Are Stripes Beneficial? Dazzle Camouflage Influences Perceived Speed and Hit Rates

In the animal kingdom, camouflage refers to patterns that help potential prey avoid detection. Mostly camouflage is thought of as helping prey blend in with their background. In contrast, disruptive or dazzle patterns protect moving targets and have been suggested as an evolutionary force in shaping the dorsal patterns of animals. Dazzle patterns, such as stripes and zigzags, are thought to reduce the probability with which moving prey will be captured by impairing predators'' perception of speed. We investigated how different patterns of stripes (longitudinal—i.e., parallel to movement direction–and vertical–i.e., perpendicular to movement direction) affect the probability with which humans can hit moving objects and if differences in hitting probability are caused by a misperception of speed. A first experiment showed that longitudinally striped objects were hit more often than unicolored objects. However, vertically striped objects did not differ from unicolored objects. A second study examining the link between perceived speed and hitting probability showed that longitudinally and vertically striped objects were both perceived as moving faster and were hit more often than unicolored objects. In sum, our results provide evidence that striped patterns disrupt the perception of speed, which in turn influences how often objects are hit. However, the magnitude and the direction of the effects depend on additional factors such as speed and the task setup.     

Tactile localisation: the function of active antennal movements in the crayfishCherax destructor

Video recordings and single frame analysis were used to study the function of the second antennae of crayfish (Cherax destructor) as a sensory system in freely behaving animals. Walking crayfish move their antennae back and forth through horizontal angles of 100 degrees and more, relative to the body long axis. At rest, animals tend to hold their antennae at angular positions between 20 and 50 degrees. Movements of the two antennae are largely independent of each other. Before and during a turn of the body the ipsilateral antenna is moved into the direction of the turn. Solid objects are explored by repeatedly moving the antennae towards and across them. Both seeing and blinded crayfish can locate stationary objects following antennal contact. On antennal contact with a small novel object, a moving animal withdraws its antenna and attacks the object. When the antenna of a blinded crayfish is lightly touched with a brush the animal turns and attacks the point of stimulation. The direction taken and the distance covered during an attack can be correlated with: the angle at which the antenna is held at the moment of contact and the distance along the antennal flagellum at which the stimulus is applied. From behavioural evidence we conclude that crayfish use information about the angular position of their antennae and about the position of stimulated mechanoreceptors along the antennal flagellum to locate objects in their environment. We suggest ways in which an active tactile system-like the crayfish's antennae--could supply animals with information about the three-dimensional layout of their environment.     

Freely flying bees discriminate between stationary and moving objects: performance and possible mechanisms

Summary Freely flying honeybees are innately attracted to moving objects, as revealed by their spontaneous preference for a moving disc over an identical, but stationary disc. We have exploited this spontaneous preference to explore the visual cues by which a bee, which is herself in motion, recognizes a moving object. We find that the moving disc is not detected on the basis that it produces a more rapidly moving image on the retina. The relevant cue might therefore be the motion of the disc relative to the visual surround. We have attempted to test this hypothesis by artificially rotating the structured environment, together with the moving disc, around the bee. Under these conditions, the image of the stationary disc rather than that of the actually moving disc is in motion relative to the surround. We find that rotation of the surround disrupts the bee's capacity not only to distinguish a moving object from a stationary one, but also to discriminate stationary objects at different ranges. Possible interpretations of these results are discussed.     

A simple strategy for detecting moving objects during locomotion revealed by animal-robot interactions

An important role of visual systems is to detect nearby predators, prey, and potential mates, which may be distinguished in part by their motion. When an animal is at rest, an object moving in any direction may easily be detected by motion-sensitive visual circuits. During locomotion, however, this strategy is compromised because the observer must detect a moving object within the pattern of optic flow created by its own motion through the stationary background. However, objects that move creating back-to-front (regressive) motion may be unambiguously distinguished from stationary objects because forward locomotion creates only front-to-back (progressive) optic flow. Thus, moving animals should exhibit an enhanced sensitivity to regressively moving objects. We explicitly tested this hypothesis by constructing a simple fly-sized robot that was programmed to interact with a real fly. Our measurements indicate that whereas walking female flies freeze in response to a regressively moving object, they ignore a progressively moving one. Regressive motion salience also explains observations of behaviors exhibited by pairs of walking flies. Because the assumptions underlying the regressive motion salience hypothesis are general, we suspect that the behavior we have observed in Drosophila may be widespread among eyed, motile organisms.     

