Biomedical subjects

Robert L Sainburg

Publications and source records attributed to Robert L Sainburg.

At least 19 recordsLinked to original sources

PubMed2007

The effect of target modality on visual and proprioceptive contributions to the control of movement distance.

Abstract The goal of this study was to determine whether the sensory nature of a target influences the roles of vision and proprioception in the planning of movement distance. Two groups of subjects made rapid, elbow extension movements, either toward a visual target or toward the index fingertip of the unseen opposite hand. Visual feedback of the reaching index fingertip was only available before movement onset. Using a virtual reality display, we randomly introduced a discrepancy between actual and virtual (cursor) fingertip location. When subjects reached toward the visual target, movement distance varied with changes in visual information about initial hand position. For the proprioceptive target, movement distance varied mostly with changes in proprioceptive information about initial position. The effect of target modality was already present at the time of peak acceleration, indicating that this effect include feedforward processes. Our results suggest that the relative contributions of vision and proprioception to motor planning can change, depending on the modality in which task relevant information is represented.

Adult↗

PubMed2006

Lateralization of motor adaptation reveals independence in control of trajectory and steady-state position.

We recently provided evidence that the dominant and nondominant arms are specialized for controlling different aspects of reaching movements. In this study, we test whether these specializations lead to qualitatively different adaptive mechanisms, when exposed to novel inertial dynamics. Two groups of six right-handed adults performed a reaching task toward a single target with either the dominant or nondominant arm. After 39 trials of task familiarization without a load, subjects performed 180 trials with a 1.5 kg mass, positioned 25 cm lateral to the forearm. To assess the time course of adaptation, we quantified changes in movement direction and linearity, and to assess the quality of adaptation, we randomly interspersed aftereffect trials and generalization trials. The former were assessed by removing the mass and the latter by changing the location of the movement in the workspace. Whereas, final position accuracy improved to the same extent for both arms, initial movement direction improved only for the dominant arm. In contrast, nondominant arm adaptation occurred mainly by making more effective corrections for persistent errors in initial direction. Consistent with these findings, aftereffect trials, an indicator of feedforward control processes, showed progressive increases in error for only the dominant arm. In addition, substantial generalization only occurred for the dominant arm. These results support our hypothesis that interlimb differences in control mechanisms produce different patterns of adaptation to novel inertial dynamics.

Adaptation, Physiological↗

PubMed2006

Hand dominance and multi-finger synergies.

Recent studies of arm movement reaching to a target have provided support for the Dynamic Dominance model of handedness, which proposes that each hemisphere/limb system is specialized for controlling different features of performance: The dominant system for control of the trajectory, and the nondominant system for control of the steady state final position. We now examine a more general form of this hypothesis by investigating differences between the right and left hands of right-handed persons in their ability to stabilize the combined action of a set of fingers. The subjects produced very quick pulses and steps of force from a low background force level while pressing with four fingers of one hand. An index of force stabilizing synergy showed a strong multi-finger synergy during steady-state force production followed by a small anticipatory drop in the index prior to the force increase. Hand differences emerged during the force increase: The left (non-dominant) hand showed a significantly larger drop in the synergy index. While our findings support the idea that the dominant system is specialized for stabilizing quick changes in performance variables, we failed to support a nondominant specialization for stabilizing steady-state isometric force. This may be a ceiling effect due to the simplicity of the task for either hand, or it might indicate that this aspect of the dynamic dominance hypothesis does not generalize to isometric conditions.

Adult↗

PubMed2006

Interlimb transfer of visuomotor rotations depends on handedness.

