Interaction of neuromuscular, spatial and visual constraints on hand-foot coordination dynamics

In the present study, we investigated the contributions of motor and perceptual processes to directional constraints as observed during hand-foot coordination. Participants performed cyclical flexion-extension movements of the right hand and foot under two coordination modes: in-phase (isodirectional) and antiphase (non-isodirectional). Those tasks were performed either with full vision or no vision of the limbs. Depending on the position of the forearm (prone or supine), the coordination patterns were performed with similar and dissimilar neuro-muscular coupling with respect to their phylogenetic origin as antigravity muscles. Results showed that the antiphase pattern was more difficult to maintain than the in-phase pattern and that neuro-muscular coupling significantly influenced the coordination dynamics. Moreover, the effect of vision differed as a function of both neuro-muscular coupling and coordination mode. Under dissimilar neuro-muscular coupling, the presence of visual feedback stabilized the in-phase pattern and destabilized the antiphase pattern. In contrast, visual feedback did not influence pattern stability during conditions of similar neuro-muscular coupling. These results shed light on the complex interactions between motor and perceptual (visual) constraints during the production of hand-foot coordination patterns.     

Plane of Motion Mediates the Coalition of Constraints in Rhythmic Bimanual Coordination

The authors hypothesized that the modulation of coordinative stability and accuracy caused by the coalition of egocentric (neuromuscular) and allocentric (directional) constraints varies depending on the plane of motion in which coordination patterns are performed. Participants (N = 7) produced rhythmic bimanual movements of the hands in the sagittal plane (i.e., up-and-down oscillations resulting from flexion—extension of their wrists). The timing of activation of muscle groups, direction of movements, visual feedback, and across-trial movement frequency were manipulated. Results showed that both the egocentric and the allocentric constraints modulated pattern stability and accuracy. However, the allocentric constraint played a dominant role over the egocentric. The removal of vision only slightly destabilized movements, regardless of the effects of directional and (neuro)muscular constraints. The results of the present study hint at considering the plane in which coordination is performed as a mediator of the coalition of egocentric and allocentric constraints that modulates coordinative stability of rhythmic bimanual coordination.     

Age-related differences in the integration of sensory information during the execution of a bimanual coordination task

Young (n = 7) and elderly (n = 7) subjects performed bimanual coordination patterns in the transverse plane according to the in-phase or antiphase mode. Sensory information was manipulated through visual (with or without vision of the limbs) and proprioceptive input (with or without vibratory stimuli on one limb). Movement patterns with vibrations showed higher deviations from the intended relative phase than did those without vibrations. This finding suggests that the proprioceptive information induced by the vibrations and the movement interfered, leading to a disruption of the coordination patterns. In addition, as compared with the elderly, the young subjects performed more stable movements under normal circumstances but were more strongly affected by vibratory stimuli during the performance of in-phase movements. During antiphase movements, both age groups experienced a decrease of pattern stability. Furthermore, the absence or presence of visual feedback influenced the performance of the young subjects more than that of the elderly. The presence of vision led to stable in-phase movements, whereas a decrease of pattern stability was observed for antiphase movements. In general, these results demonstrate that manipulation of feedback sources affects young subjects more than elderly ones, and this can be related to a reduced sensory sensitivity as a function of aging.     

Effects of correct and transformed visual feedback on rhythmic visuo-motor tracking: tracking performance and visual search behavior

The effects of correct and transformed visual feedback on rhythmic unimanual visuo-motor tracking were examined, focusing on tracking performance (accuracy and stability) and visual search behavior. Twelve participants (reduced to 9 in the analyses) manually tracked an oscillating visual target signal in phase (by moving the hand in the same direction as the target signal) and in antiphase (by moving the hand in the opposite direction), while the frequency of the target signal was gradually increased to probe pattern stability. Besides a control condition without feedback, correct feedback (representing the actual hand movement) or mirrored feedback (representing the hand movement transformed by 180 degrees) were provided during tracking, resulting in either in-phase or antiphase visual motion of the target and feedback signal, depending on the tracking mode performed. The quality (accuracy and stability) of in-phase tracking was hardly affected by the two forms of feedback, whereas antiphase tracking clearly benefited from mirrored feedback but not from correct feedback. This finding extends previous results indicating that the performance of visuo-motor coordination tasks is aided by visual feedback manipulations resulting in coherently grouped (i.e., in-phase) visual motion structures. Further insights into visuo-motor tracking with and without feedback were garnered from the visual search patterns accompanying task performance. Smooth pursuit eye movements only occurred at lower oscillation frequencies and prevailed during in-phase tracking and when target and feedback signal moved in phase. At higher frequencies, point-of-gaze was fixated at a location that depended on the feedback provided and the resulting visual motion structures. During in-phase tracking the mirrored feedback was ignored, which explains why performance was not affected in this condition. Point-of-gaze fixations at one of the end-points were accompanied by reduced motor variability at this location, reflecting a form of visuo-motor anchoring that may support the pick up of discrete information as well as the control of hand movements to a desired location.     

