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On-line Visual Control of Aiming Movements?
acta psychologica Acta Psychologica 90 (1995) 333-348
ELSEVIER
On-line visual control of aiming movements? Will Spijkers
a, * ,
Simone Spellerberg
b
• Institut fUr Arbeitsphysiologie an der Uniuersitiit Dortmund, Ardeystrasse 67, 44139 Dortmund, Germany b Institut fUr Psychologie, RWTH Aachen, Aachen, Germany
Abstract Two experiments are reported which addressed the flexibility of vi suo-motor processing by manipulating the availability of visual information while executing a discrete aiming movement. The flexibility of visuo-motor processing was tested by unexpectedly changing the proportion of the movement trajectory that visual feedback was present. Visual feedback was manipulated for a short (0.30), medium (0.60) or long (0.90) proportion of the trajectory within a block of trials. Each of these three proportions of vision occlusion (Experiment 1) or visual disclusion (Experiment 2) during the initial trajectory was examined. Within a visual condition, one of the three visual feedback proportions occurred with a high probability (p = 0.72), whereas the remaining two proportions each occurred with a low probability (p = 0.14). The results clearly indicated that spatial accuracy was determined by the actual vision period, independent of its probability of occurrence. The data are consistent with a model of continuous on-line control of movement execution.
1. Introduction
Visual information plays an important role in the spatial accuracy of discrete aiming movements (e.g., Keele and Posner, 1968; Woodworth, 1899). Since the early investigations of Woodworth (1899) on the underlying principles of movement control, it has been suggested that a rapid aiming movement consists of at least two successive phases: a first initial impulse phase, which is pre programmed and ballistic (open-loop) and a secondary part, the so-called current control phase, in which feedback based error-correction mechanisms are activated. This two-phase notion has been a prevalent theoretical framework and has dominated the theorizing on motor control (e.g., Carlton, 1981). However, recent studies have raised doubts about the appropriateness of this concept. Several experiments (Moore, 1984; Spijkers and Lochner, 1994; Blouin et • E-mail: [email protected]. 0001-6918/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0001-6918(95)00033-X
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aI., 1993a) clearly showed that visual feedback during the initial phase of a movement improved aiming accuracy in comparison to situations in which vision was absent during the initial part. To account for these results the original two-phase notion seems no longer tenable. It is suggested by several authors (e.g. Prablanc et aI., 1986; Komilis et aI., 1993) that the intake and processing of visual information is continuous during the execution of a movement. This constitutes also a main assumption of the stochastic optimized submovement model that was proposed by Meyer et al. (1990) to account for the speed-accuracy relations in discrete aiming. In short, the stochastic optimized submovement model assumes that a discrete aiming movement consists of one or more submovements. The primary submovement is programmed to hit the center of the target region and during its execution kinesthetic and visual information is continuously picked up. This information is used to program the secondary submovement, if the primary one is insufficiently accurate. If the assumption of continuous processing of visual information is valid, the implicit hypothesis must be proved, that the visuo-motor system reacts flexibly to unexpected manipulations of visual information. In most of the previous studies on visuo-motor coordination manipulation of visual feedback was carried out blockwise. Improved accuracy with short periods of vision presented in blocked conditions could therefore be a reflection of higher-order strategies adopted by the subjects, rather than flexible on-line processing of visual feedback. Factors like learning effects (Proteau, 1992), feedforward processes (Beaubaton and Hay, 1986) and preparatory states (Zelaznik et aI., 1983) might playa role in these strategies. When it can be shown that unexpected shortening or lengthening of the visual feedback period immediately affects the aiming performance, it would mean convincing evidence that movements can be controlled in an on-line mode. In such a view it is assumed that during movement execution information is continuously picked-up and processed in order to come to optimal aiming performance. How flexible the motor control system can react to sudden changes of haptic sensory information, was nicely shown in a series of experiments on grip movements by Johansson and Westling (1990). It appeared that unexpectedly changed circumstances in mass and slippery properties of the to be gripped object rapidly elicited corrective movements. For reaching movements Paulignan et ai. (1991) showed that the kinematic pattern of the hand was modified within 100 ms when the original position of the object to be reached for was shifted unexpectedly. The flexible use of visual information in the control of an aiming movement was investigated in the present study. Two experiments were carried out. In the first experiment visual information was occluded from the onset of the aiming movement and became available after the hand travelled a certain proportion of the trajectory. In the second experiment, visual feedback was available during identical proportions of the initial trajectory and visual occlusion was now applied for the remaining trajectory. In both experiments three proportions were examined, i.e., short, medium and long. Flexibility of visuomotor processing was examined by changing unexpectedly the proportion of either the occluded or visible trajectory at the outset of the movement. This was achieved by varying its probability of
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occurrence. Three conditions were realized in which each proportion was once the highly probable one (p = 0.72), while the other proportions each occurred with a low probability (p = 0.14). Extreme rigidity of the visuomotor system would imply that the point at which visual information is picked up, was programmed for the highly probable viewing condition as an invariant feature of the motor program. On the contrary, high flexibility would mean that the actual moment of dis- or occlusion of vision would determine the aiming accuracy of that particular movement, irrespective of its probability being either high or low. A secondary purpose of the study was to examine the temporal relationship between the initiation of the eye movement and hand movement. While fixating the starting point and the hand, subjects have peripheral, retinal information of the target position and after the saccade to the target, the (command to the) eye musculature provides extraretinal information of the target position. The eyes usually fixate the target before the hand movement is initiated under normal viewing conditions (e.g., Abrams et aI., 1990). In an exploratory way the question was addressed here whether the temporal relationship between the initiation of the eye and hand is affected by increasing periods of visual occlusion at movement initiation or movement termination.
