under two ball velocity conditions with normal binocular vision. Under the faster ..... initial analysis of the experiments, and Peter Beek and John Whiting for their.
Complex Movement Behaviour: The'motor-actioncontroversy, pp. 189-199 O.G. Meijer & K Roth (editors) 0 Elsevier Science Publishers B.V. (North-Holland), 1988
Chapter6
VISUAL CONTROLOF AN ATTACKING FOREHAND DRIVE INTABLETENNIS
Reinoud J. Bootsma and Piet C.W. van Wieringen
SUMMARY
An expert table tennis player was required to perform attacking forehand drives under two ball velocity conditions with normal binocular vision. Under the faster condition he also performed monocularly. Film analysis results suggest that under binocular conditions two sources of information were used for timing the initiation of the drive, viz. ball location and time to frontal eye plane (FEP) - the time period that would elapse between the moment of movement initiation and the moment the ball would cross the frontal plane through the eyes. Under the monocular condition, in which localization of the ball is supposed to be more d i f f m l t , the subject tended to slow down his movements. Moreover, while under the binocular conditions the subject was shown to execute a consistent drive, under the monocular condition he adapted his drive to the very slight variations in time to FEP. It is tentatively concluded that under normal (binocular) Conditions the player simply runs off a standard 'motor programme', while under the more uncertain (monocular) condition 'programme parameters' are adaptively varied to match the changes in environmental information.
1.Introduction In one of the many discussions during this Conference, Schmidt commented on the work of Warren (1984)on stair-climbing. Basically, his argument was that the results Warren had obtained were not in contradiction to any current theory of motor control. We believe this to be true, but, more importantly, we believe that no traditional theory of motor control would have posed the question of how the actions of a person are guided by his perception, as did Warren in the work just mentioned, and as other proponents of an ecological approach to perception and
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action have done before him (e.g., Lee, 1976 & 1980; Von Hofsten, 1983). Herein lies, perhaps, one of the most important contributions of an action systems approach to the field of motor control and learning. It has enlarged the scope of traditional motor control research and theorizing from a component-analysis approach to a holistic approach. The purpose of this Conference was to critically evaluate the present controversy in the field of motor control between what have come to be known as 'motor systems approaches' (e.g., Adams, 1971; Schmidt, 1975) and 'action systems approaches' (e.g., Kugler, Kelso & Turvey, 1980; Lee, Lishman & Thomson, 1982;Lee et al, 1983;Reed, 1982;Turvey, 1977). Such an evaluation is beyond the scope of the present study, which explores a much more restricted topic. It aims at gaining some insight into the constructive role of visual information sources for the execution and preservation of ongoing action. The approach which is followed is in line with the work of Lee, referred to earlier. However, the adoption of such an approach should not be considered to imply an a priori rejection of motor systems theories; in looking for the mechanisms subserving the guiding function of visual information such an approach might, at least in principle, be useful.
2. An experimentconcerningsources of visual information in table tennis
The experiment that will be reported here is a small pilot study, conceived to distinguish between different potential sources of visual information guiding a real-life action, viz. the attacking forehand drive in table tennis. David Lee and hie co-workers have provided a host of data, all in support of the notion that a specific perceptual variable, tau - the inverse of the rate of dilation of the retinal image - , is used to accommodate action, be it in diving gannets (Lee, 1980), long jumpers running up to the take-off board (Lee, Lishman & Thomson, 1982),car drivers braking to avoid collision (Lee, 19761,or subjects jumping up to hit a falling ball (Lee et al, 1983). This variable, it is claimed, directly specifies the time-to-contact between the actor (more specifically, the actor's eye) and the environmental object of interest, at least in situations in which the velocity of the object relative to the actor is constant. Von Hofsten (1986)argues in this respect that, although time-to-contact can indeed be used in the performance of tasks in which the eye is approached directly by an object, it cannot be the sole determinant in, for example, catching. If an object is to be contacted at a variable distance away from the eye, additional information about the object in question must be available as well.
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In the experiment to be reported the subject was an expert table tennis player. Such expert performers can be distinguished from novices by, amongst other features, highly consistent movement characteristics (Franks, Weicker & Robertson, 1985; Spaeth-Arnold, 1977; Tyldesley & Whiting, 1975). Such highly consistent movement characteristics imply that, if the action is to be successful, the performer has to be very accurately tuned to the environment. Three sources of information have, in different studies, been put forward as determining the temporal initiation point (TIP)of an action like a table tennis drive: 01. Distance between the player and the object to be hit (Eckert & Hamdorf, 1980); 02. Location in extracorporeal space of the object to be hit (Tyldesley, 1980); 03. Time to contact as specified by tau (Lee et al, 1982 & 1983).
