Intermovement interval and spatial assimilation effects in sequential bimanual and unimanual movements

This study extended earlier work by showing spatial assimilations in sequential bimanual and unimanual movements separated by 1.5-3.5 s. In Experiments 1 and 2, 30 right-handed participants (18-22 years of age) made rapid single and bimanual lever reversals of 20 degrees and 60 degrees assigned to 1.5, 2.5, or 3.5 s intermovement interval groups. Participants self-timed the intermovement interval in the first experiment, but it was provided in the second experiment using separate auditory stimuli. In the third experiment, participants performed both the 20 degree and 60 degree movement with the same hand. In all experiments, the shorter-distance limb overshot and the longer-distance limb undershot the targets in both bimanual and unimanual sequential movements relative to single movements in all three intermovement interval groups, particularly in the non-dominant left limb. The results suggest that assimilation effects in sequential movements are caused by command interactions at the planning level, but the effects are reduced by practice.     

Spatial assimilation effects in sequential movements: effects of parameter value switching and practice organization

In Experiment 1, the author extended earlier work by investigating spatial assimilations in sequential aiming movements when participants were able to preplan only the 1st movement of a 2-movement sequence. Right-handed participants (N = 20) aged 18-22 years tried unimanual rapid lever reversals of 20 degrees and 60 degrees with an intermovement interval of 2.5 s. Following the 1st movement, participants made a same-distance movement, different-distance movement, or no movement in a randomly determined order. Participants overshot the short-distance target and undershot the long-distance target for both movements in the sequence, but the errors were greater when the 2nd movement differed from the 1st one. In Experiment 2, right-handed participants (N = 20) demonstrated greater assimilation effects after random practice than after blocked practice of both same-distances (20 degrees -20 degrees and 60 degrees -60 degrees ) and different-distances (20 degrees -60 degrees and 60 degrees -20 degrees ) sequences, although spatial errors were greater in different-distances conditions than in same-distances conditions. Overall, the experiments showed that parameter-value switching and practice organization are 2 major sources of spatial inaccuracy in sequential aiming movements.     

Hand preference, practice order, and spatial assimilations in rapid bimanual movement

When subjects make rapid bimanual aiming movements over different distances, spatial assimilations are shown; the shorter distance limb overshoots when paired with a longer distance limb. Recent research has also shown spatial assimilations to be greater in the nonpreferred left limb of right-handed subjects, but it is not known whether the increased spatial assimilations represent a handedness effect or one of hemispheric lateralization of motor control. To determine the nature of the asymmetric effect, left- (n = 32) and right- (n = 60) handed subjects part practiced, then whole practiced, short (20 degrees ) and long 60 degrees ) reversal movements. During whole practice, both groups showed spatial assimilations in the shorter distance limb, particularly when the left limb performed the short movement. This asymmetry was greatest for right-handed subjects, but left-handed subjects showed smaller, but systematic effects, providing moderate support for the hypothesis that the asymmetric effect is due to hemispheric lateralization of motor control. All interlimb differences in spatial accuracy for the short and long movements were eliminated with practice, however, suggesting the asymmetric effect was temporary as well. In addition, subjects who part practiced the long movement just prior to whole practice showed greater overshooting in the short distance limb compared with subjects who followed the other practice order throughout whole practice and the no-KR retention trials. Such findings suggest that the part-practice order of bimanual tasks can directionally bias whole-task performance.     

Separate movement planning and spatial assimilation effects in sequential bimanual aiming movements

This study extended earlier work by showing spatial assimilations in sequential bimanual aiming movements when the participant preplanned only the first movement of a two-movement sequence. Right-handed participants (n=20, aged 18 to 22 years) made rapid lever reversals of 20 degrees and 60 degrees singly and sequentially with an intermovement interval of 2.5 sec. Following blocked single practice of both movements in each hand (15 trials each), two sets of 30 sequential practice trials were completed. The sequences began with either the long or the short movement and the participant always knew the goal of the first movement. During the intermovement interval, the experimenter gave instructions to complete the sequence with a short movement, a long movement, or no movement in a random order. Compared to the single trials, both movements in the sequence overshot the short-distance and undershot the long-distance goal. Spatial errors increased when a change in the movement goal was required for the second movement in the sequence. The experiment demonstrated that separate planning of sequential aiming movements can reduce spatial assimilation effects, but interference due to practice organization and switching the task's goal must also be overcome in order to produce accurate aiming movements.     

