Jeroen B. J. Smeets

Fast Responses of the Human Hand to Changes in Target Position

If a target toward which an individual moves his hand suddenly moves, he adjusts the movement of his hand accordingly. Does he use visual information on the targets velocity to anticipate where he will reach the target? These questions were addressed in the present study. Subjects (N = 6 in each of 4 experiments) were instructed to hit a disk with a rod as soon as it appeared on a screen. Trajectories of the hand toward stationary disks were compared with those toward disks that jumped leftward or rightward as soon as the subjects hand started moving toward the screen, and with those in which either the disk or the background started moving leftward or rightward. About 110 ms after the disk was suddenly displaced, the moving hand was diverted in the direction of the perturbation. When the background moved, the disks perceived position shifted in the direction in which the background was moving, but the disk appeared to be moving in the opposite direction. When hitting such disks, subjects adjusted their movement in accordance with the perceived position, rather than moving their hand in the direction of the perceived motion in anticipation of the disks future displacement. Thus, subjects did not use the perceived velocity to anticipate where they would reach the target but responded only to the change in position.

Adjustments of fast goal-directed movements in response to an unexpected inertial load

SummarySubjects made fast goal-directed elbow flexion movements against an inertial load. Target distance was 8 or 16 cm, randomly chosen. To exert a force in the direction of the movement subjects had to activate flexors of both shoulder and elbow, but shoulder flexors did not change appreciably in length during the movement. In 20% of the trials the inertial load was increased or decreased without knowledge of the subjects. Until 90–110 ms after the onset of the agonist muscle activity (about 65–85 ms after the start of movement) EMG activity was very similar in all conditions tested. The changes that occured in the EMG from that moment on were effectively a later cessation of the agonist activity and a later start of the antagonist activity if the load was increased unexpectedly. If the load was reduced unexpectedly, the agonist activity ceased earlier and the antagonist activity began earlier. The latency at which EMGs started to change was the same for muscles around shoulder and elbow, for agonists and antagonists and for both distances. All adjustments had the same latency (37 ms) relative to the point where the angular velocity of the elbow in the unexpectedly loaded movements differed by 0.6 rad/s from the expected value. We discuss why simple reflex- or servo-mechanisms cannot account for the measured EMG changes. We conclude that appropriate adjustments of motor programmes for fast goal-directed arm movements start within 40 ms of the detection of misjudgment of load.

Dependence of autogenic and heterogenic stretch reflexes on pre-load activity in the human arm.

1. Subjects held their right arm in a horizontal plane. The angle of the elbow was 90 deg. They exerted forces in several directions in the plane of the arm, varying independently the pre‐load torques about shoulder and elbow. We measured electromyographic (EMG) activity in several arm muscles in response to force perturbations which extended the shoulder, without changing the elbow angle. 2. The EMG activity in flexors of both shoulder and elbow showed reflex responses at short latency (approximately 25 ms). In all muscles the reflex activity increased with the pre‐load activity of that muscle. 3. The short‐latency reflex activity of m. brachialis, which was not stretched by the perturbations, was independent of the pre‐load activity of the muscles acting over the shoulder. 4. From these results we conclude that the force resulting from the short‐latency reflex, assessed from the EMGs, does not counteract the perturbations exactly. Having found that the short‐latency reflex is dependent on the pre‐load direction, we argue that this dependence makes the short‐latency reflex suitable for correcting fast movements for misjudgements of load. 5. At longer latencies (greater than 50 ms) the direction of the force resulting from the reflex, assessed from the EMGs, was almost independent of the direction of the pre‐load. In our experiment the force resulting from the long‐latency reflex counteracted the perturbations quite well.

Illusions in action: consequences of inconsistent processing of spatial attributes

Abstract. Many authors have performed experiments in which subjects grasp objects in illusory surroundings. The vast majority of these studies report that illusions affect the maximum grip aperture less than they affect the perceived size. This observation has frequently been regarded as experimental evidence for separate visual systems for perception and action. In order to make this conclusion, one assumes that the grip aperture is based on a visual estimate of the objects size. We believe that it is not, and that this is why size illusions fail to influence grip aperture. Illusions generally do not affect all aspects of space perception in a consistent way, but mainly affect the perception of specific spatial attributes. This applies not only to object size, but also to other spatial attributes such as position, orientation, displacement, speed, and direction of motion. Whether an illusion influences the execution of a task will therefore depend on which spatial attributes are used rather than on whether the task is perceptual or motor. To evaluate whether illusions affect actions when they influence the relevant spatial attributes we review experimental results on various tasks with inconsistent spatial processing in mind. Doing so shows that many actions are susceptible to visual illusions. We argue that the frequently reported differential effect of illusions on perceptual judgements and goal-directed action is caused by failures to ensure that the same spatial attributes are used in the two tasks. Illusions only affect those aspects of a task that are based on the spatial attributes that are affected by the illusion.