We previously reported that opposite arm adaptation to visuomotor rotations improved the initial direction of right arm movements in right-handers, whereas it only improved the final position accuracy of their left arm movements. We now investigate the pattern of interlimb transfer following adaptation to 30 degrees visuomotor rotations in left-handers to determine whether the direction of transfer depends on handedness. Our results indicate unambiguous transfer across the arms. In terms of final position accuracy, the direction of transfer is opposite to that observed in right-handers, such that transfer only occurred from the left to the right arm movements. Directional accuracy also showed the opposite pattern of transfer to that of right-handers: initial movement direction, calculated at peak tangential acceleration, transferred only from right to left arms. When movement direction was measured later in the movement, at peak tangential velocity, asymmetrical transfer also occurred, such that greater transfer occurred from right to left arms. However, a small, but significant influence of opposite arm adaptation also occurred for the left arm, which might reflect differences in the use of the nondominant arm between left- and right-handers. Overall, our results indicate that left-handers show a mirror-imaged pattern of interlimb transfer in visuomotor adaptation to that previously reported for right-handers. This pattern of transfer is consistent with the hypothesis that asymmetry in interlimb transfer is dependent on differential specialization of the dominant and nondominant hemisphere/limb systems for trajectory and positional control, respectively.

Adolescent↗

PubMed2005

Differential influence of vision and proprioception on control of movement distance.

The purpose of this study was to investigate the contribution of proprioceptive and visual information about initial limb position in controlling the distance of rapid, single-joint reaching movements. Using a virtual reality environment, we systematically changed the relationship between actual and visually displayed hand position as subjects' positioned a cursor within a start circle. No visual feedback was given during the movement. Subjects reached two visual targets (115 and 125 degrees elbow angle) from four start locations (90, 95, 100, and 105 degrees elbow angle) under four mismatch conditions (0, 5, 10, or 15 degrees). A 2 x 4 x 4 ANOVA enabled us to ask whether the subjects controlled the movement distance in accord with the virtual, or the actual hand location. Our results indicate that the movement distance was mainly controlled according to the virtual start location. Whereas distance modification was most extensive for the closer target, analysis of acceleration profiles revealed that, regardless of target position, visual information about start location determined the initial peak in tangential hand acceleration. Peak acceleration scaled with peak velocity and movement distance, a phenomenon termed "pulse-height" control. In contrast, proprioceptive information about actual hand location determined the duration of acceleration, which also scaled with peak velocity and movement distance, a phenomenon termed "pulse-width" control. Because pulse-height and pulse-width mechanisms reflect movement planning and sensory-based corrective processes, respectively, our current findings indicate that vision is used primarily for planning movement distance, while proprioception is used primarily for online corrections during rapid, unseen movements toward visual targets.

Adult↗

PubMed2005

The symmetry of interlimb transfer depends on workspace locations.

We have previously shown that when both arms learn visuo-motor tasks within the shared midline workspace, transfer becomes asymmetrical: initial direction information only transfers from the nondominant to the dominant arm, whereas the final position information only transfers from the dominant to the nondominant arm. We now examine whether symmetry of interlimb transfer depends on the location of workspace provided for the two arms, by investigating the pattern of interlimb transfer when each arm adapts to a 30 degree rotation at its ipsilateral workspace. All subjects performed center-out reaching movements while adapting to a 30 degree rotation in the visual display. Half the subjects performed with the nondominant arm first and then the dominant arm, while the other half performed with the dominant arm first and then the nondominant arm. To assess transfer, naïve performance and the performance following opposite arm adaptation were compared for each arm separately. Our results indicate unambiguous transfer that is symmetrical: both arms benefited from opposite arm training in terms of initial direction control. In terms of final position information, neither arm benefited from opposite arm training. This clearly demonstrates that symmetry of interlimb transfer depends on the location of workspace provided for the two arms. Our findings suggest that when visuo-motor tasks are performed within a shared midline workspace, the nondominant controller is selectively inhibited from access to dominant controller information, due to a certain competition between the two limb/hemisphere systems that is introduced by the shared nature of the workspace.

Adolescent↗

PubMed2005

Adaptation to visuomotor rotations remaps movement vectors, not final positions.