Speed-accuracy tradeoffs in rapid bimanual aiming movements

Two experiments reported the effect of movement time and knowledge of results on overall spatial errors in rapid simultaneous bimanual aiming movements. In Exps. 1 (n=32) and 2 (n=32), participants used light, aluminum levers oriented vertically in the sagittal plane to make reversal movements over the same distance (20 degrees - 20 degrees or 60 degrees - 60 degrees) or different distances (20 degrees - 60 degrees) in each arm in 250, 350, or 450 msec. to the reversal point. The participants in Exp. 1 were given knowledge of results on the spatial and temporal accuracy for both arms, while in Exp. 2 knowledge of results was provided for one arm only. Strong speed-accuracy tradeoffs were shown for all groups in both experiments, but errors were larger in the different distance movements compared to the same distance groups. Spatial errors were also elevated in Exp. 2 when knowledge of results was not available compared to those conditions where knowledge of results was available. Overall, bimanual speed-accuracy tradeoffs are similar to single arm movements when one moves the same distance in each arm and when knowledge of results is available.     

Parameter value switching in discrete and continuous aiming movements

The effect of practice variations on spatial and temporal accuracy was investigated in both discrete and continuous aiming movements in the preferred hand of college-aged participants (N=25). In a completely within-subject design, participants made rapid reversal movements with a lightweight lever in the sagittal plane, practicing 20 degrees and 60 degrees movements in repeated (same distance) and alternating (switching between 20 degrees and 60 degrees) conditions. Movements were also made one at a time (discretely) or in sequences of 20 movements (continuously). Spatial constant error, spatial variable error, spatial overall error, the coefficient of variation, movement time, and the relative timing were calculated for each set of 20 movements and analyzed by within-subject analyses of variance. Movements in the repeated conditions for both discrete and continuous movements were more accurate and consistent compared to the alternating condition where the short movements were overshot and the long movements were undershot. Discrete movements were more spatially and temporally variable than continuous movements. The discrete and continuous movements showed different relative timing patterns, suggesting that the temporal structure of the motor program is affected by task characteristics.     

A Visual Representation and the Control of Manual Aiming Movements

Three experiments were conducted to determine if a representation of the movement environment is functional in the organization and control of limb movements, when direct visual contact with the environment is prevented. In Experiment 1, a visual rearrangement procedure was employed to show that a representation of the environment that provides inaccurate information about the spatial location of a target can disrupt manual target aiming. In Experiment 2, we demonstrated that spatial information about the position of a target can be destroyed by a visual pattern mask, supporting our claim that the representation is visual. A target-cuing procedure was used in Experiment 3 to show that representation of target position can be useful for premovement organization in a targetaiming task. Together our findings suggest that a short-lived visual representation of the movement environment may serve a useful role in the organization and control of limb movements.     

A visual representation and the control of manual aiming movements

Three experiments were conducted to determine if a representation of the movement environment is functional in the organization and control of limb movements, when direct visual contact with the environment is prevented. In Experiment 1, a visual rearrangement procedure was employed to show that a representation of the environment that provides inaccurate information about the spatial location of a target can disrupt manual target aiming. In Experiment 2, we demonstrated that spatial information about the position of a target can be destroyed by a visual pattern mask, supporting our claim that the representation is visual. A target-cuing procedure was used in Experiment 3 to show that representation of target position can be useful for premovement organization in a target-aiming task. Together our findings suggest that a short-lived visual representation of the movement environment may serve a useful role in the organization and control of limb movements.     

The influence of movement cues on intermanual interactions

In two experiments, we studied intermanual interactions in bimanual reversal movements and bimanual aiming movements. Targets were presented on a monitor or directly on the table on which the movements were produced. Amplitudes for each hand were cued symbolically or spatially either in advance of an imperative signal or simultaneous with it. In contrast to findings of Diedrichsen et al. (Psychological Science, 12, 493–498, 2001), reaction times for different-amplitude movements were longer than for same-amplitude movements both for symbolic and spatial cues presented on the monitor and directly on the table. However, with symbolic cues the effect of the relation between target amplitudes was considerably stronger than with spatial cues, no matter where the cues were presented. Intermanual correlations of amplitudes, movement times, and reaction times were smaller with different than with same target amplitudes, and this modulation was more pronounced when targets and cues were presented on the monitor than when they were presented on the table. The findings are taken to suggest that the basic reaction-time disadvantage of different-amplitude movements results from interference between concurrent processes of amplitude specification. Additional factors like interference between concurrent processes of mapping cues on movement characteristics may add strongly to it.     

