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).     

Target location and visual feedback as variables determining accuracy of aiming movements

The accuracy of a long aiming movement was studied as a function of whether it was performed toward or away from the midline of the subject's body in the presence or absence of visual feedback. 30 right-handed, male university students (19-26 yr.) served as subjects. With movement distance and duration controlled, the mean percentage of error was 6.34% less for movements made toward the body's midline than for those performed away from the midline. The mean percentage of error was also 48% less in the presence of visual feedback than in its absence. However, contrary to our expectation, movements executed toward the body's midline were not appreciably less disrupted in the absence of visual feedback than movements performed away from the midline.     

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 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.     

Rapid visual feedback processing in single-aiming movements

A major line of behavioral support for motor-program theory derives from evidence indicating that feedback does not influence the execution and control of limited duration movements. Since feedback cannot be utilized, the motor-program is assumed to act as the controlling agent. in a classic study, Keele and Posner observed that visual feedback had no effect on the accuracy of 190-msec single-aiming movements. Therefore visual feedback processing time is greater than 190 msec, and, more importantly, limited duration movements are governed by motor programs. In the present paper, we observed that visual feedback can affect the spatial accuracy of movement with durations much less than 190 msec. We hypothesize that visual feedback can aid motor control via processes not associated with intermittent error corrections.     

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.     

Hand position as a variable determining the accuracy of aiming movements

Two experiments investigated the effect of hand position on the accuracy of short- and long-duration aiming movements in the presence and absence of visual feedback. In Experiment 1 (N = 16) short aiming movements were executed rapidly, which would require them to be predominantly programmed, whereas in Experiment 2(N = 8) these movements were performed slowly enough so that visual feedback, which implies that they were predominantly programmed. However, the long-duration, short-length movements of Experiment 2 were disrupted when visual feedback was removed, which suggests that these movements were being guided by visual feedback. Having the heel of the responding hand in contact with the target platform during the response resulted in greater accuracy than no hand contact for the short-length movements of both experiments. Taken together, these results indicated that hand contact produced greater aiming accuracy than no hand contact for both programmed- and feedback-based movements.     

The utilization of visual information in the control of reciprocal aiming movements

Two experiments examined on-line processing during the execution of reciprocal aiming movements. In Experiment 1, participants used a stylus to make movements between two targets of equal size. Three vision conditions were used: full vision, vision during flight and vision only on contact with the target. Participants had significantly longer movement times and spent more time in contact with the targets when vision was available only on contact with the target. Additionally, the proportion of time to peak velocity revealed that movement trajectories became more symmetric when vision was not available during flight. The data indicate that participants used vision not only to 'home-in' on the current target, but also to prepare subsequent movements. In Experiment 2, liquid crystal goggles provided a single visual sample every 40 ms of a 500 ms duty cycle. Of interest was how participants timed their reciprocal aiming to take advantage of these brief visual samples. Although across participants no particular portion of the movement trajectory was favored, individual performers did time their movements consistently with the onset and offset of vision. Once again, performance and kinematic data indicated that movement segments were not independent of each other.     

Spatial accuracy demand in aiming movements: kinematic analysis of subtended angle and tolerance width

Subtended angle has been assumed to be an important factor in both response programming time and kinematic characteristics of aiming movements. Support for this assumption has come mainly from studies in which circular targets have been used. However, with circular targets, the subtended angle covaries with the size of the target in the principal direction of the movement (tolerance width). The purpose of this study was to examine the effects of tolerance width and subtended angle on aiming movement with multiple targets. Participants first hit a 5-cm-diameter circular target located 8 cm to the left of a starting position and then moved another 8 cm left to hit either a 5-cm diameter circular target or a 5- x 1-cm rectangular target oriented either horizontally or vertically, depending on the condition. Analysis showed that reaction times and movement times were longer for the vertical rectangular target, which had a smaller tolerance width than the other two targets. In addition, the vertical rectangular target also showed a greater percentage of secondary-submovement trials, lower movement velocity, and higher peak vertical displacement. Overall, the results indicate that the tolerance width of the target may impose more constraints on aiming movements than subtended angle.     