When exposed to novel visuomotor rotations, subjects readily adapt reaching movements, such that the virtual display of the hand is brought to the target. Whereas this clearly reflects remapping of the relationship between hand movements and the visual display, the nature of this remapping is not well understood. We now examine whether such adaptation results in remapping of the position of the visually displayed target and the final limb position or between the target vector and the movement vector. The latter is defined relative to a starting position, whereas the former should be independent of the starting position. Subjects first adapted to a 30 degrees rotation during reaching movements made from a single starting location to four different target locations. After adaptation, generalization trials were introduced, during which reaching movements were made under the same visual rotation condition but started from one of two locations outside the practiced workspace. These trials were directed to either the previously practiced targets or new targets that reflected the direction and distance of the practiced trials. Generalization was greatest for movements made in similar directions, regardless of changes in spatial location. Most significantly, when reaching to the previously adapted targets, subjects did not reach to the previously learned limb positions but rather to positions that reflected a near 30 degrees rotation of the new target vector. These results indicate that learned visuomotor rotations remap the representations of movement vectors and not final positions of the limb in the workspace.

Acclimatization↗

PubMed2005

Handedness: differential specializations for control of trajectory and position.

Handedness is often portrayed as a nonfunctional artifact of cerebral asymmetry. Our data, however, indicate that handedness reflects cerebral specialization for specific control processes. Each hemisphere/limb system is specialized for different but complementary functions: the dominant system for controlling limb trajectory dynamics, and the nondominant system for controlling limb position.

Biomechanical Phenomena↗

PubMed2004

Interlimb transfer of load compensation during rapid elbow joint movements.

Previous research has shown that training of a novel task can improve subsequent performance in the opposite arm owing to anticipation of the previously learned task conditions. Interestingly, we recently reported preliminary evidence that such transfer might also include modulation of feedback-mediated responses. We now test interlimb transfer of load compensation responses, measured through kinematic and EMG recordings during rapid 20 degrees elbow flexion movements. Two subject groups, LR and RL, each comprising six right-handed subjects, first performed using either the left (LR) or right (RL) arm, followed by opposite arm performance. After 30 trials of consistent performance, five random trials within a background of 50 trials were loaded with a 2-kg mass prior to the "go" signal. We compared load compensation responses for naive performance with those following opposite arm exposure. Under naive conditions, the resulting load compensation responses began about 50 ms following movement onset, and were substantially more effective for the nondominant arm. Opposite arm exposure substantially improved the accuracy of only dominant arm responses. This, however, did not occur through changes in the short latency components of the load compensation response. Instead, changes in muscle activities, associated with interlimb transfer, began some 150 ms following movement onset. We expect that these changes represent transfer in the "volitional" component of the load compensation response. Because the shorter latency response was unaffected by opposite arm exposure, modulation of this component likely requires prior experience with limb specific effectors.

Adult↗

PubMed2004

Interlimb differences in control of movement extent.

The current study was designed to examine potential interlimb asymmetries in controlling movement extent. Subjects made repetitive single-joint elbow extension movements while the arm was supported on a horizontal, frictionless, air-jet system. Four targets of 10, 20, 35, and 45 degrees excursions were randomly presented over the course of 150 trials. For both arms, peak tangential hand velocity scaled linearly with movement distance. There was no significant difference between either peak velocities or movement accuracies for the two arms. However, the mechanisms responsible for achieving these velocities and extents were quite distinct for each arm. For the dominant arm, peak tangential finger acceleration varied systematically with movement distance. In contrast, nondominant-arm peak tangential acceleration varied little across targets and, as such, was a poor predictor of movement distance. Instead the velocities of the nondominant arm were determined primarily by variation in the duration of the initial acceleration impulse, which corresponds to the time of peak velocity. These different strategies reflect previously identified mechanisms in controlling movement distance: pulse-height control and pulse-width control. The former is characterized by a variation in peak acceleration and has been associated with preplanning mechanisms. The latter occurs after peak acceleration and has been shown to depend on peripheral sensory feedback. Our findings indicate that the dominant-arm system controls movement extent largely through planning mechanisms that specify pulse-height control, whereas the nondominant system does so largely through feedback mediated pulse-width control.

