Discrete vs. continuous visual control of manual aiming

393

and R Chua, 1991. Discrete vs. continuous visual control Science l&393-418. criments were conducted to

of instructional set and vision on the and end-point accuracy of a simple target-aiming movement. While instructional se’! o&y md acceleration patterns of the movements, the availability of t was the best predictor of accuracy. Although subjects were than in two visually degraded conditions, subjects made ovement trajectory. These data suggest that the visual . g may occur in a continuous fashion. examine

the

influence

(1899), it has been clear ioneering WQ vision plays a major ion of goal-directed limb odels of limb coc*r01 have emphasized nts. Convention ee of vision for discrete ceria-’ red xPkm (e.g., Crossman 1963/1983). Specifically, visua ink;nnation about the

Since the at

research was supported by the Natural Sciences ad Engineering Research Council of . We thank Linda Robinson for her help with experiment 2 and Clifford Storlund for his programming skill. Requests for reprints should be sent to D. Elliott, %fotor Behaviour Laboratory, Dept. of ucation, l&Master University, Hamilton, Ontario, Canada L8S 4KI. Physical 0167-9457,‘91/‘$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

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of the limb relative to a target signals s view of th once detecte , can be reduce at least two iffering mo els of the role of vision movements. ultiple correction m

ent. Since it t tion movements are assu thus generally more accurate. An alternative ex lanation of the spee -accuracy relations oses that only a single correction occurs lton 1979; owarth et al. 197 the modification depends on the limb to the target at the time the correction is made. t for longer duration movements the li en this occurs. Thus the constraint on movem proximity of the limb to the target at the time of t not the number of modifications. Support for both the single (

stments sometimes

occur, little is

own about the role

scarce. n a recent study em laying a wrist rotation tas manipulated visual feedback by eliminating a cur t on a corn to move to an eve that secondary s oveLments occurre 1 Subjects in this work were not permitted to see their hand.

r et al. (19

D. Elliott at al. / Visocalcmtrol

395

s,unless t

e index of difficulty (Fitts 195 was very low. Furthermore, there was no fference in the frequency of submovements feedback and no feedback conditions, in spite of an vantage for the former. of the two studies reported here was to examine the role control of a more traditional target-aiming task (cf. d to move a stylus from a ante away. Vision was manipus on or turning them off upon movement s well, we included a condition in which the room lights were extinguished from 2 s rior to movement initiation until the completion of the movement. s latter condition was employed since indicates that reasonably precise ent environment may persist for a 1988; Elliott et al. 1990; on delay prior to pointing r this information to decay. As well as manipulating vision; we varied instructional set by asking hasize either speed or accuracy. This manipulation was an attempt to determine how subjects’ strategies for achieving a particular goal t vary with the three visual conditions. By changing instructional set, we were also able to exercise indirect control over movement time while avoiding the pitfalls associated with a dual-task situation (cf., Elliott and adalena 1987; ele and Posner 1968; Zelaznik et al. 1983). 2 Our aim was to determine the manner in which the kinematics of aiming movement vary as a function of visual condition and the -t. Iate,rmittent correction mod& of perceived goals of the movemWyA movement control, emphasizing the important of vision (e.g., et al. 1931; Keele 1968, 1981) predict both a greater frequency of adjustments and greater terminal accuracy in full vision conditions than in situations in which vision is occluded. * Rather than manipulate movement time directly by having subjects practice making movements of particular durations prior to the experiment (e.g., Elliott 1988; Zelaznik et al. 1983), we decided to manipulate time indirectly by varying instructional set. We adopted this approach because requiring subjects to produce a movement of a particular duration, while maintaining accuracy, essentially puts the subject in a dual-task situation. Since specific attentional requirements could vary with vision condition, the dual-task nature of that approach could impact on the independent variable of primary interest-vision.

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In inferring the presence of adjustments from the kinematics, discontinuities can be identified from the acceleration profile (e.g., Brooks 1974; Crossman and Goodeve 1963/1983). Following an intermittent correction model, the frequency of these discontinuities is expected to increase with increasing demands for accuracy, provided vision is available to mediate the comparison of limb and target positions. interest was whether or not the presence/frequency of adjustments would also increase with accuracy demands in no vision conditions. was our expectation that these data would provide us with information pertaining to the relative contributions of vision and kinesthesis in the closed loop control of aiming. In this paper we rep rt two experiments that address these issues. entical, except that in experiment 1 visual ation of information about target position, while in experiment 2 a small dot of phosphorescent paint allowed subjects to see the target when all other visual information was eliminated. Elsewhere we have demonstrated that visual target information is particularly important for accuracy when vision is occluded prior to movement initiation (Elliott and adalena 1987). ur expectation here was that across experiment comparisons would allow us to examine the relative importance of visual information with respect to botfn the limb and target in determining movement accuracy, and the means of achieving that accuracy through discrete modifications of the movement trajectories.