2. Experiment 1
In the first experiment vision was occluded as soon as the hand started moving and this lasted for a certain proportion of the trajectory. In addition to a control condition (full vision), three conditions were examined in which vision was oc- . cluded for the following proportions of the trajectory: 0.30, 0.60 or 0.90. We will refer to these conditions as short, medium and long occlusion condition. A further important manipulation concerned the probability of occurrence of a certain occlusion period during a block of trials. The occlusion period, after which the condition is denoted was the highly probable one (p = 0.72), whereas the other two periods each occurred with a probability of p = 0.14. Thus, during an occlusion condition the length of the predominant occlusion period was unexpectedly changed. For example, if the occlusion condition was the medium one, the two periods with a low probability were the short and long occlusion period. When the short occlusion condition was the highly probable one, the unexpected occlusion periods always implied a prolongation of the expected one, whereas in the long occlusion condition, vision was available earlier as expected if the occlusion periods of low probability were presented. A main effect of occlusion was expected, that is, increasing the proportion of the occluded trajectory would increasingly impair aiming accuracy. The main question concerned the effect of unexpected shortening or prolongation of the occlusion period. It is expected that aiming performance depends on the actual occlusion period, irrespective of its probability of occurrence, when visuo-motor processing is flexibly following the actual viewing circumstances. Based on previous
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research (e.g., Spijkers and Lochner, 1994) it was hypothesized that even occluding a small proportion of the initial movement trajectory would impair aiming accuracy in comparison to the control condition (full-vision). 2.1. Method Subjects Nine subjects (3 female and 6 male adults, mean age, 24 years) participated. They were right-handed and had normal or corrected-to-normal vision. Task and apparatus Subjects performed an aiming task in which a stylus was moved laterally from a starting point (5 X 5 mm) to a target on the left (width X height: 5 X 100 mm). Distance from center of the starting area to the midline of the target was 16.75 cm. Because the starting point was directly in front of the subject the hand did not obstruct the sight on the target during execution of the movement. The movement had to be executed such that the movement times were within a 33.3% range of an imposed movement duration of 600ms. This particular time constraint was imposed in order to obtain similar movement profiles across subjects and to compare the results with other movement tasks currently under investigation. Directly above the starting point an array of three LEOs was mounted. The middle yellow LED sezved as a feedback signal, indicating whether the stylus was correctly placed on the start position. Two green coloured LEOs, each immediately adjacent to the yellow one, sezved as warning signals; the visual warning was accompanied by a tone (300 ms; 1000 Hz). After a constant foreperiod (2 s) the imperative signal, the left LED, was illuminated; it was extinguished as soon the movement started. Movement begin was defined as the stylus was moved across the boundaries of the starting position (see also procedure and design). Movement recording Seated in front of a table in which a digitizer (Kurta) was mounted flush to the table surface, subjects performed the aiming movements by sliding a pencil (10 g) across a glass plate that covered the digitizer tablet. Underneath the glass plate a cardboard was placed on which the starting point and target were drawn. The movement coordinates were recorded with a sampling rate of 100 Hz and a spatial accuracy of 0.25 mm. A PDP 11-23 computer controlled presentation of the signals, the length of the programmed visual feedback intezvals, measured reaction and movement times, and stored spatial coordinates of the movement. Manipulation of visual feedback Duration of visual feedback was varied by way of a Portable Liquid-crystal Apparatus for Tachistoscopic Occlusion (PLATO; for technical details see Milgram, 1987). The glasses of the PLATO-apparatus consist of liquid crystals which may adopt two different states depending on the induced electric potential: "open" vs. "closed". Transition from the open to the closed state and vice versa
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occurs within 5 ms. In the open state, at which 85% of the luminance is transmitted, the glasses allow normal retinal vision; the closed state scatters the incoming light and leads to a completely uniform visual field, which doesn't even allow the subject to perceive any contours. The light-scattering cholesteric liquid crystals do not become opaque in their closed state, but remain translucent. The subject's eye therefore remains substantially illuminated during the spectacles' closed state. The PLATO-apparatus switches off visual feedback without largerly affecting light adaptation; for that reason confounding effects arising from different states of dark-adaption could be excluded. The subjects wore the glasses throughout the whole experiment.
Eye-movement recording During the experiment subjects head was fixed: chin, forehead and the temples were softly, but firmly clenched, so that no head movements were possible in either the horizontal or vertical direction. Fixation of the head was individually adjusted. Eye movements were recorded by means of surface electrodes (Beckmann silverchloride; diameter 1.6 mm), which were placed on the medial portion of the orbicularis oculi at both sides of the head. A third reference electrode was fixed to the ear. The responses were amplified and continuously sampled by means of a specially developed program for Atari (1040) with a frequency of 1000 Hz (Luecke, 1993). Before the start of the experiment a calibration procedure was conducted to obtain the individual EOG-to-eye-movement ratio. This calibration procedure was repeated during the session prior to each condition change. During calibration of the EOG-signal subjects had to fixate three capital letters: one at the starting point of the hand-movement, the other two at 16.75 cm to the right and left of this point. Arm-movements were synchronized with the eye movement by means of markers in the EOG-signal in order to relate hand-position with the eye-position. On the EOG-trace the following three time-markers were superimposed: one at presentation of the imperative signal, the next one at movement begin (as the stylus left the starting position) and the last one when the hand arrived at the target. Subjects were only instructed to fixate the starting point until the imperative signal was presented. Procedure and design
A trial was initiated when the subject placed the stylus on the start position. After 1.5 s, an auditory warning signal sounded and the two green LEOs were illuminated, which was followed by the visual imperative signal after a fixed interval of 2 s. In a within-subject design, we studied three periods of occlusion. In addition to the control condition with continuous vision, the three experimental conditions were short, medium and long occlusion of the movement trajectory. These three conditions were completely counterbalanced across six subjects, while for the three remaining subjects one sequence of the conditions was repeated. Within each occlusion condition all three occlusion periods occurred: One with a probability of p = 0.72 and the two remaining ones each with a probability of 0.14. Visual
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occlusion was applied synchronous to the start of the movement. As soon the scheduled point in the trajectory was passed, the PLATO-glasses opened, so that target and hand could be viewed again. A session started with a training series of 50 trials with uninterrupted vision to get used to the task and to practice the execution of the movement within the imposed time constraints. During this training series subjects received verbal feedback on their movement time. The series was repeated (without providing feedback concerning the movement times) and this was considered as control condition; the control condition was again given at the end of the session. After the initial control condition, subjects performed the three occlusion conditions consecutively; each new condition was preceded by repetition of the calibration procedure. In each occlusion condition subjects accomplished 100 trials in two consecutive series of 50 trials. Thus, in each occlusion condition 72 trials were performed with the frequent occlusion period and 14 trials with each infrequent occlusion period. Immediately before presentation of the imperative signal, the effective size of the start position was reduced. It then extended only 1 X 1 mm around the momentary position of the tip of the stylus. By this procedure, the measurements of the reaction time was minimally inflated by the time needed to arrive at and cross the starting point borders. When subjects made premature reactions by moving the stylus inadvertently outside the starting point before presentation of the imperative signal, the trial was started again. Reaction time (RT) was recorded as the time that elapsed between the onset of the imperative signal and departure from the home position. The time between leaving the home position and first crossing of the target boundary defined movement time (MT). As subjects did not reach the target or passed over the target, these movements were recorded as under- and overshoots, respectively. Movement time was then computed as the time between leaving the starting position and the moment in the recorded movement trace that three consecutive coordinates were identical. 2.2. Results
Two factors were distinguished: occlusion period and probability of the occlusion period. Because each occlusion period occurred once with a high probability and twice with an identical low probability, it was examined first whether the effects of the low probability conditions differed from each other. For none of the dependent variables a difference was observed (p > 0.20). In addition, no interaction with occlusion period was found (p > 0.30). Hence, the values averaged across the two low probability conditions of each occlusion period were entered in the ANOVA. For each dependent measure a 3 x 2 ANOVA was conducted with occlusion period (short, medium, long) and probability (0.72 vs. 0.14) as factors. Spatial accuracy Analysis was focused on the percentage of target hits as spatial accuracy measure. A target hit was defined as an aiming movement that ended at the target
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Fig. 1. (a) Mean target hits (%) as a function of full vision (C:control), actual no-vision period (short, medium, long) and concurrently present no-vision condition. The high probability no-vision periods in which the actual period occurred are coded as follows: short (solid black), medium (hatched), long (dotted). (b) Mean target hits (%) as a function of length and probability of the actual no-vision period.