Adult↗

PubMed2004

Interlimb transfer of novel inertial dynamics is asymmetrical.

Mechanisms underlying interlimb transfer of adaptation to visuomotor rotations have recently been explored in depth. However, little data are available regarding interlimb transfer of adaptation to novel inertial dynamics. The present study thus investigated interlimb transfer of dynamics by examining the effect of initial training with one arm on subsequent performance with the other in adaptation to a 1.5-kg mass attached eccentrically to the forearm. Using inverse dynamic analysis, we examined the changes in torque strategies associated with adaptation to the extra mass, and with interlimb transfer of that adaptation. Following initial training with the dominant arm, nondominant arm performance improved substantially in terms of linearity and initial direction control as compared with naïve performance. However, initial training with the nondominant arm had no effect on subsequent performance with the dominant arm. Inverse dynamic analysis revealed that improvements in kinematics were implemented by increasing flexor muscle torques at the elbow to counter load-induced increases in extensor interaction torques as well as increasing flexor muscle torques at the shoulder to counter the extensor actions of elbow muscle torque. Following opposite arm adaptation, the nondominant arm adopted this dynamic strategy early in adaptation. These findings suggest that dominant arm adaptation to novel inertial dynamics leads to information that can be accessed and utilized by the opposite arm controller, but not vice versa. When compared with our previous findings on interlimb transfer of visuomotor rotations, our current findings suggest that adaptations to visuomotor and dynamic transformations are mediated by distinct neural mechanisms.

Adolescent↗

PubMed2003

Limitations in interlimb transfer of visuomotor rotations.

It has been shown that learning visuomotor rotations with multiple target directions, compared with a single target direction, leads to greater generalization to untrained targets within the same limb. This implies that multiple direction learning results in a more complete internal model of the visuomotor transform. It has also been documented that the extent of transfer of movement information regarding visuomotor adaptations between the limbs is limited, relative to that between different configurations of the same limb. The present study thus investigated the origin of this restriction in interlimb transfer, by comparing the effects of eight-direction and one-direction training conditions with one arm on the subsequent performance with the other arm. It was hypothesized that if multiple direction learning leads to a more complete model of the novel visuomotor transform, interlimb transfer should be enhanced relative to that following single direction training. However, if no differences are observed between single and multiple direction training conditions, this would suggest that such learning is effector dependent. We also tested the hypothesis that interlimb transfer of visuomotor adaptation is not obligatory, by examining the effects of visual rotation direction (same or oppositely directed visuomotor rotations for the two arms). All subjects first adapted to a 30 degrees rotation, either clockwise or counterclockwise, in the visual display during reaching movements. Following this, they adapted to a 30 degrees rotation in either the same or opposing direction with the other arm. Results showed that initial training with the non-dominant arm facilitated subsequent performance with the dominant arm in terms of initial direction control, but only under the same rotation condition. Both single and eight direction training conditions led to substantial transfer in subsequent performance with the other arm, but multiple direction training was no more beneficial than single direction training. This finding suggests that the previously reported intralimb advantages of multiple direction learning are effector specific. Our findings are discussed in the context of hierarchical models of motor control to explain the intralimb advantages of multiple direction training.

Adolescent↗

PubMed2003

Movement speed effects on limb position drift.

Previous research has shown that even when limb position drifts considerably during continuous blind performance, the topological and metrical properties of generated hand paths remain remarkably invariant. We tested two possible accounts of this intriguing effect. According to one hypothesis, position drift is due to degradation of limb-position information. This hypothesis predicted that drift of static hand positions at movement reversals should not depend on movement speed. According to the other hypothesis, position drift is due to degradation of movement information. This hypothesis predicted that drift of static hand positions at movement reversals should vary with movement speed. We tested these two hypotheses by varying the required movement speed when normal human adults performed back-and-forth manual positioning movements in the absence of visual feedback. Movement distance and direction were well preserved even though hand positions between movements drifted considerably. In accord with the movement error hypothesis, but not in accord with the position hypothesis, the rate at which hand positions drifted depended on movement speed. The data are consistent with the idea that hand position, which defines the origin of the trajectory control coordinate system, and movement trajectory are controlled by distinct neural mechanisms.