Method Subjects

Subjects were ten, experiment naive, male (8) and female (2) volunteers, each of whom were paid $7.00 for their participation. Individuals were all classified as right handers on the basis of the andedness Inventory ( Idfield 1971). -411subjectshad normal or corrected-to-normal vision. Apparatus for data collection

Subjects were seated at a table upon which was secured a digitizing tablet (Chalkowerpad, X Y resolution 2.2 mm). A

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starting position platform was located directly in front of the subject, while the target itself was located 40 cm from the start position on a virtual line extending directly forward from the subject’s midline. The target was defined by the intersection of 15 mm horizontal and vertical lines aligned with the X and Y axes of the tablet. Subjects’ task in this study was to make a target pointing movement with a stylus from the starting position to the target. All movements were recorded using a ion analysis system. A membrane microswitch was p f the stylus. A rubber coated metal tip was mounted upon the microswitch. The tip was cylindrical and mea mm in height with a diameter of 2 mm. A single WATSMART (infra-red emitting diode) was attached 7.5 mm above this tip. e stylus was covered with a tape wrapping just above the emitting diode which also served to indicate the part of the stylus Two W RT cameras were positioned 1.8 m apart, 2.5 m from the groun dicular distance 2.1 m from the display panel with the respective viewing fields converging between the subject and the digitizing tablet. During practice trial blocks, it was verified that the stylus tip could be ‘seen’ by both cameras when located at the starting position and at the termination of the movement. The ATSMART system was calibrated immediately prior to each experimental session. 3 The stylus microswitch and the digitizing tablet were inter an Apple IIe microcomputer, in which was installed a ardware Apple Clock for millisecond timing and control. When the stylus was placed upon the starting platform, the microswitch was in a closed position. On leaving the starting position, upon movement ini&ttion, the status of the switch was reversed, and remained SO until the subject made contact with the surface of the digitizing tablet. Upon contact with the digitizing tablet movement time as well as the X and Y coordinates of the stylus tip were recorded. he X and Ycoordinates of the stylus tip along with target position were used to calculate radial error, and algebraic error in the X and Y dimensions.

3 The camera positions were calibrated using a manufacturer supplied steel calibration frame upon which 24 IREDs were permanently mounted. As the frame has known dimensions it serves as reference across experimental sessions. The calibration error for camera positions was in all sessions < 1 mm.

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microcomputer generated an external trigger to initiate and terminate the collection of data by the WATS system. A E A-D unit was used to record the single channel of a change of status of the stylus microswitch which occurred wh.en the subject initiated a movement and the subsequent change of status, when the stylus tip matte contact with the tablet. In all trials, data were sampled at 400 z and were saved to disk. Ambient illumination within what was an otherwise completely dark, black walled room, was provided by a custom modified single fluorescent lamp. 4A control device allowing almost instantaneous offset of the lamp ( c 25 ms) was interfaced with the microcomputer and was triggered by a logic us the lam could be extinguished prior to vision, dela condition), upon movement movement initiatio initiation (no vision ion), or the lamp could remain on throughout the course of the movement (full vision condition). or conditions in which the lamp was ex guished, the lamp was re-illuminated upon movement termination. hus subjects were permitted to view the end result of their movement in all visual conditions. An

Apple

IIe

The design of the experiment involved a factorial arrangement of two instruction conditions (fast: movements to be made as rapidly as possible, accurate: movements to be made as accurately as possible) and three visual conditions (fuil vision, no vision with a O-s delay and no vision with a 2-s delay). There were two blocks of 10 trials for each six factorial combinations, the order of block presentation being completely randomized for each subject. Test sessions were preceded by a block of eight practice trials (4 fast, accurate) for which vision was available. All trials began when the subjects placed the stylus upon the starting platform. Upon the experimenter’s keypress command, a microcomputer-generated tone signalled that, for full vision and O-s delay conditions, subjects could initiate the movement in their own time. In the O-s dc!ay condition the room lights were extinguished upon movement initiation. In the delay condition the room lights were extinguished when the subject indicated that he/she was ready to begin. After 2 s, a 4 Goodman, D., P. Keogh, R.G. Carson and D. Elliott, A simple modification for instantaneous offset of fluorescent lamps. In preparation.

