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Attention for action: coordinating bimanual reach-to-grasp movements.

Coordination of bimanual action

Many everyday tasks involve bimanual movements. That is, they necessitate cooperative rather than separate movements of the two hands, for example when: unscrewing the lid of a jar; sweeping the floor with a broom; folding a newspaper; or pouring milk into a cup. Some bimanual movements entail reaching to single objects while others involve reaching to two separate objects. Although the topic of inter-limb coordination has been of interest for nearly a century (Woodworth, 1899), most studies of upper limb movements have examined unimanual rather than bimanual actions. In this paper the authors investigate the mechanisms involved in bimanual coordination by examining how movement kinematics become coupled during bimanual prehension (reach-to-grasp) movements to two separate objects.

When we execute a unimanual aimed movement, the duration of the movement is frequently found to depend on the ratio of movement amplitude to target size (a formula known as Fitt's law). Movement duration is shorter when the distance is small and/or the target is large (sometimes referred to as having a low index of difficulty), compared to when the distance is longer and/or the target object is smaller (referred to as having a high index of difficulty). Bimanual aiming movements having the same index of difficulty also conform to this rule. However, bimanual aiming movements of mixed difficulty do not (Kelso, Putnam & Goodman, 1983; Kelso, Southard & Goodman, 1979). While participants are typically not explicitly instructed to synchronize their hands during bimanual aiming tasks, they tend nevertheless to do so (Keele, 1986). Consequently, movement duration as well as the time of movement initiation are similar for both hands. Due to this tendency to synchronize the hands, when mixed tasks are performed Fitt's law is violated. Thus, the hand reaching for the difficult target takes less time than it would do if the other hand were also reaching to a difficult target, whereas the hand reaching to the easy target takes more time than it would do if the other hand were also moving to easy target. On the basis of evidence such as this, Kelso et al. have proposed that during bimanual movements, the two limbs are coupled together with a single coordinating structure - an organized functional group of muscles - and are thus constrained to act simultaneously.

Other authors, however, have questioned the degree of synchronization attained. Marteniuk, MacKenzie & Baba (1984) showed that the limbs are significantly less synchronized when reaching to mixed- compared to same-difficulty targets. They argue that the hands are not coupled to a single timing structure but are controlled separately; the similarity between the movements of the two hands under mixed-difficulty conditions arise as a result of neural cross-talk between the hands. It should be noted, however, that while there are significant departures from synchrony when the limbs are moving to mixed-difficulty targets, the absolute differences in movement onset times and movement duration between the limbs are relatively small ([approximately equal to] 20 ms; Marteniuk et al., 1984).

In the studies cited above, bimanual coordination was examined during simple aiming movements. In the current study the authors examine bimanual coordination during the execution of reach-to-grasp movements. The literature to date is equivocal as to whether reach-to-grasp movements involve more limb-specific control, and greater asynchrony than simple aiming movements (Castiello, Bennett & Stelmach, 1993). Jeannerod (1984) found that movement onset and duration were closely synchronized when reaching to grasp a single object (i.e. a bottle with a small top). In addition, the timing of maximum hand velocity and maximum grip aperture were also similar for each hand. However, Castiello and colleagues found no evidence of velocity synchronization in their study of reaching to grasp a single object (Castiello et al., 1993). They did, however, confirm that movement duration was synchronized across the hands. In contrast, in a later study involving reach-to-grasp movements directed towards a cylinder with a laterally displaced handle, movement duration was not found to be synchronized across the hands, and there were fewer significant correlations between the various dependent measures (Castiello & Bennett, 1997).

Models of bimanual coordination

In view of the very small number of studies reported to date, a useful starting point for understanding how bimanual prehension movements are coordinated may be to consider bimanual coordination within the context of theoretical frameworks developed to explain how transport and grasp phases are coordinated during unimanual prehension movements. Two main classes of frameworks can be distinguished: those suggesting that the coordination of movement components is planned in advance of movement onset and based upon temporal synchronization; and those proposing the coordination is achieved by the on-line control of movement parameters based upon continuous sampling of spatial information.

One of the most influential of the former frameworks has been Jeannerod's 'visuomotor channels hypothesis' (e.g. Jeannerod, 1981, 1984). Within this view, prehension consists of two independently computed components: a transport component in which the limb is transferred to the region of the target object, and a grasp component in which the hand is preshaped and oriented so as to facilitate gripping the target (Jeannerod, 1984). These components are assumed to be based upon separate visuomotor channels which provide different sources of information about the perceptual properties of objects. The transport component is thought to depend upon an egocentric representation - in which objects are represented in terms of their spatial position relative to the body - while the grasp component is thought to depend upon intrinsic properties including object size, shape and the orientation of its major axis. Physiological and anatomical demonstrations of independent neural region concerned with the programing of distal and proximal movements provide additional support for the separation of transport and grasp (Gallese, Murata, Kaseda, Niki & Sakata, 1994; Gentilucci, Fogassi, Luppino, Matelli, Camarda & Rizzolatti, 1988; Rizzolatti, Camarda, Fogassi, Gentilucci, Luppino & Matelli, 1988; Sakata & Taira, 1994). A key aspect of Jeannerod's original proposal was that the independently computed transport and grasp phases were coordinated by a common kinematic plan, which is generated centrally, and in which the temporal unfolding of the grasp phase is linked to the time frame computed for the transport phase. An important prediction of this model, however, was that experimental manipulations that affect the computation of the grasp, e.g. changes to object size, should not have consequences for transport kinematics. As several studies have reliably demonstrated that changes in object size can influence transport phase kinematics (e.g. Marteniuk, Leavitt, MacKenzie & Athenes, 1990), later reformulations of this model permit transport and grasp to interact in determining the temporal movement plan. Hoff & Arbib (1993) suggest that separate estimates of the time needed to complete the transport and grasp are relayed to a higher order control system responsible for coordinating lower level movement elements (schemas). This controller then issues a common movement duration for both components. Importantly, it is explicitly assumed within this model that, when the time estimates received by the controller differ, whichever estimate is the longer will be selected as the common movement duration, and other components will be slowed as a consequence. Two points should be noted about these models: the clear emphasis on movement planning processes rather than on-line (continuous) control; and the proposal that movement duration is the coordinating factor.

