Earlier studies have suggested that the size of an object to be grasped influences the time taken to complete a prehensile movement. However, the use of cylindrical objects in those studies confounded the effects of object size-extent orthogonal to the reach axis-and object width-extent along the reach axis. In separating these effects, the present study demonstrates that movement time is not affected by manipulation of object size, as long as the latter does not approach the maximal object size that can be grasped. Object width, on the other hand, is shown to exert a systematic influence on movement time: Smaller object widths give rise to longer movement times through a lengthening of the deceleration phase of the movement, thus reproducing the effect of target width on the kinematics of aiming movements. As in aiming, movement amplitude also affects the movement time in prehension, influencing primarily the acceleration phase (i.e. peak velocity attained). The effects of object width and movement amplitude were found to combine in a way predicted by Fitts' law, allowing a generalisation of the latter to the transport component in prehensile actions. With respect to the grasp component, both object size and object width are shown to affect peak hand aperture. Increasing object width thus lowers the spatial accuracy demands on the transport component, permitting a faster movement to emerge. At the same time, the hand opens to a larger grip in order to compensate for eventual directional errors that result.(ABSTRACT TRUNCATED AT 250 WORDS)

On the basis of a critical review of studies that examined the use of temporal information in the regulation of movement, J. P. Wann (1996) concluded that there is little evidence in favor of the use of tau. Although more experimental work is certainly needed, progress can only be made if (a) the conceptual confusion emanating from a lack of distinction between specification (i.e., information) and what is specified (i.e., relevant property of the environmentactor system) is resolved, and (b) the way in which information is used in the regulation of movement is reconsidered. It is argued that continuous control models incorporating firstorder time-to-contact related information not only explain the results obtained but also allow testable accounts of the principles involved in kinematic trajectory formation.A change in the relative distance between an observer and an object (or a surface) gives rise to a change in the optic angle subtended at the point of observation by the object. Thus, a change in the optic angle may be used by an observer to detect the existence of relative motion. Moreover, the pattern of change of the optic angle contains information about certain characteristics of the relative motion, signaling, for instance, whether the relative distance increases or decreases and, in the latter case, whether a collision is imminent (Schiff, 1965). Lee (1974Lee ( ,1976Lee ( ,1980 demonstrated that besides these qualitative aspects, the pattern of change of the optic angle also contains quantitative temporal information, signaling the time remaining until contact if the velocity of approach were to remain constant.The identification of such quantitative temporal information in the optic flow pattern opened the door for the development of a theory of the (temporal) regulation of movement, and since then a large number of studies have been reported that attempted to test and/or further develop such a theory. Today, the optical flow pattern descriptor identified by Lee (1974Lee ( , 1976Lee ( , 1980)-which he coined T (tau)-figures prominently in most textbooks on perception or movement. However, this by no means implies that the debate on the manner in which, if at all, such an optical variable is used in the regulation of movement is closed, as is exemplified by the critical article of Wann (1996). On Distinguishing Specification FromWhat Is SpecifiedBefore entering into the argument about the use of T-like variables in the timing of action, a persistent and pervasive conceptual confusion needs to be resolved first. This confusion is generated by the fact that, more often than not, students of perception and movement fail to distinguish between the property of the Environment-Actor System (EAS) that is thought to be relevant for the regulation of movement and its optical (or acoustical, mechanical, chemical, etc.) representation, that is, the distinction between what is specified (the property of the EAS) and its specification (the informative flow pattern that can be detected). In the introductory example...

