Biological movements look uniform: evidence of motor-perceptual interactions

Human-likeness and attribution of intentionality predict vicarious sense of agency over humanoid robot actions

Sense of Agency (SoA) is the feeling of being in control of one's actions and their outcomes. In a social context, people can experience a "vicarious" SoA over another human's actions; however, it is still controversial whether the same occurs in Human-Robot Interaction (HRI). The present study aimed at understanding whether humanoid robots may elicit vicarious SoA in humans, and whether the emergence of this phenomenon depends on the attribution of intentionality towards robots. We asked adult participants to perform an Intentional Binding (IB) task alone and with the humanoid iCub robot, reporting the time of occurrence of both self- and iCub-generated actions. Before the experiment, participants' degree of attribution of intentionality towards robots was assessed. Results showed that participants experienced vicarious SoA over iCub-generated actions. Moreover, intentionality attribution positively predicted the magnitude of vicarious SoA. In conclusion, our results highlight the importance of factors such as human-likeness and attribution of intentionality for the emergence of vicarious SoA towards robots.

If You Are Old, Videos Look Slow. The Paradoxical Effect of Age-Related Motor Decline on the Kinematic Interpretation of Visual Scenes

Perception and action are tightly coupled. However, there is still little recognition of how individual motor constraints impact perception in everyday life. Here we asked whether and how the motor slowing that accompanies aging influences the sense of visual speed. Ninety-four participants aged between 18 and 90 judged the natural speed of video clips reproducing real human or physical motion (SoS, Sense-of-Speed adjustment task). They also performed a finger tapping task and a visual search task, which estimated their motor speed and visuospatial attention speed, respectively. Remarkably, aged people judged videos to be too slow (speed underestimation), as compared to younger people: the Point of Subjective Equality (PSE), which estimated the speed bias in the SoS task, was +4% in young adults (<40), +12% in old adults (40–70) and +16% in elders. On average, PSE increased with age at a rate of 0.2% per year, with perceptual precision, adjustment rate, and completion time progressively worsening. Crucially, low motor speed, but not low attentional speed, turned out to be the key predictor of video speed underestimation. These findings suggest the existence of a counterintuitive compensatory coupling between action and perception in judging dynamic scenes, an effect that becomes particularly germane during aging.

Modulation of ellipses drawing by sonification

Most studies on the regulation of speed and trajectory during ellipse drawing have used visual feedback. We used online auditory feedback (sonification) to induce implicit movement changes independently from vision. The sound was produced by filtering a pink noise with a band-pass filter proportional to movement speed. The first experiment was performed in 2D. Healthy participants were asked to repetitively draw ellipses during 45 s trials whilst maintaining a constant sonification pattern (involving pitch variations during the cycle). Perturbations were produced by modifying the slope of the mapping without informing the participants. All participants adapted spontaneously their speed: they went faster if the slope decreased and slower if it increased. Higher velocities were achieved by increasing both the frequency of the movements and the perimeter of the ellipses, but slower velocities were achieved mainly by decreasing the perimeter of the ellipses. The shape and the orientation of the ellipses were not significantly altered. The analysis of the speed-curvature power law parameters showed consistent modulations of the speed gain factor, while the exponent remained stable. The second experiment was performed in 3D and showed similar results, except that the main orientation of the ellipse also varied with the changes in speed. In conclusion, this study demonstrated implicit modulation of movement speed by sonification and robust stability of the ellipse geometry. Participants appeared to limit the decrease in movement frequency during slowing down to maintain a rhythmic and not discrete motor regimen.

Bioinspired Implementation and Assessment of a Remote-Controlled Robot

Daily activities are characterized by an increasing interaction with smart machines that present a certain level of autonomy. However, the intelligence of such electronic devices is not always transparent for the end user. This study is aimed at assessing the quality of the remote control of a mobile robot whether the artefact exhibits a human-like behavior or not. The bioinspired behavior implemented in the robot is the well-described two-thirds power law. The performance of participants who teleoperate the semiautonomous vehicle implementing the biological law is compared to a manual and nonbiological mode of control. The results show that the time required to complete the path and the number of collisions with obstacles are significantly lower in the biological condition than in the two other conditions. Also, the highest percentage of occurrences of curvilinear or smooth trajectories are obtained when the steering is assisted by an integration of the power law in the robot's way of working. This advanced analysis of the performance based on the naturalness of the movement kinematics provides a refined evaluation of the quality of the Human-Machine Interaction (HMI). This finding is consistent with the hypothesis of a relationship between the power law and jerk minimization. In addition, the outcome of this study supports the theory of a CNS origin of the power law. The discussion addresses the implications of the anthropocentric approach to enhance the HMI.

