The Impact of Real and Illusory Perturbations on the Early Trajectory Adjustments of Goal-Directed Movements

Examinations of goal-directed movements reveal a process of control that operates to make adjustments on the basis of the expected visual afference associated with the limb's movement. This experiment examined the impact of perturbations to the perceived and actual velocity of aiming movements when each was presented alone or in tandem with the other. Perturbations to perceived velocity were achieved by translating the background over which aiming movements were performed. An aiming stylus that discharged air either in the direction of the movement or in the direction opposite the movement generated the actual velocity perturbations. Kinematic analyses of the aiming movements revealed that only the actual perturbation influenced the control of early movement trajectories. The results are discussed with respect to the influence that visual information has on the control exerted against physical perturbations. Speculations are raised regarding how potential for perturbations influences the strategies adopted for minimizing their impact.     

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.     

The influence of environmental context in interpersonal observation–execution

Cyclical upper-limb movements involuntarily deviate from a primary movement direction when the actor concurrently observes incongruent biological motion. We examined whether environmental context influences such motor interference during interpersonal observation–execution. Participants executed continuous horizontal arm movements while observing congruent horizontal or incongruent curvilinear biological movements with or without the presence of an object positioned as an obstacle or distractor. When participants were observing a curvilinear movement, an object located within the movement space became an obstacle, and, thus, the curvilinear trajectory was essential to reach into horizontal space. When acting as a distractor, or with no object, the curvilinear trajectory was no longer essential. For observing horizontal movements, objects were located at the same relative locations as in the curvilinear movement condition. We found greater involuntary movement deviation when observing curvilinear than horizontal movements. Also, there was an influence of context only when observing horizontal movements, with greater deviation exhibited in the presence of a large obstacle. These findings suggest that the influence of environmental context is underpinned by the (mis-)matching of observed and executed actions as incongruent biological motion is primarily coded via bottom-up sensorimotor processes, whilst the congruent condition incorporates surrounding environmental features to modulate the bottom-up sensorimotor processes.     

Real-time manipulation of visual displacement during manual aiming

This study examined the spatial and temporal limitations of the visual corrective process in the control of upper limb movements. Real-time calculation of kinematic data was used to trigger a prismatic displacement of the movement environment during manual aiming. Using an OptoTrak motion tracking system, a data acquisition unit, and a custom-made program, perturbations were triggered at peak acceleration, peak velocity, and the estimated time of peak deceleration. Movement outcome was significantly influenced only when the visual displacement occurred at peak acceleration. The results support models of visual control that posit that early visual information is required for accurate limb control.     

NO ONE IS GUILTY: CRIME, PATRIARCHY, AND INDIVIDUALISM*

Let us begin with a fundamental realization: No amount of thinking and no amount of public policy have brought us any closer to understanding and solving the problem of crime. The more we have reacted to crime, the farther we have removed ourselves from any understanding and any reduction of the problem. In recent years, we have floundered desperately in reformulating the law, punishing the offender, and quantifying our knowledge. Yet this country remains one of the most crime-ridden nations. In spite of all its wealth, economic development, and scientific advances, this country has one of the worst crime records in the world.
With such realization, we return once again—as if starting anew—to the subject of crime, a subject that remains one of our most critical indicators of the state of our personal and collective being. If what is to be said seems outrageous and heretical, it is only because it is necessarily outside the conventional wisdom both of our understanding of the problem and of our attempt to solve it.     

Goal-Directed Aiming: Correcting a Force-Specification Error With the Right and Left Hands

In 2 experiments, the authors examined manual aiming asymmetries as well as the ability of participants to adjust their aiming trajectories following an unexpected change to the inertial resistance to movement. In Experiment 1, participants (N = 11) were able to rapidly adjust their movement trajectories to conform to the new movement requirements. They were faster and more consistent when aiming with their right hand than with their left hand, regardless of whether or not the movement was perturbed. In Experiment 2, participants' (N = 11) vision of the hand was manipulated so that the role of visual feedback in the corrective process could be examined. Vision had an impact not only on performance but also on the characteristics of the movement trajectories. Manual asymmetries in aiming were associated with a right hand superiority during the final corrective stages of the movement.     

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.     

Nomadic inhibition of attention and motor responses

Klein and MacInnes [Klein, R. M., & MacInnes, W. J. (1999). Inhibition of return is a foraging facilitator in visual search. Psychological Science, 10, 346-352] posited that the function of a phenomenon known as the inhibition of return (IOR) [Posner, M. I., & Cohen, Y. (1984). Components of visual orienting. In H. Bouma, & D. G. Bouwhuis (Eds.), Attention and performance X: Control of language processes (pp. 531-554). Hillsdale, NJ: Erlbaum] is to facilitate the foraging of food and objects in the environment. Once a target object has been identified either the location of that target in space or a movement to that target is inhibited in order to allow the performer to shift his/her attention to something new. Interestingly, in the majority of IOR studies, participants begin their search from a central home position. This research examined IOR in a nomadic target-target paradigm in which the home position randomly appeared at one of three target locations and attentional shifts/movements progressed to other locations. In Experiment 1, participants executed simple manual button presses in response to the sequential presentation of a home position and then two target stimuli. In Experiment 2, participants made manual-aiming movements in response to the same type of presentation. Results obtained from both experiments implicate perceptual-motor mechanisms over and above the inhibition of a specific target location or response. Inhibitory effects appear to be associated with both perceptual and motor processes, and depend not only on the temporal and spatial relations between potential targets, but also on the actions required to detect or engage the targets.     

Sampling frequency and the study of eye-hand coordination in aiming

This study synchronized sampling of point of gaze (PG) and hand movements in a fast aiming task, using a 60- and a 120-Hz sampling frequency. The subjects moved eyes, head, hand, and trunk freely. For limb kinematics, a significant difference between sampling conditions was only found for the number of accelerations in the profile following peak velocity of the hand. For PG movements, no differences were found for initiation time, saccade angle, fixation duration, and overall number of saccades. However, significant differences were observed for saccade duration. Previously, an invariant feature was found for the ratio of PG and hand response times (50%). For both sampling frequencies, a significant correlation and, thus, temporal coupling was found between PG response time and time to peak acceleration for the hand. Depending on the measures required, a 60-Hz sampling of PG and hand movements may provide as meaningful results as a 120-Hz sampling.     

The role of target information on manual-aiming bias

Three experiments were conducted to examine the role of target information in manual aiming. The key manipulations in this experiment were the use of two target contexts (the two forms of the Müller-Lyer illusion) and the visual conditions under which subjects moved. In Experiment 1, we demonstrated that the inward- and outward-pointing arrows biased manual-aiming movements in a manner consistent with their well-known influence on perceptual judgements. The elimination of visual feedback during the aiming movement (Experiment 2), and visual information about the target-aiming layout prior to the movement (Experiment 3) increased the magnitude of the bias. Together, these results demonstrate the strong effect of target information on manual aiming, and specifically, on the movement-planning processes that precede movement.