An alternative to the temporal planning models are those proposing that coordination is based upon changing spatial position. A noteworthy feature of these models is an emphasis on the on-line or continuous control of movement variables (e.g. velocity, grip aperture etc.), and the proposal that the later stages of reach-to-grasp movements may operate within object-centred rather than body-centred coordinates. One such model proposed by Bootsma and colleagues (Bootsma & van Wieringen, 1992; Zaal, Bootsma & van Wieringen, in press) proposes that control of transport and grasp is dependent, in each case, upon a common source of perceptually derived information - the rate of change in the distance between the hand and target object (often referred to as the remaining time to contact). Coordination is not therefore planned, but instead arises as a consequence of each component sharing a common information signal.

Divided attention during bimanual action

One important aspect of the reach-to-grasp studies reviewed above is that they each involve bimanual movements directed towards a single target object. This fact may have important consequences for the question of how, and to what extent, bimanual prehension movements are coupled as there is now considerable evidence to attest to the difficulty of attending to more than one object. For example, participants find it significantly easier to report the presence of a pair of stimulus properties when these are present on a single object, compared to when they occur on different objects (Duncan, 1984). This is the case, even when both objects are presented at the same spatial location. Additional support for this view also comes from neuropsychological syndromes such as visual extinction, were individuals perceive only one of a pair of stimuli that are simultaneously presented. Moreover, it is of interest to note that visual extinction can be significantly reduced when ipsilesional and contralesional stimuli group together to form a perceptually coherent whole (Ward, Goodrich & Driver, 1994), and from the finding that patients with right parietal lesions can show specific deficits in shifting attention between different objects (Egly, Driver & Rafal, 1994). Thus, it is possible that bimanual synchronization is enhanced when the two hands must perform a single task or different actions on a single object. In the latter case, coupling may nevertheless occur because the different actions performed by each hand are integrated by their reference to a shared goal-directed movement that is executed towards a single object. To investigate this issue, we examined bimanual coupling for congruent (same action) versus incongruent (different action) bimanual reach-to-grasp movements directed to different target objects.

EXPERIMENT 1

In this experiment we manipulated factors known to reliably influence the transport phase kinematics. Specifically, we varied the distance at which the target objects were placed relative to the start position of the hand (i.e. movement amplitude). As increasing movement amplitude has been consistently shown to lead to increased movement durations and to increased values of peak velocity (Jeannerod, 1988), we can predict that in the current study, movement durations and peak velocities will be scaled for movement amplitude for both unimanual and congruent bimanual reaches. But what about incongruent bimanual trials, where each hand is required to move a different amplitude? If each hand is independently controlled then each hand should scale for the appropriate movement amplitude. If however, the hands are coupled on bimanual movements, then this should be reflected in the movement durations and peak velocity values observed for each hand.

Method

Participants

Eight participants, 6 females and 2 males, with a mean age of 25 years, 3 months were recruited from the School of Psychology, UWB's community participant panel. All were right handed (Edinburgh Handedness Inventory) and had normal or corrected to normal vision including stereopsis (Randot Stereopsis Test). All participated in both Expt 1 and Expt 2. The order in which they completed the experiments was counterbalanced across the group. Participants were paid a small honorarium for their participation.

Apparatus and stimuli

Participants were seated at a 1000 mm square table and executed prehension movements towards target objects presented in the frontal plane (red wooden dowels 50 mm high with a diameter of 22.5 mm). Reaches with the right hand were made from a foam rubber-backed metal pedal (225 mm x 120 mm), positioned approximately in line with the participant's right shoulder. Reaches with the left hand were made from a similar pedal positioned in line with the participant's left shoulder. The centre-to-centre separation between the pedals was 400 mm. Target objects could be positioned at one of two distances from the starting positions of the hands, either 200 mm (near location) or 300 mm (far location), and could be positioned 200 mm to the left or the right of the participant's mid-saggital plane [ILLUSTRATION FOR FIGURE 1 OMITTED]. Note that reaches using the right hand were always executed towards targets presented in right space, and reaches using the left hand to targets in left space. At no time therefore did participants reach with their right hand into left space or vice versa.

Participants executed four types of prehension movement: unimanual reaches using the right hand; unimanual reaches using the left hand; congruent bimanual reaches; and incongruent bimanual reaches. During unimanual reaches, they reached using either their right or left hand, for a single target object presented in the appropriate hemispace. During congruent bimanual trials, they reached using both right and left hands, for two target objects, one presented in each hemispace. Furthermore, during congruent trials only, both targets were presented at the same distance from the participant (i.e. both near or both far target locations). In contrast, the target object locations always differed from one another on incongruent bimanual trials (i.e. right hand far target and left hand near target or vice versa).

Each participant completed 6 trials of each trial type x target position permutation as follows: 24 unimanual trials (12 trials with each hand, and 12 trials to each target distance), 24 bimanual trials, 12 congruent trials and 12 incongruent trials. For bimanual trials, the number of near and far reaches with each hand was counterbalanced. Participants began each trial with their hand placed flat upon the starting pedals, oriented along the saggital plane, and with their thumb closed against their index finger. Each trial commenced with an auditory 'start' signal (note that participants could view the position of target objects prior to the arrival of the start cue). The order of presentation of trials for each block was individually randomized for each participant. Participants were instructed to reach as quickly as possible while maintaining accuracy. A short practice session was conducted prior to the presentation of the experimental trials.

Movement recording and data analysis. Hand movements were recorded using a MacReflex infra-red motion analysis system (Qualisys Inc.) with a sampling rate of 50 Hz. Reflective markers measuring 5 mm x 5 mm were placed: on the distal portion of the thumbnail; on the distal portion of the index finger; and on the wrist of each hand. Additional markers were also fixed to the target objects. The 3D spatial coordinates of these markers were analysed off-line using custom software written using Labview (National Instruments Inc.) and Matlab (Mathworks Inc.) programming environments. Data were low pass-filtered using an 4th order Butterworth filter (cut-off frequency of 10 Hz).

