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.     

Fitts’ law holds for pointing movements under conditions of restricted visual feedback

Fitts’ law robustly predicts the time required to move rapidly to a target. However, it is unclear whether Fitts’ law holds for visually guided actions under visually restricted conditions. We tested whether Fitts’ law applies under various conditions of visual restriction and compared pointing movements in each condition. Ten healthy participants performed four pointing movement tasks under different visual feedback conditions, including full-vision (FV), no-hand-movement (NM), no-target-location (NT), and no-vision (NV) feedback conditions. The movement times (MTs) for each task exhibited highly linear relationships with the index of difficulty (r² > .96). These findings suggest that pointing movements follow Fitts’ law even when visual feedback is restricted or absent. However, the MTs and accuracy of pointing movements decreased for difficult tasks involving visual restriction.     

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.     

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 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).     

Concurrent visual feedback and spatial accuracy in continuous aiming movements

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

Effects on prism adaptation of duration and timing of visual feedback during pointing

In two experiments, we investigated the effects of duration of visual feedback of the pointing limb and the time (early to late) in the movement when the limb first becomes visible (timing of visual feedback). Timing, rather than duration of visual feedback, proved to have the greater effect on the relative magnitude of visual and proprioceptive adaptation. Visual adaptation increased smoothly with feedback delay, but corresponding decreases in proprioceptive adaptation underwent an additional sharp change when feedback was delayed until about three-fourths of the way to the terminal limb position. These results are consistent with the idea that visual and proprioceptive adaptation are mediated by exclusive processes. Change in the limb position sense (i.e., proprioceptive adaptation) may be produced by visual guidance of the pointing limb, and view of the limb early in the pointing movement seems to be critical for such visual guidance. The limb may be ballistically released as it nears the terminal position, and, thereafter, any opportunity for visual guidance (i.e., view of the limb) is not effective. On the other hand, change in the eye position sense (i.e., visual adaptation) may be mediated by proprioceptive guidance of the eye; the eyes may track the imaged position of the nonvisible limb. Such proprioceptive guidance seems to be solely a function of the distance moved before the limb becomes visible.     

On the role of peripheral visual afferent information for the control of rapid video-aiming movements

It has been shown that, even for very fast and short duration movements, seeing one's hand in peripheral vision, or a cursor representing it on a video screen, resulted in a better direction accuracy of a manual aiming movement than when the task was performed while only the target was visible. However, it is still unclear whether this was caused by on-line or off-line processes. Through a novel series of analyses, the goal of the present study was to shed some light on this issue. We replicated previous results showing that the visual information concerning one's movement, which is available between 40 degrees and 25 degrees of visual angle, is not useful to ensure direction accuracy of video-aiming movements, whereas visual afferent information available between 40 degrees and 15 degrees of visual angle improved direction accuracy over a target-only condition. In addition, endpoint variability on the direction component of the task was scaled to direction variability observed at peak movement velocity. Similar observations were made in a second experiment when the position of the cursor was translated to the left or to the right as soon as it left the starting base. Further, the data showed no evidence of on-line correction to the direction dimension of the task for the translated trials. Taken together, the results of the two experiments strongly suggest that, for fast video-aiming movements, the information concerning one's movement that is available in peripheral vision is used off-line.     

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).     

Inhibition of return and manual pointing movements

To examine whether the motor inhibition of return (IOR) postulated by Taylor and Klein (1998, 2000) generalizes to manual guided movements or is restricted to saccadic responses, the following three experiments were conducted. The first experiment combined peripheral cues (which generate IOR) with four types of manual responses made to central targets (central arrow indicating the response location). The responses were made on a touch-screen and were the equivalent of either a detection keypress, a choice keypress, a detection-guided pointing movement, or a choice-guided pointing movement. No IOR was found for any of the responses. The second experiment replicated the main result under eye fixation control. In Experiment 3, peripheral cues and peripheral targets were used, and IOR was present in all responses. Overall, these finding suggest that motor-based IOR is restricted to the oculomotor system. Implications for motor-based IOR and attention-based IOR are discussed.     