Plane of motion mediates the coalition of constraints in rhythmic bimanual coordination

The authors hypothesized that the modulation of coordinative stability and accuracy caused by the coalition of egocentric (neuromuscular) and allocentric (directional) constraints varies depending on the plane of motion in which coordination patterns are performed. Participants (N = 7) produced rhythmic bimanual movements of the hands in the sagittal plane (i.e., up-and-down oscillations resulting from flexion-extension of their wrists). The timing of activation of muscle groups, direction of movements, visual feedback, and across-trial movement frequency were manipulated. Results showed that both the egocentric and the allocentric constraints modulated pattern stability and accuracy. However, the allocentric constraint played a dominant role over the egocentric. The removal of vision only slightly destabilized movements, regardless of the effects of directional and (neuro)muscular constraints. The results of the present study hint at considering the plane in which coordination is performed as a mediator of the coalition of egocentric and allocentric constraints that modulates coordinative stability of rhythmic bimanual coordination.     

Learning as change of coordination dynamics: theory and experiment

Learning of coordination patterns was investigated theoretically from the point of view of a dynamic theory of biological coordination and with reference to recent experiments on the learning of relative timing patterns. The theory is based on theoretical and experimental work showing that coordinated movement is characterized not only by the actually performed pattern of coordination but by an entire dynamics of coordination. Theoretically, such dynamics are captured as equations of motion of relevant collective variables. Experimentally, signatures of these underlying dynamics can be found in the temporal stability of coordination patterns, which can be assessed through various stability measures as well as through processes of pattern change. We argue that not only intrinsic coordination tendencies, but also specific behavioral requirements, be they perceived, memorized, or intended, must be expressed in terms of such dynamics. The concept of behavioral information captures such requirements as part of the coordination dynamics. We expound two hypotheses on the nature of learning in this framework. First, we assume that at each point during the learning process the system is governed by a well-defined coordination dynamics. This equation of motion evolves with learning so as to acquire an attractor solution near the to-be-learned pattern. Second, we hypothesize that this change of the coordination dynamics, captured by the time course of memorized behavioral information, can itself be ascribed to an additional layer of dynamics, the slower learning dynamics. Testable consequences of these views are discussed in the light of recent experimental findings on the learning of a relative phase in rhythmic movement: (a) Learning affects dynamic properties of performed coordination patterns, in particular, their stability; (b) the change of the coordination dynamics due to learning leads to specific changes of behavior also under conditions other than the learned condition, namely, to systematic deviation toward the learned patterns; (c) learning may lead to instabilities in the coordination behavior if initial and learned performance differ sufficiently; and (d) the dynamic properties of the performed coordination patterns are distinct on the two time scales of learning and of performance.     

Size-distance invariance: kinetic invariance is different from static invariance.

The static form of the size-distance invariance hypothesis asserts that a given proximal stimulus size (visual angle) determines a unique and constant ratio of perceived object size to perceived object distance. A proposed kinetic invariance hypothesis asserts that a changing proximal stimulus size (an expanding or contracting solid visual angle) produces a constant perceived size and a changing perceived distance such that the instantaneous ratio of perceived size to perceived distance is determined by the instantaneous value of visual angle. The kinetic invariance hypothesis requires a new concept, an operating constraint, to mediate between the proximal expansion or contraction pattern and the perception of rigid object motion in depth. As a consequence of the operating constraint, expansion and contraction patterns are automatically represented in consciousness as rigid objects. In certain static situations, the operation of this constraint produces the anomalous perceived-size-perceived-distance relations called the size-distance paradox.     