Neuronal processing delays are compensated in the sensorimotor branch of the visual system

Moving objects change their position until signals from the photoreceptors arrive in the visual cortex. Nonetheless, motor responses to moving objects are accurate and do not lag behind the real-world position. The questions are how and where neural delays are compensated for. It was suggested that compensation is achieved within the visual system by extrapolating the position of moving objects. A visual illusion supports this idea: when a briefly flashed object is presented in the same position as a moving object, it appears to lag behind. However, moving objects do not appear ahead of their final or reversal points. We investigated a situation where participants localized the final position of a moving stimulus. Visual perception and short-term memory of the final target position were accurate, but reaching movements were directed toward future positions of the target beyond the vanishing point. Our results show that neuronal latencies are not compensated for at early stages of visual processing, but at a late stage when retinotopic information is transformed into egocentric space used for motor responses. The sensorimotor system extrapolates the position of moving targets to allow for precise localization of moving targets despite neuronal latencies.     

Audiovisual integration: an investigation of the "streaming-bouncing" phenomenon

Temporal aspects of the perceptual integration of audiovisual information were investigated by utilizing the visual "streaming-bouncing" phenomenon. When two identical visual objects move towards each other, coincide, and then move away from each other, the objects can either be seen as streaming past one another or bouncing off each other. Although the streaming percept is dominant, the bouncing percept can be induced by presenting an auditory stimulus during the visual coincidence of the moving objects. Here we show that the bounce-inducing effect of the auditory stimulus is paramount when its onset and offset occur in temporal proximity of the onset and offset of the period of visual coincidence of the moving objects. When the duration of the auditory stimulus exceeded this period, visual bouncing disappears. Implications for a temporal window of audiovisual integration and the design of effective audiovisual warning signals are discussed.     

Perceiving spatial relations via attentional tracking and shifting

Perceiving which of a scene's objects are adjacent may require selecting them with a limited-capacity attentional process. Previous results support this notion [1-3] but leave open whether the process operates simultaneously on several objects or proceeds one by one. With arrays of colored discs moving together, we first tested the effect of moving the discs faster than the speed limit for following them with attentional selection [4]. At these high speeds, participants could identify which colors were present and determine whether identical arrays were aligned or offset by one disc. They could not, however, apprehend which colors in the arrays were adjacent, indicating that attentional selection is required for this judgment. If selection operates serially to determine which colors are neighbors, then after the color of one disc is identified, attention must shift to the adjacent disc. As a result of the motion, attention might occasionally miss its target and land on the trailing disc. We cued attention to first select one or the other of a pair of discs and found the pattern of errors predicted. Perceiving these spatial relationships evidently requires selecting and processing objects one by one and is only possible at low object speeds.     

Reliability of cardiotocography in predicting baby's condition at birth.

A prospective study of 6825 labours was undertaken to determine the relation between the Apgar scores of the babies at one minute and the cardiotocograph tracing in labour. The sensitivity of an abnormal tracing was 35.2% for babies who needed intermittent positive pressure ventilation and 20.0% for babies who did not but who had Apgar scores of less than 7. The sensitivity of an abnormal tracing for all babies with an Apgar score of less than 7 was 23.2%. The positive predictive value of an abnormal tracing was 8.7% for babies who needed intermittent positive pressure ventilation and 18.7% for babies who did not but who had an Apgar score of less than 7. The positive predictive value of an abnormal tracing was 27.4% for all babies with an Apgar score of less than 7. The specificity of the tracing was 93.4% for babies with an Apgar score of 7 or over. The relatively high incidence of false positive predictions might be explained on the grounds that abnormalities in the cardiotocograph tracing are a more sensitive indicator of hypoxia than the Apgar score. False negative predictions might have been due to adverse factors other than hypoxia--for example, fetal trauma, compression of the head, infection, and analgesia in labour. These findings suggest that the current overdependence on fetal monitoring by cardiotocography alone should be examined and that other reliable indicators for non-hypoxic fetal distress should be sought.     

Activation in posterior superior temporal sulcus parallels parameter inducing the percept of animacy

An essential, evolutionarily stable feature of brain function is the detection of animate entities, and one of the main cues to identify them is their movement. We developed a model of a simple interaction between two objects, in which an increase of the correlation between their movements varied the amount of interactivity and animacy observers attributed to them. Functional magnetic resonance imaging revealed that activation in the posterior superior temporal sulcus and gyrus (pSTS/pSTG) increased in relation to the degree of correlated motion between the two objects. This activation increase was not different when subjects performed an explicit or implicit task while observing these interacting objects. These data suggest that the pSTS and pSTG play a role in the automatic identification of animate entities, by responding directly to an objective movement characteristic inducing the percept of animacy, such as the amount of interactivity between two moving objects.     

