Visual Constraints Research Articles

DEVELOPMENTAL CHANGES IN THE DYNAMICAL STRUCTURE OF POSTURAL SWAY DURING A PRECISION FITTING TASK

Recent research using measures to assess the time-dependent structure of postural fluctuations has provided new insights into the stability and adaptability of human postural control in adults. To date, little research has examined how postural dynamics reflecting the stability and adaptability of postural control may change as a function of development, especially during supra-postural tasks. The goal of this study was to examine the dynamics of postural fluctuations during a manual-fitting task in which precision, visual and postural task constraints were altered in children and adults. Three age groups were tested: 7-, 10-year olds and college aged adults. Recurrence quantification analysis (RQA) was used to assess the regularity (percent determinism) and complexity (entropy) of the center of pressure (CoP) in the anterior–posterior (AP) and medial-lateral (ML) directions. The CoP patterns exhibited by adults were more deterministic and more complex (higher entropy) than those of the 7-year-old children under the different experimental manipulations. No differences between the adults and the 10-year-old children were observed. The increase in determinism with a corresponding increase in entropy exhibited by the adults and older-children during a manual fitting task may be a prospective mechanism over which postural movements follow a more predictable path allowing for stable and flexible task performance. Our results also support the notion that complex postural fluctuations (as measured by RQA entropy) are functional and typically increase as the precision requirements of a manual task increase.

Divergent foraging strategies of three co-occurring north Pacific flatfishes

Despite facing similar constraints imposed by the environment, significant variation in life history traits frequently exists among species generally considered to comprise a single ecological guild. For juvenile flatfishes, constraints on foraging activity include variation in light and prey availability, as well as predation risk. This paper describes the visual constraints on, and divergent foraging strategies of three co-occurring north Pacific flatfish species, northern rock sole (Lepidopsetta polyxystra), Pacific halibut (Hippoglossus stenolepis), and English sole (Pleuronectes vetulus). Visual foraging abilities measured in the laboratory decreased rapidly below 10−4 μmol photons·m−2 s−1, and were similar among species. Despite similar sensory constraints, field sampling in August 2004 at a Kodiak Island nursery site (Holiday Beach, 57o41.2′ N, 152o27.7′ W) identified species differences in diets, diel foraging patterns, and within-nursery depth distributions. Northern rock sole and English sole fed primarily on bivalve siphons and polychaetes, whereas mysids dominated the diets of Pacific halibut. Northern rock sole were geographically the most widespread but feeding activity was temporally restricted to the dusk period. Pacific halibut were rare in shallow depths (<5 m) and fed most intensively prior to dusk. English sole fed throughout the daylight hours and were abundant only in the shallowest (<5 m) habitats. These differences in diets, foraging times, and habitat use appear related to previously documented species-specific behavioral characteristics as well as general spatial (increasing with depth) and temporal (increasing during foraging activity) variations in predation risk. At one extreme, the conservative behavioral strategy of northern rock sole may permit use of a broader range of foraging habitats, whereas English sole may be restricted to shallow water by limited behavioral responses to predation threat. These observations demonstrate that the appearance of habitat partitioning is not due to differences in sensory ability, but reflects multi-faceted, species-specific responses to the ecological tradeoffs between foraging and predation risks.

Object Localization in Blurred and Jumbled Scenes: Differences between Myopic and Emmetropic Observers

To compare the effects of transient blur constraints on the perception of natural scenes in myopic and emmetropic observers. When natural images are blurred, the global structure of the scene-its context-becomes essential for decision making. The authors also aimed to control whether the difference in performance between myopes and emmetropes, if any, resulted from the ability to use contextual information or the ability to process blurred features. Twenty-four subjects (13 emmetropes, 11 myopes) participated in the study. Subjects were instructed to fixate a target in a scene. Low-pass-filtered images were briefly displayed (100 ms) with normal or jumbled structures. The experiment was repeated three times to assess the ability of subjects to adapt to the imposed blur constraints. Results showed that myopes performed better than emmetropes only when scene context was available. Myopes also adapted faster to the jumbling constraint. However, after three repetitions, both groups reached the same level of performance. The present study demonstrated that myopes did not perform better than emmetropes in a transiently blurred environment, but they exhibited better and faster ability to adapt their perceptual strategies to the visual constraints by learning to make use of the relevant information available in the images displayed.

