Neural extrapolation of motion for a ball rolling down an inclined plane

It is known that humans tend to misjudge the kinematics of a target rolling down an inclined plane. Because visuomotor responses are often more accurate and less prone to perceptual illusions than cognitive judgments, we asked the question of how rolling motion is extrapolated for manual interception or drawing tasks. In three experiments a ball rolled down an incline with kinematics that differed as a function of the starting position (4 different positions) and slope (30°, 45° or 60°). In Experiment 1, participants had to punch the ball as it fell off the incline. In Experiment 2, the ball rolled down the incline but was stopped at the end; participants were asked to imagine that the ball kept moving and to punch it. In Experiment 3, the ball rolled down the incline and was stopped at the end; participants were asked to draw with the hand in air the trajectory that would be described by the ball if it kept moving. We found that performance was most accurate when motion of the ball was visible until interception and haptic feedback of hand-ball contact was available (Experiment 1). However, even when participants punched an imaginary moving ball (Experiment 2) or drew in air the imaginary trajectory (Experiment 3), they were able to extrapolate to some extent global aspects of the target motion, including its path, speed and arrival time. We argue that the path and kinematics of a ball rolling down an incline can be extrapolated surprisingly well by the brain using both visual information and internal models of target motion

Coherence of structural visual cues and pictorial gravity paves the way for interceptive actions

Dealing with upside-down objects is difficult and takes time. Among the cues that are critical for defining object orientation, the visible influence of gravity on the object's motion has received limited attention. Here, we manipulated the alignment of visible gravity and structural visual cues between each other and relative to the orientation of the observer and physical gravity. Participants pressed a button triggering a hitter to intercept a target accelerated by a virtual gravity. A factorial design assessed the effects of scene orientation (normal or inverted) and target gravity (normal or inverted). We found that interception was significantly more successful when scene direction was concordant with target gravity direction, irrespective of whether both were upright or inverted. This was so independent of the hitter type and when performance feedback to the participants was either available (Experiment 1) or unavailable (Experiment 2). These results show that the combined influence of visible gravity and structural visual cues can outweigh both physical gravity and viewer-centered cues, leading to rely instead on the congruence of the apparent physical forces acting on people and objects in the scene

Perceptual judgments of duration of parabolic motions

In a 2-alternative forced-choice protocol, observers judged the duration of ball motions shown on an immersive virtual-reality display as approaching in the sagittal plane along parabolic trajectories compatible with Earth gravity effects. In different trials, the ball shifted along the parabolas with one of three different laws of motion: constant tangential velocity, constant vertical velocity, or gravitational acceleration. Only the latter motion was fully consistent with Newton's laws in the Earth gravitational field, whereas the motions with constant velocity profiles obeyed the spatio-temporal constraint of parabolic paths dictated by gravity but violated the kinematic constraints. We found that the discrimination of duration was accurate and precise for all types of motions, but the discrimination for the trajectories at constant tangential velocity was slightly but significantly more precise than that for the trajectories at gravitational acceleration or constant vertical velocity. The results are compatible with a heuristic internal representation of gravity effects that can be engaged when viewing projectiles shifting along parabolic paths compatible with Earth gravity, irrespective of the specific kinematics. Opportunistic use of a moving frame attached to the target may favour visual tracking of targets with constant tangential velocity, accounting for the slightly superior duration discrimination

Hand interception of occluded motion in humans: A test of model-based versus on-line control

Two control schemes have been hypothesized for the manual interception of fast visual targets. In the model-free on-line control, extrapolation of target motion is based on continuous visual information, without resorting to physical models. In the model-based control, instead, a prior model of target motion predicts the future spatio-temporal trajectory. To distinguish between the two hypotheses in the case of projectile motion, we asked participants to hit a ball that rolled down an incline at 0.2g, and then fell in air at 1g along a parabola. By varying starting position, ball velocity and trajectory differed between trials. Motion on the incline was always visible, while parabolic motion was either visible or occluded. We found that participants were equally successful at hitting the falling ball in both visible and occluded conditions. Moreover, in different trials the intersection points were distributed along the parabolic trajectories of the ball, indicating that subjects were able to extrapolate an extended segment of the target trajectory. Remarkably, this trend was observed even at the very first repetition of movements. These results are consistent with the hypothesis of model-based control, but not with on-line control. Indeed, ball path and speed during the occlusion could not be extrapolated solely from the kinematic information obtained during the preceding visible phase. The only way to extrapolate ball motion correctly during the occlusion was to assume that the ball would fall under gravity and air drag when hidden from view. Such an assumption had to be derived from prior experience