The speed-curvature power law in tongue movements of repetitive speech

The speed-curvature power law is a celebrated law of motor control expressing a relation between the kinematic property of speed and the geometric property of curvature. We aimed to assess whether speech movements obey this law just as movements from other domains do. We describe a metronome-driven speech elicitation paradigm designed to cover a wide range of speeds. We recorded via electromagnetic articulometry speech movements in sequences of the form /CV…/ from nine speakers (five German, four English) speaking at eight distinct rates. First, we demonstrate that the paradigm of metronome-driven manipulations results in speech movement data consistent with earlier reports on the kinematics of speech production. Second, analysis of our data in their full three-dimensions and using advanced numerical differentiation methods offers stronger evidence for the law than that reported in previous studies devoted to its assessment. Finally, we demonstrate the presence of a clear rate dependency of the power law's parameters. The robustness of the speed-curvature relation in our datasets lends further support to the hypothesis that the power law is a general feature of human movement. We place our results in the context of other work in movement control and consider implications for models of speech production.

Observation of Point-Light-Walker Locomotion Induces Motor Resonance When Explicitly Represented; An EEG Source Analysis Study

Understanding human motion, to infer the goal of others' actions, is thought to involve the observer's motor repertoire. One prominent class of actions, the human locomotion, has been object of several studies, all focused on manipulating the shape of degraded human figures like point-light walker (PLW) stimuli, represented as walking on the spot. Nevertheless, since the main goal of the locomotor function is to displace the whole body from one position to the other, these stimuli might not fully represent a goal-directed action and thus might not be able to induce the same motor resonance mechanism expected when observing a natural locomotion. To explore this hypothesis, we recorded the event-related potentials (ERP) of canonical/scrambled and translating/centered PLWs decoding. We individuated a novel ERP component (N2c) over central electrodes, around 435 ms after stimulus onset, for translating compared to centered PLW, only when the canonical shape was preserved. Consistently with our hypothesis, sources analysis associated this component to the activation of trunk and lower legs primary sensory-motor and supplementary motor areas. These results confirm the role of own motor repertoire in processing human action and suggest that ERP can detect the associated motor resonance only when the human figure is explicitly involved in performing a meaningful action.

The power law as behavioral illusion: reappraising the reappraisals

Marken and Shaffer (Exp Brain Res 235:1835-1842, 2017) have argued that the power law of movement, which is generally thought to reflect the mechanisms that produce movement, is actually an example of what Powers (Psychol Rev 85:417-435, 1978) dubbed a behavioral illusion, where an observed relationship between variables is seen as revealing something about the mechanisms that produce a behavior when, in fact, it does not. Zago et al. (Exp Brain Res. https://doi.org/10.1007/s0022-017-5108-z , 2017) and Taylor (Exp Brain Res, https://doi.org/10.1007/s00221-018-5192-8 , 2018) have "reappraised" this argument, claiming that it is based on logical, mathematical, statistical and theoretical errors. In the present paper we answer these claims and show that the power law of movement is, indeed, an example of a behavioral illusion. However, we also explain how this apparently negative finding can point the study of movement in a new and more productive direction, with research aimed at understanding movement in terms of its purposes rather than its causes.

Natural Translating Locomotion Modulates Cortical Activity at Action Observation

The present study verified if the translational component of locomotion modulated cortical activity recorded at action observation. Previous studies focusing on visual processing of biological motion mainly presented point light walker that were fixed on a spot, thus removing the net translation toward a goal that yet remains a critical feature of locomotor behavior. We hypothesized that if biological motion recognition relies on the transformation of seeing in doing and its expected sensory consequences, a significant effect of translation compared to centered displays on sensorimotor cortical activity is expected. To this aim, we explored whether EEG activity in the theta (4–8 Hz), alpha (8–12 Hz), beta 1 (14–20 Hz) and beta 2 (20–32 Hz) frequency bands exhibited selectivity as participants viewed four types of stimuli: a centered walker, a centered scrambled, a translating walker and a translating scrambled. We found higher theta synchronizations for observed stimulus with familiar shape. Higher power decreases in the beta 1 and beta 2 bands, indicating a stronger motor resonance was elicited by translating compared to centered stimuli. Finally, beta bands modulation in Superior Parietal areas showed that the translational component of locomotion induced greater motor resonance than human shape. Using a Multinomial Logistic Regression classifier we found that Dorsal-Parietal and Inferior-Frontal regions of interest (ROIs), constituting the core of action-observation system, were the only areas capable to discriminate all the four conditions, as reflected by beta activities. Our findings suggest that the embodiment elicited by an observed scenario is strongly mediated by horizontal body displacement.

Children's first handwriting productions show a rhythmic structure

Although much research has been concerned with the development of kinematic aspects of handwriting, little is known about the development along with age of two principles that govern its rhythmic organization: Homothety and Isochrony. Homothety states that the ratio between the durations of the single motor events composing a motor act remains invariant and independent from the total duration of the movement. Isochrony refers to the proportional relationship between the speed of movement execution and the length of its trajectory. The current study shows that children comply with both principles since their first grade of primary school. The precocious adherence to these principles suggests that an internal representation of the rhythm of handwriting is available before the age in which handwriting is performed automatically. Overall, these findings suggest that despite being a cultural acquisition, handwriting appears to be shaped by more general constraints on the timing planning of the movements.