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microcomputer-generated tone signalled that subjects could move when ready. Thus the time without vision was actually the 2-s delay plus the time subjects spent prior to movement. Trials on which the subject anticipated the signal to move or on which the digitizing tablet failed to detect stylus contact were repeated. Data reduction

Following the experimental sessions, and for each trial, the two sets of ‘raw’ 2-dimensional linearized camera data were converted to threedimensional Cartesian coordinates, by direct linear transforms, using a Northern igital CONVE program. isplacement ddta were filtered using a second-order utterworth filter with a forward and reverse pass (low pass 10 SCOPE data files were used to identify the initiation a conclusion of movements as indicated by changes in the status of the stylus microswitch. 5 isplacement data were smoothed with a 12.5 ms triangular window. The instantaneous vector (resultant) velocity was computed by means of a two-point central difference algorithm, equivalent to a second-order quadratic interpolating polynomial ( ood 1982). The velocity ‘curve’ was further smoothed with a 17.5 ms triangular window, and the highest peak velocity identified. Instantaneous vector acceleration was computed, by means of a two-point central difference algorithm and smoothed with a 22.5 ms triangular window, following Pezzack et al. (1.977). In house algorithms were developed to identify positive to negative, and negative to positive, transitions of the ‘acceleration profile’ in the period from the highest instantaneous velocity to the termination of the movement. 6 For each trial, data were therefore available relating to the peak velocity reached during movement, time to peak velocity, time from peak velocity to the end of the movement, and the number of zero crossings of the acceleration profile from the time of peak velocity until the termination of the movement. These data were merged with the corresponding movement time and terminal error data files. ’ Since it was possible for the stylus and therefore the emitter to pivot forward on the switch before a change occurred in the status ot the switch, on some trials there was a positive velocity at what we identified as movement initiation (see fig. 2). 6 Zero crossings were easily identified by noting a change in sign of the acceleration values. In order to avoid multiple counts (due to noise or small fluctuations), our counting criteria required acceleration values greater than 0.01 ms -2 for not less than 10 ms following a change in sign.

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Results

erfortnance measures The performance variables considered in this experiment were mean

movement time, mean radial error, and both constant and variable ovement error in the primary direction of the movement (Y error). time reflects the time from movement initiation until target acquisition. adial error is simply the distance between the movement end point d the target, and provides an overall measure of movement accuracy. onstant and variable error provide information about movement bias (i.e., tendency t o undershoot or overshoot target) and consistency respectively. The four dependen.t variables were analyzed separately using a 3 vision condition (lights on, lights off, 2-s delay) by 2 instructional set (fast, accurate) by 9 subjects repeated measures analysis of variance. ’ The movement time analysis revealed a main effect for vision F(2,16) = 7.0, p c 0.01, a main effect for instructional set, F(1,8) = 20.6, p < 0.01, and a vision condition by instructions interaction, s is apparent in table 1, subjects moved more F(2,16) = 4.1, p -c 0.05. slowly when instructed to move accurately as opposed to rapidly. As well, they moved more slowly in the lights on and 2-s delay conditions e interaction reflects the than they did in the lights off condition. fact that this vision effect was significant only when accuracy was stressed, although a similar trend existed when speed was emphasized. rror analysis also revealed main effects for vision, p < 0.001, and instructions, F(1,8) = 10.2, p < 0.05, as well as an interaction of the two factors, F(2,16) = 8.1, p < 0.01. nspection of fig. 1, indicates that while all 3 visual conditions were significantly different from each other (Tukey 1x, p < 0.05), the benefit of visual information was greater when accuracy as opposed to speed was stressed. As in our other work (e.g., adalena 1987), the vision condition main effect accounted for a large proportion ( a2 = 0.699) of the overall radial error variance ( o2 for instructional set = 0.088). The main effects of vision, F(2,16) = 177.7, p < 0.001, and instructions, F( 1,8) = 6.1, p c 0.05, on variable error were similar to radial 7 One subject consistently held the stylus in such a way as to obscure the WATSMART IRED. This subject’s data were eliminated from all analyses.