within the imposed time. In the control condition, a score of 87% hits was obtained. In Fig. 1 the percentages of target hits are shown. In Fig. 1a the percentage of target hits is shown as function of the actual occlusion period and its concurrent occlusion condition in which the actual occlusion period occurred. In this panel it can be observed that aiming performance depends on the actual period of no-vision and is not affected by concurrent no-vision proportions. Spatial accuracy as a function of the actual occlusion period and probability of occurrence of this period is shown in Fig. lb. Both panels clearly show that extension of the occlusion period reduced spatial accuracy, F(2,16) = 29.69; p < 0.001, and that the reduction was not influenced by the occlusion condition in which the periods occurred. Important for the main question of the study is that nor the main effect of probability, F(1,8) = 0.33; p = 0.59, neither its interaction with actual occlusion period, F(2,16) = 1.38; p = 0.28, were significant. On average, the target hits had an amplitude of 16.70 cm; this means that subjects stopped their movement 0.05 cm before the center of the target. Across the three occlusion conditions the percentages of temporally correct trials were 95.8, 91.7 and 83.3 for the short, medium and long occlusion conditions, respectively. Because the prolongation of the occlusion period was accompanied by a reduction of the number temporally correct movements, the percentages of hits were expressed as the number temporally correct trials. Additional ANOVAs which included all trials instead of only the temporally correct ones, revealed the
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same significant main effects. The latter result means that the basic result is not affected by differences in the number of temporally correct trials between occlusion conditions. Averaged across experimental conditions 35.1 % of the temporally correct movements were target misses and they consisted of 22% undershoots and 13.1% overshoots. For the short, medium and long occlusion periods, the undershooting movements ended 1.5, 3.2 and 4.6 mm before the target and the overshoots 2.5, 3.2 and 3.75 mm behind the target, respectively. In the control condition there were 10.9% undershoots (-1.0 mm) and 2.1 % overshoots (+ 1.0 mm). Fig. 2 depicts the percentages of under- and overshoots as a function of the occlusion period and its probability of occurrence. The percentage of undershoots increased as a function of occlusion period, F(2,16) = 5.45; p < 0.5, and was slightly lower for the low probability condition, F(1,8) = 5.19; p = 0.05. Overshoots also increased with occlusion period, F(2,16) = 7.51; p < 0.01, and the number of overshoots tended to be smaller for the low probability condition, F(1,8) = 4.30; p = 0.07. Reaction times Mean RTs were 363, 365, 361, 371 ms for the control and the short, medium and long occlusion conditions respectively. RT was independent from actual occlusion period or its probability (p > 0.26). Hence, foreperiod was sufficiently long to prepare the movement completely.
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Table 1 Eye-movement measures (latency, saccade duration, amplitude) and the difference between the reaction time of hand and eye as a function of the length and probability of the no-vision period Probability of no-vision period
Short
0.72 0.14 0.14
246 264 242
Length of initial no-vision period Medium
Long
Eye-movement latency (ms)
262 253 249
247 245 265
76 75
74 77 76
Saccade duration (ms)
0.72 0.14 0.14
76 75 75
72
Saccade amplitude (degrees)
0.72 0.14 0.14
17 16 15
16 17 15
15 17 16
RT minus eye movement latency (ms)
0.72 0.14 0.14
112 99 137
96 96 127
137 115 93
Movement times
Mean movement times of the temporally correct movements were 593, 576, 576 and 545 ms for the control and the short, medium and long occlusion periods, respectively. A main effect of actual occlusion period was found, F(2,16) = 6.84; p < 0.01, and this was caused by the speeding up of the movement in the trials in which a large proportion was occluded. Nor the main effect of probability neither its interaction with actual occlusion period reached significance (p > 0.18). Eye-movement measures
Eye-movement latency, saccade duration, saccade amplitude and the difference between the RT of the eye and the hand are shown in Table 1 for each occlusion period and probability. No main effect of the occlusion manipulations on the eye-movement measures was observed. The interaction between occlusion period and probability which was found for saccade amplitude revealed that subjects shortened the amplitudes if the long occlusion period was highly probable, but prolonged them if the long occlusion period had a low probablity of occurrence, F(2,16) = 4.96; p < 0.05. Fixation of the target preceded the initiation of the aiming movement by 48 ms on average and the eye-fixation leading time increased from 40 to 58 ms across the occlusion periods, F(2,16) = 4.87; p < 0.05.
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2.3. Discussion
The results unequivocally showed that" the actual vision time is flexibly used to control spatial aiming accuracy, because the effect of an occlusion period was not modulated by its probability of occurrence. A flexible use of the available visual information excludes a vi suo-motor control model that assumes one programmed point in the trajectory as anchor point for updating the arm control system. As expected aiming performance decreased with prolongation of the occlusion period. The short occlusion period impaired target aiming by reducing the accuracy from 87% (control condition) to 73.1 %. Despite a remaining viewing period of more than 10 cm, performance could not be corrected to the level of the control condition. Again this finding confirms previous results that vision during the first phase of a movement contributes significantly the end-point accuracy (e.g. Spijkers and Lochner, 1994). This is contrary to the conclusion of Blouin et al. (1993a), who stated that that the initial interval is only relevant for directional accuracy, and not for terminal accuracy.
3. Experiment 2 The second experiment examined how aiming accuracy is influenced when the available vision is restricted to the outset of the movement. In this case the positional control of the hand must be based on the memory representation of the position of the target that was formed before occlusion. In addition to the remembered position based on peripheral retinal information, target position information is then predominantly available by extraretinal information, for example, eye-position and the efference copy of the programmed eye movement. Several studies have shown that this type of extraretinal information is sufficient for accurate aiming (e.g. Hansen and Skavenski, 1977). 3.1. Method. Subjects Nine subjects (3 female and 6 male adults; mean age 2~ years) took part. Three of the male subjects had also participated in the previous experiment. They were right-handed and had normal or corrected-to-normal vision. Task and procedure The same task and procedure were used as in the previous experiment. The only change concerned the occlusion conditions. Now the glasses stayed open for a certain proportion of the trajectory as the movement started and vision was removed as soon the stylus passed the scheduled spatial point. Again three conditions were examined in each of which the probability of one vison period was high (p = 0.72), whereas the other two vision periods had a low probability (p = 0.14).
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Vision Fig. 3. (a) Mean target hits (%) as a function of full vision (C:controI), actual vision period (short, medium, long) and concurrently present vision condition. The high probability vision periods in which the actual periods occurred are coded as follows: short (solid black), medium (hatched), long (dotted). (b) Mean target hits (%) as a function of length and probability of the actual vision period.