Drawing ellipses in water: evidence for dynamic constraints in the relation between velocity and path curvature

Several types of continuous human movements comply with the so-called Two-Thirds Power Law (2/3-PL) stating that velocity (V) is a power function of the radius of curvature (R) of the endpoint trajectory. The origin of the 2/3-PL has been the object of much debate. An experiment investigated further this issue by comparing two-dimensional drawing movements performed in air and water. In both conditions, participants traced continuously quasi-elliptic trajectories (period T = 1.5 s). Other experimental factors were the movement plane (horizontal/vertical), and whether the movement was performed free-hand, or by following the edge of a template. In all cases a power function provided a good approximation to the V-R relation. The main result was that the exponent of the power function in water was significantly smaller than in air. This appears incompatible with the idea that the power relationship depends only on kinematic constraints and suggests a significant contribution of dynamic factors. We argue that a satisfactory explanation of the observed behavior must take into account the interplay between the properties of the central motor commands and the visco-elastic nature of the mechanical plant that implements the commands.

Action perception is intact in autism spectrum disorder

Autistic traits span a wide spectrum of behavioral departures from typical function. Despite the heterogeneous nature of autism spectrum disorder (ASD), there have been attempts at formulating unified theoretical accounts of the associated impairments in social cognition. A class of prominent theories capitalizes on the link between social interaction and visual perception: effective interaction with others often relies on discrimination of subtle nonverbal cues. It has been proposed that individuals with ASD may rely on poorer perceptual representations of other people's actions as returned by dysfunctional visual circuitry and that this, in turn, may lead to less effective interpretation of those actions for social behavior. It remains unclear whether such perceptual deficits exist in ASD: the evidence currently available is limited to specific aspects of action recognition, and the reported deficits are often attributable to cognitive factors that may not be strictly visual (e.g., attention). We present results from an exhaustive set of measurements spanning the entire action processing hierarchy, from motion detection to action interpretation, designed to factor out effects that are not selectively relevant to this function. Our results demonstrate that the ASD perceptual system returns functionally intact signals for interpreting other people's actions adequately; these signals can be accessed effectively when autistic individuals are prompted and motivated to do so under controlled conditions. However, they may fail to exploit them adequately during real-life social interactions.

Timing of continuous motor imagery: the two-thirds power law originates in trajectory planning

The two-thirds power law, v = γκ(-1/3), expresses a robust local relationship between the geometrical and temporal aspects of human movement, represented by curvature κ and speed v, with a piecewise constant γ. This law is equivalent to moving at a constant equi-affine speed and thus constitutes an important example of motor invariance. Whether this kinematic regularity reflects central planning or peripheral biomechanical effects has been strongly debated. Motor imagery, i.e., forming mental images of a motor action, allows unique access to the temporal structure of motor planning. Earlier studies have shown that imagined discrete movements obey Fitts's law and their durations are well correlated with those of actual movements. Hence, it is natural to examine whether the temporal properties of continuous imagined movements comply with the two-thirds power law. A novel experimental paradigm for recording sparse imagery data from a continuous cyclic tracing task was developed. Using the likelihood ratio test, we concluded that for most subjects the distributions of the marked positions describing the imagined trajectory were significantly better explained by the two-thirds power law than by a constant Euclidean speed or by two other power law models. With nonlinear regression, the β parameter values in a generalized power law, v = γκ(-β), were inferred from the marked position records. This resulted in highly variable yet mostly positive β values. Our results imply that imagined trajectories do follow the two-thirds power law. Our findings therefore support the conclusion that the coupling between velocity and curvature originates in centrally represented motion planning.

The effect of movement kinematics on predicting the timing of observed actions

The ability to predict the actions of other agents is vital for joint action tasks. Recent theory suggests that action prediction relies on an emulator system that permits observers to use a model of their own movement kinematics to predict the actions of other agents. If this is the case, then people should be more accurate at generating predictions about actions that are similar to their own. We tested this hypothesis in two experiments in which participants were required to predict the occurrence and timing of particular critical points in an observed action. In Experiment 1, we employed a self/other prediction paradigm in which prediction accuracy for recordings of self-generated movements was compared with prediction accuracy for recordings of other-generated movements. As expected, prediction was more accurate for recordings of self-generated actions because in this case the movement kinematics of the observer and observed stimuli are maximally similar. In Experiment 1, people were able to produce actions at their own tempo and, therefore, the results might be explained in terms of self-similarity in action production tempo rather than in terms of movement kinematics. To control for this possibility in Experiment 2, we compared prediction accuracy for stimuli that were matched in tempo but differed only in terms of kinematics. The results showed that participants were more accurate when predicting actions with a human kinematic profile than tempo-matched stimuli that moved with non-human kinematics. Finally, in Experiment 3, we confirmed that the results of Experiment 2 cannot be explained by human-like stimuli containing a slowing down phase before the critical points. Taken together, these findings provide further support for the role of motor emulation in action prediction, and they suggest that the action prediction mechanism produces output that is available rapidly and available to drive action control suggesting that it can plausibly support joint action coordination.

