The utilization of visual information in the control of rapid sequential aiming movements.

Two experiments were conducted to examine the role of vision in the execution of a movement sequence. Experiment 1 investigated whether individual components of a sequential movement are controlled together or separately. Participants executed a rapid aiming movement to two targets in sequence. A full vision condition was compared to a condition in which vision was eliminated while in contact with the first target. The size of the first target was constant, while the second target size was varied. Target size had an influence on movement time and peak velocity to the first target. Vision condition and target size did not affect the time spent on the first target. These results suggest that preparation of the second movement is completed before the first movement is terminated. Experiment 2 examined when this preparation occurred. A full vision condition was compared to a condition in which vision was occluded during the flight phase of the first movement. Movement initiation times were shorter when vision was continually available. Total movement time was reduced with vision in two-target condition, but not in a control one-target condition. The time spent on the first target was greater when vision was not available during the first movement component. The results indicate that vision prior to movement onset can be used to formulate a movement plan to both targets in the sequence [Fischman & Reeve (1992).     

Determining the Temporal Limits of a Visual Sample for Visual Regulation

The author examined the minimum amount of time needed for vision to increase aiming accuracy and decrease movement duration. Participants selected when they would receive a visual sample during aiming movements by pressing a switch held with the left hand. The sample was one of the following durations: 40 ms, 30 ms, 20 ms, 10 ms, or 0 ms (no vision). Decreased accuracy in the no-vision condition compared to the vision conditions was observed when the duration of the impending sample was unknown (Experiment 1). Samples 40 ms in duration were sufficient to decrease endpoint variability when the duration of the sample was known before the movement (Experiment 2). These results indicate that short visual samples can be used to decrease movement time and increase accuracy and that knowledge of the impending visual context can impact the individual's subsequent behavior.     

The contribution of peripheral and central vision in the control of movement amplitude

Past research has revealed that central vision is more important than peripheral vision in controlling the amplitude of target-directed aiming movements. However, the extent to which central vision contributes to movement planning versus online control is unclear. Since participants usually fixate the target very early in the limb trajectory, the limb enters the central visual field during the late stages of movement. Hence, there may be insufficient time for central vision to be processed online to correct errors during movement execution. Instead, information from central vision may be processed offline and utilised as a form of knowledge of results, enhancing the programming of subsequent trials. In the present research, variability in limb trajectories was analysed to determine the extent to which peripheral and central vision is used to detect and correct errors during movement execution. Participants performed manual aiming movements of 450 ms under four different visual conditions: full vision, peripheral vision, central vision, no vision. The results revealed that participants utilised visual information from both the central and peripheral visual fields to adjust limb trajectories during movement execution. However, visual information from the central visual field was used more effectively to correct errors online compared to visual information from the peripheral visual field.     

Target velocity effects on manual interception kinematics

Participants generated manual interception movements toward a target cursor that moved across a computer screen. The target reached its peak velocity either during the first third, at the midpoint, or during the last third of the movement. In Experiment 1 the view of the target was available for either the first 316, 633, 950, or 1267 ms, after which it disappeared. Results showed that for all viewing conditions, the timing of the interception velocity was related to the temporal properties of the target's trajectory. In Experiment 2, when the portion of the target trajectory that was viewed was reversed (such that participants did not see the first 316, 633, 950, or 1267 ms of the trajectory, but instead saw only the later portions of the trajectory), there was no clear relationship between the target trajectory and the timing of the aiming trajectory. These results suggest that participants use visual information early in the target's trajectory to form a representation of the target motion that is used to facilitate manual interception.     

