition, participants in the extend, pause, and flex movement condition were encouraged to make a brief (

To begin the trial, participants were presented with the stimulus cursor on the left side of the oscilloscope screen (designated as -22.5 degrees). When the participants had positioned their response cursor underneath the stimulus cursor, they said “ready.” A low pitched warning tone of 100 Hz indicated the start of a trial. The computer was programmed to occlude the spectacles at specific time intervals and this was used as the imperative stimulus. This variable foreperiod was randomly determined between the limits of 1000 to 2500 ms. In order to maintain an element of event uncertainty during the task, there was a 20% chance of a catch trial occurring. The error rate associated with these catch trials was only 2% and was evenly distributed among conditions. In addition, the error rate associated with anticipatory reactions (less than 100 ms) and inattentive reactions (greater than 500 ms) was 5% and was also evenly distributed among trials.

Upon completion of the trial, participants were given visual feedback on a colour monitor (Zenith “Flat Screen” ZCM1490) which was positioned directly underneath the oscilloscope. An example of the type of feedback given is displayed in Figure 2. In addition, participants were informed of the acceptability of the trial on the basis that reaction time had to be between 100 and 500 milliseconds. During a catch trial, the foreperiod was extended and the spectacles did not occlude for five seconds. The experimenter would then report the catch trial to the participant and record any movement as error. Dependent variables. Reaction time was determined using the following algorithm. Displacement data for 200 ms before the imperative stimulus were analysed and the mean and standard deviation were calculated. The minimum standard deviation permitted was 0.05 degrees, where the resolution of the angular potentiometer-A/D converter system was 0.05 degrees. The data collected following the imperative stimulus were scanned forward until the point where the participant had moved more than five degrees from the starting position. The data were then scanned backwards until the point where the displacement profile was within the bandwidth of one standard deviation. The reaction time point was the next point forward in time. This algorithm was successfully used on all but five trials, in which the participants displayed some positional drift before and after the imperative stimulus. Those trials were either rejected from analysis or visually edited, which allowed three independent observers to determine the onset time of movement. Both the inter- and intraobserver reliability coefficient of these observers exceeded 0.87.

The first movement time (for the forearm extension) was calculated as the time interval between the initiation of movement and the acquisition of the reversal point. In contrast, the total movement time was calculated as the time interval between the initiation of movement and the acquisition of the final displacement target. The first target accuracy was measured from the displacement data as degrees less than or greater than the centre of the first target. Data ana*sis. After the displacement and acceleration profile of each trial was visually inspected and the results of the algorithms were verified by three independent observers, an analysis profile program was run to compile detailed results for each trial as listed above. These results were then used for the construction of descriptive graphs and tables for statistical analysis. The statistical tables were imported into SYSTAT 4 and a pause groups by movement conditions ANOVA was used on each dependent variable. Subsequent post-hoc comparisons of means utilized the Tukey Honestly Significant Difference (HSD) test (Rosenberg, 1990, p. 352). The alpha level for the experiment was set at .05. RESULTS AND DISCUSSION

The reaction time and movement time means and standard deviations for the variables pause and movement are given in Table 1. The reaction time analysis revealed that there was no significant main effect for movement. However, there was a significant interaction effect between Pause and Movement, F(1,28) = 11.2,p

As was expected, the total movement time for the extension/flexion movements without a pause was found to be significantly shorter than the total movement time for the extension/flexion movement with a pause. These results are however contrary to the findings of numerous investigations of the effect of total movement time on response programming. Such studies have consistently found that RT increases with increasing movement duration (e.g., Klapp, 1981; Klapp & Erwin, 1976; Quinn et al., 1980; Siegel, 1986). However, it should be noted that previous studies have not used movement sequences with distinct response elements. Nevertheless, results of this present experiment do offer some support for Henry’s original contention that movement duration per se would not be expected to cause an increase in RT (Henry, 1980).

Consideration should be given to the possibility that the pause duration at movement reversal was the locus of on-line control for the upcoming flexion movement. Evidence for this position is somewhat indirect. Few research studies have failed to find an increase in reaction time when concatenated movements have: (a) been completed at maximal rates; Co) had a second response element added to the initial movement, (moving from one to two response elements); (c) had movement complexity manipulated by means of a reversal in direction; and (d) had the entire movement completed in less than 1 second. Whereas the complexity effect (RT increases with the addition of more response elements) was evident when no pause was allowed at the reversal, no such increase was observed when participants were required to pause. If indeed the flexion movement was responsible for this complexity effect in the extension/ flexion movement with no pause, the programming for the flexion when a pause was required should have occurred either during the execution of the extension movement or at the reversal (a combination of both is also possible). However, because there were no significant main or interaction effects for the dependent variable first movement time, it is unlikely that programming for the flexion movement occurred during execution of the extension movement. Therefore, the time spent at the reversal of the movement does appear to be a critical factor in determining the amount of programming a participant is willing to enter into prior to overall movement initiation. Similar to the suggestions made by Ostry (1980, 1983), Franks et al. (1985), and Rosenbaum et al. (1983), the inter-response interval is an ideal candidate for the locus of on-line control exhibited by participants during the production of a sequence of discrete movements (extension/flexion movements in the present study).

