Hand, Eye, and Head Coordination While Pointing to Perturbed Targets

Dynamic Changes in Upper-Limb Corticospinal Excitability during a 'Pro-/Anti-saccade' Double-Choice Task

Under natural behavioral conditions, visually guided eye movements are linked to direction-specific modulations of cortico-spinal system (CSS) excitability in upper-limb muscles, even in absence of a manual response. These excitability changes have been shown to be compatible with a covert motor program encoding a manual movement toward the same target of the eyes. The aim of this study is to investigate whether this implicit oculo-manual coupling is enforced following every saccade execution or it depends on the behavioral context. Twenty-two healthy young adults participated in the study. Single-pulse transcranial magnetic stimulation was applied to the motor cortex at nine different time epochs during a double-choice eye task, in which the decision to execute a prosaccade or an antisaccade was made on the color of a peripheral visual cue. By analyzing the amplitude of the motor evoked potentials (MEP) in three distal muscles of the resting upper-limb, a facilitation peak of CSS excitability was found in two of them at 120 ms before the eyes begin to move. Furthermore, a long-lasting, generalized reduced corticomotor excitability develops following the eye response. Finally, a quite large modulation of MEP amplitude, depending on the direction of the saccade, is observed only in the first dorsal interosseous muscle, in a narrow time window at about 150 ms before the eye movement, irrespective of the type of the ocular response (pro-/anti-saccade). This change in CSS excitability is not tied up to the timing of the occurrence of the visual cue but, instead, appears to be tightly time-related to the saccade onset. Observed excitability changes differ in many respects from those previously reported with different behavioral paradigms. A main finding of our study is that the implicit coupling between eye and hand motor systems is contingent upon the particular motor set determined by the cognitive aspects of the performed oculomotor task. In particular, the direction-specific modulation in CSS excitability described in this study appears to be related to perceptual and decision-making processes rather than representing an implicit upper-limb motor program, coupled to the saccade execution.

The role of goal representations in moving to temporal and spatial targets

Traditionally, movement kinematics are thought to reflect physical properties (e.g., position and time) of movement targets. However, targets may also evoke intentional goals like “to be in a certain position at a given time”. Therefore, kinematics may be viewed not as a reaction to stimuli, but rather as the means to attain intended goals. In the present study participants performed continuous reversal movements. It was first shown that kinematics towards temporal and spatial targets differ from kinematics away from those targets. Further, kinematics are different for movements to temporal (relatively short movement times, high and late peak velocity) and spatial (relatively long movement times, early peak velocity) targets (Experiments 1 and 2). In order to obtain evidence for the influence of goal representations on kinematics, combinations of temporal and spatial targets were investigated in Experiments 3 and 4. Specifically, the conditions were: spatial targets always present with varying temporal targets, temporal targets always present with varying spatial targets, and combined and separate spatial and temporal targets. Not only the physical features, but also how the targets were represented as movement goals, were important. Thus, movement kinematics do not simply reflect stimulus properties, but rather the representation of the intended goal.

Covert preparation of a manual response in a 'go'/'no-go' saccadic task is driven by execution of the eye movement and not by visual stimulus occurrence

It has been recently demonstrated that visually guided saccades are linked to changes in muscle excitability in the relaxed upper limb, which are compatible with a covert motor plan encoding a hand movement toward the gaze target. In this study we investigated whether these excitability changes are time locked to the visual stimulus, as predicted by influential attention models, or are strictly dependent on saccade execution. Single-pulse transcranial magnetic stimulation was applied to the motor cortex at eight different time delays during a 'go'/'no-go' task, which involved overt or covert orienting of attention. By analyzing the time course of excitability in three hand muscles, synchronized with the onset of either the attentional cue or the eye movement, we demonstrated that side- and muscle-specific excitability changes were strictly time locked to the saccadic response and were not correlated to the onset of the visual attentive stimulus. Furthermore, muscle excitability changes were absent following a covert shift of attention. We conclude that a sub-threshold manual motor plan is automatically activated by the saccade decision-making process, as part of a covert eye-hand coordination program. We found no evidence for a representation of spatial attention within the upper limb motor map.