Partial visual feedback and spatial end-point accuracy of discrete aiming movements

Five experiments are reported in which the effect of partial visual feedback on the accuracy of discrete target aiming was investigated. Visual feedback was manipulated through a spectacle-mounted liquid-crystal tachistoscope. The length of the visual feedback interval was varied as a percentage of the instructed movement time. In Experiment 1, the length of the vision interval was manipulated symmetrically at the beginning- and end-phase of the movement, whereas in the remaining experiments, the vision time was varied with respect to the end-phase only. The variations at the end were examined for different distances (Experiment 2), different movement speeds at the same distance (Experiment 3), and in small interstep intervals (Experiment 4). A vision time of more than 150 ms at the end-phase of the movement enhanced aiming performance in all experiments. Longer vision times monotonously improved aiming accuracy; the fifth experiment showed that a vision time of about 275 ms was sufficient for near-perfect aiming. Furthermore, the significance of vision during the first phase of a movement was demonstrated again. The results of the five experiments pointed to shorter visuomotor processing times. To explain the beneficial effects of short vision times for aiming accuracy, we propose a model of visuomotor processing that is based on the stochastic optimized submovement model of Meyer, Abrams, Kornblum, Wright, and Smith (1988).     

The dual role of vision in sequential aiming movements

Previous research has demonstrated that movement times to the first target in sequential aiming movements are influenced by the properties of subsequent segments. Based on this finding, it has been proposed that individual segments are not controlled independently. The purpose of the current study was to investigate the role of visual feedback in the interaction between movement segments. In contrast to past research in which participants were instructed to minimize movement time, participants were set a criterion movement time and the resulting errors and limb trajectory kinematics were examined under vision and no vision conditions. Similar to single target movements, the results indicated that vision was used within each movement segment to correct errors in the limb trajectory. In mediating the transition between segments, visual feedback from the first movement segment was used to adjust the parameters of the second segment. Hence, increases in variability that occurred from the first to the second target in the no vision condition were curtailed when visual feedback was available. These results are discussed along the lines of the movement constraint and movement integration hypotheses.     

The effect of viewing the moving limb and target object during the early phase of movement on the online control of grasping

Two experiments were conducted to investigate (1) during which phase of the movement vision is most critical for control, and (2) how vision of the target object and the participant's moving limb affect the control of grasping during that movement phase. In Experiment 1, participants, wearing liquid crystal shutter goggles, reached for and grasped a cylinder with a diameter of 4 or 6 cm under a shutting paradigm (SP) and a re-opening paradigm (RP). In SP, the goggles closed (turned opaque) 0 ms, 150 ms, 350 ms, 500 ms, or 700 ms after movement onset, or remained open (transparent) during the prehension movements. In RP, the goggles closed immediately upon movement onset, and re-opened 0 ms (i.e., without initially shutting), 150 ms, 350 ms, 500 ms, or 700 ms after the initial shutting, or remained opaque throughout the prehension movements. The duration of the prehension movements was kept relatively constant across participants and trials at approximately 1100 ms, i.e., the duration of prehension movements typically observed in daily life. The location of the target object was constant during the entire experiment. The SP and RP paradigms were counter-balanced across participants, and the order of conditions within each session was randomized. The main findings were that peak grip aperture (PGA) in the 150 ms-shutting condition was significantly larger than in the 350 ms-shutting condition, and that PGA in the 350 ms-re-opening condition was significantly larger than in the 150 ms-re-opening condition. These results revealed that online vision between 150 ms and 350 ms was critical for grasp control on PGA in typical, daily-life-speeded prehension movements. Furthermore, the results obtained for the time after maximal deceleration (TAMD; movement duration-time to maximal deceleration) demonstrated that early-phase vision contributed to the temporal pattern of the later movement phases (i.e., TAMD). The results thus demonstrated that online vision in the early phase of movement is crucial for the control of grasping. In addition to the apparatus used in Experiment 1, two liquid shutter plates placed in the same horizontal plane (25 cm above the experimental table) were used in Experiment 2 to manipulate the visibility of the target and the participant's moving limb. The plate closest to the participant altered vision of the limb/hand, while the more distant plate controlled vision of the object. The conditions were as follows: (1) both plates were open during movement (full vision condition); (2) both plates were closed 0, 150, or 350 ms following onset of arm movement (front-rear condition: FR); or (3) only the near plate closed 0, 150, or 350 ms following the onset of the arm movement (front condition: F). The results showed that shutting at 0 and 150 ms in the FR condition caused a significantly larger PGA, while the timing of shutting in the F condition had little influence on the PGA. These findings indicated that online vision, especially of the target object, during the early phase of prehension movements is critical to the control of grasping.     