Compositionality in neural control: an interdisciplinary study of scribbling movements in primates

This article discusses the compositional structure of hand movements by analyzing and modeling neural and behavioral data obtained from experiments where a monkey (Macaca fascicularis) performed scribbling movements induced by a search task. Using geometrically based approaches to movement segmentation, it is shown that the hand trajectories are composed of elementary segments that are primarily parabolic in shape. The segments could be categorized into a small number of classes on the basis of decreasing intra-class variance over the course of training. A separate classification of the neural data employing a hidden Markov model showed a coincidence of the neural states with the behavioral categories. An additional analysis of both types of data by a data mining method provided evidence that the neural activity patterns underlying the behavioral primitives were formed by sets of specific and precise spike patterns. A geometric description of the movement trajectories, together with precise neural timing data indicates a compositional variant of a realistic synfire chain model. This model reproduces the typical shapes and temporal properties of the trajectories; hence the structure and composition of the primitives may reflect meaningful behavior.

Segmenting sign language into motor primitives with Bayesian binning

The endpoint trajectories of human movements fulfill characteristic power laws linking velocity and curvature. The parameters of these power laws typically vary between different segments of longer action sequences. These parameters might thus be exploited for the unsupervised segmentation of actions into movement primitives. For the example of sign language we investigate whether such segments can be identified by Bayesian binning (BB), using a Gaussian observation model whose mean has a polynomial time dependence. We show that this method yields good segmentation and correctly models ground truth kinematics composed of consecutive segments derived from wrist trajectories recorded from users of Israeli Sign Language (ISL). Importantly, polynomial orders between 3 and 5 yield an optimal trade-off between complexity and accuracy of the trajectory approximation, in accordance with the minimum acceleration and minimum jerk models. Comparing the orders of the polynomials best approximating natural kinematics against those needed to fit the power law ground truth data suggests that kinematic properties not compatible with power laws are also not adequately represented by low order polynomials and require higher order polynomials for a good approximation.

The relation between geometry and time in mental actions

Mental imagery is a cognitive tool that helps humans take decisions by simulating past and future events. The hypothesis has been advanced that there is a functional equivalence between actual and mental movements. Yet, we do not know whether there are any limitations to its validity even in terms of some fundamental features of actual movements, such as the relationship between space and time. Although it is impossible to directly measure the spatiotemporal features of mental actions, an indirect investigation can be conducted by taking advantage of the constraints existing in planar drawing movements and described by the two-thirds power law (2/3PL). This kinematic law describes one of the most impressive regularities observed in biological movements: movement speed decreases when curvature increases. Here, we compared the duration of identical actual and mental arm movements by changing the constraints imposed by the 2/3PL. In the first two experiments, the length of the trajectory remained constant, while its curvature (Experiment 1) or its number of inflexions (Experiment 2) was manipulated. The results showed that curvature, but not the number of inflexions, proportionally and similarly affected actual and mental movement duration, as expected from the 2/3PL. Two other control experiments confirmed that the results of Experiment 1 were not attributable to eye movements (Experiment 3) or to the perceived length of the displayed trajectory (Experiment 4). Altogether, our findings suggest that mental movement simulation is tuned to the kinematic laws characterizing actions and that kinematics of actual and mental movements is completely specified by the representation of their geometry.

Effect of temporal organization of the visuo-locomotor coupling on the predictive steering

Studies on the direction of a driver’s gaze while taking a bend show that the individual looks toward the tangent-point of the inside curve. Mathematically, the direction of this point in relation to the car enables the driver to predict the curvature of the road. In the same way, when a person walking in the street turns a corner, his/her gaze anticipates the rotation of the body. A current explanation for the visuo-motor anticipation over the locomotion would be that the brain, involved in a steering behavior, executes an internal model of the trajectory that anticipates the completion of the path, and not the contrary. This paper proposes to test this hypothesis by studying the effect of an artificial manipulation of the visuo-locomotor coupling on the trajectory prediction. In this experiment, subjects remotely control a mobile robot with a pan-tilt camera. This experimental paradigm is chosen to manipulate in an easy and precise way the temporal organization of the visuo-locomotor coupling. The results show that only the visuo-locomotor coupling organized from the visual sensor to the locomotor organs enables (i) a significant smoothness of the trajectory and (ii) a velocity-curvature relationship that follows the “2/3 Power Law.” These findings are consistent with the theory of an anticipatory construction of an internal model of the trajectory. This mental representation used by the brain as a forward prediction of the formation of the path seems conditioned by the motor program. The overall results are discussed in terms of the sensorimotor scheme bases of the predictive coding.