Perception of relative phase and phase variability may play a fundamental role in interlimb coordination. This study was designed to investigate the perception of relative phase and of phase variability and the stability of perception in each case. Observers judged the relative phasing of two circles rhythmically moving on a computer display. The circles moved from side to side, simulating movement in the frontoparallel plane, or increased and decreased in size, simulating movement in depth. Under each viewing condition, participants observed the same displays but were to judge either mean relative phase or phase variability. Phase variability interfered with the mean-relative-phase judgments, in particular when the mean relative phase was 0°. Judgments of phase variability varied as a function of mean relative phase. Furthermore, the stability of the judgments followed an asymmetric inverted U-shaped relation with mean relative phase, as predicted by the Haken-Kelso-Bunz model.In the early eighties, Kugler, Turvey, and Kelso, among others, introduced the concepts of nonlinear dynamics into the study of human movement, thereby building on Bernstein's (1967) important insight that perception-action systems should be regarded as coordinative structures that are task specific and soft molded (Kelso, Holt, Rubin, & Kugler, 1981;Kugler, Kelso, & Turvey, 1980;Kugler & Turvey, 1987). Taking rhythmicity as paradigmatic of human movement, they noted that cyclical movements are sustained in spite of the universal tendency for order to diminish and cease as described by the second law of thermodynamics (cf. Yates, 1982). They suggested therefore that coordinative structures are best studied as ensembles of nonlinear coupled oscillators, exhibiting limit-cycle properties (Kugler et al., 1980;Turvey, 1990). The energy for sustaining cyclical motion was hypothesized to be regulated using information in an autonomous fashion. Indeed, cyclical limb movement has been shown to exhibit limitcycle properties, and coordinated rhythmic limb movement has been modeled successfully as a system of coupled nonlinear oscillators (e.g., see Haken, Kelso, & Bunz, 1985;Kay, Kelso, Saltzman, & Schoner, 1987;Kelso et al., 1981). As a result of these developments, dynamic systems theory has had an enormous impact on human movement science (for reviews, see Beek, Peper, & Stegeman, 1995;Haken, 1996;Kelso, 1995 We thank Betty Tuller and Claudia Carello for helpful comments on an earlier version of this article.Correspondence concerning this article should be addressed to Frank T. J. M. Zaal, who is now at the Faculty of Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT Amsterdam, the Netherlands. Electronic mail may be sent to fzaal@fbw.vu.nl. Schoner, 1990;Schmidt & Turvey, 1995;Turvey, 1990). However, a brief review of this research reveals that the perceptual variables used in coordinated limb movement are not yet well understood, although the prominent role of perceptual information is widely recognized. Coordinat...

Studies of bimanual coordination have found that only two stable relative phases (0°and 180°) are produced when a participant rhythmically moves two joints in different limbs at the same frequency. Increasing the frequency of oscillation causes an increase in relative phase variability in both of these phase modes. However, relative phasing at 180°is more variable than relative phasing at 0°, and when the frequency of oscillation reaches a critical frequency, a transition to 0°occurs. These results have been replicated when 2 people have coordinated their respective limb movements using vision. This inspired us to investigate the visual perception of relative phase. In Experiment 1, recordings of human interlimb oscillations exhibiting different frequencies, mean relative phases, and different amounts of phase variability were used to generate computer displays of spheres oscillating either side to side in a frontoparallel plane or in depth. Participants judged the stability of relative phase. Judgments covaried with phase variability only when the mean phase was 0°or 180°. Otherwise, judgments covaried with mean relative phase, even after extensive instruction and demonstration. In Experiment 2, mean relative phase and phase variability were manipulated independently via simulations, and participants were trained to perceive phase variability in testing sessions in which mean phase was held constant. The results of Experiment 1 were replicated. The HKB model was fitted to mean judgment standard deviations.Phase refers to the proportion of the cycle traveled at a given time in a rhythmic motion. If the motion is represented as a trajectory on the phase plane (i.e., a plot ofvelocity vs. position), then the phase is the angular coordinate of the motion (measured in degrees or radians). The relative phase of two motions (e.g., the swinging of two legs) is the difference ofthe two phases. A number ofstudies have shown that, when a person oscillates two equivalent limbs at a common frequency and each about a single joint, then one of only two stable relative phase relations is exhibited, either 0°or 180°relative phase (Kelso. 1984(Kelso. , 1995Kelso, Schoner, Scholz, & Haken, 1987;Schoner & Kelso, 1988;Tuller & Kelso, 1989;Yaminishi, Kawato, & Suzuki, 1979, 1980. For instance, Tuller and Kelso (1989) used two metronomes Portions of this work were presented at the annual meeting ofthe Psychonomic Society. November 1996 (Bingham, Schmidt, & Zaal, 1996;Zaal, Bingham, & Schmidt, 1996). The authors wish to acknowledge the assistance of Michael Stevens in the data collection of Experiment I, Michael Stassen in programming the simulations of Experiment 2, and James Craig, Tim Lee, and an anonymous reviewer for valuable suggestions about methodology and analysis. Correspondence to should be addressed to G. P. Bingham, Department of Psychology, Indiana University, Bloomington, IN 47405 (e-mail: gbingham@indiana.edu).to help participants to try to oscillate their left and right index fingers at relative phases other than 0°an...

Psychophysical studies reveal distortions in perception of distance and shape. Are reaches calibrated to eliminate distortions? Participants reached to the front, side, or back of a target sphere. In Experiment 1, feedforward reaches yielded distortion and outward drift. In Experiment 2, haptic feedback corrected distortions and instability. In Experiment 3, feedforward reaches with only haptic experience of targets replicated the shape distortions but drifted inward. This showed that outward drift in Experiment 1 was visually driven. In Experiment 4, visually guided reaches were accurate when participants used binocular vision but when they used monocular vision, reaches were distorted. Haptic feedback corrected inaccuracy and instability of distance but did not correct monocular shape distortions. Dynamic binocular vision is representative and accurate and merits further study.