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Table 1 Mean movement time (ms), constant error (mm) and variable error (mm) in experiment 1 as a function of vision condition and instructional set. Vision condition

Movement time

Constant error

Variable error

Fast

Accurate

Fast

Fast

Lights on Lights off upon initiation 2-s lights off delay

334

980

1.34

- 0.15

5.42

1.66

302

748

- 0.14

2.17

8.57

6.91

330

925

- 1.38

- 0.96

11.53

10.33

Accurate

Accurate

error (see table 1 and fig. 1). For constant error, the results were more complex. A vision condition by instructional set interaction, F(2,16) = 3.6, p < 0.05, indicates that while subjects undershot the target in both the fast and accurate delay conditions, they overshot the target when they were moving accurately with the lights off and quickly with the lights on. In the other two conditions subjects were reasonably accurate (table 1). Kinematic measures The kinematic variables analyzed in this experiment were peak velocity, time to peak velocity, the time from peak velocity until the end of the movement and the number of positive zero crossings in the acceleration function following peak velocity. ollowing the logic of other investigators (e.g., rooks 1974; see Jeannerod 1988, for review),

‘4 1 12 1 10 -

8642O!

I

FAST

I

,

ACCURATE

Fig. 1. Mean radial error in experiment 1 as a function of instructional set and vision condition.

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402

Table 2 Mean peak velocity (m/s) and time to peak velocity (ms) and time following peak velocity (ms) and their proportions In experiment 1 as a function of vision condition and instructional set. Vision condition/ instructional set Lights on Fast Accurate Lights off upon initiation Fast Accurate 2-s lights off delay Fast Accurate

PV

TPV

TPV/MT

TFPV

TFPV/MT

2.16 1.28

127 229

0.39 0.25

205 750

0.61 0.75

2.30 1.35

121 227

0.41 0.32

179 524

0.59 0.68

2.21 1.24

132 237

0.41 0.27

196 687

0.59 0.73

we interpreted zero crossings as a measure of the relative continuity of the movement. As such, a positive zero crossing (second acceleration) is interpreted as a discrete adjustment to the movement. All the kinematic variables other than zero crossings were analyzed using a 3 vision condition by 2 instructional set by 9 subjects repeated measures analysis of variance. The extreme positive skewness (i.e., mode of 0 or 1 in most conditions) associated with the zero crossing data made it necessary for us to employ a Friedman analysis of variance by ranks to examine differences between instructional set and visual condition. The peak velocity analysis revealed only a main effect for instrucal set, F( 1,8) = 68.8, p < 0.001. As expected, subjects achieved a higher peak velocity in the fast condition (2.22 m/s) than in the accurate condition (1.29 m/s). ed a main effect for The time to peak velocity analysis’ able 2). ’ The absence instructionalal set, F(1,8) = 21.3, p < 0 of a visual condition effect (p > 0.20) suggests that movement time differences between visual conditions are a function of the time from peak velocity until the end of the movement. An analysis of time from peak velocity to end of movement confirmed this hypothesis, yielding a main effect for vision condition, F(2,16) = 7.9, p < 0.01. As well, the ’ The slight discrepancy between mean movement times and the sum of mean time to peak velocity and mean time following peak velocity is due to movement times being recorded with a temporal resolution of 1 ms, while kinematic parameters were recorded with a temporal resolution of 2.5 ms.

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Table 3 Mean number of positive zero crossings, and the proportion of total trials with zero, one, two and three or greater positive zero crossings in experiment 1 as a function of vision condition and instructional set. Vision condition/ instructional set

Mean number of zero crossings

Lights on Fast 0.10 Accurate 0.41 Lights off upon initiation Fast 0.12 Accurate 0.44 2-s lights off delay Fast 0.13 Accurate 0.75

Proportion of zero crossings Zero

One

Two

Three or more

0.908 0.624

0.088 0.348

0.004 0.028

0.000 0.000

0.889 0.607

0.111 0.328

0.000 0.048

0.000 0.017

0.886 0.511

0.080 0.352

0.034 0.085

OBO9 0.057

Note: Mean number of zero crossings is based on an unweighted means approach, giving equal

weight to each subject’s data. Proportion completed trials in each condition.

of zero crossings is weighted by the number of

analysis revealed a main effect for instructional set, F(l,g) = 19.1, p < 0.01, and an instructional set by vision condition interaction, F(2,16) = 4.9, p < 0.05. As is evident in table 2, subjects spent more time after peak velocity, in the li&ts on condition than the lights off upon movement initiation condition with the 2-s delay condition being intermediate. ‘This pattern was more pronounced for accurate movements than fast movements, presumably, because of the longer absolute times in the former situation (see proportions in table 2). As Jeannerod (1988) has suggested, the proportionately longer time spent in deceleration (i.e., time after peak velocity) for accurate movements under visual guidance might reflect the time it takes subjects to use visual information to reduce the discrepancy between limb and target position. 0ur analysis of zero crossings was to determine if a greater number of adjustments to the movement trajectory occurred. The mean number of zero crossings per condition and the frequency of 0, 1, 2 and 3 or more zero crossings are presented in table 3. An riedman’s analysis of variance by ranks revealed that subjects exhibited more secondary accelerations in the accurate condition than in the fast condition, x2(1, iV = 9) = 72.0, p < 0.01. Subsequent analyses were carried out for each instructional set in order to examine