Forward to the past

Our daily experience shows that the CNS is a highly efficient machine to predict the effect of actions into the future; are we so efficient also in reconstructing the past of an action? Previous studies demonstrated we are more effective in extrapolating the final position of a stimulus moving according to biological kinematic laws. Here we address the complementary question: are we more effective in extrapolating the starting position (SP) of a motion following a biological velocity profile? We presented a dot moving upward and corresponding to vertical arm movements that were masked in the first part of the trajectory. The stimulus could either move according to biological or non-biological kinematic laws of motion. Results show a better efficacy in reconstructing the SP of a natural motion: participants demonstrate to reconstruct coherently only the SP of the biological condition. When the motion violates the biological kinematic law, responses are scattered and show a tendency toward larger errors. Instead, in a control experiment where the full motions were displayed, no-difference between biological and non-biological motions is found. Results are discussed in light of potential mechanisms involved in visual inference. We propose that as soon as the target appears the cortical motor area would generate an internal representation of reaching movement. When the visual input and the stored kinematic template match, the SP is traced back on the basis of this memory template, making more effective the SP reconstruction.

Movement kinematics affect action prediction: comparing human to non-human point-light actions

The influence of movement kinematics on the accuracy of predicting the time course of another individual's actions was studied. A human point-light shape was animated with human movement (natural condition) and with artificial movement that was more uniform regarding velocity profiles and trajectories (artificial condition). During brief occlusions, the participants predicted the actions in order to judge after occlusion whether the actions were continued coherently in time or shifted to an earlier or later frame. Error rates and reaction times were increased in the artificial compared to the natural condition. The findings suggest a perceptual advantage for movement with a human velocity profile, corresponding to the notion of a close interaction between observed and executed movement. The results are discussed in the framework of the simulation account and alternative interpretations are provided on the basis of correlations between the velocity profiles of natural and artificial movements with prediction performance.

People post-stroke perceive movement fluency in virtual reality

We investigated the visual perception of biological movement by people post-stroke, using minimal kinematic displays. A group of twenty patients and a group of twelve age-matched healthy controls were asked to judge movement fluency. The movements to judge were either displayed as an end-point dot or as a stick-figure of the arm and trunk. It was found that the perception of movement fluency was preserved post-stroke, however, with an increase in the variability of judgment. Moreover, the end-point dot representation ameliorated what was perceived and judged, presumably by directing attention to the important kinematic cues: smoothness and directness of the trajectory. We conclude that, despite perception of actions is influenced by the ability of the observer to execute the observed movement, hemiparesis has a mild effect on the perception of biological movement. Yet, a valuable virtual learning environment for upper-limb rehabilitation should be implemented to provide the observer with neither too much, nor too little information to maximize learning.

Continuation tapping to triggered melodies: motor resonance effects of melodic motion

Common Coding theory predicts that perceived action should resonate in produced action to which it bears some resemblance. Here we show that the qualities of motion commonly attributed to melodies are instantiated in motor plans that control timed movements. Participants attempted to tap a steady beat. Each tap triggered a sounded tone, and successive tones were systematically varied in pitch to form short melodies. Tapping behavior was monitored with motion capture. Although instructed to ignore them, triggered tones systematically affected timing and finger movement. When slower melodic motion was implied by a contour change or a smaller pitch displacement, the interval-tap interval (ITI) was longer. When faster melodic motion was implied by a preserved pitch contour or a larger pitch displacement, ITI was shorter. Kinematic recordings suggested that ITI Error arose from an initial failure to disambiguate perception (i.e., velocity implied by melodic motion) from action (i.e., finger velocity [FV]). Early in the tap trajectory, slower FV was associated with longer ITI and faster FV was associated with shorter ITI. These associations were reversed near mid-trajectory, suggesting a transition from execution of motor planning to online control (Glover et al. in Exp Brain Res 154:103-108, 2004).

The perception of visible speech: estimation of speech rate and detection of time reversals

Four experiments investigated the perception of visible speech. Experiment 1 addressed the perception of speech rate. Observers were shown video-clips of the lower face of actors speaking at their spontaneous rate. Then, they were shown muted versions of the video-clips, which were either accelerated or decelerated. The task (scaling) was to compare visually the speech rate of the stimulus to the spontaneous rate of the actor being shown. Rate estimates were accurate when the video-clips were shown in the normal direction (forward mode). In contrast, speech rate was underestimated when the video-clips were shown in reverse (backward mode). Experiments 2-4 (2AFC) investigated how accurately one discriminates forward and backward speech movements. Unlike in Experiment 1, observers were never exposed to the sound track of the video-clips. Performance was well above chance when playback mode was crossed with rate modulation, and the number of repetitions of the stimuli allowed some amount of speechreading to take place in forward mode (Experiment 2). In Experiment 3, speechreading was made much more difficult by using a different and larger set of muted video-clips. Yet, accuracy decreased only slightly with respect to Experiment 2. Thus, kinematic rather then speechreading cues are most important for discriminating movement direction. Performance worsened, but remained above chance level when the same stimuli of Experiment 3 were rotated upside down (Experiment 4). We argue that the results are in keeping with the hypothesis that visual perception taps into implicit motor competence. Thus, lawful instances of biological movements (forward stimuli) are processed differently from backward stimuli representing movements that the observer cannot perform.