Visual information: The control of aiming movements

The study examined the contribution of various sources of visual information utilised in the control of discrete aiming movements. Subjects produced movements, 15.24 cm in amplitude, to a 1.27 cm target in a movement time of 330 ms. Responses were carried out at five vision-manipulation conditions which allowed the subject complete vision, no vision, vision of only the target or stylus, and a combination of stylus and target. Response accuracy scores indicated that a decrement in performance occurred when movements were completed in the absence of visual information or when only the target was visible during the response. The stylus and the target plus stylus visual conditions led to response accuracy which was comparable to movements produced with complete vision. These results suggest that the critical visual information for aiming accuracy is that of the stylus. These findings are consistent with a control model based on a visual representation of the discrepancy between the position of the hand and the location of the target.     

Optimal control strategies under different feedback schedules: kinematic evidence

Two experiments were conducted in which participants (N = 12, Experiment 1; N = 12, Experiment 2) performed rapid aiming movements with and without visual feedback under blocked, random, and alternating feedback schedules. Prior knowledge of whether vision would be available had a significant impact on the strategies that participants adopted. When they knew that vision would be available, less time was spent preparing movements before movement initiation. Participants also reached peak deceleration sooner but spent more time after peak deceleration adjusting limb trajectories. Consistent with those findings, analysis of spatial variability at different points in the trajectory indicated that variability increased up to peak deceleration but then decreased from peak deceleration to the end of the movement.     

The influence of advance information about target location and visual feedback on movement planning and execution.

This study was designed to determine if movement planning strategies incorporating the use of visual feedback during manual aiming are specific to individual movements. Advance information about target location and visual context was manipulated using precues. Participants exhibited a shorter reaction time and a longer movement time when they were certain of the target location and that vision would be available. The longer movement time was associated with greater time after peak velocity. Under conditions of uncertainty, participants prepared for the worst-case scenario. That is, they spent more time organizing their movements and produced trajectories that would be expected from greater open-loop control. Our results are consistent with hierarchical movement planning in which knowledge of the movement goal is an essential ingredient of visual feedback utilization.     

Visual Feedback of Target Position Affects Accuracy of Sequential Movements at Even Spaces

The role of visual feedback during movement is attributed to its accuracy, but findings regarding the utilization of this information are inconsistent. We developed a novel dot-placing task to investigate the role of vision in arm movements. Participants conducted pointing-like movements between two target stimuli at even spaces. In Experiment 1, visual feedback of targets and response positions was manipulated. Although visual loss of target stimuli hindered accuracy of movements, the absence of the position of previously placed dots had little effect. In Experiment 2, the effect of movement time on accuracy was assessed, as the relationship between these has been traditionally understood as a speed/accuracy trade-off. Results revealed that duration of movement did not impact movement accuracy.     

Aiming at a far target under different viewing conditions: visual control in basketball jump shooting

Most research on visual search in aiming at far targets assumes preprogrammed motor control implying that relevant visual information is detected prior to the final shooting or throwing movements. Eye movement data indirectly support this claim for stationary tasks. Using the basketball jump shot as experimental task we investigated whether in dynamic tasks in which the target can be seen until ball release, continuous, instead of preprogrammed, motor control is possible. We tested this with the temporal occlusion paradigm: 10 expert shooters took shots under four viewing conditions, namely, no vision, full vision, early vision (vision occluded during the final +/-350 ms before ball release), and late vision (vision occluded until these final +/-350 ms). Late-vision shooting appeared to be as good as shooting with full vision while early-vision performance was severely impaired. The results imply that the final shooting movements were controlled by continuous detection and use of visual information until ball release. The data further suggest that visual and movement control of aiming at a far target develop in close correspondence with the style of execution.     

Visual regulation of manual aiming: A comparison of methods

Visual regulation of upper limb movements occurs throughout the trajectory and is not confined to discrete control in the target area. Early control is based on the dynamic relationship between the limb, the target, and the environment. Despite robust outcome differences between protocols involving visual manipulations, it remains difficult to identify the kinematic events that characterize these differences. In this study, participants performed manual aiming movements with and without vision. We compared several traditional approaches to movement analysis with two new methods of quantifying online limb regulation. As expected, participants undershot the target and their movement endpoints were more variable when vision was not available. Although traditional measures such as reaction time, time after peak velocity, and the presence of discontinuities in acceleration were sensitive to the visual manipulation, measures quantifying the trial-to-trial spatial variability throughout the trajectory were the most effective in isolating the time course of online regulation.     