Dependent measures

Kinematic parameters were initially calculated for each hand separately. Movement onset was defined as the first frame in which the wrist marker exceeded a consistent velocity (in the direction of movement) of 25 mm/s. Movement end-point was defined as the first frame in which horizontal displacement of the target marker exceeded 1.0 mm. Movement duration (MD) was defined as movement end-point minus movement onset. The following dependent measures were computed from the 3D coordinates for the markers placed on the thumb, index finger and wrist, and were used to analyse the kinematics of the grasp phase of the prehension task: (1) peak grip aperture (PGA) between index finger and thumb (measured in mm); (2) the time taken to reach PA as a percentage of total movement duration (TTPA percentage). The following dependent measures were computed from the 3D coordinates for the wrist marker, and were used to analyse the kinematics of the transport phase: (3) peak velocity in the direction of movement (PV); (4) absolute time between movement onset and the point where peak velocity was achieved (msecs); and (5) deceleration time (i.e. the time after PV) expressed as a percentage of total movement duration (DT percentage).

In addition to the above measures, we also computed for bimanual movements only, a series of relative measures in which the kinematics of the left hand were indexed to those of the right (dominant) hand. These measures were organized around a set of key questions as follows: (1) Do the hands begin to move at the same point in time? Movement onset lag (RH-LH): A positive difference would indicate that the right hand began to move after the left hand while a negative difference would indicate that the right hand moved first; (2) Do the hands reach peak velocity at the same point? Peak velocity lag (RH-LH); (3) Do the hands reach peak grip aperture at the same point? Peak grip lag (RH-LH); (4) Do the hands make contact with the target objects at the same time ? Movement end-point lag (RH-LH).

Results

Transport phase kinematics

Data were initially analysed using a 2 x 2 x 3 repeated measures ANOVA containing the following factors: hand (left vs. right); distance (near vs. far); and condition (unimanual vs. congruent bimanual vs. incongruent bimanual).

Movement duration. The ANOVA revealed significant main effects of distance (F(1,7) = 77.9, MSE = 1268, p [less than] .0005) and condition (F(2,14) = 24.6, MSE = 3253, p [less than] .00025). The main effect of hand, however, was not statistically significant (E(1,7) [less than] 1, p = .8). Movement durations were scaled for movement amplitude, with reaches of increased amplitude producing longer movement durations (far = 573 (143) ms vs. near = 509 (134) ms). The main effect of condition was further examined in a series of planned comparisons using linear contrast procedures. Relevant means were unimanual = 484 (127) ms; congruent bimanual = 568 (140) ms; incongruent bimanual = 572 (144) ms. These analyses revealed that movement durations were significantly longer for bimanual reaches compared to unimanual reaches (F(1) = 49.0, p [less than] .0001). In contrast, movement durations for congruent bimanual reaches did not differ from those for incongruent bimanual reaches (F(1) [less than] 1, p = .7).

The ANOVA also revealed a significant distance x condition interaction effect (F(2,14) = 8.8, MSE = 1275, p [less than] .01). Relevant means are presented in Fig. 2 (upper panel). Planned comparisons (using linear contrasts) were carried out to examine the effects of target distance for each type of movement condition. These analyses revealed that movement durations were scaled for movement amplitude for both unimanual (F(1) = 54.1, p [less than] .0001) and congruent (F(1) = 37.9, p [less than] .0001) bimanual movements. In contrast, movement durations did not show significant distance scaling for incongruent bimanual movements (F(1) = 3.0, p [greater than] .1). Inspection of Fig. 2 shows that for the incongruent bimanual condition, mean movement durations for both near and far target locations were approximately midway between the mean movement durations observed for the far and near target locations in the congruent bimanual condition. This finding suggests that on incongruent bimanual trials, where each hand is required to travel a different distance, a movement duration is selected which is a compromise between the preferred duration for the near and far locations. All other interaction effects failed to reach statistical significance.

Peak velocity. The ANOVA revealed significant main effects of distance (F(1,7) = 322.4, MSE = 2002, p [less than] .001) and condition (F(2,14) = 16.5, MSE = 6509, p [less than] .00025). The main effect of hand was not statistically significant (F(1,7) [less than] 1, p = .8). Peak wrist velocities were scaled for movement amplitude, with reaches of increased amplitude producing higher velocities (far = 1259.1 (234) mm/s vs. near = 1095.2 (242) mm/s). The main effect of condition was again examined in a series of planned comparisons using linear contrasts. Relevant means were unimanual = 1239.9 (270) mm/s; congruent bimanual = 1165.7 (239) mm/s; incongruent bimanual = 1125.9 (236) mm/s. These analyses revealed that peak velocities were significantly greater for unimanual compared to bimanual reaches (F(1) = 29.0, p [less than] .0005). Peak velocities for congruent bimanual reaches did not differ from those for incongruent bimanual reaches, although the difference between the means approached statistical significance (F(1) = 3.9, p = .08).

Importantly, and in contrast to the findings for movement duration, the distance x condition interaction effect was not significant (F(2,14) = 2.6, p [greater than] .1). Relevant means are presented in Fig. 2 (lower panel). Planned comparisons (using linear contrasts) were again carried out to examine the effects of target distance for each type of movement condition. These analyses revealed that peak velocities were scaled for movement amplitude in each type of movement condition, including the incongruent bimanual condition (minimum E(1) = 6.5, p [less than] .05). These results confirm that on incongruent bimanual trials - where each hand was required to move to a different target distance participants were able to scale independently the velocity of each hand. All other interaction effects failed to reach statistical significance.

Time to reach peak velocity. The ANOVA revealed no significant main or interaction effects. Thus, the time that elapsed between movement onset and peak wrist velocity did not differ between the hands, or vary across movements of different amplitude, or between unimanual and bimanual movements.