Hitting moving targets. Continuous control of the acceleration of the hand on the basis of the target's velocity

Abstract Previous studies on how we hit moving targets have revealed that the direction in which we move our hand is continuously adjusted on the basis of the target’s perceived position, with a delay of about 110 ms. In the present study we show that the acceleration of the hand is also under such continuous control. Subjects were instructed to hit moving targets (running spiders) as quickly as possible with a rod. We found that changing the velocity of the target influenced the speed with which the rod was moved. The influence was noticeable about 200 ms after the target’s velocity changed. The extent of the influence was consistent with a direct dependence of the acceleration of the hand on the target’s velocity. We conclude that the acceleration of the hand is continuously adjusted on the basis of the speed of the target, with a delay of about 200 ms.

Multiple information sources in interceptive timing

This study was designed to explore the limitations of tau (τ) as an explanatory construct for the timing of interceptive action. This was achieved by examining the effects of environmental structure and binocular vision on the timing of the grasp in a simple one-handed catch. In two experiments, subjects were required to catch luminous balls of different diameters (4, 6, 8 and 10 cm) in a completely darkened room. In the first experiment the influence of the presence vs. absence of an environmental background structure (both under monocular viewing) was tested, and in the second experiment the influence of monocular vs. binocular vision was examined. It was found that irrespective of the presence of environmental structure, an effect of ball size occurred in the monocular viewing conditions. That is, in monocular viewing conditions the grasp was initiated and completed earlier for the larger balls as compared to the smaller ones, while in the binocular viewing condition subjects behaved in accordance with a constant time to contact strategy: no effects of ball size were found. It is concluded that under binocular viewing a binocular information source is used, while in the monocular viewing condition a lower order information source like image size or image velocity is probably involved.

Perception of acceleration with short presentation times: Can acceleration be used in interception?

To investigate whether visual judgments of acceleration could be used for intercepting moving targets, we determined how well subjects can detect acceleration when the presentation time is short. In a differential judgment task, two dots were presented successively. One dot accelerated and the other decelerated. Subjects had to indicate which of the two accelerated. In an absolute judgment task, subjects had to adjust the motion of a dot so that it appeared to move at a constant velocity. The results for the two tasks were similar. For most subjects, we could determine a detection threshold even when the presentation time was only 300 msec. However, an analysis of these thresholds suggests that subjects did not detect the acceleration itself but that they detected that a target had accelerated on the basis of the change in velocity between the beginning and the end of the presentation. A change of about 25% was needed to detect acceleration with reasonable confidence. Perhaps the simplest use of acceleration for interception consists of distinguishing between acceleration and deceleration of the optic projection of an approaching ball to determine whether one has to run backward or forward to catch it. We examined the results of a real ball-catching task (Oudejans, Michaels, & Bakker, 1997) and found that subjects reacted before acceleration could have been detected. We conclude that acceleration is not used in this simple manner to intercept moving targets.

10 years of illusions

A decade ago, S. Aglioti, J. F. X. DeSouza, and M. A. Goodale (1995) published an experiment that has had a big influence on the way that visual information is thought to control human behavior. Their findings have often been simplified as suggesting that action is immune to perceptual illusions. The current authors critically analyze the 4 steps involved in this simplification and argue that research during the last 10 years has shown that the validity of 3 of the 4 steps is doubtful. They conclude that this experiment cannot be regarded as firm support for the 2-visual-systems hypothesis (i.e., that the ventral stream is for perception and the dorsal stream is for visually guided actions).

The difference between the perception of absolute and relative motion: A reaction time study

We used a reaction-time paradigm to examine the extent to which motion detection depends on relative motion. In the absence of relative motion, the responses could be described by a simple model based on the detection of a fixed change in position. If relative motion was present, the responses could be modelled using characteristics of motion detectors. Comparing reaction times when relative and absolute velocity are equal with ones when relative velocity is twice the absolute velocity reveals that these detectors measure relative motion.

Fast corrections of movements with a computer mouse

When we reach out for an object with our hand, we transform visual information about the objects position into muscle contractions that will bring our digits to that position. If we reach out with a tool the transformation is different, because the muscle contractions must bring the critical part of the tool to the object, rather than the digits. The difference between the motion of the hand and that of the tool can be quite large, as when moving a computer mouse across a table to bring a cursor to a position on a screen. We examined the responses to unpredictable visual perturbations during such movements. People responded about as quickly to changes in the position of the target when pointing with the mouse as when doing so with their hand. They also responded about as quickly when the cursor was displaced as when the target was displaced. We show that this is not because the visually perceived separation between target and cursor is transformed into a desired displacement of the hand. Our conclusion is that our actions are controlled by the judged positions of the end-effector and the target, even when the former is quite detached from the muscles and joints that are involved in the action.