The role of vision in repetitive circle drawing

acta psychologica

ELSEVIER Acta Psychologica 92 (1996) 105-118

The role of vision in repetitive circle drawing

Howard N. Zelaznik *, Dawn Lantero Department of Health, Kinesiology and Leisure Studies, Purdue Uniuersity, West Lafayette, Indiana, 47907.

USA

Received 1 July 1994; revised 7 January 1995; accepted 10 February 1995

Abstract

In the present experiment the role of vision in the control of repetitive circular movements was examined. Subjects drew circles at a 600 ms per circle rate. During the first nine seconds of the trial subjects moved with full vision and were paced by a metronome. During the latter 15 seconds, vision could be removed and/or the pacing signal could be removed. There were no effects of the pacing signal on the temporal and spatial characteristics of the circle. Withdrawal of vision did not affect the shape of the circle, but did change its scaler quality. The circles became smaller and the center drifted in a systematic fashion. Furthermore, the loss of vision produced an increase in variability in the circle shape, size and location. It is clear that in a simple task such as circle drawing, vision serves not as a source of information about form, but to maintain a stable and consistent form.

PsyclNFO classification: 2330

Keywords: Vision; Motor control; Continuous tasks

1. Introduct ion

Vision clearly has an important role in the control of movement. In everyday tasks

such as intercepting objects or in impending collision with a surface, vision via the optical parameter tau determines the timing of many actions (Lee et al., 1982). In laboratory tasks, vision has been shown to have an important role in the control of pointing, aiming and reaching movements (see Jeannerod, 1988). In research on all of

* Corresponding author. E-mail: [email protected], Tel.: + 1 317 494-5601.

0001-6918/96/$15.00 © 1996 Elsevier Science B.V. SSDI 0001-6918(95 )00007-0

106 H.N. Zelaznik, D. Lantero / Acta Psychologica 92 (1996) 105-118

these tasks the role of vision can be considered a metrical one. Vision serves to coordinate the actions of the performer with the environment, but does not appear to be involved in the specification of the form of the movement.

The distinction between the form of a movement, sometimes called the structural prescription, from the scalar qualities of a movement, called the metrical prescription (Turvey, 1977), is a recurring theme in motor control. This distinction serves as the basis for the notion of generalized motor programs (Schmidt, 1975, 1988) as well as the search for invariant aspects of motor control (see Wright, 1990). Recent research on handwriting and drawing movements have examined the role of vision in the production of letters and movement shapes. In this literature the shape of the movement, or its pattern of motion, has been called the morphocinetic aspects of movement, while the size and location of the motion are called the topocinetic aspects (Teasdale et al., 1993). In the present paper we ask questions about the role of vision in controlling the pattern of motion.

Theories conceming the control of movement state imply that the spatial-temporal structure of a movement is driven by central processes. Information processing theorists would posit that these central processes are motor programs, while other accounts would deny that there exists a central representation but would not deny that there are endogenous processes that produce form and structure to a movement (see Stelmach and Requin, 1992, for a recent set of papers). How might vision operate within this type of framework? Based upon research from the handwriting and drawing domain it seems clear that vision should serve to scale the aspects of the motion but not to determine its form. For example, Teasdale et al. (1993) show that a deafferented patient can produce cursive letters and ellipses of the same quality as non-deafferented control subjects, but that the scalar aspects of their movements appear to be affected by the loss of proprioception. Furthermore, the form, i.e., morphocinetic aspects of their movements, was not affected by the withdrawal of vision. In models of handwriting production, it often is posited that the processes of letter selection are independent from the processes of letter size and duration (see Van Galen and Teulings, 1983; Van Galen et al., 1994).

In the present study we were interested in examining the persistence of the form of a well-known shape, produced repetitively when timing guidance and /or visual information was withdrawn. If in fact the processes producing the spatial and temporal pattern of a movement are driven largely by endogenous processes, then the withdrawal of visual information might alter the scalar, i.e., topocinetic, aspects of the movement but leave the structural, morphocinetic, aspects unaffected.

The task chosen was circle drawing. We chose a shape that would be very easy for subjects to produce and probably would not require much learning by the subject to perform the task well. We did not want to choose a difficult task because the effect of withdrawal of visual feedback might be due to the fact that subjects have not yet developed an intemal control structure to govern the movement. Although we had no real a priori hypothesis for the role of timing guidance, we were curious as to the nature of visual control when an external timing signal was present or absent. Many views of skilled motor performance stress the importance of timing processes (Hancock and Newell, 1985; Keele and Ivry, 1987), and as such, the role of temporal guidance in trajectory control of repetitive circles drawing was examined.