-1.If this kind of information (distance between the player and the object to be hit) is, in fact, used for initiating the drive, the performer would, necessarily, have to alter his movement velocity if objects were to approach with different velocities; -2. The same applies if extracorporeal location of the object to be hit is used, unless the performer is able to adjust his position in space prior to the execution of the drive itself in such a way that the time to contact from the extracorporeal location of the object is kept constant; -3.If a particular time to contact, as specified by tau, is used to determine the temporal initiation point of the drive, a constant movement velocity and movement time would be expected. Conflicting results with respect to any one of these variables have been reported in the literature but, it is maintained here, such differences may be more artefactual than real in the sense that they may be brought about by the differing constraints imposed upon the player in the different studies. In the Lee et a1 (1983)study, for example, environmental information was reduced, and in the Tyldesley (1980) study the use of only one approach velocity meant that all three information sources previously mentioned were confounded. In the present study a top table tennis player was required to execute an extensive series of attacking forehand drives under conditions differing with respect to vision and ball velocity. 2.1. Method
Subject The subject was a former Dutch national table tennis champion. He was 23 years of age, right handed, and volunteered for participation in the experiment.
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Task The experimental set-up was a normal table tennis situation with a Sitco RII-s ball projection machine standing opposite the player behind the table. Balls were delivered to the subject at a constant rate of 40 per minute. The subject was required to smash the balls as hard and as accurately as possible onto a circular target (55 cm diameter) on the opposite side of the net, utilizing an attacking forehand drive technique.
Procedure The subject was tested under three conditions, viz. 1. BILO (binocular vision, low ball velocity - horizontal ball velocity at TIP 4.1 dsec), 2. BIHI (binocular vision, high ball velocity - horizontal ball velocity at TIP 4.5 dsec), 3. MOHI (monocular vision, high ball velocity - horizontal ball velocity at TIP 4.5 dsec). In the latter condition the non-dominant (left) eye was covered. Under both ball velocity conditions the flight paths were almost identical; comparable flight times (preview times about 550 msec) were obtained by moving the robot backwards under the higher velocity condition. The three conditions were presented blockwise. After a number of acclimatization trials, which were repeated under each of the three conditions, film recordings were made of, respectively, 25 trials in condition BILO, 25 trials in condition BIHI, and 12 trials in condition MOHI. These recordings were made using a Teledyne camera running at 150 frames per sec. Following development, the films (Kodak 4 x Reversal 400 ASA) were projected by means of a NAC (DF-16b) projector onto an opaque screen. Frame by frame, the coordinates of ball, bat, and right eye were digitized, using a SAC 14" XY tablet, Connected to an Apple 11microcomputer. After smoothing, using a second order recursive Butterworth filter (cut-off frequency 8.0 Hz),'information' parameters, such as location of the ball in space relative to the table, distance between the ball and the eye, and time to Frontal Eye Plane (FEP) were calculated for the temporal initiation point of the drive. Time to FEP refers to the time period which would elapse between TIP and the moment the ball, when maintaining constant velocity, would cross the frontal plane which passed through the eye at TIP. This.'virtual' time (virtual, because the ball is intercepted by the bat) is considered to be very close to 'time to contact' as defined by Lee (1980). The latter term, however, might lead to confusion with the time period between TIP and the moment of ballhat contact, and will, therefore, be avoided. In addition to the aforementioned 'information' parameters, a number of 'execution' parameters, such as spatial location of the bat at the moment of initiation and at ballhat contact, movement duration, and mean bat velocity, were derived.
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From the trials which were recorded, some, because of imperfect registration, were unsuitable for analysis. Nineteen trials under the BILO condition, 21 trials under the BIHI condition, and 10 trials under the MOHI condition, were available for analysis.
Table 1. Means, standard deviations, t and F values, and levels of significance for selected variables from the cine-analysis of a top player under two different ball velocity conditions with normal binocular vision. Means were compared by way of a t-test, standard deviations by means of an F-test. In the case where the F-test would reveal significant differences in standard deviations, a t-test with separate variance estimates was used. TTF-TIP: estimated time to FEP at the temporal initiation point (TIP) of the drive; BAL-TIP: horizontal position of the ball in space relative to the leading edge of the table at TIP; DIST-TIP: distance between the eye and the ball at TIP; DURAT: duration of the drive; BAT-TIP: horizontal position of the bat relative to the leading edge of the table at TIP; BAT-HIT: horizontal position of the bat relative to the leading edge of the table at the moment of contact between bat and ball; MBV: mean bat velocity during the drive.