3.2. Results
A preceding analysis showed that neither the difference between the data of the low probability levels nor its interaction with vision period was significant for any of the investigated variables (p > 0.16). For each dependent measure a 3 X 2 ANOVA was conducted with vision period (short, medium, long) and probability (0.72 vs. 0.14) as factors. Spatial accuracy
In the control condition a score of 84.9% hits was obtained. The percentage of hits increased significantly with length of the vision period, F(2,16) = 14.07; p < 0.001, but again was not affected by its probability of occurrence, F(1,8) = 0.32; p = 0.58. Fig. 3a shows that aiming performance for a certain actual vision period remained the same irrespective of the length of the concurrently present vision periods. In Fig. 3b the percentage of target hits is depicted as a function of vision period and its probability of occurrence. Inspection of the mean amplitudes per vision condition revealed that the maximal deviation from the target center was 0.085 cm. In 41.1% of the correctly timed movements the target was missed: 29.1% undershoots and 12.0% overshoots. Undershooting of the target decreased from -3.9 mm for the short vision period to -2.1 mm for the long vision period. On average, the overshoots ended + 2.5, + 1.5, and + 1.25 mm beyond the target
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boundary in the short, medium and long vision periods, respectively. The percentage of undershoots was not affected by any of the manipulated variables, whereas the number of overshoots decreased as the vision period increased, F(2,16) = 8.74; p < 0.01 (see Fig. 4). Under- and overshoots in the control condition were 13.4% (-0.95 mm) and 1.7% (+ 1.25 mm). Across the three vision conditions the percentages of temporally correct trials were similar: 99.1, 97.9 and 95.7% for the short, medium and long vision conditions, respectively. Reaction times Mean RTs were 311, 325, and 309 ms for the short, medium and long vision periods, respectively (control: 309 ms). RT was not affected by manipulation of either the vision period (p = 0.74) or its probability (p = 0.11). Movement times MT was prolonged as the vision period increased, F(2,16) = 13.46; p < 0.001; averaged movement times were 562, 575 and 587 ms for the short, medium and long vision periods, respectively (control: 621 ms). Eye-movement measures None of theses measures was affected by any of the VISIon or probability conditions. Eye-movement data are shown in Table 2. The fact that eye-movement
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Table 2 Eye-movement measures (latency, saccade duration, amplitude) and the difference between reaction time of hand and eye as a function function of the length and probability of the vision period Probability of vision period
Length of the initial vision period Short
Medium
Long
Eye-movement latency (ms)
0.72 0.14 0.14
234 231 248
243 251 250
241 245 241
82 81 79
80 82 79
Saccade duration (ms)
0.72 0.14 0.14
85 82 80
Saccade amplitude (degrees)
0.72 0.14 0.14
17 17 17
17 17 17
17 17 17
RT minus eye movement latency (ms)
0.72 0.14 0.14
80 90 63
82 59 59
67 65 88
properties were not modulated by visual conditions argues against different higher-order strategies for different visual feedback conditions in carrying out the movement task. The eye fixated the target 9 ms later than the hand movement was initiated. This was opposed to the results of the first experiment where the eye preceded the hand by 112 ms. This can be explained by the longer RT of the hand in the first experiment, because the eye-latencies were about the same in both experiments.
3.3. Discussion Again, actual vision time was found to be the most important parameter that determined spatial accuracy. When for a longer proportion of the initial movement trajectory vision is provided aiming precision increases accordingly. However, lack of vision for the last 1.675 cm, as in the 90% vision condition, still prevents performance being as accurate as in the control condition (84.9% versus 66.3%) which indicates that the final assessment of the discrepancy between hand and target is important for aiming accuracy. Making many overshoots when the target and a large proportion of the trajectory is not visible any longer whatever the highly probable vision period is, suggests that vision in the early part is used to estimate the necessary adjustments of the limb and that this estimation process is rather imprecise i.e. in many cases the distance is overestimated. When vision continues to be available the necessary adjustments are attuned increasingly better.
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4. General discussion The most important result of both experiments is that aiming accuracy was found to be tightly related to the vision period which is actually present during the execution of the movement and that this aiming performance is not affected by other vision periods which concurrently may occurr in a series. This was found both for movement task conditions in which different proportions of the initial trajectory were occluded (Experiment 1) and for task conditions in which vision was removed during different proportions of the terminal trajectory (including the target; Experiment 2). In both experiments aiming accuracy increased as a larger proportion of the trajectory could be viewed as previously has been reported by others (e.g., Carlton, 1981; Moore, 1984; Spijkers et aI., 1988; Spijkers and Lochner, 1994). These findings are in agreement with a model of continuous on-line control of movement execution in which control is attuned to the momentarily available visual information, like the stochastic optimized submovement model (Meyer et aI., 1990). Yet, the results do not definitely exclude an interpretation in terms of an intermittent visuo-motor control system with a short sampling cycle duration. Because the grain-size of the chosen vision/no-vision periods of the present experiments was quite crude, an intermittent control model with a sampling cycle duration of about 150 ms also would account for the data fairly well. However, several studies suggest that vi suo-motor cycle times are much shorter than than 150 ms (Zelaznik et aI., 1983,1987; Spijkers and Lochner, 1994). In the study of Spijkers and Lochner (1994) it was shown that aiming accuracy was impaired by brief interruptions of vision (50 ms) during the middle portion of the movement trajectory and that small (50 ms) increments in vision time improved aiming accuracy monotonously. Together with the flexibility of the vi suo-motor process that was shown in the present study, these findings support a model of continuous on-line control of movement execution. Blouin et aI. (1993b) also concluded that the visual feedback-loop system is rapid and that higher cognitive processes, such as involved in feedforward and learning processes, only may have small effects on its efficiency. Blouin et ai. (1993b) argued that the initial part of the movement (125 ms) was important for making corrections of the directional component of pointing movements. The present finding that end-point accuracy was affected by removal of vision during the initial part (see also Spijkers and Lochner, 1994), suggests that visual feedback during this initial phase is also beneficial for reducing positional error at the final phase. In general, the eye-movement pattern (latency, saccade amplitude and duration) was not affected by the manipulations of the viewing conditions. The time differences between the RT and eye-latency suggest that the aiming movement was mostly triggered by the same internal signal that also controls the programmed eye movement. The initially programmed size of the arm amplitude is then based on peripheral retinal information of the position of the target and necessary adjustments are carried out during the execution. The expectation that the target and
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hand become invisible after a certain point seems to speed up the initiation of the hand (see Experiment 1), which indicates that the coupling between the eye and hand is not fixed. No evidence was obtained that extraretinal information of the target position alone did suffice to control the arm movement accurately.