The visual control of aimed hand movements to stationary and moving targets.

The present study attempted to determine if during short-duration movements visual feedback can be processed in order to make adjustments to changes in the environment. The effect that varying the importance of monitoring target position has on the relative importance of vision of hand and vision of target (Carlton 1981a; Whiting and Cockerill 1974) was also examined. Subjects performed short- (150 ms) and longer-duration (330 ms) aimed hand movements under four visual feedback conditions (lights-on/lights-off by target-on/target-off) to stationary and moving targets. For the lights-off and target-off conditions, the lights and target, respectively, were extinguished 50 ms after movement initiation. For all moving-target conditions, the target started to move as the movement was initiated. Subjects were able to process visual information in 165 ms, as movement endpoints were biased in the direction of target motion for movements of this duration. Removing visual feedback 50 ms after movement initiation did not alter this finding. Subjects performed equally well with target and lights on or off, independent of whether the target remained stationary or moved. Presumably, during the first 50 ms of the movement subjects received sufficient visual information to aid in movement control.     

The effect of the Müller-Lyer illusion on the planning and control of manual aiming movements

Two experiments used Müller-Lyer stimuli to test the predictions of the planning-control model (S. Glover, 2002) for aiming movements. In Experiment 1, participants aimed to stimuli that either remained the same or changed upon movement initiation. Experiment 2 was identical except that the duration of visual feedback for online control was manipulated. The authors found that the figures visible during movement planning and online control had additive effects on endpoint bias, even when participants had ample time to use visual feedback to modify their movements (Experiment 2). These findings are problematic not only for the planning-control model but also for A. D. Milner and M. A. Goodale's (1995) two visual system explanation of illusory bias. Although our results are consistent with the idea that a single representation is used for perception, movement planning, and online control (e.g., V. H. Franz, 2001), other work from our laboratory and elsewhere suggests that the manner in which space is coded depends on constraints associated with the specific task, such as the visual cues available to the performer.     

The utilization of visual feedback information during rapid pointing movements

Three experiments were conducted to determine how variables other than movement time influence the speed of visual feedback utilization in a target-pointing task. In Experiment 1, subjects moved a stylus to a target 20 cm away with movement times of approximately 225 msec. Visual feedback was manipulated by leaving the room lights on over the whole course of the movement or extinguishing the lights upon movement initiation, while prior knowledge about feedback availability was manipulated by blocking or randomizing feedback. Subjects exhibited less radial error in the lights-on/blocked condition than in the other three conditions. In Experiment 2, when subjects were forced to use vision by a laterally displacing prism, it was found that they benefited from the presence of visual feedback regardless of feedback uncertainty even when moving very rapidly (e.g. less than 190 msec). In Experiment 3, subjects pointed with and without a prism over a wide variety of movement times. Subjects benefited from vision much earlier in the prism condition. Subjects seem able to use vision rapidly to modify aiming movements but may do so only when the visual information is predictably available and/or yields an error large enough to detect early enough to correct.     

Eye-hand coordination in goal-directed aiming.

In a number of studies, we have demonstrated that the spatial-temporal coupling of eye and hand movements is optimal for the pickup of visual information about the position of the hand and the target late in the hand's trajectory. Several experiments designed to examine temporal coupling have shown that the eyes arrive at the target area concurrently with the hand achieving peak acceleration. Between the time the hand reached peak velocity and the end of the movement, increased variability in the position of the shoulder and the elbow was accompanied by a decreased spatial variability in the hand. Presumably, this reduction in variability was due to the use of retinal and extra-retinal information about the relative positions of the eye, hand and target. However, the hand does not appear to be a slave to the eye. For example, we have been able to decouple eye movements and hand movements using Müller-Lyer configurations as targets. Predictable bias, found in primary and corrective saccadic eye movements, was not found for hand movements, if on-line visual information about the target was available during aiming. That is, the hand remained accurate even when the eye had a tendency to undershoot or overshoot the target position. However, biases of the hand were evident, at least in the initial portion of an aiming movement, when vision of the target was removed and vision of the hand remained. These findings accent the versatility of human motor control and have implications for current models of visual processing and limb control.     