I
VARIABLE
LOW VELOCITY iIGH VELOCITY N=19 N=21
lTF-TIP (msec) M SD
STATISTIC
P
126.9 11.3
122.7 7.7
t = 1.38 F = 2.13
0.175 0.104
BAL-TIP (cm)
M SD
0.3 8.3
2.5 4.7
t = 1.29 F = 3.05
0.206 0.01 8
DIST-TIP (cm)
M SD
62.3 4.7
65.5 4.7
t = 2.12 F = 1.01
0.040 0.982
DURAT (msec) M SD
90.6 4.0
91.8 4.1
t = 0.95 F = 1.06
0.347 0.907
BAT-TIP (cm)
M SD
117.7 7.1
121.8 5.7
t = 2.02 F = 1.55
0.051 0.344
BAT-HIT (cm)
M SD
50.7 8.1
56.6 4.6
t = 2.80 F = 3.12
0.009 0.016
MBV(m/sec)
M SD
7.89 0.38
7.66 0.51
t = 1.66 F = 1.81
0.106 0.21 1
R.J. Bootsma h P.C. W. van Wieringen
194 2.2. Results
The results from the normal (binocular) vision conditions are summarized in Table 1. They indicate that mean times to FEP did not differ significantly between Table 2. Means, standard deviations, t and F values, and levels of significance for selected variables from the cine-analysis of a top player under two different vision conditions, the velocity of the ball being the same under both conditions. Means were compared by way of a t-test, standard deviations by way of an F-test. In the case where the F-test would reveal significant differences in standard deviations, a t-test with separate variance estimates was used. TTF-TIP: estimated time to FEP at the temporal initiation point (TIP) of the drive; BAL-TIP: horizontal position of the ball in space relative to the leading edge of the table at TIP; DIST-TIP: distance between the eye and the ball at TIP; DURAT: duration of the drive; BAT-TIP: horizontal position of the bat relative to the leading edge of the table at TIP; BAT-HIT: horizontal position of the bat relative to the leading edge of the table at the moment of contact between bat and ball; MBV: mean bat velocity during the drive.
STATISTIC
P
VARIABLE
MONOCULAR N=10
BINOCULAR N=21
llF-TIP (rnsec) M SD
123.6 8.6
122.7 7.7
t = 0.29 F = 1.24
0.772 0.656
M SD
-6.8 5.1
2.5 4.7
t = 4.98 F = 1.17
0.001 0.729
SD
M
67.9 3.4
65.5 4.7
t = 1.44 F 1.91
0.160 0.318
DURAT (rnsec) M SD
97.2 8.5
91.8 4.1
t = 1.91 F = 4.26
0.082 0.007
BAT-TIP (cm)
M SD
114.1 6.6
121.8 5.7
t = 3.33 F = 1.33
0.002 0.566
BAT-HIT (crn)
M SD
50.1 4.9
56.6 4.6
t = 3.63 F = 1.13
0.001 0.772
MBV(m/sec)
M SD
7.01 0.58
7.66 0.51
t = 3.18 F = 1.29
0.004 0.608
BAL-TIP (crn) DIST-TIP (cm)
5:
-
Viswl Control in Table Tennis
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the conditions BILO and BIHI, although time to FEP was somewhat more variable under the former condition than under the latter. With respect to the extracorporeal location of the ball at TIP (relative to the leading edge of the table in the direction of the player) no significant difference between the conditions BILO and BIHI was found. The distance between the subject and the ball at TIP, however, was significantly different under the BILO and BIHI conditions. Given the non-significant difference between the times to FEP under both conditions, the latter result was, of course, to be expected. The 'execution' parameter, movement duration, showed no significant difference between the two conditions. The very low variability of the parameter is remarkable. An explanation of the simultaneous finding of non-significant differences in times to FEP and ball locations at TIP between the two velocity conditions is found in the fact that the subject stepped backwards a little under the high velocity condition prior to the drive proper, thus transposing his entire body position with respect to the leading edge of the table. This is reflected in the differences in position of the bat at TIP and at ballhat contact between both conditions. In Table 2 the values of the dependent variables under the BIHI condition are further compared with those obtained under the MOHI condition. Again, the mean times to FEP did not differ significantly. An interesting finding concerned the mean location of the ball in space at TIP, which differed significantly under the two vision conditions, while ball velocity did not change. Although the length of the drive remained unchanged, mean bat velocity decreased and movement duration increased under the monocular situation. Movement time also became more variable.