Covert oculo-manual coupling induced by visually guided saccades

Hand pointing to objects under visual guidance is one of the most common motor behaviors in everyday life. In natural conditions, gaze and arm movements are commonly aimed at the same target and the accuracy of both systems is considerably enhanced if eye and hand move together. Evidence supports the viewpoint that gaze and limb control systems are not independent but at least partially share a common neural controller. The aim of the present study was to verify whether a saccade execution induces excitability changes in the upper-limb corticospinal system (CSS), even in the absence of a manual response. This effect would provide evidence for the existence of a common drive for ocular and arm motor systems during fast aiming movements. Single-pulse TMS was applied to the left motor cortex of 19 subjects during a task involving visually guided saccades, and motor evoked potentials (MEPs) induced in hand and wrist muscles of the contralateral relaxed arm were recorded. Subjects had to make visually guided saccades to one of 6 positions along the horizontal meridian (±5°, ±10°, or ±15°). During each trial, TMS was randomly delivered at one of 3 different time delays: shortly after the end of the saccade or 300 or 540 ms after saccade onset. Fast eye movements toward a peripheral target were accompanied by changes in upper-limb CSS excitability. MEP amplitude was highest immediately after the end of the saccade and gradually decreased at longer TMS delays. In addition to the change in overall CSS excitability, MEPs were specifically modulated in different muscles, depending on the target position and the TMS delay. By applying a simple model of a manual pointing movement, we demonstrated that the observed changes in CSS excitability are compatible with the facilitation of an arm motor program for a movement aimed at the same target of the gaze. These results provide evidence in favor of the existence of a common drive for both eye and arm motor systems.

Humans use visual and remembered information about object location to plan pointing movements

We investigated whether humans use a target's remembered location to plan reaching movements to targets according to the relative reliabilities of visual and remembered information. Using their index finger, subjects moved a virtual object from one side of a table to the other, and then went back to a target. In some trials, the target shifted unnoticed while the finger made the first movement. We regressed subjects' movement trajectories against the initial and shifted target locations to infer the weights that subjects gave to remembered and visual locations. We measured the reliability of vision and memory by adding conditions in which the target only appeared after subjects made the first movement (vision only) and in which the target was initially present but disappeared during the first movement (memory only). When both visual and remembered information were available, movement trajectories were biased to the remembered target location. The different weights that subjects gave to memory and visual information on average matched the weights predicted by the variance associated with the use of vision and memory alone. This suggests that humans integrate remembered information about object locations with peripheral visual information by taking into account the relative reliability of the two sources of information.

Reprogramming of Interceptive Actions: Time Course of Temporal Corrections for Unexpected Target Velocity Change

The authors investigated the time course of reprogramming of the temporal dimension of motor acts in a task requiring interception of a moving target. The target moved at a constant velocity on a monitor screen; in part of the trials, target velocity was unexpectedly increased or decreased. Those modifications were produced at different moments during target displacement, leaving periods of time from 100 to 800 ms for movement timing correction. The authors assessed the effects of probability of target velocity change (25% vs. 50%), uncertainty about direction of velocity change (unidirectional vs. bidirectional), and direction of velocity change (increase vs. decrease). Analysis of 24 participants' arm acceleration showed that fast adjustments took place between 100 and 200 ms after target velocity change similarly for all uncertainty conditions. Analysis of temporal error indicated that the combination of high probability of target velocity change and certainty on direction of target velocity change led to the most successful movement timing reprogramming. For the other experimental conditions, temporal accuracy was still poor when a period of 800 ms was available for correction. Movement reprogramming was a continuous process that was more efficient for target velocity increase than for target velocity decrease.