H.N. Zelaznik, D. Lantero / Acta Psychologica 92 (1996) 105-118 107

2. Methods

2.1. Overview

Subjects drew circles to the beat of a computer-driven metronome. Each circle was to be completed 'on the beat' such that the cycle time was 600 ms. On every trial there were 16 pacing signals, producing 15 circles (assuming that the subject was in rhythm with the metronome) followed by a time interval where the subject was to continue to draw circles at the same pace. On half of the trials the metronome was not engaged during this time. On the other half of the trials the pacing signal would stay on for an additional 26 beats. Vision also was manipulated after the first 16 pacing signals of each trial. Subjects wore liquid crystal goggles (PLATO goggles, Translucent Technology). When a current was passed through these glasses, the goggles were 'clear' and subjects had normal visual information. When the current was turned off, the goggles would become 'opaque' such that the same amount of light reached the retina, but the light was scattered. The phenomenon is very much like being in a dense fog. Thus, subjects could not see their hand, nor the pattern of motion produced on the desk. Thus, on half of the trials the goggles remained clear during the latter 25 movements and on the other half of the trials the goggles became cloudy. The manipulation of the pacing signal was independent from the manipulation of vision. Thus, there were four distinct trial types, which were distinguished by what occurred after the first 16 paced movements with the goggles clear. These trials were clear paced, clear unpaced, opaque paced, and opaque unpaced.

The trajectories of the circles were analyzed for their shape, i.e., how circular they were, by utilizing the circle-ratio metric developed by Franz et al. (1991). On a cycle by cycle basis the diameter of the trajectory along the x and y dimensions of a Cartesian coordinate system was computed. If the subject drew a perfect circle for that cycle, the ratio of the shorter diameter to the longer diameter would be 1.0. Changes in the value of the ratio measure reflect changes in the shape of the circle and changes in the length of each diameter reflect changes in the scale, i.e., topocinetic aspects of the circle. Although it is true that the ratio metric is determined by the diameter of a circle, the size of any diameter does not determine the value of the ratio. The ratio is independent of the average size of any circle. We have defined the measure of shape as the ratio of the diameters. If only one of the two diameters changes size, then of course, the circle becomes more elliptical and of course changes its shape.

We also were interested in the location of the circle. To measure this aspect of performance, the center of each circle was computed.

2.2. Subjects

Twelve right-handed undergraduate students (eight female and four males) volun- teered to be subjects. All subjects had normal or corrected to normal vision with contact lens. The requirement that subjects wear the Plato goggles precluded subjects from wearing corrective eye glasses.


2.3. Apparatus

A 79 cm high desk served as the table for the drawing movements. A sheet of white computer printer paper (38.1 x 21.6 cm) was secured near the center of the desk top. A Mars-Staedtler lead holder with 2 mm, 2H hardness lead served as the subject's writing instrument. The lead holder was wrapped in white adhesive tape to improve the comfort of the subject gripping the instrument. An infrared light emitting diode (IRED) from a Watsmart kinematic recording system was attached near the tip of the lead holder. During the entire data collection the subject wore the Plato goggles. The speaker from a Compaq 386/16 computer provided the metronome signal. This computer also con- trolled the computer running the Watsmart system.

2.4. Task

The task was to draw circles about 12 cm in diameter in rhythm to a metronome. The metronome period was 600 ms. The subject was instructed to keep the circles 'on time' and to keep the circles consistent in terms of shape and size.

2.5. Procedure

Upon entering the laboratory and being seated the subject was provided with instructions and a demonstration of the operation of the metronome and liquid crystal goggles. After providing informed consent the testing session began.

A trial commenced with the subject initiating a circle drawing movement upon command of the experimenter. The Watsmart system was then engaged and 1 second later the metronome pacing signal was initiated. Sixteen beats were produced (producing 15 paced intervals). After the sixteenth beat the goggles could become opaque and /or the metronome could be disengaged. The subject's task did not change. They were to continue drawing these circles as though the metronome remained on and to keep the circles the same size and in the same location as in the earlier portion of the trial. The trial ended when the metronome produced five 16 ms in duration 50 ms cycle time beeps. The experimenter then replaced the drawn upon paper with an unmarked clean sheet of paper and the subject checked to see that there was still enough graphite exposed in the lead holder. The interval between trials was about 30 s.

