The control of sequential aiming movements: the influence of practice and manual asymmetries on the one-target advantage

The present experiment was conducted to explore the effect of practice on the one-target advantage in manual aiming, as well as asymmetries in intermanual transfer of training. Reaction and movement times for the first movement were longer in the
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  THE CONTROL OF SEQUENTIAL AIMING MOVEMENTS:THE INFLUENCE OF PRACTICE AND MANUALASYMMETRIES ON THE ONE-TARGET ADVANTAGE Ann Lavrysen 1 , Werner F. Helsen 1 , Luc Tremblay 2 , Digby Elliott 2 , Jos J. Adam 3 ,Peter Feys 1 and Martinus J. Buekers 1 ( 1 Katholieke Universiteit Leuven, Leuven, Belgium; 2 Mc Master University, Hamilton,Ontario, Canada; 3 Maastricht University, Maastricht, The Netherlands) A BSTRACT The present experiment was conducted to explore the effect of practice on the one-target advantage in manual aiming, as well as asymmetries in intermanual transfer of training. Reaction and movement times for the first movement were longer in the 2-targetthan in the 1-target task, regardless of the amount of practice, hand preference and practicehand. When two movements were required, peak velocity was lower and, proportionally,more time was spent after peak velocity. Our kinematic results suggest that the one-targetadvantage is related to both predefined strategies as well as movement implementationprocesses during execution. Therefore, an integration of advance planning and on-lineexplanations for the one-target advantage is suggested. Regarding manual asymmetries,right-handers showed more transfer of training from the left to the right hand than viceversa. Left-handers exhibited a reversed pattern of asymmetric transfer of training to right-handers, but they were more disadvantaged using their non-dominant hand. These lattertwo findings have implications for models of manual asymmetry and upper limb control.Key words: manual aiming, one-target advantage, manual asymmetries, handedness I NTRODUCTION The One-Target Advantage When an individual performs a fast two-component aiming movement, thetime to initiate and execute the first component of the movement is generallylonger than if the movement is performed in isolation. The ‘one-target advantage’(OTA) for reaction time is generally attributed to the greater motor programmingdemands associated with the number of elements in a movement sequence(Henry and Rogers, 1960; Klapp, 1995; Sternberg et al., 1978; Christina et al.,1985; Fischman, 1984). 1 However, there is less agreement on the source of themovement time effect (see Adam et al., 2000; Adam et al., 1995, for a review).Possible explanations vary in the extent to which they emphasize advanceplanning processes, on-line control and strategic considerations. Fischman and Reeve (1992) have suggested that the additional accuracydemands associated with a two-component movement require the first movement Cortex, (2003) 39, 307-325 1 There are situations in which reaction time does not increase with complexity. In these situations, the movements arecharacterized by more discontinuities in the trajectory, suggesting a greater dependence on on-line control.  in a movement sequence to be completed in a more controlled, constrainedmanner (‘constrained movement hypothesis’). Presumably, the performer reducesthe accelerative impulse associated with the first movement in order to reduceimpulse variability, and therefore the endpoint variability, associated with thefirst movement (Schmidt et al., 1979; see also Smiley-Oyen and Worringham,1996). Thus strategic planning processes prior to the initiation of the movementsequence lead to the one-target movement time advantage. Although thissuggestion has intuitive appeal and is consistent with one-target reaction timeeffects, the kinematic data required to adequately examine this idea are scarce(cf. Smiley-Oyen and Worringham, 1996). Another possible explanation for the one-target advantage is the ‘on-lineprocessing hypothesis’. This view holds that the temporal cost for movementtime-1 is related to the commitment of processing resources to the preparation of the second movement during the first movement (Chamberlin and Magill, 1989).It has also been hypothesized that movement planning for the second movementoccurs both prior to and during the movement to the first target (Ricker et al.,1999; see also Ketelaars et al., 1999). In this regard, Adam et al. (2000) haveforwarded the ‘movement integration hypothesis’ to explain the one-targetmovement time advantage. Like the constrained movement hypothesis, this viewholds that the two movements are prepared in advance. However, the responseelements associated with the second movement are held in a buffer untilimplementation is necessary. This implementation process occurs during the firstmovement, and carries with it a temporal cost. While the processes associatedwith the on-line processing hypothesis and the movement integration hypothesisare slightly different, these two explanations of the one-target advantage formovement time include both prior planning and on-line