I

Time

600

600

r

-20

- 10

0

Movement

I 400

1 200 600

I

ACCuftlle

I 600

n

0

r i

a

r

I e

0

0

__

-20

-10

0

10

20

30

05

1

15

2

25

200

200

Movement

400

D

Movement

400

c

Time

600

600

ACCW(llR

Time

-

-

-

I FOSI

Fast

600

800

Fig. 2. Sample velocity (m/s) and acceleration (m/s2) b,$movement time (ms) functions in experiment 1. The panels on the left depict the velocity (A) and acceleration (B) profiles of an accurate and a fast movement without a zero crossing, while the panels on the tight (C and D) depict movements in which one zero crossing was in evidence.

t i 0 n

a

10

Time

-

C C e

B

A

20

400

Movement

C C e I e

30

200

A

Y Y

I 0 C :

0 P

I

v e

V e

A

D. Eiliol? et al. / Visual control

the effect of visual co were foun when subjects were mstruc ingly, when subjects secondary accelerations in the 2 situations, x*(2, N accurate in the full tions, but made no more secon advantages do not a iscrete corrections. pitted in fig. 2. C~2rrelationalanalysis ne the relationship

variables, and how those relatio tion, a number of within-subject c lated. Specifically, correlations betw radial error and ti,me following peak velocity, and zero cross time following peak velocity were c condition (i.e., over the twenty trial, tion coefficients were then trans by a 2 instructional set by 3 measures analysis of variance. 9 The zero crossins-radial error correlations were instruct;i.onal set

continuous fashio atible with what is seemingly a between-c@ radial error and zero crossings. T kinesthetic feed

’ In the coirelational analyses involving zero-crossings, the actual number of zero crossings trial was used. Biserial correlations, for which trials without zero crossings were coded as zero trials with one or more zero crossings coded as one, ore essentially identical.

D. EIIiott et al. / Visual control

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hile zero cross’

condition there

iscussion

vision and instructional set on manual aiming trajectories. lar, it was thought that an examin (primarily zero crossings of the a es of control under s information to ev ur performance findin accurate instructional set ( eous when full visual information was av movement. ur visual mani ess of instructional set, act error variance. ese large vision condition zffects roblems for 1s that mi accuracy of speeded limb movements (e.g., n the last few years, the more d control has been the single correctio

nant visual model of model (e.g., Carlton 1

park’ movement undershoots

corrective impulse, based on visual out limb position, adjusts for this undershooting ( odeI, the frequency of rmation is eh a ematic dat a number of aspects of the movement r visual manipulations was velocity and the end of the visual information available rtion of the movement than tiation. While traditional of visual-motor control (e.g., detect and cations in evidence when vision was there were no more discrete the movement. As well, information was avaiIable, the presence or frequency acceleration (i.e., a discrete adjustment) did not predict movement error. It is these latter two findings that are not consistent with a discrete correction model of visual limb control (e.g., 1968, 1981). b in the 2-s no vision delay condition, in which visual information was the most degraded, subjects also spent more time after velocity than in the lights off condition. Perhaps when the lights were extinguished upon movement initiation, subjects were simply ting to acquire the target before the representation of its position has had time to decay (Elliott et al. 1990). In the most visually esthetic ed condition (2-s delay), subjects may depend more on to control their movements. Thus the greater number of discrete adjustments in this condition than in the other two conditions from nonvisual sources. These adjustments e based on feedb velocity do not appear to be spent following pe however, since error in the 2-s delay condition is large lights off upon movement initiation condition. In essence then, we are suggesting that in the 2-s delay condition, subjecti’ were, by default, adopting an ineffective kinesthetic control strat *OWeth

Ron Marteniuk for his suggestions and helpful comments on this point.

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aeger 1988; nt 1 and else here (Elliott 198 adalena 1987), we have suggest that brief visual representation of the movement environment may irect visual contact with the environment is s 1984; and olding 1968). tion was responsible for the large accuracy lights off upon movement initiation conditi tion. As well, we have suggested that the for the lights off upon movement initiati et area before this ts attempt to rea may reflect th an opportunity t these proposals are representation correct then these effects should be reduced or eliminated if visual information about target position is made available in all con experiment 2 then, a small dot of phosphorescent paint information even in the lights out conditions. Methods Subjects

Subjects were ten, experiment naive, male (7) an teers, each of whom were paid $7.00 for their participatior:. individuals were right handers (Oldfield 1971). All subjects had normal or corrected- to-normal vision. pparatus and procedure

The apparatus and procedures in this study were identical to expe was isi ment 1, except that in all three conditions t e target the entire course of a trial. square of phosphorescent paint (a 15 tion of the vertical and horizontal target lines. dures were identical to those used in experiment 1, l1 $1

” One subject con? .tently responded in a fashion such that the heel of the hand contacted the digitizing tablet ,+or to the stylus tip. This subject’s data were eliminated from all analyses.