References Abrams, R.A., D.E. Meyer and S. Kornblum, 1990. Eye-hand-coordination: Oculomotor control in rapid aimed limb movements. Journal of Experimental Psychology: Human Perception and Performance 16 2, 248-267. Beaubaton, D. and L. Hay, 1986. Contribution of visual information to feedforward and feedback processes in rapid pointing movements. Human Movement Science 5, 9-14. Blouin, J., N. Teasdale, C. Bard and M. Fleury, 1993a. Directional control of rapid arm movements; The role of the kinetic visual feedback system. Canadian Journal of Experimental Psychology 47, 678-696. Blouin, J., C. Bard, N. Teasdale and M. Fleury, 1993b. On-Line versus off-line control of rapid movements. Journal of Motor Behavior 25, 275-279. Cariton, L.G., 1981. Processing visual feedback information for movement control. Journal of Experimental Psychology: Human Perception and Performance 7, 1019-1030. Hansen, R.M. and A.A. Skavenski, 1977. Accuracy of eye position information for motor control. Vision Research 17,919-926. Johansson, R.S. and Westling, G., 1990. 'Tactile afferent signals in the control of precision grip'. In: M. Jeannerod (Ed.), Attention and Performance 13, Motor Representation and Control (pp. 677-713). Hillsdale, N.J.: Erlbaum. Keele, S.W.and M.I. Posner, 1968. Processing of visual feedback in rapid movements. Journal of Experimental Psychology 77, 155-158. Komilis, E., D. Pelisson and C. Prablanc, 1993. Error processing in pointing at randomly feedback-induced double-step stimuli. Journal of Motor Behavior 25, 299-308. Luecke, S., 1993. (fuer die Wirkung geringer Alkoholdosen auf die sakkadischen Augenbewegungen in Leistungsaufgaben. Aachen: Verlag Shaker. Meyer, D.E., J.E.K. Smith, S. Kornblum, R.A. Abrams and C.E. Wright, 1990. 'Speed-accuracy trade-offs in aimed movements: Toward a theory of rapid voluntary action'. In: M. Jeannerod (ed.), Attention and performance 13: Motor representation and control (pp. 173-226). Hillsdale, NJ: Erlbaum. Milgram, P.A., 1987. A spectable-mounted liquid-crystal tachistoscope. Behavior Research Methods, Instruments, and Computers 19,449-456. Moore, S.P., 1984. Systematic removal of visual feedback. Journal of Human Movement Studies 10, 165-173. Paulignan, Y., C. McKenzie, R. Marteniuk and M., Jeannerod, 1991. Selective perturbation of visual input during prehension movements. Experimental Brain Research 83, 502-512. Prablanc, c., D. Pelisson and M.A. Goodale, 1986. Visual control of reaching movements without vision of the limb. I. Role of retinal feedback of target position in guiding the hand. Experimental Brain Research 62, 293-302. Proteau, L., 1992. 'On the specificity of learning and the role of visual information for movement control'. In: L. Proteau and D. Elliott (eds.), Vision and motor control (pp. 67 103). Amsterdam: North-Holland. Spijkers, W.A.c., K. Albracht and P.M. Lochner, 1988. Zur Bedeutung des partiellen visuellen Feedback bei diskreten Zielbewegungen [On the significance of partial visual feedback for discrete aiming movements]. Zeitschrift flir Experimentelle und Angewandte Psychologie 3, 463-475. Spijkers, W.A.C. and P.M. Lochner, 1994. Partial visual feedback and spatial end-point accuracy of discrete aiming movements. Journal of Motor Behavior 26, 283-295.
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Woodworth, R.S., 1899. The accuracy of voluntary movements. Psychological Monographs 3 (2, Whole No. 13). Zelaznik, H.N., B. Hawkins and L. Kisselburgh, 1983. Rapid visual feedback processing in single aiming movements. Journal of Motor Behavior 15, 217-236. Zelaznik, H.N., B. Hawkins and L. Kisselburgh, 1987. The effects of movement distance and movement time on visual feedback processing in aimed hand movements. Acta Psychologica 65, 181-191.
#
ELSEVIER
On-line visual control of aiming movements? Will Spijkers
a, * ,
Simone Spellerberg
b
• Institut fUr Arbeitsphysiologie an der Uniuersitiit Dortmund, Ardeystrasse 67, 44139 Dortmund, Germany b Institut fUr Psychologie, RWTH Aachen, Aachen, Germany
Abstract Two experiments are reported which addressed the flexibility of vi suo-motor processing by manipulating the availability of visual information while executing a discrete aiming movement. The flexibility of visuo-motor processing was tested by unexpectedly changing the proportion of the movement trajectory that visual feedback was present. Visual feedback was manipulated for a short (0.30), medium (0.60) or long (0.90) proportion of the trajectory within a block of trials. Each of these three proportions of vision occlusion (Experiment 1) or visual disclusion (Experiment 2) during the initial trajectory was examined. Within a visual condition, one of the three visual feedback proportions occurred with a high probability (p = 0.72), whereas the remaining two proportions each occurred with a low probability (p = 0.14). The results clearly indicated that spatial accuracy was determined by the actual vision period, independent of its probability of occurrence. The data are consistent with a model of continuous on-line control of movement execution.
1. Introduction
Visual information plays an important role in the spatial accuracy of discrete aiming movements (e.g., Keele and Posner, 1968; Woodworth, 1899). Since the early investigations of Woodworth (1899) on the underlying principles of movement control, it has been suggested that a rapid aiming movement consists of at least two successive phases: a first initial impulse phase, which is pre programmed and ballistic (open-loop) and a secondary part, the so-called current control phase, in which feedback based error-correction mechanisms are activated. This two-phase notion has been a prevalent theoretical framework and has dominated the theorizing on motor control (e.g., Carlton, 1981). However, recent studies have raised doubts about the appropriateness of this concept. Several experiments (Moore, 1984; Spijkers and Lochner, 1994; Blouin et • E-mail: [email protected]. 0001-6918/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0001-6918(95)00033-X
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aI., 1993a) clearly showed that visual feedback during the initial phase of a movement improved aiming accuracy in comparison to situations in which vision was absent during the initial part. To account for these results the original two-phase notion seems no longer tenable. It is suggested by several authors (e.g. Prablanc et aI., 1986; Komilis et aI., 1993) that the intake and processing of visual information is continuous during the execution of a movement. This constitutes also a main assumption of the stochastic optimized submovement model that was proposed by Meyer et al. (1990) to account for the speed-accuracy relations in discrete aiming. In short, the stochastic optimized submovement model assumes that a discrete aiming movement consists of one or more submovements. The primary submovement is programmed to hit the center of the target region and during its execution kinesthetic and visual information is continuously picked up. This information is used to program the secondary submovement, if the primary one is insufficiently accurate. If the assumption of continuous processing of visual information is valid, the implicit hypothesis must be proved, that the visuo-motor system reacts flexibly to unexpected manipulations of visual information. In most of the previous studies on visuo-motor coordination manipulation of visual feedback was carried out blockwise. Improved accuracy with short periods of vision presented in blocked conditions could therefore be a reflection of higher-order strategies adopted by the subjects, rather than flexible on-line processing of visual feedback. Factors like learning effects (Proteau, 1992), feedforward processes (Beaubaton and Hay, 1986) and preparatory states (Zelaznik et aI., 1983) might playa role in these strategies. When it can be shown that unexpected shortening or lengthening of the visual feedback period immediately affects the aiming performance, it would mean convincing evidence that movements can be controlled in an on-line mode. In such a view it is assumed that during movement execution information is continuously picked-up and processed in order to come to optimal aiming performance. How flexible the motor control system can react to sudden changes of haptic sensory information, was nicely shown in a series of experiments on grip movements by Johansson and Westling (1990). It appeared that unexpectedly changed circumstances in mass and slippery properties of the to be gripped object rapidly elicited corrective movements. For reaching movements Paulignan et ai. (1991) showed that the kinematic pattern of the hand was modified within 100 ms when the original position of the object to be reached for was shifted unexpectedly. The flexible use of visual information in the control of an aiming movement was investigated in the present study. Two experiments were carried out. In the first experiment visual information was occluded from the onset of the aiming movement and became available after the hand travelled a certain proportion of the trajectory. In the second experiment, visual feedback was available during identical proportions of the initial trajectory and visual occlusion was now applied for the remaining trajectory. In both experiments three proportions were examined, i.e., short, medium and long. Flexibility of visuomotor processing was examined by changing unexpectedly the proportion of either the occluded or visible trajectory at the outset of the movement. This was achieved by varying its probability of
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occurrence. Three conditions were realized in which each proportion was once the highly probable one (p = 0.72), while the other proportions each occurred with a low probability (p = 0.14). Extreme rigidity of the visuomotor system would imply that the point at which visual information is picked up, was programmed for the highly probable viewing condition as an invariant feature of the motor program. On the contrary, high flexibility would mean that the actual moment of dis- or occlusion of vision would determine the aiming accuracy of that particular movement, irrespective of its probability being either high or low. A secondary purpose of the study was to examine the temporal relationship between the initiation of the eye movement and hand movement. While fixating the starting point and the hand, subjects have peripheral, retinal information of the target position and after the saccade to the target, the (command to the) eye musculature provides extraretinal information of the target position. The eyes usually fixate the target before the hand movement is initiated under normal viewing conditions (e.g., Abrams et aI., 1990). In an exploratory way the question was addressed here whether the temporal relationship between the initiation of the eye and hand is affected by increasing periods of visual occlusion at movement initiation or movement termination.