components. This makesthese two explanations difficult to distinguish empirically 2 . As well, there is noneed for advance planning and on-line explanations of the one-target movementtime advantage to be mutually exclusive (Smiley-Oyen and Worringham, 1996;Lavrysen et al., 2002). The Influence of Practice on Sequential Aiming An issue that deserves more attention in the sequential aiming literature is theimpact of practice on the one-target advantages. For single aiming movements,Khan et al. (1998) have demonstrated that both movement planning and on-lineprocesses become more efficient with practice. In this context, it is possible thatthe information processing events responsible for the one-target advantage changeas an individual receives more practice performing the two movements together.The primary goal of this experiment was to extend the performance-basedresearch on the one-target advantage to an examination of motor learning. Of particular interest was how movement preparation, as indexed by reaction time, 308  Ann Lavrysen and Others 2 Adam et al. (2000; cf. Lajoie and Franks, 1997) have demonstrated that the one-target advantage for movement timedisappears when the second movement in the sequence is simply a reversal of the first movement. This is because thesame muscular forces that decelerate the first movement are also used to propel the limb back towards the homeposition/second target. The idea is that, in the case of a reversal, movement integration is not a factor during the firstmovement because the two movements are implemented as one package.  and the temporal and spatial characteristics of the movement trajectories changewith practice. By examining the stability of the one-target advantage overpractice, as well as the kinematics of single-target and two-target aiming, we hopeto determine the relative importance of advance and on-line processes.In this context, one view of motor learning is that, with practice, a performeris able to chunk response elements together that once were separate responseunits (Brown and Carr, 1989; Verwey, 1996; Klapp, 1995). If this is the case,one might expect an increase in advance preparation time compared to on-lineprocessing time during the performance of a two-element sequence. Specifically,an increase in reaction time might coincide with a reduction in the temporal costassociated with movement time-1. As well, the actual movement trajectoriesmight be expected to become more stereotyped and symmetric over practice,because these characteristics are typically associated with greater central control(see Elliott et al., 1999 for a review). Interestingly, research conducted with single-target movements appears tosupport quite a different view of the processing changes taking place with practice.Specifically, it has been shown (Proteau et al., 1987; see also Elliott et al., 1995,and Khan et al., 1998) that part of skill development involves learning to useafferent information more rapidly and efficiently in order to regulate goal-directedmovements on-line. If this is also the case with sequential aiming movements, thenadvance planning of the two-movement elements may give way to greaterimportance of on-line control with practice. In this case, the one-target reactiontime advantage may decrease with practice while the one-target movement timeadvantage persists. Greater on-line control would of course be characterized byless stereotyped movement trajectories. Adam et al. (2001) have recentlydemonstrated the persistence of the one-target movement time advantage throughseveral practice trials. Unfortunately, as a participant-paced paradigm was used,this study failed to show how prior processing events are influenced by practice.  Manual Asymmetries in Limb Control The second goal of this research was to examine the manual asymmetries inlimb control and to determine if they remain constant with practice. In thisexperiment, both right- and left-handers practiced sequential aiming movementswith either their preferred or non-preferred hand. There is substantial evidencefrom manual aiming experiments that the two cerebral hemispheres arespecialized in certain aspects of movement preparation and control (Carson etal., 1995; Winstein and Pohl, 1995; Elliott and Chua, 1996; Buekers and Helsen,2000; Helsen et al., 1998). This specialization is reflected in hand advantages forparticular tasks or task components. For example, the left hand usually enjoys areaction time advantage in goal-directed aiming (Mieschke et al., 2001), whilethe right hand is superior at movement execution (see Elliott and Chua, 1996,for a review). Of interest here was how these initial performance asymmetriesare mediated by practice (e.g., Taylor and Heilman, 1980), and how they reflectthemselves in the one-target advantages for reaction and movement time.In this study, we examined intermanual transfer of learning. In severalexperiments involving right-handers (e.g., Edwards and Elliott, 1987; Taylor and Control of aiming movements 309  Heilman, 1980), it has been demonstrated that there is more transfer of trainingfrom the left hand to the right hand than vice