Subjects performed 24 trials, six in each condition for two sessions. The time between sessions was usually two days, but never more than four days. For scheduling reasons each testing session was to be no more than 40 minutes of total time (including set up time for each subject) and because we required data from more than six trials within each condition subjects were tested over two sessions.

The type of trial (clear paced, opaque paced, clear unpaced, opaque unpaced) was randomly chosen with the restriction that each trial type occur only three times within the first 12 trials and within the second 12 trials of each session. The subject was not aware of this restriction, but was told that the type of trial following the first set of clear-paced movements was chosen at random by the computer.


2.6. Data collection and reduction

The location of the IRED attached to the lead holder was sampled at 256 Hz. After conversion to three dimensions, the x and y dimensions of the IRED were converted into a format suitable for Matlab tm, a mathematical toolbox and programming environment. Within this environment the displacement data were filtered with an 8 Hz, 5th order, Butterworth filter in the forward and backward directions. The filtered data were differentiated via a 3-point central difference technique. For each trial the kinematic record was t r immed to capture the 9-s interval for the clear-paced movements, as well as the following 15-s interval in which the manipulation of the two independent variables (vision and pacing signal) occurred.

2.7. Design

There were two main independent variables in the present experiment. Vision (goggles clear or goggles opaque) and pacing (paced or unpaced). These two variables were manipulated after the first 9 s of a trial. In our statistical analyses, we analyzed the effects of these variables within days (1 and 2), trials (1 to 6). In general there were no interactions between days, trials and the pacing and vision variables. We only report interactions between days, trials and the variables of interest when they were significant and more importantly, meaningful.

3. Results

The tr immed 24-s trial data were blocked into 3-s intervals. If a subject were exactly on time with the metronome and continued that behavior during the latter 15 seconds, 40 circles, or five circles within each 3-s interval, would have been drawn. ~ The ratio of the two diameters, the center and the cycle duration was computed on a cycle by cycle basis and then averaged within each interval (see Franz et al., 1991, for further details). Consistency in the circle trajectory was ascertained by analyzing the within-subject standard deviation for each of these variables within each 3-s interval. We also computed the center of each circle in relation to the x and y center of the entire record. This computation is described in detail in a subsequent section of the paper.

In Figs. 1 and 2 a set of typical trials from one subject on the first day of testing is presented. The first of the four panels in each set represent the x - y trajectory of the pencil during the first 9 s during which the goggles remained 'c lear ' and the circles were ' paced ' by the metronome. The latter three panels (s 10-14, s 15-19, s 20-24)

It was important that there were an equal number of cycles per interval, so that the estimates of means and standard deviations would not be influenced by the number of cycles across conditions and intervals. Although this method made it much easier to conduct statistical analysis, it is true that interval three, for some subjects who on average moved slower than 600 ms per circle, will contain movements in which the metronome already was disengaged and/or the goggles. Given that this study is not conducting a fine-grained temporal resolution of the effect of vision, the small problem between interval three and four was not a major concern.


500 / 1 St

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3rd

0 500

500 subj. 5 day 1 tr 1

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500

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500 500

Fig. 1. A typical trial for one subject in the opaque paced and opaque unpaced conditions. Each panel plots the x - y trajectory (in mm) of the pencil for the time period specified. The top four panels represents performance in the opaque-paced condition and the bottom four panels represents performance in the opaque-unpaced conditions. Printed over the third and fourth panels of each set of four are the file name and the subject, day and trial information. Seconds 1-9 in all trials were performed with full vision and the metronome engaged. The type of trial was manipulated over seconds 10-24. Please note that we used a 1/1 aspect ratio for each panel, so that if the subject were producing a circle, the graph would show a circle.

represent the next 15 s o f the trial in which the goggles could have b e c o m e opaque

a n d / o r the pacing signal could have turned of f ( ' unpaced ' ) . As can be seen in both

figures, the c i rc le was not affected in a major qual i ta t ive fashion when the m e t r o n o m e

turned of f or when the goggles b e c a m e opaque. In addit ion, there does not appear to be

a large ef fec t o f the combina t ion o f the loss o f the pacing signal and the loss o f vision.