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).     

Visual aiming movements in Alzheimer's disease

This study examined whether AD affects the control of pointing movements. Sixteen older adults with probable AD and 10 age-matched healthy adults pointed at targets varying in size (3 and 7 mm in diameter) and located at three positions (at the midline and 33 degrees to the left and right). Results revealed the patients exhibited longer movement time, lower peak velocity and more time in deceleration, although they exhibited effects of target size and location comparable to the healthy controls. AD, then, would appear to slow down movement without affecting the motor system's ability to respond to task demands.     

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.     

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.     

Eye movements disrupt spatial but not visual mental imagery

It has long been known that eye movements are functionally involved in the generation and maintenance of mental images. Indeed, a number of studies demonstrated that voluntary eye movements interfere with mental imagery tasks (e.g., Laeng and Teodorescu in Cogn Sci 26:207–231, 2002). However, mental imagery is conceived as a multifarious cognitive function with at least two components, a spatial component and a visual component. The present study investigated the question of whether eye movements disrupt mental imagery in general or only its spatial component. We present data on healthy young adults, who performed visual and spatial imagery tasks concurrently with a smooth pursuit. In line with previous literature, results revealed that eye movements had a strong disruptive effect on spatial imagery. Moreover, we crucially demonstrated that eye movements had no disruptive effect when participants visualized the depictive aspects of an object. Therefore, we suggest that eye movements serve to a greater extent the spatial than the visual component of mental imagery.     

Contribution of visual information to feedforward and feedback processes in rapid pointing movements

The purpose of this study was to investigate what types of visual cues may contribute to improving movement accuracy in a pointing task, and to determine in what kind of control processes these cues are involved. During the experiment, subjects had to point their finger at visual targets as accurately as possible making rapid movements. Subjects were required to perform a movement with an amplitude of 40 cm within a series of times ranging from 110 to 270 msec. Five visual feedback conditions were applied: no feedback (NF), dynamic ongoing feedback on the complete hand trajectory (CF), static error feedback on the movement end-point (EF), and two partial feedback conditions in which dynamic feedback was available from either the initial (IF) or the terminal (TF) part of the trajectory. The results showed that under the NF and IF conditions accuracy was lowest; constant error was not speed-dependent, whereas dispersion increased with movement speed. Accuracy was highest under the CF and TF conditions and was speed-dependent, as shown by both constant error and dispersion. Under the EF condition, the accuracy level was intermediate, and was also speed-dependent. The time course of performance during the series was analyzed by comparing the mean error of the first and the last five-trial blocks in the series under the three feedback conditions resulting in accuracy improvement and speed-dependence (CF, TF and EF). The block effect was significant overall, with the last block showing greatest accuracy. The block effect was found to be significant for rapid movements only under the CF and TF conditions (with a Block × Speed interaction under the CF condition), and for all movement speeds under the EF condition. But the feedback and speed effects turned out to be significant for each block. The results are discussed in terms of the interchange between ‘corrected’ ongoing responses vs ‘amended’ delayed responses within the motor regulatory processes, the preponderance of one or the other type of response being dependent on feedback availability and movement speed.     

Developmental trends in speed accuracy trade-off in 6-10-year-old children performing rapid reciprocal and discrete aiming movements

Cyclic tasks are performed better than discrete tasks in adults but it is unknown whether this advantage is present in children as well. Three age groups of participants (6, 8, and 10 years old) executed cyclic and discrete aiming movements to two differently sized target using a Fitts task to examine the developmental effects on speed/accuracy trade-off. Children showed the same advantage of cyclic over discrete movements as previously demonstrated for adults but at a slower speed. The slope of the speed accuracy trade-off was similar in the three age groups in the cyclic as compared to the discrete control mode, suggesting that children learn both tasks equally well in this age range. The index of performance (IP) increased with age but not differently for the two control modes. Children showed clear differences between the kinematics of discrete and cyclic movements and these differences were similar to those seen in adults. Cyclic movements were faster, had higher IP, showed fewer changes in velocity and were more ballistic. Thus movement execution was different between the two tasks, consistent with the hypothesis that cyclic tasks make use of neural oscillators. The slower movement speed in young children is consistent with their limited ability to use open loop control.     