Size-distance invariance: Kinetic invariance is different from static invariance

The static form of the size-distance invariance hypothesis asserts that a given proximal stimulus size (visual angle) determines a unique and constant ratio of perceived-object size to perceived object distance. A proposed kinetic invariance hypothesis asserts that a changing proximal stimulus size (an expanding or contracting solid visual angle) produces a constant perceived size and a changing perceived distance such that the instantaneous ratio of perceived size to perceived distance is determined by the instantaneous value of visual angle. The kinetic invariance hypothesis requires a new concept, an operating constraint, to mediate between the proximal expansion or contraction pattern and the perception of rigid object motion in depth. As a consequence of the operating constraint, expansion and contraction patterns are automatically represented in consciousness as rigid objects. In certain static situations, the operation of this constraint produces the anomalous perceived-size-perceived-distance relations called the size-distance paradox.     

Structural salience and the nonaccidentality of a gestalt

We perceive structure through a process of perceptual organization. Here we report a new perceptual organization phenomenon-the facilitation of visual grouping by global curvature. Observers viewed patterns that they perceived as organized into collections of curves. The patterns were perceptually ambiguous such that the perceived orientation of the patterns varied from trial to trial. When patterns were sufficiently dense and proximity was equated for the predominant perceptual alternatives, observers tended to perceive the organization with the greatest curvature. This effect is tantamount to visual grouping by maximal curvature and thus demonstrates an unprecedented effect of global structure on perceptual organization. We account for this result with a model that predicts the perceived organization of a pattern as function of its nonaccidentality, which we define as the probability that it could have occurred by chance. Our findings demonstrate a novel relationship between the geometry of a pattern and the visual salience of global structure.     

Spatial orientation from optic flow in the central visual field

Previous research has shown that stimulation of the central visual field with radial flow patterns (produced by forward motion) can induce perceived self-motion, but has failed to demonstrate effects on postural stability of either radial flow patterns or lamellar flow patterns (produced by horizontal translation) in the central visual field. The present study examined the effects of lamellar and radial flow on postural stability when stimulation was restricted to the central visual field. Displays simulating observer motion through a volume of randomly positioned points were observed binocularly through a window that limited the field of view to 15 degrees. The velocity of each display varied according to the sum of four sine functions of prime frequencies. Changes in posture were used to measure changes in perceived spatial orientation. A frequency analysis of postural sway indicated that increased sway occurred at the frequencies of motion simulated in the display for both lamellar and radial flow. These results suggest that both radial and lamellar optic flow are effective for determining spatial orientation when stimulation is limited to the central visual field.     

Perceptual organization and attention

It is widely assumed that the grouping of the visual field first described by the Gestalt psychologists and the related phenomenon of texture segregation occur very early in the processing of visual information and involve preattentive processes. All the recent evidence supporting this assumption comes from visual search experiments in which the subject is actively looking for a target and attending to the stimulus. The question at issue is whether these kinds of patterns are perceived under conditions of inattention, i.e., when observers are not searching for them. We performed six experiments to determine whether texture segregation and grouping by similarity or proximity are perceived under conditions of inattention. On the first two trials subjects were asked to report the longer arm of a briefly presented cross which was surrounded by a pattern of ungrouped small elements. On the third trial and subsequent control trials these elements were configured into grouping patterns and subjects queried about them immediately following their line length reports. The results establish that neither texture segregation nor grouping by similarity of lightness or proximity are perceived under conditions of inattention. They support the conclusion that there is an earlier stage of processing than that referred to as preattentive.     

Changes in coordination and variability during running as a function of head stability demands

The purpose of this study was to identify changes in segment/joint coordination and coordination variability in running with increasing head stability requirements. Fifteen strides from twelve recreational runners while running on a treadmill at their preferred speed were collected. Head stability demands were manipulated through real-time visual feedback that required head-gaze orientation to be contained within boxes of different sizes, ranging from 21 to 3 degrees of visual angle in 3-degree decrements. Coordination patterns and coordination variability were assessed between head and trunk segments, hip and knee joints, and knee and ankle joints in the three cardinal planes, respectively. Mean coupling angles and the standard deviation of the coupling angles at each individual point of the stance phase were calculated using a modified vector coding technique and circular statistics. As head stability demands increased, transverse plane head-trunk coordination was more anti-phase and showed increased head‑leading and decreased trunk‑leading patterns; for the lower extremity, there was increased in-phase and decreased anti-phase sagittal plane coordination. Increased head stability demands also resulted in an increase in coordination variability for both upper body and lower extremity couplings during the second half of the stance phase. Overall, the results provide evidence that coordinative adaptations to increasing head stability demands occur throughout the entire body: 1) through more independent control of the head relative to the trunk; 2) by increasing in-phase coordination between lower extremity joints, and 3) through increased coordination variability in the second half of stance in both upper body segmental and lower extremity joint couplings. These adaptations likely contribute to the reduction of the range of motion of the trunk in vertical direction.     