2. Experiment 1
In the first experiment vision was occluded as soon as the hand started moving and this lasted for a certain proportion of the trajectory. In addition to a control condition (full vision), three conditions were examined in which vision was oc- . cluded for the following proportions of the trajectory: 0.30, 0.60 or 0.90. We will refer to these conditions as short, medium and long occlusion condition. A further important manipulation concerned the probability of occurrence of a certain occlusion period during a block of trials. The occlusion period, after which the condition is denoted was the highly probable one (p = 0.72), whereas the other two periods each occurred with a probability of p = 0.14. Thus, during an occlusion condition the length of the predominant occlusion period was unexpectedly changed. For example, if the occlusion condition was the medium one, the two periods with a low probability were the short and long occlusion period. When the short occlusion condition was the highly probable one, the unexpected occlusion periods always implied a prolongation of the expected one, whereas in the long occlusion condition, vision was available earlier as expected if the occlusion periods of low probability were presented. A main effect of occlusion was expected, that is, increasing the proportion of the occluded trajectory would increasingly impair aiming accuracy. The main question concerned the effect of unexpected shortening or prolongation of the occlusion period. It is expected that aiming performance depends on the actual occlusion period, irrespective of its probability of occurrence, when visuo-motor processing is flexibly following the actual viewing circumstances. Based on previous
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research (e.g., Spijkers and Lochner, 1994) it was hypothesized that even occluding a small proportion of the initial movement trajectory would impair aiming accuracy in comparison to the control condition (full-vision). 2.1. Method Subjects Nine subjects (3 female and 6 male adults, mean age, 24 years) participated. They were right-handed and had normal or corrected-to-normal vision. Task and apparatus Subjects performed an aiming task in which a stylus was moved laterally from a starting point (5 X 5 mm) to a target on the left (width X height: 5 X 100 mm). Distance from center of the starting area to the midline of the target was 16.75 cm. Because the starting point was directly in front of the subject the hand did not obstruct the sight on the target during execution of the movement. The movement had to be executed such that the movement times were within a 33.3% range of an imposed movement duration of 600ms. This particular time constraint was imposed in order to obtain similar movement profiles across subjects and to compare the results with other movement tasks currently under investigation. Directly above the starting point an array of three LEOs was mounted. The middle yellow LED sezved as a feedback signal, indicating whether the stylus was correctly placed on the start position. Two green coloured LEOs, each immediately adjacent to the yellow one, sezved as warning signals; the visual warning was accompanied by a tone (300 ms; 1000 Hz). After a constant foreperiod (2 s) the imperative signal, the left LED, was illuminated; it was extinguished as soon the movement started. Movement begin was defined as the stylus was moved across the boundaries of the starting position (see also procedure and design). Movement recording Seated in front of a table in which a digitizer (Kurta) was mounted flush to the table surface, subjects performed the aiming movements by sliding a pencil (10 g) across a glass plate that covered the digitizer tablet. Underneath the glass plate a cardboard was placed on which the starting point and target were drawn. The movement coordinates were recorded with a sampling rate of 100 Hz and a spatial accuracy of 0.25 mm. A PDP 11-23 computer controlled presentation of the signals, the length of the programmed visual feedback intezvals, measured reaction and movement times, and stored spatial coordinates of the movement. Manipulation of visual feedback Duration of visual feedback was varied by way of a Portable Liquid-crystal Apparatus for Tachistoscopic Occlusion (PLATO; for technical details see Milgram, 1987). The glasses of the PLATO-apparatus consist of liquid crystals which may adopt two different states depending on the induced electric potential: "open" vs. "closed". Transition from the open to the closed state and vice versa
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occurs within 5 ms. In the open state, at which 85% of the luminance is transmitted, the glasses allow normal retinal vision; the closed state scatters the incoming light and leads to a completely uniform visual field, which doesn't even allow the subject to perceive any contours. The light-scattering cholesteric liquid crystals do not become opaque in their closed state, but remain translucent. The subject's eye therefore remains substantially illuminated during the spectacles' closed state. The PLATO-apparatus switches off visual feedback without largerly affecting light adaptation; for that reason confounding effects arising from different states of dark-adaption could be excluded. The subjects wore the glasses throughout the whole experiment.
Eye-movement recording During the experiment subjects head was fixed: chin, forehead and the temples were softly, but firmly clenched, so that no head movements were possible in either the horizontal or vertical direction. Fixation of the head was individually adjusted. Eye movements were recorded by means of surface electrodes (Beckmann silverchloride; diameter 1.6 mm), which were placed on the medial portion of the orbicularis oculi at both sides of the head. A third reference electrode was fixed to the ear. The responses were amplified and continuously sampled by means of a specially developed program for Atari (1040) with a frequency of 1000 Hz (Luecke, 1993). Before the start of the experiment a calibration procedure was conducted to obtain the individual EOG-to-eye-movement ratio. This calibration procedure was repeated during the session prior to each condition change. During calibration of the EOG-signal subjects had to fixate three capital letters: one at the starting point of the hand-movement, the other two at 16.75 cm to the right and left of this point. Arm-movements were synchronized with the eye movement by means of markers in the EOG-signal in order to relate hand-position with the eye-position. On the EOG-trace the following three time-markers were superimposed: one at presentation of the imperative signal, the next one at movement begin (as the stylus left the starting position) and the last one when the hand arrived at the target. Subjects were only instructed to fixate the starting point until the imperative signal was presented. Procedure and design
A trial was initiated when the subject placed the stylus on the start position. After 1.5 s, an auditory warning signal sounded and the two green LEOs were illuminated, which was followed by the visual imperative signal after a fixed interval of 2 s. In a within-subject design, we studied three periods of occlusion. In addition to the control condition with continuous vision, the three experimental conditions were short, medium and long occlusion of the movement trajectory. These three conditions were completely counterbalanced across six subjects, while for the three remaining subjects one sequence of the conditions was repeated. Within each occlusion condition all three occlusion periods occurred: One with a probability of p = 0.72 and the two remaining ones each with a probability of 0.14. Visual