Observing human movements helps decoding environmental forces

Vision of human actions can affect several features of visual motion processing, as well as the motor responses of the observer. Here, we tested the hypothesis that action observation helps decoding environmental forces during the interception of a decelerating target within a brief time window, a task intrinsically very difficult. We employed a factorial design to evaluate the effects of scene orientation (normal or inverted) and target gravity (normal or inverted). Button-press triggered the motion of a bullet, a piston, or a human arm. We found that the timing errors were smaller for upright scenes irrespective of gravity direction in the Bullet group, while the errors were smaller for the standard condition of normal scene and gravity in the Piston group. In the Arm group, instead, performance was better when the directions of scene and target gravity were concordant, irrespective of whether both were upright or inverted. These results suggest that the default viewer-centered reference frame is used with inanimate scenes, such as those of the Bullet and Piston protocols. Instead, the presence of biological movements in animate scenes (as in the Arm protocol) may help processing target kinematics under the ecological conditions of coherence between scene and target gravity directions

Body orientation contributes to modelling the effects of gravity for target interception in humans

It is known that interception of targets accelerated by gravity involves internal models coupled with visual signals. Non-visual signals related to head and body orientation relative to gravity may also contribute, although their role is poorly understood. In a novel experiment, we asked pitched observers to hit a virtual target approaching with an acceleration that was either coherent or incoherent with their pitch-tilt. Initially, the timing errors were large and independent of the coherence between target acceleration and observer's pitch. With practice, however, the timing errors became substantially smaller in the coherent conditions. The results show that information about head and body orientation can contribute to modelling the effects of gravity on a moving target. Orientation cues from vestibular and somatosensory signals might be integrated with visual signals in the vestibular cortex, where the internal model of gravity is assumed to be encoded

Identification and Updating of AB 204 Helicopter Blade F.E. Model by Means of Static and Dynamic Tests

In this paper, the structural properties of the AB 204 helicopter rotor blade, whose characteristics are completely unavailable, have been identified through both static and dynamic tests, and by a further structural updating of the corresponding Finite Element numerical model. Under the hypothesis that the dynamic behavior of the blade is well described by a beam representation, this "reverse engineering" analysis estimates first the stiffness distribution, along the blade span, by measuring the displacements of the structure when known static loads are applied. Then, the mass distribution is further estimated using the modal parameters achieved from experimental tests. The 1-D finite element model, obtained using these - experimentally evaluated - elastic and mass characteristics, has been further improved by an updating procedure based on the sensitivity of frequency-response-based correlation functions to changes in the design parameters adopted for the updating process

Motion direction, luminance contrast, and speed perception: An unexpected meeting

Motion direction and luminance contrast are two central features in the representation of visual motion in humans. In five psychophysical experiments, we showed that these two features affect the perceived speed of a visual stimulus. Our data showed a surprising interaction between contrast and direction. Participants perceived downward moving stimuli as faster than upward or rightward stimuli, but only at high contrast. Likewise, luminance contrast produced an underestimation of motion speed, but mostly when the stimuli moved downward. We explained these novel phenomena by means of a theoretical model, accounting for prior knowledge of motion dynamics

InclinedPlaneExperimentalData

<p>Experimental Data of paper:</p> <p>La Scaleia B, Lacquaniti F, Zago M (2014) Neural Extrapolation of Motion for a Ball Rolling Down an Inclined Plane. PLoS ONE 9(6): e99837. doi:10.1371/<br>journal.pone.0099837</p