Limiting the recruitment of degrees of freedom reduces the stability of perception-action patterns

The goal of the present study was to examine how recruiting/suppressing degrees of freedom affects the (differential) stability of two rhythmic perception-action patterns. In particular, participants synchronized either adduction-on-the-beat or abduction-on-the-beat movements of the right index finger with an auditory metronome. The stability of both patterns as performed in the horizontal plane was predicted to depend on the utilization, or recruitment, of the vertical plane of motion. Movements of the index finger were either free or physically restricted to the horizontal plane. The results showed that in the free condition, the vertical plane was recruited more in the more stable pattern (abduction-on-the-beat) than in the less stable pattern (adduction-on-the-beat). In the constrained condition, abduction-on-the-beat pattern was destabilized, whereas the stability of the adduction-on-the-beat pattern was preserved. These results suggest that the recruitment of the vertical plane of motion in the abduction-on-the-beat movements brought about an increase in the stability of this pattern. More generally, the trade-off between the stability of coordination patterns in the horizontal plane is based on a self-organizing process of recruitment in which neuromuscular factors are an intrinsic aspect.     

Visual apparent motion can be modulated by task-irrelevant lexical information

Previous studies have repeatedly demonstrated the impact of Gestalt structural grouping principles upon the parsing of motion correspondence in ambiguous apparent motion. Here, by embedding Chinese characters in a visual Ternus display that comprised two stimulus frames, we showed that the perception of visual apparent motion can be modulated by activation of task-irrelevant lexical representations. Each frame had two disks, with the second disk of the first frame and the first disk of the second frame being presented at the same location. Observers could perceive either "element motion," in which the endmost disk is seen as moving back and forth while the middle disk at the central position remains stationary, or "group motion," in which both disks appear to move laterally as a whole. More reports of group motion, as opposed to element motion, were obtained when the embedded characters formed two-character compound words than when they formed nonwords, although this lexicality effect appeared to be attenuated by the use of the same characters at the overlapping position across the two frames. Thus, grouping of visual elements in a changing world can be guided by both structural principles and prior world knowledge, including lexical information.     

Age-related differences and the role of augmented visual feedback in learning a bimanual coordination pattern

The purpose of this study was to investigate the effects of aging and the role of augmented visual information in the acquisition of a new bimanual coordination pattern, namely a 90° relative phase pattern. In a pilot study, younger and older adults received augmented visual feedback in the form of a real-time orthogonal display of both limb movements after every fifth trial. Younger adults acquired this task over three days of practice and retained the task well over periods of one week and one month of no practice while the older adults showed no improvement at all on the task. It was hypothesized that the amount of augmented information was not sufficient for the older adults to overcome the strong tendency to perform natural, intrinsically stable coordination patterns, which consequently prevented them from learning the task. The present study evaluated the age-related role of augmented visual feedback for learning the new pattern. Participants were randomly assigned within age groups to receive either concurrent or terminal visual feedback after every trial in acquisition. In contrast to the pilot study, all of the older adults learned the pattern, although not to the same level as the younger adults. Both younger and older adults benefitted from concurrent visual feedback, but the older adults gained more from the concurrent feedback than the younger adults, relative to terminal feedback conditions. The results suggest that when learning bimanual coordination patterns, older adults are more sensitive to the structure of the practice conditions, particularly the availability of concurrent visual information. This greater sensitivity to the learning environment may reflect a diminished capacity for inhibitory control and a decreased ability to focus attention on the salient aspects of learning the task.     

Interactions between intrinsic principles of similarity and proximity and extrinsic principle of common region in visual perception

The interactions between intrinsic grouping principles (by proximity or by similarity in shape or luminance) and extrinsic grouping by common region were examined by presenting both principles acting alone or conjoined with another principle in visual patterns. The procedure used in our study closely mirrored that of Quinlan and Wilton (1998 Perception 27 417-430). However, in the present experiment, the intrinsic and extrinsic grouping principles involved had similar relative salience in order to avoid their possible effects on interactions. The results showed that the grouping effect of conjoined cooperating principles was greater than that of either principle acting alone, and the grouping effect of conjoined competing principles was lower than that of either principle operating alone. Compatibility of data with additive effects of grouping principles has been examined.     