However, inferences about movement programming from simple reaction time (SRT) has its problems due to the imprecise nature of the dependent variable. It is possible to partial out the programming aspects of movement preparation from the mechanical aspects of movement initiation using electromyography (EMG). SRT can be fractionated into premotor and motor time components (Botwinick & Thompson, 1966). Premotor reaction time is the time from the onset of the imperative stimulus to the beginning of EMG and has typically been associated with delays in central programming activity. On the other hand, motor reaction time is the time from the beginning of EMG activity to the start of external limb movement. This period is believed to reflect the duration of nonprogramming events, such as the electromechanical delay in the muscle and development of sufficient torque to initiate movement (Anson, 1982). Thus the use of premotor time as an indicator of movement programming enables valid interpretations of SRT differences across levels of movement complexity (Anson, 1989; Christina & Rose, 1985). Furthermore, although the findings of this first experiment point toward the possibility that the reversal of the extension/flexion movement is the locus of on-line control when there is sufficient time available to program the flexion portion of the movement, the median pause time of 262 ms was well outside the limits of movement programming. As such, it is not surprising that participants chose to wait until movement reversal to program the flexion movement. Experiment 2

A second experiment was undertaken in order to address some of the issues that were raised in the first experiment. First, premotor reaction time was used to ensure that changes in SRT that were observed in Experiment 1 were indeed reflective of delays associated with central programming alone and not muscle mechanics associated with the movement. Second, the pause duration at the reversal was reduced to approximately 100 ms. It was expected, that under these conditions, the participants’ ability to program the flexion movement on-line would be restricted. This would cause them to adopt a programming strategy similar to that exhibited in Experiment 1 by the participants who were forced to make a continuous extension/flexion movement and thus increase premotor reaction time.

A repeated measures design was used in Experiment 2, in which all participants completed three movement conditions. A simple extension movement (E) was compared to two extension/flexion movements. The continuous extension/flexion movement with no pause at the reversal was termed EFC while an extension flexion movement that required participants to pause for less than 100 ms at movement reversal was named EFP. METHOD

Participants. Ten university students who had not taken part in Experiment 1 volunteered as participants for this experiment. Once again, all were naive as to the hypothesis being tested and were inexperienced at the experimental task. The particpants were paid 10 dollars for their participation in the study. The experiment was carried out in accord with the ethical guidelines of the University of British Columbia. Apparatus. In addition to the apparatus described in Experi ment 1, this experiment required a multichannel electromyography system (model 544, Therapeutics Unlimited Inc.) to measure the electromyographical activity of the biceps and triceps muscles. Electrical activity from the medial head of the right Biceps muscle and the lateral head of the right Triceps muscle was monitored using Ag/AgCl surface electrodes (8 mm diameter). The electrical signal from the two sets of surface electrodes was amplified by a multichannel electromyographic (EMG) system (model 544, Therapeutics Unlimited Inc.) and raw amplified EMG signals (maximum + 10 V) were sampled at a frequency of 1000 lIz and stored for subsequent analysis.

The flexion movement used in this experiment was 22.5 degrees and therefore the start position was also the final flexion target. Each of the 10 participants completed all three movement conditions (E, EFC and EFP) in separate blocks with the order of presentation of each block being randomized across participants. In Experiment 2, participants performed 15 trials in each of the movement conditions, 5 practice trials and 10 test trials. As in Experiment 1, participants were asked to make either: an extension movement (E); a continuous extension-flexion movement, consisting of an extension of 22.5 degrees followed by a flexion of 22.5 degrees (EFC); and to extend 22.5 degrees and pause before flexing 22.5 degrees (EFP). Participants in the EFP condition were encouraged to make a short (approximately 100 ms) and consistent pause at the reversal of the movement. This is in contrast to the more flexible pause time of “less than 500 ms” made by participants in Experiment 1. Finally, in Experiment 2, changes were made to the procedure because the occlusion of the PLATO glasses at specific time intervals during the trials (imperative stimulus) showed electrical interference (crosstalk) with the EMG signal. In this experiment, the disappearance of the stimulus and response cursors from the left side of the oscilloscope screen served as the imperative stimulus. Catch trials were presented 20% of the time and the procedure was the same as in Experiment 1. Less than 3% of trials were discarded due to movement during catch trials and only 6% errors were due to anticipation (RT 500 ms). EMC ANALYSIS

The raw EMG signals were first full wave rectified and then low pass filtered using a fourth-order zero-phase-shift Butterworth filter with a cut-off frequency set at 30 Hz. Following this procedure, the experimenter was presented with a raw, rectified EMG signal (inverted) and a raw, rectified and filtered EMG signal on the computer screen. The experimenter placed a cursor at the first indication of heightened EMG activity above the baseline for each raw, rectified and filtered EMG signal and compared the placement of the cursor to the raw, rectified profile. This method provided reliable inter- (coefficient = 0.86) and intra-observer (coefficient = 0.88) results and therefore was used to detect the onset times of muscle activation. Premotor reaction time was calculated by measuring the time interval between the onset of the imperative stimulus and the first sign of heightened EMG activity of the triceps muscle above baseline. Motor reaction time was calculated by measuring the time interval between the first sign of heightened EMG activity and the initiation of overt movement, as measured from the displacement data.