Adult↗

PubMed2003

Differential contributions of vision and proprioception to movement accuracy.

We examined the relative roles of visual and proprioceptive information about initial hand position on movement accuracy. A virtual reality environment was employed to dissociate visual information about hand position from the actual hand position. Previous studies examining the effects of such dissociations on perception of hand location have indicated a bias toward the visually displayed position. However, an earlier study, which employed optical prisms to dissociate visual and proprioceptive information prior to targeted movements, suggested a bias in movement direction toward that defined by the actual hand position. This implies that visual and proprioceptive information about hand position may be differentially employed for perceptual judgments and movement planning, respectively. We now employ a virtual reality environment to systematically manipulate the visual display of the hand start position from the actual hand position during movements made to a variety of directions. We asked whether subjects would adjust their movements in accord with the virtual or the actual hand location. Subjects performed a series of baseline movements toward one of three targets in each of three blocks of trials. Interspersed among these trials were "probe" trials in which the cursor location, but not the hand location, was displaced relative to the baseline start position. In all cases, cursor feedback was blanked at movement onset. Our findings indicated that subjects systematically adjusted the direction of movement in accord with the virtual, not the actual, start location of the hand. These findings support the hypothesis that visual information about hand position predominates in specifying movement direction.

Adolescent↗

PubMed2003

Nondominant arm advantages in load compensation during rapid elbow joint movements.

This study was designed to examine interlimb asymmetries in responding to unpredictable changes in inertial loads, which have implications for our understanding of the neural mechanisms underlying handedness. Subjects made repetitive single joint speed constrained 20 degrees elbow flexion movements, while the arm was supported on a horizontal, frictionless, air-jet system. On random trials, a 2-kg mass was attached to the arm splint prior to the "go" signal. Subjects were not given explicit information about the mass prior to movement nor were they able to view their limb or the mass. Accordingly, muscle activity, recorded prior to peak tangential finger acceleration, was the same for loaded and baseline trials. After this point, substantial changes in muscle activity occurred. In both limbs, the load compensation response was associated with a reduction in extensor muscle activity, resulting in a prolonged flexion phase of motion. For the nondominant arm, this resulted in effective load compensation, such that no differences in final position accuracy occurred between loaded and baseline trials. However, the dominant arm response also included a considerable increase in flexor muscle activity. This substantially prolonged the flexor acceleration phase of motion, relative to that of the nondominant arm. As a result, the dominant arm overcompensated the effects of the load, producing a large and systematic overshoot of final position. These results indicate more effective load compensation responses for the nondominant arm; supporting a specialized role of the nondominant arm/hemisphere system in sensory feedback mediated error correction mechanisms. The results also suggest that specialization of the dominant arm system for controlling limb and task dynamics is specifically related to feedforward control mechanisms.

Adult↗

PubMed2003

Mechanisms underlying interlimb transfer of visuomotor rotations.