Deceleration time (percentage). The ANOVA revealed significant main effects of hand (F(1,7) = 7.0, MSE = 11, p [less than] .05) and condition (F(2,14) = 10.0, MSE = 54, p [less than] .01). The main effect of distance was not significant (F(1,7) = 1.2, p = .3). Overall, deceleration times were significantly longer when executing reaches with the left (non-dominant) hand (left = 49.9 (10.0) % vs. right = 48.2 (8.3) %). The main effect of condition was again examined in a series of planned comparisons (linear contrasts). Relevant means were unimanual = 44.3 (12.4) %; congruent bimanual = 51.4 (6.4) %; incongruent bimanual = 51.1 (5.5) %. These analyses revealed that participants spent significantly less time decelerating for unimanual compared to bimanual reaches (F(I) = 19.9, p [less than] .005). Deceleration times for congruent bimanual reaches did not differ from those for incongruent bimanual reaches (F(1) [less than] 1, p = .9). All interaction effects failed to reach statistical significance.

Grasp phase kinematics

Maximum grip aperture (MGA). The ANOVA revealed a significant main effect of condition (F(2,14) = 8.4, MSE = 34.1, p [less than] .01). All other main and interaction effects failed to reach conventional levels of statistical significance. The effect of condition was examined in a series of planned comparisons using linear contrasts. Relevant means were unimanual = 94.2 (11.3) mm; congruent bimanual = 100.1 (12.1) mm; incongruent bimanual = 98.0 (11.2) min. These analyses revealed that MGAs were significantly greater for bimanual compared to unimanual reaches (F(1) = 14.7, p [less than] .005). MGAs for congruent bimanual reaches did not differ however from those for incongruent bimanual reaches (F(1) = 2.1, p = .2). These findings demonstrate that, although the size of the target objects was not varied in this experiment, grasp safety margins as reflected by MGA increased when participants were required to execute bimanual movements.

Time taken to reach MGA. The ANOVA revealed significant main effects of condition (F(2,14) = 5.1, MSE = 3822, p [less than] .05) and distance (F(1,7) = 17.7, MSE = 3512, p [less than] .005). Participants reached MGA earlier when reach to the near compared to the far target location (means: far = 478.8 (102) ms vs. near 427.9 (106) ms). All other main and interaction effects failed to reach conventional levels of statistical significance. The effect of condition was examined in a series of planned comparisons using linear contrasts. Relevant means were unimanual = 427.5 (111) ms; congruent bimanual = 476.6 (111) ms; incongruent bimanual = 455.9 (94) ms. These analyses revealed that participants reach MGA significantly later for bimanual compared to unimanual reaches (F(1) = 8.4, p [less than] .025). The time taken to reach MGA for congruent bimanual reaches did not differ from the observed for incongruent bimanual reaches (F(1) = 1.7, p = .2). These findings confirm the patter of results observed for MGA. While the size of the target objects was not varied in this experiment, time to MGA increased when participants were required to execute bimanual movements.

Measures of bimanual coupling

As noted above, we also computed for bimanual movements only, a set of relative measures in which the kinematics of the left hand were indexed to those of the right (dominant) hand. These measures were organized to test the extent to which the hands reached important kinematic markers at the same point in time. Fig. 3 shows two representative trials from a single participant executing congruent bimanual reaches to the far target locations.

Movement onset lag. The mean movement onset lag (onset of RH-onset of LH) was 11 ms. Given the 50 Hz sampling rate (20 ms) this demonstrates that movements of the left and right hands started moving within one sampling frame of one another. Movement onset lag did not differ significantly between congruent and incongruent bimanual trials (means: congruent = 10 (10) ms vs. incongruent = 12 (8) ms; F(1,7) [less than] 1.0, p = .4).

Peak velocity lag. The mean peak velocity lag (RH-LH) was 12 ms, demonstrating that, on average, the left hand achieved maximum velocity within the same sampling frame as the right hand. The peak velocity lag did not differ for congruent and incongruent bimanual trials, although it did approach conventional levels of statistical significance (means: Congruent = 8 (16) ms vs. incongruent = 16 (24) ms; F(1,7) = 5.3, p = .06).

Peak grip lag. The mean peak grip lag (RH-LH) was 2.5 (30) ms, thereby demonstrating that the left and right hands achieved maximum grip aperture at around the same time. Once again this lag measure did not differ significantly between congruent and incongruent bimanual trials (means: congruent = 12 (25) ms vs. incongruent = -7 (34) ms; F(1,7) = 1.6, p = .3).

Movement end-point lag. The mean movement end-point (RH-LH) was 62 ms, demonstrating that on average, the left hand makes contact with its target slightly before the right hand. Once again, however, both hands achieved contact with their targets at a relatively similar point in time, and movement end-point did not differ significantly between congruent and incongruent bimanual trials (means: congruent = 75 (87) ms vs. incongruent = 46 (61) ms; F(1,7) = 1.5, p = .3).

Discussion

This experiment revealed several interesting findings. First, as one might expect, there is a clear cost in carrying out two movements simultaneously. Movement durations are longer; velocities lower; deceleration phases longer; grip apertures are wider and reached earlier, when participants carry out bimanual compared to unimanual movements. A somewhat unexpected finding, however, is that it does not seem to matter whether the movements to be executed by each hand are the same or different from one another. Thus, movement durations, peak velocities, deceleration phase lengths, grip apertures, and the time taken to reach grip aperture are each equivalent for congruent and incongruent bimanual reaches. This finding is surprising, not least because several authors have suggested, when considering the problem of selective reaching (how does one choose to direct an action towards one object when many objects are simultaneously present within the workspace) that the 'costs' of selecting between two objects which afford different actions ought to be greater than between two objects which afford the same action (e.g. Castiello, 1996; Chieffi, Gentilucci, Allport, Sasso & Rizzolatti, 1993; Howard & Tipper, 1997; Jackson, Jackson & Rosicky, 1995; Jackson, Jones, Pritchard & Newport, 1997; Tipper, Howard & Jackson, 1997; Tipper, Howard & Meegan, 1997; Tipper, Lortie & Baylis, 1992). However, it should be noted that in the current study participants did not have to select between two different objects, as each object had to be grasped. We will return to this issue in Expt 2.