RESULTS AND DISCUSSION

The movement time and reaction time means and standard deviations are given in Table 2. A significant difference was found between the SRT of the extension movement condition and both of the extension-flexion movement conditions, F(2,18) = 8.6, p

Median pause duration between the extension and flexion movements in the pause condition in Experiment 1 (262 ms; SD 56 ms) was compared with the median pause duration for EFP in Experiment 2 (75 ms; SD 50 ms). If this duration was less than 100 ms, the entire extension/flexion movement had to be prepared in advance, similar to the EFC movement condition. The strategy employed by participants prior to movement is one of cognitive efficiency. That is, if sufficient time is available between response elements in a sequence of movements, then they program only the first element and deal with the programming of subsequent movements at a later time. However, in the EFP condition it was evident that the entire extension/flexion movement required programming, because the duration of the pause did not allow extra programming of the flexion movement. It appears that the limit of programming duration at inter-response intervals in this simple task lies between 75 and 260 ms. An interesting finding that arose from the first experiment and was also replicated in the second experiment was that longer movement sequences were not associated with longer reaction times. For example, the consequences of adding the flexion movement in both EF conditions was not only to increase the complexity of the movement but also to increase total movement time. As discussed earlier, several researchers (Klapp, 1981; Klapp & Erwin, 1976; Quinn et al., 1980; Siegel, 1986) have concluded that there is a direct relationship between total movement time and reaction time, and differences observed in reaction time are probably due to programming movement duration. In this experiment, total movement time for completing the EFC condition (mean = 580 ms) was significantly less than the EFP condition (mean = 657 ms), F(1,9) = 22.65,p

General Discussion

The effects of varying response complexity have been reported for a SRT task involving extension and flexion movements at the elbow. When the number of elements in a movement sequence were varied, complexity affected the central programming time as measured by premotor reaction time. These results are consistent with Klapp’s (1996) explanation of the response complexity effect for protocols involving simple as opposed to choice reaction time. Klapp proposed that the increase in SRT can be attributed to the time taken for the additional process of sequential ordering of the two movements. This process is not required when programming a single response element (e.g., extension movement). In the present experiment the extension movement remained invariant across all conditions (both extension and extension/flexion) and therefore, the process of retrieving, selecting, and computing the appropriate movement parameters for this first movement was held constant and did not affect SR’I’. However, this was only the case when time at movement reversal did not allow for on-line control of the upcoming flexion movement (Experiment 2, EC and EFP conditions). When the pause duration in the extension/flexion movement was less than 100 ms, the “to be produced movement” was considered a sequence composing two elements. These results are in contrast to those of Experiment 1 (see footnote 1 for a description of the differences in the two experiments) in which SRT was not affected by variations in the number of response elements (extension/flexion pause condition). In this experiment, when the pause duration at movement reversal was extended to approximately 250 ms, the response complexity effect was not evident. Furthermore, the absence of any differences in extension movement times between all conditions appears to rule out the possibility that participants were engaging in some form of on-line control during the execution of the first movement. This was somewhat to be expected due to the fact that all movements were made in the absence of vision.

It is our contention that participants used the extended time at reversal to program the subsequent flexion movement and therefore negated the need to sequentially order the two movements prior to movement initiation and after the onset of the imperative stimulus. In fact, it is likely that the participants viewed the task as the production of two separate movements and not a sequence of movements. Unfortunately it is not possible to determine the exact nature of the processing completed during the pause duration. Further studies will be required to determine what internal features (Klapp, 1996) of the flexion movement are being programmed at movement reversal. This research was supported by a grant from the Natural Sciences and Engineering Research Council of Canada awarded to Ian M. Franks.

Please send correspondence to: University of British Columbia, School of Human Kinetics, University of British Columbia, 6081 University Blvd, Vancouver, B.C. Canada, V6’T IZ1. Email address ifranks(unixg.ubc.ca

There were several differences between the two experiments that should be acknowledged. These differences include: the design of the two experiments (Experiment 1, randomized group; Experiment 2, repeated measures design); the change in imperative stimulus (Experiment 1, occluding spectacles; Experiment 2, withdrawal of visual display from oscilloscope); and the magnitude of the flexion movement (Experiment 1, 11.25 degrees; Experiment 2, 22.5 degrees). We do not believe these differences would have differentially affected the dependent variables that were compared across the two experiments. The number of acceptable trials completed by participants in both experiments were equivalent (30 trials) and should not have been subject to any practice effect. On the whole SRT was faster in Experiment 2 than in Experiment 1 (e.g. extension movement SRTs were 189 ms vs. 215 ms) possibly due to the change in the nature of the stimulus. However, there was no reason to believe that these changes had a differential effect across independent variables within experiments. The increase in movement flexion of 11.25 degrees would not be expected to affect SRT or first movement time, but would obviously increase the total movement time for the extension flexion movements in Experiment 2. This dependent variable was not compared across experiments.

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