"Biological geometry perception": visual discrimination of eccentricity is related to individual motor preferences

Background

In the continuum between a stroke and a circle including all possible ellipses, some eccentricities seem more “biologically preferred” than others by the motor system, probably because they imply less demanding coordination patterns. Based on the idea that biological motion perception relies on knowledge of the laws that govern the motor system, we investigated whether motorically preferential and non-preferential eccentricities are visually discriminated differently. In contrast with previous studies that were interested in the effect of kinematic/time features of movements on their visual perception, we focused on geometric/spatial features, and therefore used a static visual display.

Methodology/Principal Findings

In a dual-task paradigm, participants visually discriminated 13 static ellipses of various eccentricities while performing a finger-thumb opposition sequence with either the dominant or the non-dominant hand. Our assumption was that because the movements used to trace ellipses are strongly lateralized, a motor task performed with the dominant hand should affect the simultaneous visual discrimination more strongly. We found that visual discrimination was not affected when the motor task was performed by the non-dominant hand. Conversely, it was impaired when the motor task was performed with the dominant hand, but only for the ellipses that we defined as preferred by the motor system, based on an assessment of individual preferences during an independent graphomotor task.

Conclusions/Significance

Visual discrimination of ellipses depends on the state of the motor neural networks controlling the dominant hand, but only when their eccentricity is “biologically preferred”. Importantly, this effect emerges on the basis of a static display, suggesting that what we call “biological geometry”, i.e., geometric features resulting from preferential movements is relevant information for the visual processing of bidimensional shapes.

Incorporating prediction in models for two-dimensional smooth pursuit

A predictive component can contribute to the command signal for smooth pursuit. This is readily demonstrated by the fact that low frequency sinusoidal target motion can be tracked with zero time delay or even with a small lead. The objective of this study was to characterize the predictive contributions to pursuit tracking more precisely by developing analytical models for predictive smooth pursuit. Subjects tracked a small target moving in two dimensions. In the simplest case, the periodic target motion was composed of the sums of two sinusoidal motions (SS), along both the horizontal and the vertical axes. Motions following the same or similar paths, but having a richer spectral composition, were produced by having the target follow the same path but at a constant speed (CS), and by combining the horizontal SS velocity with the vertical CS velocity and vice versa. Several different quantitative models were evaluated. The predictive contribution to the eye tracking command signal could be modeled as a low-pass filtered target acceleration signal with a time delay. This predictive signal, when combined with retinal image velocity at the same time delay, as in classical models for the initiation of pursuit, gave a good fit to the data. The weighting of the predictive acceleration component was different in different experimental conditions, being largest when target motion was simplest, following the SS velocity profiles.

Visual learning shapes the processing of complex movement stimuli in the human brain

Recognition of actions and complex movements is fundamental for social interactions and action understanding. While the relationship between motor expertise and visual recognition of body movements has received a vast amount of interest, the role of visual learning remains largely unexplored. Combining psychophysics and functional magnetic resonance imaging (fMRI) experiments, we investigated neural correlates of visual learning of complex movements. Subjects were trained to visually discriminate between very similar complex movement stimuli generated by motion morphing that were either compatible (experiments 1 and 2) or incompatible (experiment 3) with human movement execution. Employing an fMRI adaptation paradigm as index of discriminability, we scanned human subjects before and after discrimination training. The results of experiment 1 revealed three different effects as a consequence of training: (1) Emerging fMRI-selective adaptation in general motion-related areas (hMT/V5+, KO/V3b) for the differences between human-like movements. (2) Enhanced of fMRI-selective adaptation already present before training in biological motion-related areas (pSTS, FBA). (3) Changes covarying with task difficulty in frontal areas. Moreover, the observed activity changes were specific to the trained movement patterns (experiment 2). The results of experiment 3, testing artificial movement stimuli, were strikingly similar to the results obtained for human movements. General and biological motion-related areas showed movement-specific changes in fMRI-selective adaptation for the differences between the stimuli after training. These results support the existence of a powerful visual machinery for the learning of complex motion patterns that is independent of motor execution. We thus propose a key role of visual learning in action recognition.