A second finding of interest concerns those kinematic parameters which become coupled during bimanual movements. Inspection of Fig. 3 clearly indicates that, even when they are not given explicit instructions to do so, participants readily couple bimanual prehension movements so that they begin, reach peak velocity, reach peak grip aperture, and make contact with their target objects at approximately the same point in time. How is this achieved in circumstances where each hand is required to carry out a different action? The answer appears to be that while the movement velocity of each hand is free to vary, a movement duration is chosen which is common for across the two hands. This can be seen most clearly in Fig. 2, which illustrates how the distance scaling for movement duration, but not for movement velocity, is removed when participants execute incongruent bimanual reaches. Furthermore, the movement duration that is selected does not appear to match either of the durations observed on congruent bimanual reaches, i.e. to the near on the far locations. Instead, the duration selected appears to approximate the mean of these two values, as if a compromise were reached between the temporal demands of each hand. These findings suggest that a key aspect of bimanual coupling is the selection of a common movement duration within which the actions to be performed by each hand must be scaled accordingly (see Hoff & Arbib, 1993 and Jeannerod, 1997 for a similar account of how the transport and grasp phases of unimanual prehension movements may be temporally coordinated).

EXPERIMENT 2

In Expt 1 we manipulated factors known to influence transport phase kinematics, i.e. movement amplitude. In Expt 2 we manipulate a factor which is known to reliably influence grasp phase kinematics - target object width. We can predict that maximum grip apertures will be scaled for target object width for both unimanual and congruent bimanual reaches. But what about incongruent bimanual trials, where each hand is required to select a different sized grip? As was noted above, several authors have proposed that objects that afford different hand actions would be expected to lead to the parallel activation of different grasp patterns (e.g. Castiello, 1996; Chieffi et al., 1993). Furthermore, neurophysiological data obtained from nonhuman primates support the view that specific action repertoires can be triggered by the visual properties of objects located within peripersonal space (Fogassi & Gallese, in press; Sakata & Taira, 1994). If this view is correct, then grasp kinematics should be impaired on incongruent bimanual trials compared to congruent bimanual trials (where both hands must select the same grip).

Method

Apparatus and stimuli

Most aspects of the experimental task were unchanged from Expt 1. Participants were seated at the same 1000 mm square table and executed prehension movements in the same manner as previously described, towards target objects presented in the frontal plane. One difference was that target objects were now presented at a single distance (250 mm) from the starting positions of the hands. A second was that dowels of two widths were used as target objects; referred to hereafter as the large (50 mm) and the small (19 mm) targets. On congruent bimanual trials, participants reached using both right and left hands, for two target objects, of an identical width, one presented in each hemispace. On incongruent bimanual trials, the two target objects were of different widths.

Results

Transport phase kinematics

Data were initially analysed using a 2 x 2 x 3 repeated measures ANOVA containing the following factors: hand (left vs. right); size (large vs. small); and condition (unimanual vs. congruent bimanual vs. incongruent bimanual).

Movement duration. The ANOVA revealed significant main effects of size (F(1,7) = 10.4, MSE = 2374, p[less than].025) and condition (F(2,14) = 26.8, MSE = 2743, p [less than] .0005). The main effect of hand was not statistically significant (F(1,7) = 1.2, p = .3). Movement durations varied with object size, but in a rather unexpected manner. Reaches directed toward smaller targets produced shorter rather than longer movement durations (large = 569 (129) ms vs. small = 537 (128) ms). However, it should be noted that the main effect of size interacted with other manipulated factors (see below). The main effect of condition was examined in a series of planned comparisons (linear contrasts). Relevant means were unimanual = 498 (102) ms; congruent bimanual = 573 (131) ms; incongruent bimanual = 588 (136) ms). These analyses revealed that movement durations were significantly longer for bimanual reaches compared to unimanual reaches (F(1) = 52.2, p [less than] .0001). In contrast, movement durations for congruent bimanual reaches did not differ from those for incongruent bimanual reaches (F(1) = 1.4, p = .3).

The ANOVA revealed a significant hand x size x condition interaction (F(2,14) = 5.8, MSE = 1262, p [less than] .025). However, as this effect was not predicted in advance, it will not be discussed further. All other interaction effects failed to reach statistical significance.

Peak velocity. The ANOVA revealed significant main effects of size (F(1,7) = 5.8, MSE = 2032,p [less than] .05)and condition (F(2,14) = 13.9, MSE = 2524, p [less than] .0025). The analyses revealed no statistically significant interaction effects. There was a tendency for reaches executed using the left hand to be faster than those using the right hand (means: left = 1188 ms vs. right = 1144 ms). However, the main effect of hand did not quite reach conventional levels of statistical significance (F(1,7) = 4.4, p = .07). Peak wrist velocities were scaled for object size, with reaches to large targets producing higher movement velocities (large = 1178 (214) mm/s vs. small = 1155 (199) mm/s). The main effect of condition was examined in a series of planned comparisons (linear contrasts). Relevant means were (unimanual = 1204 (211) mm/s; congruent bimanual = 1147 (207) mm/s; incongruent bimanual = 1147 (200) mm/s). These analyses revealed that peak velocities were significantly greater for unimanual compared to bimanual reaches (F(1) = 27.7,p [less than] .001). Peak velocities for congruent bimanual reaches did not differ from those for incongruent bimanual reaches although the difference between the means approached statistical significance (F(1) [less than] 1, p = .9).

Time to reach peak velocity. The ANOVA revealed no significant main effects of hand or size, and no significant interaction effects. The main effect of condition approached statistical significance (F(2,14) = 3.6, MSE = 596, p = .07), and planned comparisons revealed a similar pattern of results as was observed for other measures. Thus, the time taken to reach peak velocity was significantly longer for bimanual (275 ms) compared to unimanual (261 ms) reaches (F(1) = 6.6, p [less than] .05). In contrast, time taken to reach peak velocity did not differ between congruent and incongruent bimanual reaches (F(1) [less than] 1, p = .5).