An Empirical Explanation of the Speed-Distance Effect

Understanding motion perception continues to be the subject of much debate, a central challenge being to account for why the speeds and directions seen accord with neither the physical movements of objects nor their projected movements on the retina. Here we investigate the varied perceptions of speed that occur when stimuli moving across the retina traverse different projected distances (the speed-distance effect). By analyzing a database of moving objects projected onto an image plane we show that this phenomenology can be quantitatively accounted for by the frequency of occurrence of image speeds generated by perspective transformation. These results indicate that speed-distance effects are determined empirically from accumulated past experience with the relationship between image speeds and moving objects.     

鲶鱼电感觉中枢对静止和运动偶极子电场的反应

在7尾鲶鱼所记录到的11个电感觉中枢神经元中,6个只对运动的偶极子电场有反应;4个对运动和静止的偶极子电场均有反应,但以前者反应最强;另一个神经元不仅对运动的偶极子电场,而且对运动的塑料棒也有反应。研究表明,鲶鱼的电感应中枢最适合于察觉移动的电场目标。     

The Role of Spatial Configuration in Multiple Identity Tracking

Background

The simultaneous tracking and identification of multiple moving objects encountered in everyday life requires one to correctly bind identities to objects. In the present study, we investigated the role of spatial configuration made by multiple targets when observers are asked to track multiple moving objects with distinct identities.

Methodology/Principal Findings

The overall spatial configuration made by the targets was manipulated: In the constant condition, the configuration remained as a virtual convex polygon throughout the tracking, and in the collapsed condition, one of the moving targets (critical target) crossed over an edge of the virtual polygon during tracking, destroying it. Identification performance was higher when the configuration remained intact than when it collapsed (Experiments 1a, 1b, and 2). Moreover, destroying the configuration affected the allocation of dynamic attention: the critical target captured more attention than did the other targets. However, observers were worse at identifying the critical target and were more likely to confuse it with the targets that formed the virtual crossed edge (Experiments 3–5). Experiment 6 further showed that the visual system constructs an overall configuration only by using the targets (and not the distractors); identification performance was not affected by whether the distractor violated the spatial configuration.

Conclusions/Significance

In sum, these results suggest that the visual system may integrate targets (but not distractors) into a spatial configuration during multiple identity tracking, which affects the distribution of dynamic attention and the updating of identity-location binding.     

Barotrauma is a significant cause of bat fatalities at wind turbines

Bird fatalities at some wind energy facilities around the world have been documented for decades, but the issue of bat fatalities at such facilities — primarily involving migratory species during autumn migration — has been raised relatively recently [1] and [2]. Given that echolocating bats detect moving objects better than stationary ones [3], their relatively high fatality rate is perplexing, and numerous explanations have been proposed [1]. The decompression hypothesis proposes that bats are killed by barotrauma caused by rapid air-pressure reduction near moving turbine blades [1], [4] and [5]. Barotrauma involves tissue damage to air-containing structures caused by rapid or excessive pressure change; pulmonary barotrauma is lung damage due to expansion of air in the lungs that is not accommodated by exhalation. We report here the first evidence that barotrauma is the cause of death in a high proportion of bats found at wind energy facilities. We found that 90% of bat fatalities involved internal haemorrhaging consistent with barotrauma, and that direct contact with turbine blades only accounted for about half of the fatalities. Air pressure change at turbine blades is an undetectable hazard and helps explain high bat fatality rates. We suggest that one reason why there are fewer bird than bat fatalities is that the unique respiratory anatomy of birds is less susceptible to barotrauma than that of mammals.     

Efficiency of the Human Visual System in Recognition of Moving Objects

The relationships studied between the efficiency of the human visual system for recognition of moving objects in the presence of dynamic additive quasi-white Gaussian noise and the duration of object presentation are studied. Different conditions of observation and the parameters of moving objects were analyzed. The efficiency-presentation duration relationship was shown to be a function with two extrema (a minimum and a maximum, at 120 and 400 ms, respectively). The study offers an interpretation of the findings and a functional model to explain this phenomenon.     