versa. It has been suggested thatthis occurs because of the special role the left cerebral hemisphere plays inmovement organization (Taylor and Heilman, 1980). Specifically during left-hand training, both cerebral hemispheres are involved. The right cerebralhemisphere because it controls the distal musculature of the left hand, and theleft hemisphere due to its overall role in praxis. For left-handers however, thereis considerable debate about the relationship between hand preference, manualasymmetries in performance, and cerebral specialization for language and praxis(i.e. the cerebral specialization for the organization and control of movements).One view is that this cerebral specialiazation determines both hand preferenceand performance asymmetries that favor the preferred hand (Kimura, 1993). Thisview predicts that asymmetries in intermanual transfer of training in left-handerswill be the reverse of those of right-handers. That is, left-handers will exhibitmore transfer of training from the right hand to the left hand than vice versa.Another view is that cerebral specialization for speech, language and praxisdepend on the same neural mechanisms. Because the majority of left-handersdisplay left hemisphere specialization for language (see Bryden et al., 1996), thisview of asymmetry predicts left-handers to demonstrate greater left hand to righthand transfer of training. Given that hand preference influences manualasymmetries in performance, we were also interested in examining how it mightmediate prior planning and on-line processes associated with the one-targetadvantages for reaction time and movement time. M ATERIALS AND  M ETHODS Participants Forty-eight undergraduate students (mean age 20.3 +1.6 years) volunteeredto participate. They were equally divided into two groups by gender and handpreference. According to an adaptation of Bryden’s (1977) handednessquestionnaire, 24 participants (12 male and 12 female) were strongly right-handed (Mean = 29.9, where 30 is maximal right-hand preference). According tothe same scale, the remaining 24 participants (12 male and 12 female) werestrongly left-handed (Mean = 6.1, where 6 is maximal left-hand preference). Allparticipants had normal or corrected-to-normal vision. Participants were naiveregarding the hypotheses being tested. Informed consent was obtained from eachsubject prior to participation. The experiment has been conducted in accordancewith the ethical standards laid down in the 1964 Declaration of Helsinki, as hasbeen assessed and approved by the Committee for Ethical Considerations inHuman Experimentation of the Faculty of Physical Education and Physiotherapyat Katholieke Universiteit Leuven.  Apparatus The aiming apparatus consisted of a 1 m aluminium cylinder (4 cm diameter)fastened on an artificial platform with an angle of 15°in relation to the table 310  Ann Lavrysen and Others  surface. All targets were 1.2 cm circular push buttons mounted on top of thecylinder. The distance between the centre of each adjacent target was 20 cm (allcentre-to-centre), resulting in an index of difficulty of 4.94 bits (Fitts, 1954). Allthe push buttons were white and connected to micro-switches requiring a verticaltravel distance of 1 mm and a 750 g pressure to be activated. Using an electricalcircuit, the aiming apparatus was connected to a standard microcomputer thatrecorded the performance measures (i.e., reaction times, movement times, andcontact times). Sampling frequency was 500 Hz. Throughout the experiment,participants wore a magnetic sensor attached to the tip of the index finger. A 3-dimensional analysis system, sampling at 100 Hz and covering a test area of 1 m 3 , provided information about the position of this sensor with a spatialaccuracy of 2 mm. Task  The task was similar to an aiming task used previously (Helsen et al., 2000).Participants sat on an adjustable chair in front of the table on which theapparatus was mounted. Before each trial they were instructed to place the indexfinger on a push button in front of them (Figure 1). When the red light behindthis home button was illuminated, the participant was required to, either move tothe first target and push the button (i.e., the 1-target movement), or to push thefirst target button and move on to the second target button (i.e., the 2-targetmovement). Each of these actions was to be performed as fast as possible.Throughout the experiment, participants were able to move the eyes, head andtrunk freely. Procedure Prior to testing, each participant received standardized instructions concerningthe general nature of the experiment. They were then equipped with the minibirdsensor and positioned so that the body midline was aligned with the home button.When responding with their right hand, the first and second target were located inright hemispace; when responding with their left hand, the first and second targetwere located in left hemispace. In other words, the first and second movementswere always movements away from the body midline, and thus extension Control of aiming movements 311Fig. 1 – The test set-up.
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