Anchoring in a novel bimanual coordination pattern

Anchoring in cyclical movements has been defined as regions of reduced spatial or temporal variability [Beek, P. J. (1989). Juggling dynamics. PhD thesis. Amsterdam: Free University Press] that are typically found at movement reversal points. For in-phase and anti-phase movements, synchronizing reversal points with a metronome pulse has resulted in decreased anchor point variability and increased pattern stability [Byblow, W. D., Carson, R. G., & Goodman, D. (1994). Expressions of asymmetries and anchoring in bimanual coordination. Human Movement Science, 13, 3-28; Fink, P. W., Foo, P., Jirsa, V. K., & Kelso, J. A. S. (2000). Local and global stabilization of coordination by sensory information. Experimental Brain Research, 134, 9-20]. The present experiment examined anchoring during acquisition, retention, and transfer of a 90 degrees phase-offset continuous bimanual coordination pattern (whereby the right limb lags the left limb by one quarter cycle), involving horizontal flexion about the elbow. Three metronome synchronization strategies were imposed: participants either synchronized maximal flexion of the right arm (i.e., single metronome), both flexion and extension of the right arm (i.e., double metronome within-limb), or flexion of each arm (i.e., double metronome between-limb) to an auditory metronome. In contrast to simpler in-phase and anti-phase movements, synchronization of additional reversal points to the metronome did not reduce reversal point variability or increase pattern stability. Furthermore, practicing under different metronome synchronization strategies did not appear to have a significant effect on the rate of acquisition of the pattern.     

Concurrent visual feedback and spatial accuracy in continuous aiming movements

The effect of concurrent visual feedback (CVF) on continuous aiming movements was investigated in the preferred hand of participants of college age (ns = 12 men, 8 women). Participants made continuous rapid reversal movements with a lightweight lever in the sagittal plane. Participants attempted to reach a short target (20 degrees) and a long target (60 degrees) in separate constant practice conditions, but alternated between the two targets in a variable practice condition. Four blocks of practice trials were provided in each condition, with 40 movements made in each. CVF of the position-time trace was provided for the first 20 movements of each block, but was removed for the remaining 20 movements in each block. Movements were more accurate and consistent during constant practice compared to variable practice where the short target was overshot and the long target was undershot. CVF reduced errors in all conditions, compared to movements without CVF, particularly for the short target during variable practice. The results suggest that the interference generated by alternating targets can be modulated by providing visual feedback, but once the visual feedback was removed, errors increased markedly.     

Moving to Directly Cued Locations Abolishes Spatial Interference During Bimanual Actions

Interference is frequently observed during bimanual movements if the two hands perform nonsymmetric actions. We examined the source of bimanual interference in two experiments in which we compared conditions involving symmetric movements with conditions in which the movements were of different amplitudes or different directions. The target movements were cued either symbolically by letters or directly by the onset of the target locations. With symbolic cues, reaction times were longer when the movements of the two hands were not symmetric. With direct cues, reaction times were the same for symmetric and nonsymmetric movements. These results indicate that directly cued actions can be programmed in parallel for the two hands. Our results challenge the hypothesis that the cost to initiate nonsymmetric movements is due to spatial interference in a motor-programming stage. Rather, the cost appears to be caused by stimulus identification, response-selection processes connected to the processing of symbolic cues, or both.     

Kinetic attraction during bimanual coordination

Two experiments examined the effects of independent variations in kinetic and kinematic requirements on interlimb coupling during a bimanual task. The goal of the investigation was to provide preliminary evidence regarding one general class of physical variables that constrains discrete bimanual movements. Subjects attempted to execute a smooth unidirectional movement with the left arm, along with a three-segment reversal movement with the right arm. The first experiment manipulated the torque required to produce the reversal action, while movement duration and average angular velocity were held constant for both limbs. Several indications of increased interlimb coupling, due to the kinetic variation, were evident. The converse manipulation was used in the second experiment, with movement time and kinematics (velocity, acceleration) changed independently of joint torque requirements for the reversal limb. No clear effect of kinematics on coupling strength was noted. The results suggest that one variable influencing interlimb attraction toward common spatiotemporal trajectories may be kinetic in nature.     

Kinetic Attraction During Bimanual Coordination

Two experiments examined the effects of independent variations in kinetic and kinematic requirements on interlimb coupling during a bimanual task. The goal of the investigation was to provide preliminary evidence regarding one general class of physical variables that constrains discrete bimanual movements. Subjects attempted to execute a smooth unidirectional movement with the left arm, along with a three-segment reversal movement with the right arm. The first experiment manipulated the torque required to produce the reversal action, while movement duration and average angular velocity were held constant for both limbs. Several indications of increased interlimb coupling, due to the kinetic variation, were evident. The converse manipulation was used in the second experiment, with movement time and kinematics (velocity, acceleration) changed independently of joint torque requirements for the reversal limb. No clear effect of kinematics on coupling strength was noted. The results suggest that one variable influencing interlimb attraction toward common spatiotemporal trajectories may be kinetic in nature.     