Visual perception of writing and pointing movements

Studies of movement production have shown that the relationship between the amplitude of a movement and its duration varies according to the type of gesture. In the case of pointing movements the duration increases as a function of distance and width of the target (Fitts' law), whereas for writing movements the duration tends to remain constant across changes in trajectory length (isochrony principle). We compared the visual perception of these two categories of movement. The participants judged the speed of a light spot that portrayed the motion of the end-point of a hand-held pen (pointing or writing). For the two types of gesture we used 8 stimulus sizes (from 2.5 cm to 20 cm) and 32 durations (from 0.2 s to 1.75 s). Viewing each combination of size and duration, participants had to indicate whether the movement speed seemed "fast", "slow", or "correct". Results showed that the participants' perceptual preferences were in agreement with the rules of movement production. The stimulus size was more influential in the pointing condition than in the writing condition. We consider that this finding reflects the influence of common representational resources for perceptual judgment and movement production.     

Origins of submovements during pointing movements

Submovements that are frequently observed in the final portion of pointing movements have traditionally been viewed as pointing accuracy adjustments. Here we re-examine this long-lasting interpretation by developing evidence that many of submovements may be non-corrective fluctuations arising from various sources of motor output variability. In particular, non-corrective submovements may emerge during motion termination and during motion of low speed. The contribution of these factors and the factor of accuracy regulation in submovement production is investigated here by manipulating movement mode (discrete, reciprocal, and passing) and target size (small and large). The three modes provided different temporal combinations of accuracy regulation and motion termination, thus allowing us to disentangle submovements associated with each factor. The target size manipulations further emphasized the role of accuracy regulation and provided variations in movement speed. Gross and fine submovements were distinguished based on the degree of perturbation of smooth motion. It was found that gross submovements were predominantly related to motion termination and not to pointing accuracy regulation. Although fine submovements were more frequent during movements to small than to large targets, other results show that they may also be not corrective submovements but rather motion fluctuations attributed to decreases in movement speed accompanying decreases in target size. Together, the findings challenge the traditional interpretation, suggesting that the majority of submovements are fluctuations emerging from mechanical and neural sources of motion variability. The implications of the findings for the mechanisms responsible for accurate target achievement are discussed.     

The accuracy of aimed movements to visual targets during development: the role of visual information

Children aged 1.5 to 8 years were required to touch accurately an illuminated target lamp located on a vertical board. Movements were made when visual information was complete (target lit for 3 s, room illuminated; partial (target lit for 3 s, room dark, and reduced (target lit for 0.7 s, room dark). Dependent variables were response accuracy, reaction time, and movement time. Accuracy decreased with decreasing availability of visual information and improved with age under all conditions. Reaction times were shorter in the dark (Conditions 2 and 3) than in the light; they decreased with age up to age 5 and did not continue to decrease thereafter. Movement time did not change with age under Conditions 1 and 3 but tended to increase with age under Condition 2. Slower movements were more accurate at all ages, provided visual feedback could be utilized. Increased reliance on the strategy "slower movements yield higher accuracy" was held to account for developmental changes under Condition 2, whereas in Conditions 1 and 3 improvement in the efficiency of motor preprogramming was implicated.     

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.     

Contribution of somatosensory information to perception of the visual vertical with body tilt and rotating visual field

This study was designed to explore the role of somatosensory information from the trunk in the perception of the visual vertical. Twelve normal subjects and 1 subject with no somatosensory function below the neck attempted to set a line to the true vertical in the sitting and lying positions, first with a static visual background and then with rotation of the background about the line of sight. The absence of somatosensory information did not affect accuracy when the subjects were in the upright position. When lying horizontally, all control subjects experienced a substantial perceived tilt of the vertical in the direction of body tilt (the A effect), but, in contrast, the subject lacking somatosensory function exhibited a small but consistent apparent tilt of the vertical in the opposite direction (the E effect). This finding is discussed in relation to two competing hypotheses regarding the mechanisms subserving apparent displacement of the subjective vertical in tilted subjects.     

Deceleration requirements and the control of pointing movements

When a limb is moved from one position to a target object, the limb and the target frequently collide. Often, the goal of the movement is to strike the target with a particular magnitude of impact. For single-aiming movements, impact forces have been shown to increase systematically with both an increased movement amplitude and a decreased movement time, thus providing deceleration to the moving limb. Models of speed-accuracy trade-off, however, have neglected to account for the contribution of these impact forces in the control of accurate movements. The aim of this experiment was to examine the modifications in the control strategy as a function of the amount of impact force a subject is allowed to use in decelerating his or her limb. Results showed that the structure of the acceleration-time functions was dictated by the amount of impact force subjects were allowed to use in decelerating the limb. Movement endpoint variability decreased as more impact force was used. The experiment suggests that the impact with a target is an important contributor to the deceleration of the moving limb and a critical determinant of movement organization.