H.N. Zelaznik, D. Lantero /Acta Psychologica 92 (1996) 105-118

500 500 opoque poced

500 500

111

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4th

0 500 500

opoque unpoced

500 0 /1 st ,500 i 0 12nd

500 500

500 cilxyO02 500 subi" 5 doy 1 tr 1

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Fig. 2. A typical trial for one subject in the clear-paced and clear-unpaced conditions. Each panel plots the x - y trajectory (in mm) of the pencil for the time period specified. The top four panels represents performance in the clear-paced condition and the bottom four panels represents performance in the clear-unpaced conditions. Printed over the third and fourth panels of each set of four are the file name and the subject, day and trial information. Seconds 1-9 in all trials were performed with full vision and the metronome engaged. The type of trial was manipulated over seconds 10-24. Please note that we used a 1 / l aspect ratio for each panel, so that if the subject were producing a circle, the graph would show a circle.

3.1. Temporal analysis

T h e ave rage du ra t ion o f a c i rc le was c o m p u t e d as the t ime in te rva l b e t w e e n the y

d i m e n s i o n zero ve loc i ty c ros s ing n and zero ve loc i ty c ross ing n + 2. It was c lea r f rom

this ana lys i s tha t sub jec t s m a i n t a i n e d the p r o p e r ave rage cyc le dura t ion in all cond i t ions .

In the gogg l e s c lea r cond i t i ons the ave rage c i rc le dura t ion was 594 m s and in the

gogg le s o p a q u e cond i t i ons the ave r age c i rc le du ra t ion was 590 ms, F ( 1 , 1 1 ) = 8.10,


p < 0.02. This small difference is similar to the small temporal effects we have previously observed in discrete aimed hand movements when vision is removed (Zelaznik et al., 1983). There were no other main effects nor interactions between the visual manipulation, the pacing signal and days or trials.

In terms of temporal consistency, the overall within-subject standard deviation in timing was about 16 ms. This is about 2.5% of the mean interval. In other words, our subjects were very precise at producing the mean interval duration. On Day 1 subjects exhibited about 1 ms more variability than on Day 2, F(1,11) = 11.39, p < 0.01. The first interval, on each day, was 2 ms more variable than any of the other intervals, F(7,77) = 12.29, p < 0.001. Subjects required a few cycles to get in rhythm with the metronome and therefore they were more variable over the first 3 s. There was an interaction between the interval and the pacing variable, F(7 ,77)= 5.24, p < 0.001. During the first three intervals, consistency was about the same between the paced and unpaced trials. This should be the case because the first three intervals always were paced and the goggles always were clear. Over the final five intervals the temporal performance in the unpaced condition was more consistent (2 ms) than in the paced condition. The increase in variability for the paced intervals reflects the subjects' attempts to be coincident with the beat, while in the unpaced conditions the slight reduction in variability probably reflects a more autonomous level of control.

3.2. Spatial analysis

3.2.1. Circle shape Fig. 3 depicts the average ratio values for the four conditions over the eight 3-s

intervals. Clearly there were no differences in the shape of the circles, as indexed by the ratio measure, that were related to the pacing signal, F(1,11) < 1, nor due to the vision manipulation, F(1,11) = 1.50, p > 0.05. The only significant effects were that of the

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Fig. 3. Average value of ratio measure for the four conditions, across the eight, 3-s intervals. Intervals 1 through 3 represent the first nine seconds in vision was afforded and the pacing signal was engaged.


interval, F ( 7 , 1 1 ) = 13.06, p < 0.001. The value of the ratio measure decreased from 0.89 to 0.86 over the 24 s that were analyzed. Finally, there was an interaction between trials and the pacing variable, F(5,55) = 3.40, p < 0.05. On the first trial, the unpaced condition had a slightly larger ratio, 0.88 compared to 0.86 for the first trial of the paced condition. Given the small nature of these effects they were not pursued with any additional statistical zealousness. 2 Based upon the ratio measure, it is clear that the average shape of the circle was unaffected by the withdrawal of the pacing signal and the withdrawal of vision.

There were small effects of vision on the consistency of the shape of the circles as measured by the within-subject standard deviation in the ratio measure. First, there was a small but reliable difference between the clear and opaque conditions, F(1,11) = 7.45, p < 0.05. The clear conditions exhibited a within-subject standard deviation of 0.044 and the opaque conditions exhibited a value of 0.047. The variabili ty of the ratio measure increased from 0.042 for the first 3-s interval to 0.049 on the last 3-s interval, F(7,77) = 9.32, p < 0.001. There was an interaction between days and trials, F(5,55) = 6.33, p < 0.001. This interaction was the result of the much greater variabili ty for the first trial in each condition on the first day compared to the second day and other trials. None of the other interactions were significant.