Effect of knee extensors muscles fatigue on bilateral force accuracy,variability, and coordination

The aim of this study was to test the effect of fatigue of the knee extensors muscles on bilateral force control accuracy, variability, and coordination in the presence and absence of visual feedback. Twenty-two young physically active subjects (18 males, 4 females) were divided into two groups and performed 210 submaximal sustained bilateral isometric contractions of knee extensors muscles with and without visual feedback. One group performed a symmetrical task—both legs were set at identical positions (60° knee flexion)—while the other group performed an asymmetrical task (60° and 30° knee flexion). We used the framework of the uncontrolled manifold hypothesis to quantify two variance components: one of them did not change total force (V_UCM), while the other did (V_ORT). Performance of bilateral isometric contractions reduced voluntary and electrically induced force without changes in bilateral force control variability and accuracy. Bilateral force production stability and accuracy were higher in both tasks with visual feedback. Synergistic (anti-phase) structure of force control between the lower limbs occurred and the values of synergy index were higher only during the performance of the asymmetrical task with visual feedback. In addition, greater bilateral force control accuracy was observed during the performance of the asymmetrical task (with and without visual feedback), despite no differences in within-trial variability of both tasks.     

Motion structure in five-dot patterns as a determinant of perceptual grouping

The effect of basic motion structures on perceptual grouping was studied with five-dot motion patterns. Four basic motion structures were identified in terms of proximal common and relative motion vectors. In a forced-choice situation, the observers had to decide to which of two pairs of dots a fifth critical dot seemed to belong. Thus, one of two possible three-dot units was chosen by the observers. The two possible three-dot units defined different motion structures, and the chosen motion structure was considered to have stronger grouping power than the alternative structure had. It was found that parallel common motions (perceived translation in the plane) had the strongest grouping power; these were followed by circular common motions (perceived rotation in the plane), concurrent relative motions (perceived translation in depth), and, finally, parallel relative motions (perceived rotation in depth). The results also suggested effects of proximity and orientation of axis of rotation. It is further argued that the relative grouping power of the motion structures could not solely be interpreted in terms of changes of directions and distances between the dots. Instead it is suggested that vector analysis is a fundamental perceptual activity and that basic motion structures determine grouping power.     

The effects of temporal modulation and spatial location on the perceived spatial frequency of visual patterns

The perceived spatial frequency of a visual pattern can increase when a pattern drifts or is presented at a peripheral visual field location, as compared with a foveally viewed, stationary pattern. We confirmed previously reported effects of motion on foveally viewed patterns and of location on stationary patterns and extended this analysis to the effect of motion on peripherally viewed patterns and the effect of location on drifting patterns. Most central to our investigation was the combined effect of temporal modulation and spatial location on perceived spatial frequency. The group data, as well as the individual sets of data for most observers, are consistent with the mathematical concept of separability for the effects of temporal modulation and spatial location on perceived spatial frequency. Two qualitative psychophysical models suggest explanations for the effects. Both models assume that the receptive-field sizes of a set of underlying psychophysical mechanisms monotonically change as a function of temporal modulation or visual field location, whereas the perceptual labels attached to a set of channels remain invariant. These models predict that drifting or peripheral viewing of a pattern will cause a shift in the perceived spatial frequency of the pattern to a higher apparent spatial frequency.     

The adaptation to sensory information in the production of bimanual movement patterns

Bimanual in-phase and anti-phase patterns were performed in the transverse plane under optimal and degraded proprioceptive conditions, i.e., without and with tendon vibration. Moreover, proprioceptive information was changed midway into each trial to examine on-line reorganization. In addition to the proprioceptive perturbation, the availability of visual information was manipulated to study to which degree sensory information from different modalities interact. Movement patterns performed under identical sensory conditions were compared, i.e., the first 15 s (control) and the 15 s following a change in afferent input (transfer). In the control and transfer conditions, movements with vibrations were less accurate than those without vibrations indicating the influence of optimal proprioceptive information in the calibration and recalibration of intrinsic bimanual movement patterns. Furthermore, pattern stability was affected by the nature of the transfer condition. This indicated that the degree of fluctuations in a sensory transfer situation depended upon the quality of the proprioceptive information experienced in the initial conditions. The influence of visual information was not without importance, although the nature of the coordination mode must be taken into account. In the control conditions, in-phase movements were less stable when vision was absent, whereas anti-phase movements were more stable when vision was not present. This observation was made independent of the available proprioceptive information revealing differences in visual guidance between both coordination modes. In the transfer conditions, pattern stability was similar during the vision and no-vision conditions suggesting a limited influence of visual information in the recalibration process.