Movement timing and invariance arise from several geometries

Human movements show several prominent features; movement duration is nearly independent of movement size (the isochrony principle), instantaneous speed depends on movement curvature (captured by the 2/3 power law), and complex movements are composed of simpler elements (movement compositionality). No existing theory can successfully account for all of these features, and the nature of the underlying motion primitives is still unknown. Also unknown is how the brain selects movement duration. Here we present a new theory of movement timing based on geometrical invariance. We propose that movement duration and compositionality arise from cooperation among Euclidian, equi-affine and full affine geometries. Each geometry posses a canonical measure of distance along curves, an invariant arc-length parameter. We suggest that for continuous movements, the actual movement duration reflects a particular tensorial mixture of these canonical parameters. Near geometrical singularities, specific combinations are selected to compensate for time expansion or compression in individual parameters. The theory was mathematically formulated using Cartan's moving frame method. Its predictions were tested on three data sets: drawings of elliptical curves, locomotion and drawing trajectories of complex figural forms (cloverleaves, lemniscates and limaçons, with varying ratios between the sizes of the large versus the small loops). Our theory accounted well for the kinematic and temporal features of these movements, in most cases better than the constrained Minimum Jerk model, even when taking into account the number of estimated free parameters. During both drawing and locomotion equi-affine geometry was the most dominant geometry, with affine geometry second most important during drawing; Euclidian geometry was second most important during locomotion. We further discuss the implications of this theory: the origin of the dominance of equi-affine geometry, the possibility that the brain uses different mixtures of these geometries to encode movement duration and speed, and the ontogeny of such representations.

No existing theory successfully accounts for several amazing properties of biological movements: dependence of movement speed on path curvature, isochrony (movement duration is nearly independent of its size) and the construction of more complex movements from simpler building blocks. Here we present a new theory of movement generation, based on movement invariance with respect to geometrical transformations. Several types of transformations are considered. Euclidian transformations preserve lengths and angles; affine transformations, which are less restricted, preserve parallelisms between lines, while equi-affine transformations preserve both parallelism and area. Each geometry is associated with a different measure of distance along curves. Movement timing is continuously prescribed by the brain by combining different “geometrical times” each assumed to be proportional to the measure of distance of the corresponding geometry. Movements are constructed by using a series of instantaneous (Cartan) coordinate frames. The predictions of the theory compared well with experimental observations of human drawing and walking. Equi-affine geometry was found to play a dominant role in both tasks and is complemented by affine geometry during drawing and by Euclidian geometry during locomotion. The proposed theory has far reaching implications with respect to brain representations of motion for both action production and perception.

A compact representation of drawing movements with sequences of parabolic primitives

Some studies suggest that complex arm movements in humans and monkeys may optimize several objective functions, while others claim that arm movements satisfy geometric constraints and are composed of elementary components. However, the ability to unify different constraints has remained an open question. The criterion for a maximally smooth (minimizing jerk) motion is satisfied for parabolic trajectories having constant equi-affine speed, which thus comply with the geometric constraint known as the two-thirds power law. Here we empirically test the hypothesis that parabolic segments provide a compact representation of spontaneous drawing movements. Monkey scribblings performed during a period of practice were recorded. Practiced hand paths could be approximated well by relatively long parabolic segments. Following practice, the orientations and spatial locations of the fitted parabolic segments could be drawn from only 2-4 clusters, and there was less discrepancy between the fitted parabolic segments and the executed paths. This enabled us to show that well-practiced spontaneous scribbling movements can be represented as sequences ("words") of a small number of elementary parabolic primitives ("letters"). A movement primitive can be defined as a movement entity that cannot be intentionally stopped before its completion. We found that in a well-trained monkey a movement was usually decelerated after receiving a reward, but it stopped only after the completion of a sequence composed of several parabolic segments. Piece-wise parabolic segments can be generated by applying affine geometric transformations to a single parabolic template. Thus, complex movements might be constructed by applying sequences of suitable geometric transformations to a few templates. Our findings therefore suggest that the motor system aims at achieving more parsimonious internal representations through practice, that parabolas serve as geometric primitives and that non-Euclidean variables are employed in internal movement representations (due to the special role of parabolas in equi-affine geometry).

Models for the extrapolation of target motion for manual interception

Intercepting a moving target requires a prediction of the target's future motion. This extrapolation could be achieved using sensed parameters of the target motion, e.g., its position and velocity. However, the accuracy of the prediction would be improved if subjects were also able to incorporate the statistical properties of the target's motion, accumulated as they watched the target move. The present experiments were designed to test for this possibility. Subjects intercepted a target moving on the screen of a computer monitor by sliding their extended finger along the monitor's surface. Along any of the six possible target paths, target speed could be governed by one of three possible rules: constant speed, a power law relation between speed and curvature, or the trajectory resulting from a sum of sinusoids. A go signal was given to initiate interception and was always presented when the target had the same speed, irrespective of the law of motion. The dependence of the initial direction of finger motion on the target's law of motion was examined. This direction did not depend on the speed profile of the target, contrary to the hypothesis. However, finger direction could be well predicted by assuming that target location was extrapolated using target velocity and that the amount of extrapolation depended on the distance from the finger to the target. Subsequent analysis showed that the same model of target motion was also used for on-line, visually mediated corrections of finger movement when the motion was initially misdirected.