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occlusion was applied synchronous to the start of the movement. As soon the scheduled point in the trajectory was passed, the PLATO-glasses opened, so that target and hand could be viewed again. A session started with a training series of 50 trials with uninterrupted vision to get used to the task and to practice the execution of the movement within the imposed time constraints. During this training series subjects received verbal feedback on their movement time. The series was repeated (without providing feedback concerning the movement times) and this was considered as control condition; the control condition was again given at the end of the session. After the initial control condition, subjects performed the three occlusion conditions consecutively; each new condition was preceded by repetition of the calibration procedure. In each occlusion condition subjects accomplished 100 trials in two consecutive series of 50 trials. Thus, in each occlusion condition 72 trials were performed with the frequent occlusion period and 14 trials with each infrequent occlusion period. Immediately before presentation of the imperative signal, the effective size of the start position was reduced. It then extended only 1 X 1 mm around the momentary position of the tip of the stylus. By this procedure, the measurements of the reaction time was minimally inflated by the time needed to arrive at and cross the starting point borders. When subjects made premature reactions by moving the stylus inadvertently outside the starting point before presentation of the imperative signal, the trial was started again. Reaction time (RT) was recorded as the time that elapsed between the onset of the imperative signal and departure from the home position. The time between leaving the home position and first crossing of the target boundary defined movement time (MT). As subjects did not reach the target or passed over the target, these movements were recorded as under- and overshoots, respectively. Movement time was then computed as the time between leaving the starting position and the moment in the recorded movement trace that three consecutive coordinates were identical. 2.2. Results
Two factors were distinguished: occlusion period and probability of the occlusion period. Because each occlusion period occurred once with a high probability and twice with an identical low probability, it was examined first whether the effects of the low probability conditions differed from each other. For none of the dependent variables a difference was observed (p > 0.20). In addition, no interaction with occlusion period was found (p > 0.30). Hence, the values averaged across the two low probability conditions of each occlusion period were entered in the ANOVA. For each dependent measure a 3 x 2 ANOVA was conducted with occlusion period (short, medium, long) and probability (0.72 vs. 0.14) as factors. Spatial accuracy Analysis was focused on the percentage of target hits as spatial accuracy measure. A target hit was defined as an aiming movement that ended at the target
t
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within the imposed time. In the control condition, a score of 87% hits was obtained. In Fig. 1 the percentages of target hits are shown. In Fig. 1a the percentage of target hits is shown as function of the actual occlusion period and its concurrent occlusion condition in which the actual occlusion period occurred. In this panel it can be observed that aiming performance depends on the actual period of no-vision and is not affected by concurrent no-vision proportions. Spatial accuracy as a function of the actual occlusion period and probability of occurrence of this period is shown in Fig. lb. Both panels clearly show that extension of the occlusion period reduced spatial accuracy, F(2,16) = 29.69; p < 0.001, and that the reduction was not influenced by the occlusion condition in which the periods occurred. Important for the main question of the study is that nor the main effect of probability, F(1,8) = 0.33; p = 0.59, neither its interaction with actual occlusion period, F(2,16) = 1.38; p = 0.28, were significant. On average, the target hits had an amplitude of 16.70 cm; this means that subjects stopped their movement 0.05 cm before the center of the target. Across the three occlusion conditions the percentages of temporally correct trials were 95.8, 91.7 and 83.3 for the short, medium and long occlusion conditions, respectively. Because the prolongation of the occlusion period was accompanied by a reduction of the number temporally correct movements, the percentages of hits were expressed as the number temporally correct trials. Additional ANOVAs which included all trials instead of only the temporally correct ones, revealed the
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same significant main effects. The latter result means that the basic result is not affected by differences in the number of temporally correct trials between occlusion conditions. Averaged across experimental conditions 35.1 % of the temporally correct movements were target misses and they consisted of 22% undershoots and 13.1% overshoots. For the short, medium and long occlusion periods, the undershooting movements ended 1.5, 3.2 and 4.6 mm before the target and the overshoots 2.5, 3.2 and 3.75 mm behind the target, respectively. In the control condition there were 10.9% undershoots (-1.0 mm) and 2.1 % overshoots (+ 1.0 mm). Fig. 2 depicts the percentages of under- and overshoots as a function of the occlusion period and its probability of occurrence. The percentage of undershoots increased as a function of occlusion period, F(2,16) = 5.45; p < 0.5, and was slightly lower for the low probability condition, F(1,8) = 5.19; p = 0.05. Overshoots also increased with occlusion period, F(2,16) = 7.51; p < 0.01, and the number of overshoots tended to be smaller for the low probability condition, F(1,8) = 4.30; p = 0.07. Reaction times Mean RTs were 363, 365, 361, 371 ms for the control and the short, medium and long occlusion conditions respectively. RT was independent from actual occlusion period or its probability (p > 0.26). Hence, foreperiod was sufficiently long to prepare the movement completely.
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Table 1 Eye-movement measures (latency, saccade duration, amplitude) and the difference between the reaction time of hand and eye as a function of the length and probability of the no-vision period Probability of no-vision period
Short
0.72 0.14 0.14
246 264 242
Length of initial no-vision period Medium
Long
Eye-movement latency (ms)
262 253 249
247 245 265
76 75
74 77 76
Saccade duration (ms)
0.72 0.14 0.14
76 75 75
72
Saccade amplitude (degrees)
0.72 0.14 0.14
17 16 15
16 17 15
15 17 16
RT minus eye movement latency (ms)
0.72 0.14 0.14
112 99 137
96 96 127
137 115 93
Movement times
Mean movement times of the temporally correct movements were 593, 576, 576 and 545 ms for the control and the short, medium and long occlusion periods, respectively. A main effect of actual occlusion period was found, F(2,16) = 6.84; p < 0.01, and this was caused by the speeding up of the movement in the trials in which a large proportion was occluded. Nor the main effect of probability neither its interaction with actual occlusion period reached significance (p > 0.18). Eye-movement measures
Eye-movement latency, saccade duration, saccade amplitude and the difference between the RT of the eye and the hand are shown in Table 1 for each occlusion period and probability. No main effect of the occlusion manipulations on the eye-movement measures was observed. The interaction between occlusion period and probability which was found for saccade amplitude revealed that subjects shortened the amplitudes if the long occlusion period was highly probable, but prolonged them if the long occlusion period had a low probablity of occurrence, F(2,16) = 4.96; p < 0.05. Fixation of the target preceded the initiation of the aiming movement by 48 ms on average and the eye-fixation leading time increased from 40 to 58 ms across the occlusion periods, F(2,16) = 4.87; p < 0.05.
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2.3. Discussion
The results unequivocally showed that" the actual vision time is flexibly used to control spatial aiming accuracy, because the effect of an occlusion period was not modulated by its probability of occurrence. A flexible use of the available visual information excludes a vi suo-motor control model that assumes one programmed point in the trajectory as anchor point for updating the arm control system. As expected aiming performance decreased with prolongation of the occlusion period. The short occlusion period impaired target aiming by reducing the accuracy from 87% (control condition) to 73.1 %. Despite a remaining viewing period of more than 10 cm, performance could not be corrected to the level of the control condition. Again this finding confirms previous results that vision during the first phase of a movement contributes significantly the end-point accuracy (e.g. Spijkers and Lochner, 1994). This is contrary to the conclusion of Blouin et al. (1993a), who stated that that the initial interval is only relevant for directional accuracy, and not for terminal accuracy.