Deceleration time (percentage). The ANOVA revealed significant main effects of size (F(1,7) = 12.3, MSE = 18, p [less than] .01) and condition (F(2,14) = 24.8, MSE = 13, p [less than] .0001). The main effect of condition was again examined in a series of planned comparisons (linear contrasts). Relevant means were unimanual = 47 (7) %; congruent bimanual = 52 (6) %; incongruent bimanual = 53 (6) %. These analyses revealed that participants spent significantly less time decelerating for unimanual compared to bimanual reaches (F(1) = 49.4, p [less than] .0001). Deceleration times for congruent bimanual reaches did not differ from those for incongruent bimanual reaches (F(1) [less than] , p = .6).

The ANOVA also revealed a significant hand x size x condition interaction. Once again, as this effect was not predicted in advance, it will not be discussed further. All other interaction effects failed to reach statistical significance.

Grasp phase kinematics

Maximum grip aperture (MGA). The ANOVA revealed significant main effects of hand (F(1,7) = 16.9, MSE = 56, p[less than].005), size (F(1,7) = 82.5, MSE = 93, p [less than] .0001), and condition (F(2,14) = 15.9, MSE = 29, p [less than] .005). Reaches executed with the left hand produced a wider MGA than reaches executed with the right hand (left = 104 (14) mm vs. right = 98 (14) mm). Likewise, reaches executed to large targets produced wider MGAs than reaches executed to small targets (large = 110 (9) mm vs. small = 92 (12) mm). The effect of condition was examined in a series of planned comparisons (linear contrasts). Relevant means were unimanual = 97 (15) mm; congruent bimanual = 104 (12) mm; incongruent bimanual = 103 (14) ram. These analyses revealed that MGAs were significantly greater for bimanual compared to unimanual reaches (F(1) = 31.2, p [less than] .0025). MGAs for congruent bimanual reaches did not differ, however, from those for incongruent bimanual reaches (F(1) [less than] 1, p = .4). The ANOVA revealed a marginal hand x size interaction effect (F(1,7) = 5.3, MSE = 8, p [less than] .06). However, analyses of the simple effects of this interaction revealed statistically significant effects of both hand and size. All other interaction effects failed to reach statistical significance.

The effects of object size were examined for each type of movement (condition) in a series of planned comparisons using linear contrasts. Relevant means are presented in Fig. 4 (upper panel). These analyses confirmed that maximum grip aperture was reliably scaled for object size across all types of movement examined (minimum F(1) = 47.2, p [less than] .0001). This result clearly demonstrates that on incongruent bi-manual reaches - where each hand is required to form a different grip - independent control of grip size is achieved for each hand. This finding can be directly contrasted with that from Expt 1, where transport phase kinematics were manipulated, in which the scaling of movement duration for movement amplitude disappeared on incongruent bimanual trials.

Time taken to reach MGA. The ANOVA revealed no significant main of hand or size, and no interaction effects. The main effect of condition (F(2,14) = 3.2, MSE = 4108, p [less than] .08) was marginal, however. Planned comparisons (linear contrasts) revealed the following. First, MGA was reached significantly later on bimanual (447 ms) reaches compared to unimanual (413 ms) reaches (F(1) = 6.0,p [less than] .5). However, the difference between congruent and incongruent bimanual reaches was not significant (F(1) [less than] 1, p = .5). Second, the effects of object size were examined for each condition. While there was marginal effect of size for unimanual reaches (F(1) = 4.1,p = .07), no such effect was observed for bimanual reaches (maximum F(1) [less than] 1, p [greater than] .4). These results again provide support for the suggestion that the hands become temporally coupled during bimanual reaches, with movements for each hand unfolding within a common time frame.

Measures of bimanual coupling

A set of relative measures were computed for bimanual movements only, in which the kinematics of the left hand were indexed to those of the right (dominant) hand. Fig. 5 shows representative trials (grip and wrist velocity profiles) from a single participant executing congruent bimanual reaches to both large and small target objects. Fig. 6 shows representative trials taken from the same participant executing incongruent bimanual reaches. Inspection of these figures confirms that, while independent control of grip size is achieved for each hand on incongruent trials, movements of each hand unfold along an identical time frame.

Movement onset lag. The mean movement onset lag (RH-LH) was 11 ms, again demonstrating that movements of the left and right hands started moving within one time sample of each other. Movement onset lag did not differ significantly between congruent and incongruent bimanual trials (means: congruent = 13 (10) ms vs. incongruent = 9 (13) ms; F(1,7) = 3.8, p = .09).

Peak velocity lag. The mean peak velocity lag (RH-LH) was 8.5 ms, demonstrating that the left hand achieved maximum velocity within one time sample of the right hand. The peak velocity lag did not differ for congruent and incongruent bimanual trials (means: congruent = 11 (20) ms vs. incongruent = 6 (14) ms; F(1,7) = 1.3, p = .3).

Peak grip lag. The mean peak grip lag (RH-LH) was 22.5 ms, confirming that both hands achieved MGA within approximately one time sample of each other. This measure did not differ significantly between congruent and incongruent bimanual trials (means: congruent = 21 (34) ms vs. incongruent = 24 (38) ms; F(1,7)[less than] 1, p = .8).

Movement end-point lag. The mean movement end-point (RH-LH) was 50.5 ms, again demonstrating that on average, the left hand makes contact with its target slightly before the right hand. As in Expt 1, however, both hands achieved contact with their targets at a similar point in time, and movement end-point did not differ significantly between congruent and incongruent bimanual trials (means: congruent = 64 (61) ms vs. incongruent = 37 (49) ms; F(1,7) [less than] 1, p = .5).

Discussion

The results of this experiment are very similar to those observed in Expt 1. It was again observed that there is a clear cost in carrying out two movements simultaneously. As in the previous experiment, movement durations are longer, velocities lower, deceleration phases long, and grip apertures are wider and reached earlier when participants carry out bimanual movements compared to unimanual movements. Furthermore, the unexpected finding in Expt 1 - that it does not seem to matter whether the movements to be executed by each hand are the same or differ from one another - was replicated in this experiment. Thus, for each of the dependent measures (movement duration, peak velocity, deceleration phase length, maximum grip aperture and time taken to reach maximum grip aperture) means for congruent and incongruent bimanual reaches did not differ from one another.