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409

Results

movement time, radial error, and constant and variable error in the primary direction of the movement were analyzed using a 3 vision condition by 2 instructional set by 9 subjects repeated easures analysis of variance. e movement time analysis again revealed main effects for vision, F(2,16) = 10.0, p < 0.001, and instructions, F(l, = 32.88, p < 0.001, as well as an interaction, F(2,16) = 7.2, p < 0.01. s expected, subjects moved more slowly when given instructions to move accurately. The influence of visual condition was the result of subjects moving more slowly in 2-s delay situation than in the two other conditions (see table 4). difference was only significant (Tukey ac, p < 0.05) for the accurate instructional set. Presumably the absence of a difference between the lights on and lights off upon movement initiation condition is related to the fact that in the latter condition subjects were no longer dependent on a rapidly decaying representation of the target position to guide movement (cf. experiment 1). The r&al error analysis revealed main effects for instructional set, F&8) = 21.7, p x 0.001, and vision, F(2,16) = 7.8, p < 0.01, as well as an interaction, F(2,16) = 3.7, p < 0.05. Overall, subjects exhibited more error in the fast than accurate condition, and were more accurate when vision was available than in the two no vision conditions (Tukey LY, esumably, the reduction of the delay effect (cf. experiment p < 0.05). to the continuous availability of target information 1) is relat lights off conditions (EWott 1988; adalena 1987).

Table 4 Mean movement time (ms), constant error (mm) and variable error (mm) in experiment 2 as a function of vision condition and instructional set. Vision condition

Movement time

Constant error

Variable error

Accurate

Fast

Accurate

0.90

- 0.19

5.11

2.38

676

- 0.61

- 0.74

6.19

6.29

787

0.23

0.70

9.61

4.39

Fast

Accurate

Lights on

365

693

Lights off upon initiation

393

2-s lights off delay

391

Fast

D. Hiiott et al. / Visual control

410

8 6

FAST

ACCURAlE

Fig. 3. Mean radial error in experiment 2 as a function of instructional set and vision condition.

the main effect for vision accounts for far less variance in radial error 3) than it id in experiment 1. The interaction of vision and ( ret (see fig. 3). instructional set is a little more difficult to i Specifically, while differences between the light upon movement initiation condi ion and the ition were eliminated when subjects were instructed to strezs accuracy, the delay continued to interfere with performance in the fast instructional set. haps this continued effect may be related to the deterioration of non-target visual information which is more important in a time constrained movement. As in experiment 1, vision condition, F(2,16) = -7, p < 0.05,an instructional set, F(1,8) = 7.9, p < 0.05, findings for var:Ale error undant with radial error effects (see table 4 an Of greater interest was the absence of any instructional set or vision effects on constant error ( p > 0.20, see table 4). n contrast to experiment 1, there was no tendency for subjects to either undershoot or overshoot the target in this experiment (gran mean = 0.05 mm). Presumably thi situation exists because continuous target information is available in 0th the no vision conditions thereby freeing subjects from dependence on a memory representation of target location. nematic measures nce again, peak velocity, time to peak velocity, and time following peak velocity were analyzed using a 2 instructional set by 3 vision condition by 9 subjects repeated measures analysis of variance.

D. Elliott et al. / Visual control

41

Table 5 Mean peak velocity (m/s) and time to peak velocity (ms) and time following peak veEocity (mc) and their proportions in experiment 2 as a function of vision condition and instructional set. Vision condition/ Instructional set

PV

Lights on Fast 1.96 Accurate 1.63 Lights off upon initiation Fast 1.95 Accurate 1.64 2-s lights off delay Fast 2.02 Accurate 1.57