We previously reported that opposite arm training improved the initial direction of dominant arm movements, whereas it only improved the final position accuracy of non-dominant arm movements. We now ask whether each controller accesses common, or separate, short-term memory resources. To address this question, we investigated interlimb transfer of learning for visuomotor rotations that were directed oppositely [clockwise (CW)/counterclockwise (CCW)] for the two arms. We expected that if information obtained by initial training was stored in the same short-term memory space for both arms, opposite arm training of a CW rotation would interfere with subsequent adaptation to a CCW rotation. All subjects first adapted to a 30 degrees rotation (CW) in the visual display during reaching movements. Following this, they adapted to a 30 degrees rotation in the opposite direction (CCW) with the other arm. In contrast to our previous findings for interlimb transfer of same direction rotations (CCW/CCW), no effects of opposite arm adaptation were indicated in the initial trials performed. This indicates that interlimb transfer is not obligatory, and suggests that short-term memory resources for the two limbs are independent. Through single trial analysis, we found that the direction and final position errors of the first trial of movement, following opposite arm training, were always the same as those of naive performance. This was true whether the opposite arm was trained with the same or the opposing rotation. When trained with the same rotation, transfer of learning did not occur until the second trial. These findings suggest that the selective use of opposite arm information is dependent on the first trial to probe current movement conditions. Interestingly, the final extent of adaptation appeared to be reduced by opposite arm training of opposing rotations. Thus, the extent of adaptation, but not initial information transfer, appears obligatorily affected by prior opposite arm adaptation. According to our findings, it is plausible that the initiation and the final extent of adaptation involve two independent neural processes. Theoretical implications of these findings are discussed.

Adult↗

PubMed2003

Limb position drift: implications for control of posture and movement.

In the absence of visual feedback, subject reports of hand location tend to drift over time. Such drift has been attributed to a gradual reduction in the usefulness of proprioception to signal limb position. If this account is correct, drift should degrade the accuracy of movement distance and direction over a series of movements made without visual feedback. To test this hypothesis, we asked participants to perform six series of 75 repetitive movements from a visible start location to a visible target, in time with a regular, audible tone. Fingertip position feedback was given by a cursor during the first five trials in the series. Feedback was then removed, and participants were to continue on pace for the next 70 trials. Movements were made in two directions (30 degrees and 120 degrees ) from each of three start locations (initial shoulder angles of 30 degrees, 40 degrees, 50 degrees, and initial elbow angles of 90 degrees ). Over the 70 trials, the start location of each movement drifted, on average, 8 cm away from the initial start location. This drift varied systematically with movement direction, indicating that drift is related to movement production. However, despite these dramatic changes in hand position and joint configuration, movement distance and direction remained relatively constant. Inverse dynamics analysis revealed that movement preservation was accompanied by substantial modification of joint muscle torque. These results suggest that proprioception continues to be a reliable source of limb position information after prolonged time without vision, but that this information is used differently for maintaining limb position and for specifying movement trajectory.

Adult↗

PubMed2003

Effects of altering initial position on movement direction and extent.

The purpose of this study was to examine the relative influence of initial hand location on the direction and extent of planar reaching movements. Subjects performed a horizontal-plane reaching task with the dominant arm supported above a table top by a frictionless air-jet system. A start circle and a target were reflected from a horizontal projection screen onto a horizontally positioned mirror, which blocked the subject's view of the arm. A cursor, representing either actual or virtual finger location, was only displayed between each trial to allow subjects to position the cursor in the start circle. Prior to occasional "probe trials," we changed the start location of the finger relative to the cursor. Subjects reported being unaware of the discrepancy between cursor and finger. Our results indicate that regardless of initial hand location, subjects did not alter the direction of movement. However, movement distance was systematically adjusted in accord with the baseline target position. Thus when the hand start position was perpendicularly displaced relative to the target direction, neither the direction nor the extent of movement varied relative to that of baseline. However, when the hand was displaced along the target direction, either anterior or posterior, movements were made in the same direction as baseline trials but were shortened or lengthened, respectively. This effect was asymmetrical such that movements from anterior displaced positions showed greater distance adjustment than those from posterior displaced positions. Inverse dynamic analysis revealed substantial changes in elbow and shoulder muscle torque strategies for both right/left and anterior/posterior pairs of displacements. In the case of right/left displacements, such changes in muscle torque compensated changes in limb configuration such that movements were made in the same direction and to the same extent as baseline trials. Our results support the hypothesis that movement direction is specified relative to an origin at the current location of the hand. Movement extent, on the other hand, appears to be affected by the workspace learned during baseline movement experience.

Acceleration↗