3.2.2. Circle location

Because each individual subject centered his or her circle a little differently, a coordinate system based upon the movement of each subject within each trial was determined. Over the entire 24-s record the mean values of the x and y dimensions of movement were computed. The averages were subtracted from each and every sample value for the entire 24-s record. Thus, the origin of this new coordinate system was the 'grand center ' of all of the circles. On a cycle by cycle basis, the center of each circle was computed. The difference in each dimension of the center of the circle for that cycle and its grand mean value was computed. This measure we call the center error. If, on a particular trial the center of the circle did not change much, or changed randomly, then the center error would be close to zero for the interval and the plot of the center error measure over the 8 intervals would be a straight line with a slope of zero.

On the other hand, a systematic drift in the center of the circle would appear as a straight line with a positive or negative slope. Because the center error is computed with respect to the average center for the entire 24-s record, the entire record will exhibit the trend in the center error measure. In Fig. 4 we present the results of these calculations. In the y dimension a positive value for this score indicates that the center moved toward the subject in the anterior-posterior dimension. In the x dimension a positive value signifies a movement toward the left.

2 We did not rely solely upon these statistics and numerical computations to determine the importance of these effects, or lack thereof. Every trial was plotted and inspected to determine whether the circle ratio would be a fair indicant of the shape of the trajectory. For all trials across all subjects in all conditions, we are confident that ratio measure is capturing the basic shape of the trajectory.


o o o

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150

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1 2 3 4- 5 6 7 8

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Fig. 4. Center error for the four conditions, across the eight 3-s intervals. Intervals 1 through 3 represent the first nine seconds in vision was afforded and the pacing signal was engaged. The top panel depicts the center error in the y dimension and the bottom panel represents center error in the x dimension.

For example, examine the y dimension (top panel of Fig. 4). One might expect that over the first three intervals, when all subjects produced circles with the metronome engaged, their center errors should be the equal. However, because the opaque conditions versus the clear conditions during intervals four through eight produced opposite movements in the center and because the center error is derived from the difference in the grand average, it seems as though the clear versus opaque conditions started off differently. They did not. A gradual shift in the center of the circle toward the subject in the clear conditions, but an abrupt shift away from the subject in the opaque conditions was observed, as indicated by the cross-over interaction between vision and interval, F(7,77) = 2.81, p < 0.05.

In terms of consistency of the center error measure within an interval, the clear conditions were more consistent than the opaque ones, F(1,11) = 10.22, p < 0.01, and the interaction between vision and interval for the variability measure just failed significance, F(7,77) = 2.13, p < 0.06. The trend was for the variability conditions in which vision was removed to increase during intervals four through eight. There were no other main effects or interactions for the variability measure that approached significance.

In the x dimension the picture was not as clear although the pattern of results was similar to the y dimension. The center of the circle moved to the right when vision was taken away (opaque) and the x center moved to the left during the full vision (clear)

H.N. Zelaznik, D. Lantero /Acta Psychologica 92 (1996) 105-118 115

trials. However, the effect of vision, interval and the interaction between the two was not significant. In this dimension that there was an effect of withdrawal of vision, but only in the unpaced condition, F(7,77) = 3.10, p < 0.01. The consistency of the center was affected by day, F(1,11) = 7.8, p < 0.05. On Day 1 subjects exhibited an average within-subject standard deviation in the center error measure of 0.33 cm and on Day 2 this value was 0.30 cm. Subjects were less consistent on trial 1 than on the other trials, F(5,55) = 5.83, p < 0.01. Subjects were less consistent in the opaque conditions, F(1,11) = 25.74, p < 0.001, and this inconsistency grew over intervals when vision was withdrawn, F(7,77) = 3.83, p < 0.01.

3.2.3. Circle diameter The diameters of these circles in the x and y dimensions are presented in Fig. 5.

These results are clear and consistent in both dimensions. The diameters of the circles becomes progressively smaller as the subject spent more time in the absence of vision (opaque conditions). The interaction between vision and interval was significant both in the y dimension, F(7,77) = 44.90, p < 0.001, and in the x dimension, F(7,77) = 51.66, p < 0.001.