Performance of unimanual and bimanual multiphased prehensile movements

By manipulating task action demands in 2 experiments, the author investigated whether the context-dependent effects seen in unimanual multiphase movements are also present in bimanual movements. Participants (N = 14) in Experiment 1 either placed or tossed objects into targets. The results indicated that the intention to perform a subsequent action with an object could influence the performance of an earlier movement in a sequence in both unimanual and bimanual tasks. Furthermore, assimilation effects were found when the subsequent tasks being performed by the 2 hands were incongruent. In Experiment 2, the author investigated in 12 participants whether planning in a multiphase movement includes some representation of the accuracy demands of the subsequent task. The accuracy demands of a subsequent task did not appear to influence initial movement planning. Instead, the present results support the notion that it is the action requirements of the subsequent movement that lead to context-dependent effects.     

Bimanual coupling effects during arm immobilization and passive movements

When humans simultaneously perform different movements with both hands, each limb movement interferes with the contralateral limb movement (bimanual coupling). Previous studies on both healthy volunteers and patients with central or peripheral nervous lesions suggested that such motor constraints are tightly linked to intentional motor programs, rather than to movement execution. Here, we aim to investigate this phenomenon, by using a circles-lines task in which, when subjects simultaneously draw lines with the right hand and circles with the left hand, both the trajectories tend to become ovals (bimanual coupling effect). In a first group, we immobilized the subjects’ left arm with a cast and asked them to try to perform the bimanual task. In a second group, we passively moved the subjects’ left arm and asked them to perform voluntary movements with their right arm only. If the bimanual coupling arises from motor intention and planning rather than spatial movements, we would expect different results in the two groups. In the Blocked group, where motor intentionality was required but movements in space were prevented by immobilization of the arm, a significant coupling effect (i.e., a significant increase of the ovalization index for the right hand lines) was found. On the contrary, in the Passive group, where movements in space were present but motor intentionality was not required, no significant coupling effect was observed. Our results confirmed, in healthy subjects, the central role of the intentional and predictive operations, already evidenced in pathological conditions, for the occurrence of bimanual coupling.     

Goal congruency in bimanual object manipulation

In 3 experiments, the authors investigated the impact of action goals on the production of discrete bimanual responses. Similar to a bartender putting 2 glasses simultaneously on a shelf, participants placed 2 objects into either parallel or opposite orientations by carrying out either mirror-symmetrical or mirror-asymmetrical movements. In Experiment 1, performance was strongly affected by the congruency of the intended object orientations but was essentially unaffected by movement symmetry. Experiment 2 replicated this instrumental goal-congruency effect (and the absence of motor-symmetry effects) when actions were cued in advance. Experiment 3 revealed substantial motor-symmetry effects, provided the movements themselves became the action goal. The authors concluded that performance in bimanual choice reaction tasks is constrained by the creation and maintenance of goal codes rather than by properties inherent in the neuromuscular system that carries out these responses. These goals can relate to either body-intrinsic states or to body-extrinsic states according to the actor's current intentions.     

The role of acceleration information in prism adaptation

Two experiments were performed in which the acceleration component of limb movement information during prism exposure was manipulated, by controlling the trajectory and visibility of arm movement. When limb movements were confined to a lateral motion on a linear track, adaptation was evident when arm movement reversal at the end of the trajectory could be viewed (nonoccluded arm-movement reversal conditions). No adaptation occurred in the occluded arm-movement reversal condition. When movements were made on a curved track, adaptation was evident in both the nonoccluded and the occluded arm-movement reversal conditions. The results indicate that the acceleration component of reafferent stimulation may be critical in prism adaptation when no error information is available.     

Bimanual interference associated with the selection of target locations

Four experiments were conducted to identify the locus of interference observed during the preparation of bimanual reaching movements. Target locations were specified by color, and the right-hand and left-hand targets could be either the same or a different color. Movements of different amplitudes (Experiment 1) or different directions (Experiment 2) to targets of the same color were initiated more quickly than symmetric movements to targets of different colors. These results indicate that costs observed during bimanual movements arise during target selection rather than during motor programming. Experiments 3 and 4 further examined the interference associated with target selection. Reaction time costs were found with unimanual movements when the target was presented among distractors associated with responses for the other hand. Interference observed during bimanual reaching appears to reflect difficulty in segregating the response rules assigned to each hand.