3. Discussion
The results obtained under the binocular conditions suggest that the subject made use of two kinds of information to guide his actions, notably time to FEP and balllocation. By stepping backwards under the higher ball velocity condition, the drive could be initiated at the moment the ball occupied about the same location in space, while time to FEP at TIP was also held constant. The very low variability of movement duration, indicating high consistency in movement execution, confirms the findings in expert players reported by Tyldesley and Whiting (1975) and Franks et a1 (1985).
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According to Schmidt (1982), such consistency might be the consequence of the 'running off' of a well established 'motor programme'. Within such a framework, the 'triggering' of this programme would appear to be controlled by two information sources at the same time: time to FEP and ball-location. Instead of relying totally on one perceptual source of information, as might be implied by recent experimental work (Eckert & Hamdorf, 1980; Lee et al, 1982 & 1983; Tyldesley, 1980; Wagner, 1982), the subject appears to employ a multiple source strategy. If the latter conjecture is, in fact, correct, it would follow that preventing adequate use of one of these information sources would render movement execution more difficult and, consequently, less consistent. This prediction was borne out under the monocular condition. In this condition, information about location and trajectory of the ball would be more difficult to obtain because of the lack of binocular disparity cues (Regan, Beverley & Cynader, 1979), whereas time to FEP (specified by the rate of dilation of the retinal image of the uncovered eye) should still be accessible (McLeod, McLaughlin & Nimmosmith,in press). The results obtained here did, indeed, indicate that under the monocular condition movement execution was more variable than under the binocular condition with the same ball velocity. Moreover, movement execution seemed to be more 'cautious' under the monocular than under the binocular condition, as can be inferred from the finding that mean bat velocity was significantly lower under the former condition. With respect to the results obtained under the binocular conditions, it was speculated that the 'triggering' of the motor programme specifying the forehand drive might be under control of visual information. Post-hoc considerations led to further explorations. These explorations were motivated by the possibility that, although the low variability in movement execution may indeed be seen as one more example of the consistency of top-players, it may, nevertheless, be taken as signalling the influence of visual control factors. This latter point of view motivated the assessment of correlation coefficients between time to FEP at TIP on the one hand, and peak bat velocity and peak bat acceleration on the other. Under the BILO condition time to FEP correlated positively with peak bat velocity (rp = .42, p < ,051and peak bat acceleration (rp = .43, p < .05).Under the BIHI condition these correlations were again positive, although somewhat smaller - the correlation coefficients being .43 (p < .05)and .36 (p c .lo), respectively. It should be noted, however, that under the latter condition variations in time to FEP were extremely small; therefore, significant correlations would be diflcult to obtain (restriction of range).
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Thus, it appears justified to suggest that in the binocular condition the player operates within a specific bandwidth with regard to time to FEP (in the sense of starting his drive within the period defined by this bandwidth), and runs off his drive in the same, ballistic, way from there on. That such a control strategy would indeed give rise to positive correlations, as have been found, is clear if it is realized that the bat is continuously accelerating until the ball is hit, implying that a longer time to FEP at TIP is associated with a higher peak ball velocity and acceleration. Interestingly, in the monocular, high ball velocity condition negative correlations between time to F'EP at TIP and both peak bat velocity (rp = -.75,p c .Ol> and acceleration (rp = -.77,p c .01)were found. Such negative correlations would be expected if time to FEP at TIP is not used for triggering the same motor programme time and again, but for 'guiding' the action in eliciting a slower drive when having a relatively large, and eliciting a faster drive when having a relatively small value. Following these lines of argument, it is tentatively concluded that the top player under the normal (binocular) conditions resorts to highly consistent forehand drives, supposedly governed by a motor programme of which the parameters are kept constant. When the situation is less familiar and uncertainty about ball location and trajectory increases (monocular condition), the player appears to adapt the execution of his drives to the characteristics of the visual information at TIP. This might be achieved by specifying different values for the force parameter of the motor programme from trial to trial. Whether or not a motor programme, or some other mechanism, is subserving the perception-action coupling can not, however, be deduced from the results of the present experiment as it is not known whether the observed trial-to-trial adaptation is accomplished prior to or during the drive. It should be borne in mind, though, that Lee et a1 (1983)report evidence against the utilization of a motor programme in subjects jumping up to punch a falling ball. A n alternative mechanism has, however, not yet been put forward. It goes without saying that the interpretation of the results obtained in this N=l study is still speculative. Replication of the experiment on a larger scale is momentarily under way. At least, the tentative conclusions, drawn about the modes of control exemplified under the normal (binocular) and more uncertain (monocular) conditions, serve as hypotheses for these replications. The way in which they were framed illustrates the point, hinted at in the introduction, that looking for relations between visual information and actions attuned to it, does not a priori exclude explanations for such relations in 'motor systems' terms.