In terms of within-subject variability there was a significant interaction between vision and interval for the x dimension, F ( 7 , 7 7 ) = 2.32, p < 0.05, but not in the y dimension, F(7,77) = 1.30, p > 0.05. In the x dimension, there was a large increase in variability for interval three, when vision was going to be withdrawn. The reason for this

13

o o clear paced e - - e clear unpaced

z ~ - - A opaque paced A - - A opaque unpaced

E 0 g

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(1) 12

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9

8

7

I I I I l I I I

1 2 3 4 5 6 7 8

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1 2 3 4 5 6 7 B

Fig. 5. Diameters of the circle for the four conditions, across the eight 3-s intervals. Intervals 1 through 3 represent the first nine seconds in vision was afforded and the pacing signal was engaged. The top panel depicts the y dimension and the bottom panel represents the x dimension.


large increase was the result of several subjects moving too slowly to the pacing signal, so that the goggles became opaque before they completed the first 15 cycles of movement.

4. Discussion

The results of the present experiment allow us to infer that visual information is not crucial in the maintenance of the overall spatial-temporal pattern of a well-learned, simple or autonomous movement. Furthermore, vision had small but reliable effects on the scaling of the autonomous movement, in terms of the size of the circle as measured by the diameters and the location of the circle center as indexed by the center error measure vision was important. However, vision is clearly important in terms of the precision of movement. When the goggles became opaque, subjects became less consistent in the ratio of the circle diameters as well as the center of the circle.

Newell (1985) has discussed the differences between coordination and control. Coordination can be thought of as an equation of constraint between various effector systems, either at the joint level or at the trajectory. Circle movements can be described as a constraint between two oscillatory processes driving the pencil trajectory in the x and y dimensions. The one possible constraint between these oscillatory processes would be a 90 degree phase difference between the two oscillations. Once these proposed oscillatory constraints are established, perhaps with vision, the average value is not determined by visual processes. If such were not the case, there would have been an effect on the shape of the circle as indexed by the ratio measure of circularity.

Vision appears to be used in the regulation of control which involves the setting of the appropriate parameters on the coordination function. In the present study control would be the setting of the amplitudes of the oscillatory processes responsible for driving the pencil in each dimension. We cannot determine in the present study whether vision is utilized to continuously regulate the circle size and location, or if vision is used in a more intermittent fashion to keep the circle on its course.

This experiment does not allow us to infer the role of vision in the setting up of the trajectory. Subjects began their movement with full vision on all occasions. Work by Elliott has shown the importance of vision for setting up and planning aimed hand movements (Elliott et al., 1990). Additional work is necessary to determine whether in well conceived shapes, vision is necessary to set up the coordinative processes.

These results cannot be generalized to the drawing of other shapes. The circle is a shape that is well known to subjects and although it might not have been extensively practiced by our subjects, as their handwriting might be, the circle is clearly a trajectory that can be considered as autonomous. If subjects were given a more difficult shape to produce we expect that vision would serve a role in the maintenance of its shape.

Recent work in trajectory formation has posited that curved movements can be conceptualized as being driven by a set of small, but linear, movement vectors that are concatenated to form a 'curved' trajectory (Lukashin and Georgopoulos, 1993). The present experiment does not address this issue directly, but we believe does provide evidence not in support of this framework. If circles were produced by a set of little


straight line trajectories and if vision were used to aid in the curving and /or bending of the series of movements, then withdrawal of vision would have resulted in change in shape. There was no qualitative change in shape. The ratio measure as well as visual inspection of every trial lead us to conclude that the shape was unaffected by vision. We believe that these data are problematic for a piece-meal trajectory formation approach.

It is interesting that the temporal guidance of the metronome had almost no effect on the temporal and spatial aspects of the circle task. Previous work on timing shows that individuals can in fact maintain a rhythm well after a pacing signal has been utilized to set up the timing task (see Franz et al., 1992; Keele et al., 1987). However, in those tasks, the subject has no spatial demands placed upon them and the spatial aspects of the tapping tasks studied in those situations were not very demanding. The lack of an effect of pacing signal withdrawal on either temporal or spatial parameters provides consistency for the notion that space and time are linked in motor control (Hancock and Newell, 1985).

These results support the independence of form and scale in drawing movements. Vision had reliable effects on the scale of these movements, but did not affect the form. For more complicated coordinated movements vision might be important early in practice to establish the coordinative processes that determine form. After considerable practice, vision would be used to maintain the topocinetic aspects of a movement trajectory, but vision is not as important to maintain the morphocinetic aspects of a trajectory.

Acknowledgements

Special thanks to A. Brock, H. Burkle, B. Fritz, C. Mikesell and L. Silence for data collection and analysis.

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