Influence of visual constraints in the trajectory formation of grasping movements

The main objective of the present study is to show that the visual context can influence the trajectory formation of grasping movements. We asked participants to reach and grasp a cylinder disposed at three different positions: −20°, 0° and 20° of eccentricity with respect to the midsagittal axis. Grasping movements were performed in a direct and in an indirect visual feedback condition (i.e., controlled through a vertical video display). Results revealed that for grasping movements directed toward objects located at −20° and 0°, path curvatures of the wrist, the thumb and the index finger were significantly straighter in the indirect visual feedback condition. However, no significant difference concerning hand path curvature was observed when the movement was directed toward the object located at 20°. This suggests that grasping movements controlled through a remote visual feedback tend to be planned in extrinsic space and that the effect of the visual context on movement planning appears to be not isotropic over the workspace.

Task and Effector Constraints on the Kinematics of Pointing Movements

Recent work has emphasized the distinction between task and effector constraints underlying performance in Fitts' type discrete pointing tasks. We explored the relative contributions of these constraints in a cursor-pointing task by manipulating the control-display scale, thereby dissociating movement scale at the level of the hand from movement scale at the level of the cursor. Using linear regressions to predict movement time, we found that effector constraints best predict the primary transport phase of the movement, bringing the hand near the target. Visual task constraints underlie the secondary target acquisition phase of the movement. We present a reformulation of Fitts' (1954) index of difficulty, capturing the relative contributions of effector and visual task constraints.

Visual constraints in written word recognition: evidence from the optimal viewing‐position effect

In this paper we review the literature on visual constraints in written word processing. We notice that not all letters are equally visible to the reader. The letter that is most visible is the letter that is fixated. The visibility of the other letters depends on the distance between the letters and the fixation location, whether the letters are outer or inner letters of the word, and whether the letters lie to the left or to the right of the fixation location. Because of these three factors, word recognition depends on the viewing position. In languages read from left to right, the optimal viewing position is situated between the beginning and the middle of the word. This optimal viewing position is the result of an interplay of four variables: the distance between the viewing position and the farthest letter, the fact that the word beginning is usually more informative than the word end, the fact that during reading words have been recognised a lot of times after fixation on this letter position and the fact that stimuli in the right visual field have direct access to the left cerebral hemisphere. For languages read from right to left, the first three variables pull the optimal viewing position towards the right side of the word (which is the word beginning), but the fourth variable counteracts these forces to some extent. Therefore, the asymmetry of the optimum viewing‐position curve is less clear in Hebrew and Arabic than in French and Dutch.

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.

Visual and motor constraints on trajectory planning in pointing movements

The aim of the present study was to show that planning and controlling the trajectory of a pointing movement is influenced not solely by physical constraints but also by visual constraints. Subjects were required to point towards different targets located at 20°, 40°, 60° and 80° of eccentricity. Movements were either constrained (i.e. two-dimensional movements) or unconstrained (i.e. three-dimensional movements). Furthermore, movements were carried out either under a direct or a remote visual control (use of a video system). Results revealed that trajectories of constrained movements were nearly straight whatever the eccentricity of the target and the type of visual control. A different pattern was revealed for unconstrained movements. Indeed, under direct vision the trajectory curvature increased as the eccentricity augmented, whereas under indirect vision, trajectories remained nearly straight whatever the eccentricity of the target. Thus, movements controlled through a remote visual feedback appear to be planned in extrinsic space as constrained movements.

The Information Resource Matrix

SUMMARY Librarians are often hampered by the limitations of online catalog systems because of a lack of flexibility in how they present relationships among serial information objects and their bibliographic surrogates. Therefore librarians have to look outside the box and consider other methods for enhancing bibliographic clarity and access. They also have to be forward thinking and develop an information environment that is less encumbered by machine-imposed strictures such as linearity, polarization of viewpoint and visual constraint.

Robust Trajectory Planning for Target Following Vision-based Autonomous Land Vehicle Guidance

We propose a robust trajectory planning method for vision-based autonomous land vehicle (ALV) guidance by target following using a visual field model. The proposed trajectory planning method aims to guide the ALV to achieve the goal that the targeted person will always appear in the image. It is found in this study that the goal can be achieved if three visual contact constraints in the visual field model are satisfied. The first constraint postulates that the target has appeared in the image. The second constraint asks that the vehicle head at the next position points straight towards to the target's current position. The third constraint sets bounds between the next position of the ALV and the current position of the target. A formula for the trajectory of the vehicle that satisfies the second visual contact constraint is derived. The steps for generating a speed and a turn angle of the ALV for real-time navigation are described as a trajectory-planning algorithm. Finally, the approach is tested on a real ALV and successful navigation sessions confirm the feasibility and the robustness of the approach.