The role of motor simulation in action perception: a neuropsychological case study

Research on embodied cognition stresses that bodily and motor processes constrain how we perceive others. Regarding action perception the most prominent hypothesis is that observed actions are matched to the observer's own motor representations. Previous findings demonstrate that the motor laws that constrain one's performance also constrain one's perception of others' actions. The present neuropsychological case study asked whether neurological impairments affect a person's performance and action perception in the same way. The results showed that patient DS, who suffers from a frontal brain lesion, not only ignored target size when performing movements but also when asked to judge whether others can perform the same movements. In other words DS showed the same violation of Fitts's law when performing and observing actions. These results further support the assumption of close perception action links and the assumption that these links recruit predictive mechanisms residing in the motor system.

Velocity control in Parkinson's disease: a quantitative analysis of isochrony in scribbling movements

An experiment was conducted to contrast the motor performance of three groups (N = 20) of participants: (1) patients with confirmed Parkinson Disease (PD) diagnose; (2) age-matched controls; (3) young adults. The task consisted of scribbling freely for 10 s within circular frames of different sizes. Comparison among groups focused on the relation between the figural elements of the trace (overall size and trace length) and the velocity of the drawing movements. Results were analysed within the framework of previous work on normal individuals showing that instantaneous velocity of drawing movements depends jointly on trace curvature (Two-thirds Power Law) and trace extent (Isochrony principle). The motor behaviour of PD patients exhibited all classical symptoms of the disease (reduced average velocity, reduced fluency, micrographia). At a coarse level of analysis both isochrony and the dependence of velocity on curvature, which are supposed to reflect cortical mechanisms, were spared in PD patients. Instead, significant differences with respects to the control groups emerged from an in-depth analysis of the velocity control suggesting that patients did not scale average velocity as effectively as controls. We factored out velocity control by distinguishing the influence of the broad context in which movement is planned--i.e. the size of the limiting frames--from the influence of the local context--i.e. the linear extent of the unit of motor action being executed. The balance between the two factors was found to be distinctively different in PD patients and controls. This difference is discussed in the light of current theorizing on the role of cortical and sub-cortical mechanisms in the aetiology of PD. We argue that the results are congruent with the notion that cortical mechanisms are responsible for generating a parametric template of the desired movement and the BG specify the actual spatio-temporal parameters through a multiplicative gain factor acting on both size and velocity.

Dynamics of visuo-spatial remembering: a study of information structuring in memory

We studied the process by which learning a pattern of motor activity reaches a steady-state characterized by a reduction in fluctuations. The stimuli consisted of eight visually presented dots that appeared sequentially. In a 20-trial learning phase, participants reproduced the positions of the eight dots after each presentation. Next, they reproduced the pattern 40 times without renewed presentation. In one condition, spatial distances between the dots were proportional to the intervals between their appearances; in the other they were not proportional. We analyzed how the reproduction stabilized at the configuration and dot levels. In proportional as well as non-proportional conditions, stabilization occurs at different time scales for the configuration and dot levels. The stabilization rate differed between proportional and non-proportional conditions. These results are discussed in the framework of dynamical systems.

Adaptation of a reaching model to handwriting: how different effectors can produce the same written output, and other results

This report shows how a model initially developed for the control of reaching can be adapted for the control of handwriting. The main problem addressed by the model is how people can produce essentially the same written output with different effectors (e.g., the preferred or nonpreferred hand, the foot, or even the mouth). The model is based on the assumption that writers strive for invariant graphic outputs when they write with different effectors, when they write on surfaces with different orientations, or when they write large or small script; such output invariance is an essential requirement for later recognition of the written result. Given this assumption, the question is how the motor system enables the relevant effectors to generate the necessary pen strokes. The adapted model provides one possible answer to this question. It is first fully working model of multijoint activity underlying writing and related graphic tasks. We describe how the model differs from other models developed in the past, and we review the model's strengths and weaknesses.

Visual tuning to kinematics of biological motion: the role of eye movements

The visual system is particularly sensitive to the covariation between velocity and curvature that constraint biological motion. Previous work showed that, when this biological constraint is satisfied, simple elliptical motion of a dot looks constant, although its velocity is highly non-uniform. This paper addresses the hypothesis that such a dynamic illusion is dependent upon smooth pursuit eye movements. Subjects had to adjust the kinematics of a dot moving along elliptical trajectories until they perceived a constant velocity. Different pursuit and fixation conditions were tested. The research shows that the dynamic illusion is largely independent of eye movements, suggesting that the visual system has access to implicit knowledge of motor constraints regardless of the concurrent oculomotor commands.