Cost-effective control of chronic viral diseases: Finding the optimal level of screening and contact tracing

Chronic viral diseases such as human immunodeficiency virus (HIV) and hepatitis B virus (HBV) afflict millions of people worldwide. A key public health challenge in managing such diseases is identifying infected, asymptomatic individuals so that they can receive antiviral treatment. Such treatment can benefit both the treated individual (by improving quality and length of life) and the population as a whole (through reduced transmission). We develop a compartmental model of a chronic, treatable infectious disease and use it to evaluate the cost and effectiveness of different levels of screening and contact tracing. We show that: (1) the optimal strategy is to get infected individuals into treatment at the maximal rate until the incremental health benefits balance the incremental cost of controlling the disease; (2) as one reduces the disease prevalence by moving people into treatment (which decreases the chance that they will infect others), one should increase the level of contact tracing to compensate for the decreased effectiveness of screening; (3) as the disease becomes less prevalent, it is optimal to spend more per case identified; and (4) the relative mix of screening and contact tracing at any level of disease prevalence is such that the marginal efficiency of contact tracing (cost per infected person found) equals that of screening if possible (e.g., when capacity limitations are not binding). We also show how to determine the cost-effective equilibrium level of disease prevalence (among untreated individuals), and we develop an approximation of the path of the optimal prevalence over time. Using this, one can obtain a close approximation of the optimal solution without having to solve an optimal control problem. We apply our methods to an example of hepatitis B virus.     

Behavioral discrimination of water motions caused by moving objects.

We tested whether goldfish, Carassius auratus, discriminate hydrodynamic stimuli caused by moving objects. Blindfolded goldfish responded to a passing object with changes in inter-gill-movement intervals. To learn whether goldfish can discriminate water motions caused by different moving objects we habituated them to a certain object stimulus. If the stimulus was altered, e.g., by altering speed, direction of motion, or size or shape of the object, fish again showed a temporary suspension of breathing when the object passed by. If animals failed to respond to an altered stimulus, we paired this stimulus with a weak electric shock during training. Goldfish discriminated object motion direction. In addition, in two choice experiments goldfish discriminated water motions caused by objects which moved with different speeds (e.g., 5 cm s(-1) versus 6 cm s(-1)), or by objects which differed in size (e.g., 1 cm x 1 cm versus 1.4 cm x 1.4 cm cross section), or shape (e.g., a round versus a triangular object). If object size and/or shape was varied quasi-randomly such that the faster moving object not always caused the greatest water velocities, fish still discriminated object speed.     

Responses of medulla neurons to illumination and movement stimuli in the tiger beetle larvae

Intracellular responses of medulla neurons (second-order visual interneurons) have been examined in the tiger beetle larva. The larva possesses six stemmata on either side of the head, two of which are much larger than the remaining four. Beneath the cuticle housing the stemmata an optic neuropil complex occurs consisting of lamina and medulla neuropils. Response patterns of medulla neurons to illumination and moving objects varied from neurons to neurons. For movement stimuli black discs and a black bar were moved in the rostro-caudal direction above the larva. Comparison of responses to the discs and the bar suggested a spatial summation of responses in some neurons, and tuning to small objects in some neurons. The majority of neurons responded to objects moving at heights of 10 mm and 50 mm with the same discharge pattern. A few neurons, however, showed distance sensitivities responding with an increase of spike discharges to moving objects only at either of the two heights. Such distance sensitivities still remained in one-stemma larvae, three of the four stemmata being occluded. These data are discussed in relation to distinct visual behavior of the larva and with special reference to perception of the hunting range.     

Visual processing of moving single objects and wide-field patterns in flies: Behavioural analysis after laser-surgical removal of interneurons

A laser micro-beam unit was used to reproducibly and selectively eliminate the large horizontal and vertical motion sensitive neurons (H- and V-cells) of the lobula plate on one side of the brain of house fliesMusca domestica. This was achieved by ablating the precursors of these cells deep in the larval brain without damaging other cells in the brain or other tissues. The individually reared flies were tested for their behaviour. Three tests were performed: (i) visual fixation of a single stripe, (ii) the optomotor turning and thrust response to a stripe moving clockwise and counterclockwise around the fly, (iii) the monocular turning response to a moving grating. The responses to a moving single object were normal on both sides, the control side and the one lacking the H- and V-cells. However, the responses to a moving grating were reduced on the side lacking H- and V-cells for progressive (front to back) and regressive (back to front) motion. From this we conclude that the response to single objects is controlled mainly by cells other than the H- and V-cells. We also suggest two separate pathways for the processing of single object motion and wide field pattern motion respectively (Fig. 8). Furthermore, the H- and V-cells might function as visual stabilizers and background motion processors.     

Impacts du mouvement sur le flux d'air dans une enceinte ventilée type bloc opératoire

The risk of nosocomial infection during surgical operations is one of the main reasons leading to the development of powerful techniques of ventilation within the framework of the operating theatre. During the two last decades, computational fluids dynamics (CFD) have being rapidly expanding. Nowadays it is becoming a tool usually requested to predict the airflow into furnished enclosures. However, CFD is seldom used to study the effects of moving objects and/or operators on the airflow. The effect of a person motion is described in this paper: a nurse is moving under an unidirectionnal flow into an operation room occuped by a patient and a surgical staff.