Visual and Postural Constraints in Coordinated Movements of the Head in Hand Reaching Tasks

The purpose of the present study is to investigate movements of the head spatially and temporally coordinated with hand reach movements simulating industrial assembly tasks. The motions recorded from thirty subjects performing reach movements with the right hand toward eccentric targets indicate that 1) hand movement onset lags head movement onset with a duration proportional to target eccentricity; 2) the head does not aim directly at a target, but travels only a fraction of target eccentricity and often deviates away from the target substantially; and 3) head movements are constrained by the strategy of either controlling the head position in space or controlling head rotation about the torso. These results indicate that head movements are constrained by both visual and non-visual factors. While the major function of the head is to displace the visual gaze toward the target, non-visual constraints, which include postural coordination with whole body movements, also significantly affect head movements.

Seeing more than meets the eye: processing of illusory contours in animals.

This review article illustrates that mammals, birds and insects are able to perceive illusory contours. Illusory contours lack a physical counterpart, but monkeys, cats, owls and bees perceive them as if they were real borders. In all of these species, a neural correlate for such perceptual completion phenomena has been described. The robustness of neuronal responses and the abundance of cells argue that such neurons might indeed represent a neural correlate for illusory contour perception. The internal state of an animal subject (i.e., alert and behaving) seems to be an important factor when correlating neural activity with perceptual phenomena. The fact that the neural network necessary for illusory contour perception has been found in relatively early visual brain areas in all tested animals suggests that bottom-up processing is largely sufficient to explain such perceptual abilities. However, recent findings in monkeys indicate that feedback loops within the visual system may provide additional modulation. The detection of illusory contours by independently evolved visual systems argues that processing of edges in the absence of contrast gradients reflects fundamental visual constraints and not just an artifact of visual processing.

When insight just won't come: the failure of visual cues in the nine-dot problem.

The nine-dot problem is a classic in the field of human problem solving. Cognitive accounts of the problem's difficulty have been criticized on the grounds that the experimental methods on which they rely for support involve a qualitative change to the task requirements of the problem. The three experiments reported here utilize visual and visual-procedural hints to examine the notion that its difficulty is rooted in a mismatch between problem shape and solution shape. Experiment 1 demonstrated that a perceptual cue to the shape of the solution in the form of shading gave rise to only minimal improvements in performance; an additional hint about the relevance of the shading gave rise to modest, but not statistically significant, improvements. Experiment 2 replicated these findings against an additional control condition in which a solely verbal hint to violate the perceptual boundary of the problem shape was given. Furthermore, when both the verbal and visual hints were provided, performance improved only slightly. Experiment 3 provided participants with experience in producing the shape of the correct solution in problem variants closely related to the nine-dot problem. Performance on the transfer task, the basic nine-dot problem, remained at floor, however. These data suggest that visual constraint relaxation is unlikely to be the sole process by which the insight required to find a solution is achieved. The results are interpreted in terms of a previously proposed computational model of performance.

Building Visual Vocabulary for Image Indexation and Query Formulation

With advances in digital imaging, the amount of digital images will increase tremendously. To locate relevant images in a large collection of images presents a challenging and genuine problem for content-based retrieval research. This paper presents a novel framework called visual keywords for image indexation and query formulation. Visual keywords are flexible and intuitive visual prototypes specified perceptually from sample domain images. A visual content is described and indexed by flexible spatial aggregation of the soft presence of visual keywords. A new query method based on visual constraints is also proposed to allow direct and explicit content specification. Last but not least, we have developed a digital album prototype to demonstrate query and retrieval on both home photos and stock photos based on visual keywords.

Visual constraints in foraging bumblebees: flower size and color affect search time and flight behavior.

In optimal foraging theory, search time is a key variable defining the value of a prey type. But the sensory-perceptual processes that constrain the search for food have rarely been considered. Here we evaluate the flight behavior of bumblebees (Bombus terrestris) searching for artificial flowers of various sizes and colors. When flowers were large, search times correlated well with the color contrast of the targets with their green foliage-type background, as predicted by a model of color opponent coding using inputs from the bees' UV, blue, and green receptors. Targets that made poor color contrast with their backdrop, such as white, UV-reflecting ones, or red flowers, took longest to detect, even though brightness contrast with the background was pronounced. When searching for small targets, bees changed their strategy in several ways. They flew significantly slower and closer to the ground, so increasing the minimum detectable area subtended by an object on the ground. In addition, they used a different neuronal channel for flower detection. Instead of color contrast, they used only the green receptor signal for detection. We relate these findings to temporal and spatial limitations of different neuronal channels involved in stimulus detection and recognition. Thus, foraging speed may not be limited only by factors such as prey density, flight energetics, and scramble competition. Our results show that understanding the behavioral ecology of foraging can substantially gain from knowledge about mechanisms of visual information processing.