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ACKNOWLEDGEMENTS We gratefully acknowledge the assistance of Hans Savelberg in carrying out the initial analysis of the experiments, and Peter Beek and John Whiting for their helpful commenta and discussions. This study was supported by the Netherlands Organization for the Advancement of Pure Science, project #560-259-024.
REFERENCES Adams, J.A. (1971). A closed-loop theory of motor learning. Journal of Motor Behavior, 3, 111-149. Eckert, H. & Hamdorf, K (1980). Excitatory and inhibitory response components in the landing response of the blowfly, Calliphora erythmephala. Journal of Comparative Physiology, 138, 253-264. Franks, I.M., Weicker, D. & Robertaon, D.G.E. (1985). Kinematics, movement phasing and timing of a skilled action in response to varying conditions of uncertainty. Human Movement Science, 4, 91-105. Kugler, P.N., Kelso, J.A.S. & Turvey, M.T. (1980).On the concept of coordinative structures a s dissipative structures. I. Theoretical lines of convergence. In G.E. Stelmach & J. Requin (Eds.), Tutorials in Motor Behavior (pp. 3-47). Amsterdam: North-Holland. Lee, D.N. (1976).A theory of visual control of braking based on information about time-to-collision.Perception, 5, 437-459. Lee, D.N. (1980).Visuo-motor coordination in space-time. In G.E. Stelmach & J. Requin (Eds.), Tutorials in Motor Behavior (pp. 281-295). Amsterdam: North-Holland. Lee, D.N., Lishman, J.R. & Thomson, J.A. (1982). Regulation of gait in long jumping. Journal of Experimental Psychology: Human Perception and Performance, 8, 448-459. Lee, D.N., Young, D.N., Reddish, P.E., Lough, S. & Clayton, T.M.H. (1983). Visual timing in hitting an accelerating ball. Quarterly Journal of Experimental Psychology, 35A, 333-346. McLeod, P., McLaughlin, C. & Nimmo-Smith, I. (in press). Information encapsulation and automaticity: Evidence from the visual control of finely timed actions. Attention and Performance. Reed, E.S. (1982). An outline of a theory of action systems. Journal of Motor Behavior, 14, 98-134. Regan, D., Beverley, K & Cynader, M. (1979).The visual perception of movement in depth. Scientifik American, 241-1,122-133. Schmidt, R.A. (1975). A echema theory of discrete motor skill learning. Psychokgical Review,82, 225-260. Schmidt, R.A. (1982). Motor Control and Laming: A behavioral emphasis. Champaign, a:Human Kinetics Publishers. Spaeth-hold, R.K.(1977). Skill acquisition under variable temporal constraints: Cinematographic analysis of movement organization. Journal of Human
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Movement Studies, 2, 98-113. Turvey, M.T. (1977).Preliminaries to a theory of action with reference to vision. In R. Shaw and J. Bransford (Eds.), Perceiving, Acting and Knowing (pp. 211265). Hillsdale, NJ Erlbaum. Tyldesley, D.A. (1980). Movement structure in anticipatory timing. In G.E. Stelmach & J. Requin (Eds.), Tutorials in Motor Behavior (pp. 511-523). Amsterdam: North-Holland. Tyldesley, D.A. & Whiting, H.T.A. (1975). Operational timing. Journal of Human Movement Studies, 1, 172-177. Von Hofsten, C. (1983). Catching skills in infancy. Journnl of Experimental Psychology: Human Perception and Performance, 9,75-85. Von Hofsten, C. (1985).Catching. Report # 28/l985 Research Group on Perception and Action, Zentrum fdr interdiszipliniire Forschung, Bielefeld. Wagner, H. (1982). Flow field variables triggering landing in flies. Nature, 297, 147-148. Warren, W.H.(1984). Perceiving afl'ordances: Visual guidance of stair climbing. Journal of Experimental Psychology: Human Perception and Performance, 10, 683-703.
Reinoud J. Bootsma
The Netherlands.
r
Piet C.W. van Wieringen Dept. of Psychology, Facul of Movement Sciences Free niversity, P.0.Box 7161, 1007 MC Amsterdam, The Netherlands
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