The continuous nature of timing reprogramming in an interceptive task

The time course of movement timing reprogramming was examined in a task requiring temporal coincidence of the conclusion of a forehand drive with the arrival of a moving luminous target at the end of an electronic trackway. The moving target departed from one end of the trackway at a constant velocity of 2 m (.) s(-1), and for a part of the trials its velocity was increased to 3 m (.) s(-1). Target velocity was modified at different moments during stimulus displacement, producing times-to-arrival after velocity increment (TAVIs) from 100 to 600 ms. The effect of specific practice on movement reprogramming was also examined. The results showed early adjustments to the action (TAVIs = 100 - 200 ms) that seemed to be stereotyped, while feedback-based corrections were implemented only at TAVIs of 300 ms or longer. Temporal accuracy was progressively increased as longer TAVIs were provided up to 600 ms. Skill training led to an overall increment of temporal accuracy, but no effect of specific practice was found. The results indicate that timing reprogramming in interceptive actions is a continuous process limited mainly by intrinsic factors: latency to initiate more effective adjustments to the action, and rate-of-movement timing reprogramming.

A century later: Woodworth's (1899) two-component model of goal-directed aiming

In 1899, R. S. Woodworth published a seminal monograph, "The Accuracy of Voluntary Movement." As well as making a number of important empirical contributions, Woodworth presented a model of speed-accuracy relations in the control of upper limb movements. The model has come to be known as the two-component model because the control of speeded limb movements was hypothesized to entail both a central and a feedback-based component. Woodworth's (1899) ideas about the control of rapid aiming movements are evaluated in the context of current empirical and theoretical contributions.

Coupling of eye, finger, elbow, and shoulder movements during manual aiming

Temporal and spatial coupling of point of gaze (PG) and movements of the finger, elbow, and shoulder during a speeded aiming task were examined. Ten participants completed 40-cm aiming movements with the right arm, in a situation that allowed free movement of the eyes, head, arm, and trunk. On the majority of trials, a large initial saccade undershot the target slightly, and 1 or more smaller corrective saccades brought the eyes to the target position. The finger, elbow, and shoulder exhibited a similar pattern of undershooting their final positions, followed by small corrective movements. Eye movements usually preceded limb movements, and the eyes always arrived at the target well in advance of the finger. There was a clear temporal coupling between primary saccade completion and peak acceleration of the finger, elbow, and shoulder. The initiation of limb-segment movement usually occurred in a proximal-to-distal pattern. Increased variability in elbow and shoulder position as the movement progressed may have served to reduce variability in finger position. The spatial-temporal coupling of PG with the 3 limb segments was optimal for the pick up of visual information about the position of the finger and the target late in the movement.

Prehension with trunk assisted reaching

For prehensile tasks, where objects are located beyond the normal reaching space, the trunk is bent forward to assist in the transport of the wrist to the object. Such task behaviors raise complex motor control issues such as how is the trunk movement incorporated into the motor plan. In this experiment, seated subjects were asked to reach and grasp a small and a large object placed on a table located beyond their maximal reach. Forward trunk bending was required to extend the reach distance. For such reaching movements, the wrist velocity consisted of a bell shape profile similar to those seen when the arm is the sole transport agent. In most trials, the trunk was the first to initiate movement, although there was no strict pattern of initiation order. The transport data showed that trunk and arm movement components were decoupled at the end of the reach. While the object was being grasped and lifted, the trunk continued moving for approximately 180 ms after the grasp. Wrist deceleration time expressed in absolute and relative values was sensitive to object size. The time from maximum peak aperture to the end of wrist movement also was significantly longer for grasping the small compared to the large object. No such relationships were observed for the trunk. Temporal coupling was only observed between the grip and wrist transport component. Time to maximum aperture was significantly correlated with time to peak wrist deceleration and only rarely with time to trunk deceleration peak. When the trunk participates in the transport of the wrist to an object, these findings suggest that only the wrist component is directly related to the achievement of the grasp. While the trunk assisted the arm to reach the object, the kinematic parameter recorded did not reveal any evidence of direct coupling. The presented data suggests that the planning takes place at the level of the hand and that endpoint is the primary variable controlled.