3. Experiment 2 The second experiment examined how aiming accuracy is influenced when the available vision is restricted to the outset of the movement. In this case the positional control of the hand must be based on the memory representation of the position of the target that was formed before occlusion. In addition to the remembered position based on peripheral retinal information, target position information is then predominantly available by extraretinal information, for example, eye-position and the efference copy of the programmed eye movement. Several studies have shown that this type of extraretinal information is sufficient for accurate aiming (e.g. Hansen and Skavenski, 1977). 3.1. Method. Subjects Nine subjects (3 female and 6 male adults; mean age 2~ years) took part. Three of the male subjects had also participated in the previous experiment. They were right-handed and had normal or corrected-to-normal vision. Task and procedure The same task and procedure were used as in the previous experiment. The only change concerned the occlusion conditions. Now the glasses stayed open for a certain proportion of the trajectory as the movement started and vision was removed as soon the stylus passed the scheduled spatial point. Again three conditions were examined in each of which the probability of one vison period was high (p = 0.72), whereas the other two vision periods had a low probability (p = 0.14).
,
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3.2. Results
A preceding analysis showed that neither the difference between the data of the low probability levels nor its interaction with vision period was significant for any of the investigated variables (p > 0.16). For each dependent measure a 3 X 2 ANOVA was conducted with vision period (short, medium, long) and probability (0.72 vs. 0.14) as factors. Spatial accuracy
In the control condition a score of 84.9% hits was obtained. The percentage of hits increased significantly with length of the vision period, F(2,16) = 14.07; p < 0.001, but again was not affected by its probability of occurrence, F(1,8) = 0.32; p = 0.58. Fig. 3a shows that aiming performance for a certain actual vision period remained the same irrespective of the length of the concurrently present vision periods. In Fig. 3b the percentage of target hits is depicted as a function of vision period and its probability of occurrence. Inspection of the mean amplitudes per vision condition revealed that the maximal deviation from the target center was 0.085 cm. In 41.1% of the correctly timed movements the target was missed: 29.1% undershoots and 12.0% overshoots. Undershooting of the target decreased from -3.9 mm for the short vision period to -2.1 mm for the long vision period. On average, the overshoots ended + 2.5, + 1.5, and + 1.25 mm beyond the target
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50
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boundary in the short, medium and long vision periods, respectively. The percentage of undershoots was not affected by any of the manipulated variables, whereas the number of overshoots decreased as the vision period increased, F(2,16) = 8.74; p < 0.01 (see Fig. 4). Under- and overshoots in the control condition were 13.4% (-0.95 mm) and 1.7% (+ 1.25 mm). Across the three vision conditions the percentages of temporally correct trials were similar: 99.1, 97.9 and 95.7% for the short, medium and long vision conditions, respectively. Reaction times Mean RTs were 311, 325, and 309 ms for the short, medium and long vision periods, respectively (control: 309 ms). RT was not affected by manipulation of either the vision period (p = 0.74) or its probability (p = 0.11). Movement times MT was prolonged as the vision period increased, F(2,16) = 13.46; p < 0.001; averaged movement times were 562, 575 and 587 ms for the short, medium and long vision periods, respectively (control: 621 ms). Eye-movement measures None of theses measures was affected by any of the VISIon or probability conditions. Eye-movement data are shown in Table 2. The fact that eye-movement
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Table 2 Eye-movement measures (latency, saccade duration, amplitude) and the difference between reaction time of hand and eye as a function function of the length and probability of the vision period Probability of vision period
Length of the initial vision period Short
Medium
Long
Eye-movement latency (ms)
0.72 0.14 0.14
234 231 248
243 251 250
241 245 241
82 81 79
80 82 79
Saccade duration (ms)
0.72 0.14 0.14
85 82 80
Saccade amplitude (degrees)
0.72 0.14 0.14
17 17 17
17 17 17
17 17 17
RT minus eye movement latency (ms)
0.72 0.14 0.14
80 90 63
82 59 59
67 65 88
properties were not modulated by visual conditions argues against different higher-order strategies for different visual feedback conditions in carrying out the movement task. The eye fixated the target 9 ms later than the hand movement was initiated. This was opposed to the results of the first experiment where the eye preceded the hand by 112 ms. This can be explained by the longer RT of the hand in the first experiment, because the eye-latencies were about the same in both experiments.
3.3. Discussion Again, actual vision time was found to be the most important parameter that determined spatial accuracy. When for a longer proportion of the initial movement trajectory vision is provided aiming precision increases accordingly. However, lack of vision for the last 1.675 cm, as in the 90% vision condition, still prevents performance being as accurate as in the control condition (84.9% versus 66.3%) which indicates that the final assessment of the discrepancy between hand and target is important for aiming accuracy. Making many overshoots when the target and a large proportion of the trajectory is not visible any longer whatever the highly probable vision period is, suggests that vision in the early part is used to estimate the necessary adjustments of the limb and that this estimation process is rather imprecise i.e. in many cases the distance is overestimated. When vision continues to be available the necessary adjustments are attuned increasingly better.
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4. General discussion The most important result of both experiments is that aiming accuracy was found to be tightly related to the vision period which is actually present during the execution of the movement and that this aiming performance is not affected by other vision periods which concurrently may occurr in a series. This was found both for movement task conditions in which different proportions of the initial trajectory were occluded (Experiment 1) and for task conditions in which vision was removed during different proportions of the terminal trajectory (including the target; Experiment 2). In both experiments aiming accuracy increased as a larger proportion of the trajectory could be viewed as previously has been reported by others (e.g., Carlton, 1981; Moore, 1984; Spijkers et aI., 1988; Spijkers and Lochner, 1994). These findings are in agreement with a model of continuous on-line control of movement execution in which control is attuned to the momentarily available visual information, like the stochastic optimized submovement model (Meyer et aI., 1990). Yet, the results do not definitely exclude an interpretation in terms of an intermittent visuo-motor control system with a short sampling cycle duration. Because the grain-size of the chosen vision/no-vision periods of the present experiments was quite crude, an intermittent control model with a sampling cycle duration of about 150 ms also would account for the data fairly well. However, several studies suggest that vi suo-motor cycle times are much shorter than than 150 ms (Zelaznik et aI., 1983,1987; Spijkers and Lochner, 1994). In the study of Spijkers and Lochner (1994) it was shown that aiming accuracy was impaired by brief interruptions of vision (50 ms) during the middle portion of the movement trajectory and that small (50 ms) increments in vision time improved aiming accuracy monotonously. Together with the flexibility of the vi suo-motor process that was shown in the present study, these findings support a model of continuous on-line control of movement execution. Blouin et aI. (1993b) also concluded that the visual feedback-loop system is rapid and that higher cognitive processes, such as involved in feedforward and learning processes, only may have small effects on its efficiency. Blouin et ai. (1993b) argued that the initial part of the movement (125 ms) was important for making corrections of the directional component of pointing movements. The present finding that end-point accuracy was affected by removal of vision during the initial part (see also Spijkers and Lochner, 1994), suggests that visual feedback during this initial phase is also beneficial for reducing positional error at the final phase. In general, the eye-movement pattern (latency, saccade amplitude and duration) was not affected by the manipulations of the viewing conditions. The time differences between the RT and eye-latency suggest that the aiming movement was mostly triggered by the same internal signal that also controls the programmed eye movement. The initially programmed size of the arm amplitude is then based on peripheral retinal information of the position of the target and necessary adjustments are carried out during the execution. The expectation that the target and
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hand become invisible after a certain point seems to speed up the initiation of the hand (see Experiment 1), which indicates that the coupling between the eye and hand is not fixed. No evidence was obtained that extraretinal information of the target position alone did suffice to control the arm movement accurately.
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