Control strategies when intercepting slowly moving targets

In 3 experiments, the authors investigated and described how individuals control manual interceptive movements to slowly moving targets. Participants (N = 8 in each experiment) used a computer mouse and a graphics tablet assembly to manually intercept targets moving across a computer screen toward a marked target zone. They moved the cursor so that it would arrive in the target zone simultaneously with the target. In Experiment 1, there was a range of target velocities, including some very slow targets. In Experiment 2, there were 2 movement distance conditions. Participants moved the cursor either the same distance as the target or twice as far. For both experiments, hand speed was found to be related to target speed, even for the very slowly moving targets and when the target-to-cursor distance ratios were altered, suggesting that participants may have used a strategy similar to tracking. To test that notion, in Experiment 3, the authors added a tracking task in which the participants tracked the target cursor into the target zone. Longer time was spent planning the interception movements; however, there was a longer time in deceleration for the tracking movements, suggesting that more visually guided trajectory updates were made in that condition. Thus, although participants scaled their interception movements to the cursor speed, they were using a different strategy than they used in tracking. It is proposed that during target interception, anticipatory mechanisms are used rather than the visual feedback mechanism used when tracking and when pointing to stationary targets.     

Coding processes in preselected and constrained movements: effect of vision

Three experiments were conducted in which visual information was manipulated either at the endpoint or during preselected, subject defined and constrained, experimenter-defined movements. In Experiments 1 and 2 the subject's task was to reproduce the movement in the absence of vision. Augmenting the terminal location of the criterion movement with vision had no differential effect on reproduction in Experiment 1, although preselected movement accuracy was significantly superior to constrained. Providing vision throughout the criterion movement in Experiment 2 not only failed to improve the accuracy of constrained movements but decreased reproduction performance in preselected movements. In Experiment 3 procedures were adopted to control the allocation of the subjects' attention during the criterion movement. The subjects reproduced by vision alone, movement alone, or with both visual and movement information available. When subjects were informed of the modality of reproduction prior to criterion presentation, they were able to ignore concurrent input from vision and attend to movement information. In the absence of precues visual information was spontaneously attended. The data were interpreted as contrary to closed-loop assumptions that additional information necessarily enhances the strength of a motor memory representation. Rather, they can be accommodated in terms of Posner, Nissen and Klein's (1976) theoretical account of visual dominance and serve to illustrate the importance of selective attention effects in movement coding.     

The Effects of Objectives and Constraints on Motor Control Strategy in Reciprocal Aiming Movements

The goal of this study was to examine how the kinematics of reciprocal aiming movements were affected by both the objective of the movement and the constraints operating on that movement. In Experiment 1, the objective of the movement was indirectly manipulated by capitalizing on the fact that subjects determine their own accuracy and speed limits, despite uniform task instructions to move as quickly and accurately as possible. A Fitts' type reciprocal aiming paradigm was employed, in which 69 subjects were asked to move a stylus repetitively between two spatially separated targets. Four target widths were orthogonally combined with four movement amplitudes, resulting in 16 conditions. Movements were made on an X-Y digitizing tablet. Based on the mean variable error produced on both targets, subjects were differentiated post hoc into three movement objective groups: speed, accuracy, and speed-plus-accuracy. Kinematic analyses revealed that the programming and execution of movements were systematically influenced by both the movement objective and the movement constraints. That is, movement time, peak velocity, dwell time, acceleration and deceleration time, normalized acceleration and normalized deceleration varied systematically as a function of both the speed-accuracy movement objective and the movement constraints of target size and movement distance. Moreover, the consequences of changing the constraints of the movement were affected by an interaction with the objective of the movement. In Experiment 2, the objective of the movement was directly manipulated by varying speed and/or accuracy instructions to subjects. The basic results of Experiment 1 were substantiated. Overall, the results were consistent with the view that motor control is dependent upon sensory consequences.