This experiment also confirmed that while the hands appear to become temporally coupled during bimanual movements - achieving temporal markers such as peak velocity, peak grip etc. at very similar points in time - many features of the movements of each hand appear free to vary. In Expt 1 it was demonstrated that, on incongruent bimanual trials, each hand could independently achieve a peak velocity scaled for movement amplitude. In the current experiment, we observed that, while peak grip aperture was achieved at round the same time by both hands, on incongruent bimanual trials, each hand was able to independently achieve a maximum grip aperture that was scaled appropriately for object size.

GENERAL DISCUSSION

Earlier the authors outlined two general frameworks which have been proposed to explain how the movement coordination might be brought about during the execution of prehension movements. The first of these frameworks, which we refer to as temporal planning models, proposes that the coordination of movement components is planned in advance of movement onset and is based upon their temporal synchronization. This can be contrasted with a second class of models, which we refer to here as continuous control models, which argue instead for the on-line control of movement parameters, based upon the continuous sampling of spatial information. Here we examine how good an account each of these frameworks provide for the bimanual movement data obtained in the current study.

Continuous control models

A key aspect of the continuous control model proposed by Bootsma and colleagues is the proposal that movement components are not explicitly coordinated by a higher level controller, but that coordination arises from the fact that transport and grasp each rely on a common information source - change in hand-target separation. Support for this model was recently obtained in a study reported by Zaal et al. (in press), in which reaching movements directed to static targets presented at different distances from the hand were compared to reaches directed to target objects which were continuously moving away from the participant at different velocities. The key finding from this study was that movement duration was shown to be scaled to the initial distance between hand and target and not to the resultant movement amplitude of the reach. Thus, movement duration did not differ in the moving target condition for objects moving at different velocities and thus producing different reach amplitudes. In contrast, movement velocity was shown to scale for the initial distance between hand and target and for target object velocity. On the basis of their findings, the authors concluded that movement duration appears to be planned in advance of movement onset, within an egocentric frame of reference, whereas movement velocity is subject to on-line control, based upon perceptual information signalling the change in position and hand velocity relative to the target object (allocentric frame of reference).

How good an account does the continuous control model provide for the bimanual movement data obtained in the current study? Applying this model to bimanual movements where each hand is directed to a different target object, we obtain the prediction for Expt 1 of the current study (target distance manipulation), that both movement duration and movement velocity should scale for movement amplitude. This prediction was clearly supported for unimanual reaches, and for congruent bimanual reaches, but it was not supported for incongruent bimanual reaches where distance scaling of movement duration was not observed.

Applying this model together with Fitts law, we obtain the prediction for Expt 2 of the current study (target size manipulation), that movement duration and movement velocity should scale for task difficulty, with longer movement durations and lower movement velocities occurring for reaches directed to smaller target objects. This was not the pattern of results observed, however, and confirms previous results obtained for aiming movements that Fitts law does not apply for bimanual movements directed to different targets, each having a different index of difficulty (Kelso et al., 1979, 1983). As this model has not been applied to describing the grasp phase, no predictions can be derived to account for grasp phase kinematics in the present study.

Temporal planning models

Within the temporal planning framework reviewed above, it is assumed that a common movement duration is planned prior to movement onset, and the kinematics of each movement component are scaled to that common time frame. Once again, applying this model to bimanual movements where each hand is directed to a different target object, we obtain the following predictions for Expt 1 (target distance manipulation): (a) movement velocity should be scaled for movement amplitude in all cases; (b) movement duration should be scaled for movement amplitude on unimanual and congruent bimanual trials, but not on incongruent bimanual trials where a common movement duration should be observed for near and far reaches; (c) according to the Hoff & Arbib (1993) model, the movement duration selected should be that associated with the longest time estimate, this should therefore approximate the movement duration observed for the far location in the congruent bimanual condition; (d) finally, where the movement duration selected may not be optimal for both hands, one can speculate that grip safety margins would be varied to compensate, as suggested by the increase in right/left asymmetries previously reported for bimanual aiming movements (Fowler, Duck, Mosher & Mathieson, 1991). These predictions were almost all confirmed by the results of Expt 1. Movement velocity was found to be scaled for distance (prediction (a)), however, distance scaling of movement duration was not observed on incongruent bimanual trials (prediction (b)). While object size was not varied in Expt 1, grip aperture (safety margins) increased significantly on bimanual trials (prediction (d)). This was particularly noticeable for reaches with the left (non-dominant) hand on congruent bimanual trials. One prediction that was not confirmed, however, was prediction (c), that the movement duration should approximate the movement duration observed for the far location in the congruent bimanual condition. Instead, as inspection of Fig. 2 makes clear, our data suggests that the movement duration selected more closely approximates the mean of the movement durations observed for reaches to the near and far locations on congruent bimanual trials.

As before, this model can also be applied to Expt 2 (target size manipulation). Key predictions here are that: (a) grip apertures should scale for object size in all conditions, (b) movement time, and (c) time to peak group aperture should not differ for large and small objects on incongruent bimanual trials. As in Expt 1, each of these predictions were all confirmed by the results of Expt 2.

Is competitive processing between objects a limiting factor?

It should, of course, be acknowledged that neither of the above frameworks were developed to specifically account for bimanual prehension movements. However, in the authors' view it has proved informative to test these models against the data observed in the bimanual case. One question raised by this comparison, however, is why the continuous control model should offer such a poor account of bimanual prehension movements given its success in describing the kinematics of unimanual prehension. One possible answer to this question may revolve around the processing demands required in the continuous control case during bimanual movements.