TPV

TPV/MT

TFPV

TFPV/MT

128 164

0.35 0.26

247 519

0.65 0.74

137 167

0.37 0.27

247 520

0.63 0.73

127 169

0.35 0.23

257 619

0.65 0.77

As in experiment 1, the peak velocity analysis revealed only a main effect for instructional set, F&8) = 23.8 p < 0.001. As expected higher peak velocities were attained in the fast (1.98 m/s) as opposed to accurate (1.61 m/s) instructional set. These differences were not as pronounced as those found in experiment 1 (see table S), and were also in smaller movement time differences. me to peak velocity analysis also yielded only an instructional set main effect, F(1,8) = 17.1, p < 0.01. Again subjects took more time to reach peak velocity in the accurate condition than in the fast condition (see table 5), and vision had no influence on these times ( p > 0.20). The time following peak velocity analysis once again yielded both main effects for instructional set, F(1,8) = 30.9, p < 0.001, and vision condition, F(2,16) = 8.5, p < 0.01, and an interaction, F(2,16) = 6.4, p < 0.01. As is apparent in table 5, subjects spent more time following peak velocity in the 2-s delay condition than in the other two conditions. These differences were larger in the accurate instructional set. hus the time following peak velocity findings mirror the movement time findings, and suggest that vision has its primary influence during the deceleration phase of the primary movement. It should be noted that particularly for slow movem the velocity profiles are far from symmetrical (fig. 2 and table 5, er et al. 1988). The mean number of zero c gs per condition, and frequency riedman’s analysis of variance by data are presented in table 6. ranks revealed that there were more secondary accelerations in the

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Table 6 Mean number of positive zero crossings, and the proportion of total trials with zero, one, two and three or greater positive zero crossings in experiment 2 as a function of vision condition and instructional set. Vision condition/ instructional set

Mean number of zero crossings

Lights on Fast 0.07 Accurate 0.30 Lights off upon initiation Fast 0.03 Accurate 0.30 2-s lights off delay Fast 0.12 Accurate 0.40

Proportion of zero crossings Zero

One

Two

Three or more

0.922 0.779

0.073 0.200

0.000 0.014

0.000 0.007

0.959 0.781

0.041 0.187

0.000 0.032

0.000 0.000

0.877 0.671

0.110 0.271

0.013 0.058

0.000 o.OOc!

Note: Mean number of zero crossings is based on an unweighted means approach, giving equal weight to each subject’s data. Proportions of zero crossings is weighted by the number of completed trials in each condition.

accurate than in the fast condition, x2(1, N = 9) = 68.4, p c 0.01. Analyses involving each instructional set indicated that although there were no differences between visual conditions for accurate movements, subjects exhibite more zero crossings in the 2-s delay condition than when the lights went off upon movement initiation, x2 (2, N = 9) p c 0.05. The lights on condition was intermediate (see table 6). advantages associated with full vision appear number and frequency of discrete modifications to the movement. Correlational analyses

ithin-subject relationships between radial error, zero crossings and time following peak velocity were examined by three 2 instructional set condition by 9 subjects repeate#d measures analyses of he same calculation procedures we,re used as in experiment he zero crossing-radial error analysis again showed no overall relationship between the two variables (grand ean = - 0.03), and no influence of instructional set or visual condition on the relationship ( p > 0.20). Again this may be because accurate movements do not need

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to be corrected, or because visual-motor control proceeds in a continuous fashion. As in experiment 1 our between-condition findings for radial error and zero crossings support the latter position. n this experiment the time following peak velocity-radial error analysis failed to any influence of instructional set or vision condition ( p > 0.20). verall, there was a slight negative relationship between the two factors (grand mean = -0.17). The lack of the positive relationship for the lights off upon movement initiation condition (i.e., visual condition effect in experiment l), supports our position that in experiment 1 subjects benefited from getting to the target before the representation of the target position had decayed. n this experiment, more time was beneficial in all situations, since visual target information was always physically available. hile overall the number of zero crossings increased with time following peak velocity (grand mean = 0.295), the presence of an instructional set by vision condition interaction, F(2,16) = 5.4, p < 0.05, indicates that the strongest relationships occur in the fast 2-s delay condition (0.63) and in the accurate lights on condition (0.42, fast-on = 0.17, fast-off = 0.13, accurate-off = 0.28, accurate-delay = 0.14). The reason for this particular pattern of results is not clear. iscussion

The primary purpose of experiment 2 was to determine how the continual availability of target information might mediate the radial error and nematic effects found in experiment 1. As we have shown elsewhere (Elliott 1988; Elliott and adalena 1987), providing continuous target information essentially eliminated the accuracy differences between the lights off upon movement initiation condition, and the 2-s delay cond on, at least when subjects were instructed to perform accurately. resumably, this is because subjects were no longer dependent on a short-lived representation of the target as they were in the first experiment. t should also be noted that the difference in radial error between the full vision condition and the lights off upon movement initiation condition was reduced in this study. This finding indicates that a representation of target position is a less than substitute for irect visual target information (see Carhon 1981; rablanc et al. 1979; 1986; roteau and Coumoyer 1990; and