Manipulating Visual Informational Constraints during Practice Enhances the Acquisition of Catching Skill in Children

Previous motor learning studies examining the effects of practicing to catch one-handed under varying informational constraints on subsequent skill acquisition are equivocal, perhaps due to the use of relatively inexperienced adult participants. Ecological theory predicts that directing the learner's search for information in the perceptual-motor workspace can enhance skill acquisition. This study manipulated visual informational constraints on novice children (ages 9–10 years) learning to catch one-handed. A crossover transfer design was implemented in which one group acted as controls while two other groups practiced either without visual restrictions before transferring to full vision, or vice versa. The data indicated that learners forced to seek additional information sources under restricted viewing conditions demonstrated a greater positive, accumulative residual effect on acquiring a catching skill. The findings contradict current work on the specificity of practice hypothesis and suggest that varying visual informational constraints to encourage exploratory practice may represent a significant pedagogical approach to motor learning in sport.

Convergence of specialised behaviour, eye movements and visual optics in the sandlance (Teleostei) and the chameleon (Reptilia)

Chameleons have a number of unusual, highly specialised visual features, including telescopic visual optics with a reduced lens power, wide separation of the eye’s nodal point from the axis of rotation, a deep-pit fovea, rapid pre-calculated strikes for prey based on monocular depth judgements (including focus), and a complex pattern of partially independent alternating eye movements. The same set of features has been acquired independently by a teleost, the sandlance Limnichthyes fasciatus. Despite its underwater lifestyle, this fish displays visual behaviour and rapid strikes for prey that are remarkably similar to those of the chameleon [1]. In a direct comparison of the two species, we have revealed other, previously unsuspected, similarities, such as corneal accommodation, which was unknown in teleosts, as well as bringing together, for the first time, data collected from both species. The sandlance is the only teleost, among thousands studied, that has corneal refraction, corneal accommodation and reduced lens power, as well as sharing the other specialised optical features seen in chameleons. The independent eye movement pattern in the sandlance is also unusual and similar to that of the chameleon. The selection pressures that have produced this remarkable example of convergence may relate to common visual constraints in the life styles of these two phylogenetically disparate species.

A Sophisticated Manipulation Aid in a Virtual Environment using Dynamic Constraints among Object Faces

A natural and intuitive method is proposed to help a user manipulate an object in a virtual environment. The method does not need to assign special properties to the object faces in advance and does not require special hardware. Instead, it uses only the visual constraints of motion among object faces that are dynamically selected by a real-time collision detection method while the user manipulates the object. By constraining more than two faces during the user's manipulation, the proposed method provides an efficient tool for complicated manipulation tasks. First, the method of manipulation aid is described. Then several experiments demonstrate the effectiveness of this method, particularly when the user is requested to precisely place a virtual object in a certain location. Finally, as an application of the proposed manipulation aid, an experiment is conducted to compare the performances of a task (constructing a simple toy) in a real versus a virtual environment. Results show that the distance accuracy and completion time of the virtual task with the manipulation aid is close to that of the real task.

Automatic grasp planning for visual-servo controlled robotic manipulators

Vision based grasping holds great promise for grasping in dynamic environments where the object and/or robot are moving. The paper introduces a grasp planning approach for visual servo controlled robots with a single camera mounted at the end effector. Sensory control, mechanical, and geometrical issues in the design of such an automatic grasp planner are discussed and the corresponding constraints are highlighted. In particular, the integration of visual feature selection constraints is emphasized. Some quality measures are introduced to rate the candidate grasps. The grasp planning strategy and implementation issues in the development of an automatic grasp planner (AGP) are described. Simulation and experimental results are presented to verify the correctness of the approach and its effectiveness in dealing with dynamic situations.

Field study of subjective assessment of negative pressure half-masks. Influence of the work conditions on comfort and efficiency

The aim of this study was to assess the effects of work conditions on the acceptability and efficiency of respiratory protective devices (RPD). The subjective evaluation of comfort, protection, respiratory and visual constraint, and the acceptable duration of wear of six RPDs against dust was achieved by 30 workers during their actual work. Metabolic rate was evaluated for each worker, and dry and wet air temperatures measured in the work area. RPDs objective protection factor was measured during each of the 180 test periods. In the conditions of this study, the acceptable duration of wear was about 1 h. This duration and the comfort parameters were reduced when the air temperature increased. The younger workers and/or smokers were less sensitive to mask discomfort. Objective protection factors of the RPDs are reduced under wanner conditions, and when the metabolic rate is low. Finally, the results of this study also show the poor capacity of standardized leakage tests to assess the objective respiratory protection of workers in the field. Some hypotheses which can explain this fact are discussed.