Duncan, Humphreys & Ward (1997) put forward the following proposals as part of their integrated competition hypothesis: visual information produces activity within multiple brain systems, and an important aspect of this processing is that within such systems activations related to different objects compete with one another; in behavioural terms, such competition is characterized as interference in which the efficient processing of each object is impaired. Mechanisms therefore exist to reduce competition so that 'For the sensorimotor network as a whole, the tendency is to settle into a state in which different brain systems have converged to work on the same dominant object, analysing its multiple visual properties and implications for action ... At the neural level, there should be widespread maintenance of the selected object's representation, accompanied by widespread suppression of response to ignored objects' (p. 255). Within this view then, one obvious limiting factor during bimanual prehension movements directed toward different objects would be the visuomotor demands involved in attempting to continuously sample two independent' remaining time-to-contact' signals (i.e. the hand-target separations for each hand).

One solution to this problem might be for the sensorimotor system to adopt an intermittent sampling strategy during bimanual movements, in which the remaining time-to-contact signal is independently sampled for each hand by intermittently switching attention between target objects. Two predictions which can clearly be generated from such a model, and which are confirmed by the results of the current study, are that: there should be a clear cost in performing bimanual prehension movements compared to unimanual movements; and, there should be no additional costs of performing incongruent compared to congruent reaches, while this solution may account for the data in our study, it is difficult to reconcile with the 'integrated competition hypothesis' as an intermittent sampling strategy of two independent time-to-contact signals is clearly at odds with the spirit of this model. Second, our finding of a generalized slowing of both reaches during bimanual movements is inconsistent with the predictions of the integrated competition hypothesis which as currently formulated would be expected to predict sequential reaches. It is therefore of interest to note that we have recently studied a patient who executes bimanual reaches in exactly this manner (Jackson, Jackson, Husain & Harvey, 1998; Jackson, Jackson, Husain, Harvey & Dow, in press). In that study we examined the bimanual reaches of a female patient (DB) with a dense and complete hemianaesthesia of the left arm. Our findings showed that unimanual reaches executed by DB using her non-sensate limb were comparable to unimanual reaches executed using her sensate limb. This finding suggests that during unimanual reaches, DB was able to compensate for the absence of proprioceptive signals by using visual cues. In contrast, during bimanual trials, reaches executed by DB using her non-sensate hand showed gross directional errors and spatiotemporal irregularities. A key finding was that while movements of each hand commenced at the same time, they were completed in sequence, with one hand reaching its target in advance of the other. This result would appear to confirm the predictions of the 'integrated competition hypothesis', at least in circumstances where movement control is largely dependent upon visual signals due to the absence of proprioception.

An alternative solution, which both avoids the problem of having to concurrently monitor two remaining time-to-contact signals, and is consistent with concurrent control models, is for the sensorimotor system for reconfigure the task description so that only one time-to-contact signal need be monitored. This could be achieved by coupling the two limbs together so that they are constrained to act as a single functional unit as suggested by Kelso (e.g. Kelso et al., 1979). Within this view, each limb would commence moving at the same time, but would move at different velocities, so as to arrive at their respective targets simultaneously. An advantage of this model would be that only one object need be viewed to derive a remaining time-to-contact signal. However, within this model, time to contact could no longer be based on visual cues signalling the position of each hand relative to the target, but might instead be based upon a motor error signal between a visual target and the felt position of the limb. This model would thus suggest an important role for proprioception in coordinating bimanual movements.

The notion that movements of each limb may be unified within a single coordinating structure is also consistent with the movement planning models proposed by Jeannerod (1981, 1984) and Hoff & Arbib (1993). Within this framework, movements are synchronized by virtue of being scaled to unfold within a common movement duration, and the processes involved in generating the complex movement 'plan' are assumed to be completed prior to movement onset. Thus, the following might be predicted: During movement execution there should be minimal differences observed between unimanual and bimanual movements directed to targets objects of an equivalent index of difficulty. During movement execution there should be minimal differences observed between bimanual movements having the same index of difficulty compared to movements where each hand moves to a different index-of-difficulty target. Differences between unimanual and bimanual movements, and between congruent and incongruent bimanual movements would be likely to be reflected in changes in the time spent planning the movement in advance of movement execution. Unfortunately, these predictions are not completely borne out by our data. This prediction could not be tested in the current study for technical reasons, and while we found consistently found no differences in movement kinematics for congruent and incongruent bimanual movements, thus confirming the second prediction, we consistently observed a clear advantage for unimanual over bimanual movements, thereby disconfirming the first prediction. This finding suggests to us that a sensorimotor mechanism is being used during bimanual movements which is not required during unimanual movements, the most likely candidate being a mechanism for maintaining inter-limb coordination during movement execution. Evidence for such a mechanism was provided for unimanual movements, by Haggard & Wing (1991) who reported a study in which the forward motion of the arm during a reach-to-grasp movement was mechanically perturbed. They noted that shortly after this perturbation was applied to the arm, the opening of the grip was similarly retarded. This demonstration revealed that there must exist a mechanism for maintaining the coordination of transport and grasp during movement execution. The rapid time course of this effect ([approximately equal to] 80 ms), together with the results of our own studies of deafferented individuals performing bimanual movements (e.g. Jackson, Jackson, Husain, Harvey & Dow, in press) lead us to believe that this mechanism is largely dependent upon proprioceptive cues signalling changes in limb position.

In conclusion, the present study examined how the coordination of bimanual prehension movements varied as a function of whether movements were congruent (same action required of each hand) or incongruent (different actions required of each hand). The results indicated that there was an overall cost to performing bimanual movements compared to unimanual movements, and that the problem of executing incongruent bimanual movements was overcome by synchronizing both hands to a common movement duration. These results could not be accounted for by either a pure movement planning model or a continuous control model based upon the computation of visually derived remaining time-to-contact signals.

Acknowledgements

The authors are grateful to Llewelyn Morris and Roger Newport for their help with data analyses, and to Alan Allport, Chantal Bard and Umberto Castiello for their comments on an earlier version of this paper. This study was supported by grants from the ESRC and the Wellcome Trust to SRJ and GMJ.

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Title Annotation:Special Topic: Movement and Action
Author:Jackson, G.M.; Jackson, S.R.; Kritikos, A.
Publication:British Journal of Psychology
Date:May 1, 1999
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