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Wallace and Newell 1983, for discussion on the relative importance of limb and target information). In terms of the movement trajectories in experiment 2, subjects spent proportionately more time in the deceleration portion of the movement in the 2-s delay condition. This extra time taken by subjects to slow the movement appears to be more responsible for the improved accuracy in experiment 2, than any type of discrete modification since there were actually fewer zero crossings than in experiment 1. It would appear t at the continual availability of target information also reduced the need for subjects to move to the target quickly when the Sghts were extinguished upon movement initiation. resumably, the continuous target information reduced the necessity of subjects moving to the target before the representation of that location had had an opportunity to ecay. Certainly the disappearance of the positive relationship between time following peak velocity and radial error in the lights off upon movement initiation condition supports this position.

Traditional closed-loop models of movement control have emp f visual information for discrete reduction. warth et al. 1971) and number e 1968) of discrete vision-bas modifications have to predict liminating visual information about an ected to negate the possibility of wit ing movement would be movement adjustments bas on vision, thus resulting in an increase m movement error. hile in this work occludi vision had a major effect on move error, it had no impact on iserete adjustments that subjects made to their movement trajectories. erhaps absence of a relationship between end-point error and the presence and frequency of discrete adjustments to the movement trajectory is not su adopts a probabilistic approach to movement generation 1988). Specifically, some proportion of the petted to be precise enough as to require no a Glencross 1989).

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eyer et al. (1988) have recently incorporated this prob&il.istic approach to error generation into an explanation of speed-accuracy relationships in manual aiming. As in earlier models ( eyer et al. 1982; Schmidt et al. 1979), end-point error (variability) is thought to reflect noise in the neuro-motor systems ability to select and generate an appropriate force for a specific time. Too much force, and/or the force being applied for too much time will result in target overshooting while too little force will result in target undershooting. On a proportion of trials, however, the force-time specification will be appropriate for the target, and a relatively accurate movement will be generated that does not require modification. While there is much to be said for eyer et al.‘s (1988) probabilistic account, there are several specifics of their model that are not altogether compatible with our data. Following eyer et al. (1988) for example, it may be that discrete adjustments are simply more accurate (less variable) when visual information is available. This suggestion, however, is at odds with our correlational analyses which show no difference in the radial error-zero crossing relationship as a function of vision condition. As well, their ideal model assumes en in our fast instrucsymmetric velocity and acceleration profiles. tional set, our subjects spent more time in decelerating the primary movement than they did achieving peak velocity (see also Jeannerod 1984, 1988). At least in experiment 1, the time spent decelerating predicted movement accuracy when vision was available. Finally, a probabilistic account of impulse error assumes proportional under- and ur work overshooting of the target regardless of visual condition. indicates that subjects were more likely to undershoot the target in visually degraded conditions. Since the presence of visual information, but not the presence or frequency of a secondary acceleration (i.e., a discrete adjustment), influences movement accuracy, we contend that the visual control of y proceed in a continuous, or at least, pseudo-continuous ather than a second or third impulse being used to correct revious impulse, vision may tune the primary acceleration and braking impulse. Thus the system may function in a more analogue 1 fashion, by simply turning up or down the gain on muscle hese graded adjustments of motor system may be based on visual feedback from the movement environment. Alternatively, there may be many overlapping discrete adjustments to the movement trajectory giving the movement the appearance of continuity (i.e., pseudo-

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continuous). In any event these visual modifications to the movement proceed extremely rapidly, and are probably controlled at more peripheral levels in the central nervous system (see euer 1981). Perhaps the organization of an aiming movement includes not only specifying accelerating and braking impulses, but also environmental and feedback characteristics by which these impulses can be adjust while movements are prepared in light of feedback and envir sion can proceed without an expectations, adjustments based on blanc et al. 1986). individual’s cognitive awareness (see In making the argument that aiming movements may be controlled in a continuous fashion, we are not denying that discrete adjustments to the movement trajectory also occur. The finding however, t second and third accelerations were at least as frequent when vision was not available to subjects indicates that these adjustments may be e on the basis of kinesthetic or feedforward information. bly the system is attempting to reduce the error inherent in one impulse with a second or third corrective impulse. Although these adjustments may be more time consuming (e.g., eele 1968), our data suggest that they have limited effectiveness in reducing error.

Barrett, N.C. and D.J. Glencross, 1989. Kesponse amendments during manual aiming movements to double-step targets. Acta Psychologica 70, 205-217.

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