On re-scaleability of force and time in aiming movements

Summary Evidence for invariant relative timing is mostly based on comparisons between experimental conditions which differ in the average speed of the movements under study. The present experiment examines whether spontaneous variability of acceleration-time curves of aiming movements within a single experimental condition can also be described in terms of variable scaling parameters for both dimensions. This requires that the correlations between peak acceleration and peak deceleration, as well as those between time intervals defined by peak values and zero crossings, are high. In extensions of the elbow joint on a horizontal plane, the latter correlations turned out to be small or even negative. These results cannot be explained as being due to artifacts of several origins. To reconcile them with those based on comparisons between experimental conditions, a dual-level hypothesis of control in aiming movements is suggested. Only variability on one level can be described in terms of variable scaling parameters, and whether or not variability of acceleration-time curves can be described in this way depends on the relative contributions of both levels of control.     

Immediate spatial distortions of pointing movements induced by visual landmarks

We tested the influence of two horizontally aligned visual landmarks on pointing movements to memorized targets, to investigate whether the visuomotor system can make use of an egocentric representation unaffected by visual context. The endpoints of pointing movements were systematically distorted toward the nearest visual landmark, indicating that spatial representations included both target and nontarget information. These distortions were not due to the presence of the landmarks during the movement but, rather, to their presence in the encoding phase. Qualitatively similar distortions were present even with the shortest possible retention phase, when the target was extinguished at movement onset. Finally, we found the same pattern of distortion when participants were forced to remember the target within an allocentric frame of reference. We argue that even early memory representations for pointing movements are influenced by visual information in the surrounding visual field.     

The spatial signal for saccadic eye movements emphasizes visual boundaries

By investigating the visual processing involved when saccades are made to newly appearing targets, we show that this processing is significantly nonlinear and that texture boundary information predominates. We used the global, or center-of-gravity, effect whereby a saccadic eye movement directed to a target consisting of a pair of elements has an amplitude intermediate between that of saccades directed to the individual elements. We measured the effect using target elements with different visual characteristics, including phase-reversal checkerboard targets that had the same space-average luminance as the background. The contribution to the center-of-gravity calculation was used to measure relative salience. We found that positive and negative contrast elements contribute almost equal weightings. Thus, salience, assessed in this way, is a highly nonlinear function of luminance. The salience of checkerboard targets was found to decrease as check size was decreased and increase as the overall size of the target was increased. Checkerboards with an empty center were as effective as were full checkerboards, showing the importance of boundaries in the salience signal.     

The role of peripheral and central visual information for the directional control of manual aiming movements.

Seeing one's hand in visual periphery has been shown to optimize the directional accuracy of a sweeping hand movement, which is consistent with Paillard's (1980; Paillard & Amblard, 1985) two-channels model of visual information processing. However, contrary to this model, seeing one's hand in central vision, even for a brief period of time, also resulted in optimal directional accuracy. One goal of the present study was to test two opposing hypotheses proposed to explain the latter finding. As a second goal, we wanted to determine whether additional support could be found for the existence of a visual kinetic channel. The results indicated that seeing one's hand in central vision, even for a very short delay, resulted in the same accuracy as being permitted to see one's hand for the duration of the whole movement. This suggests that seeing one's hand around the target might enable one to code its location and that of the target within a single frame of reference and, thus, facilitate movement planning. In addition, the results of the present study indicated that seeing one's hand in motion while in visual periphery permitted a better directional accuracy than when this information was not available. This suggests that the movement vector, which is planned prior to movement initiation, can be quickly updated following movement initiation.