Patrice Senot

Visuo-motor coordination and internal models for object interception

Intercepting and avoiding collisions with moving objects are fundamental skills in daily life. Anticipatory behavior is required because of significant delays in transforming sensory information about target and body motion into a timed motor response. The ability to predict the kinematics and kinetics of interception or avoidance hundreds of milliseconds before the event may depend on several different sources of information and on different strategies of sensory-motor coordination. What are exactly the sources of spatio-temporal information and what are the control strategies remain controversial issues. Indeed, these topics have been the battlefield of contrasting views on how the brain interprets visual information to guide movement. Here we attempt a synthetic overview of the vast literature on interception. We discuss in detail the behavioral and neurophysiological aspects of interception of targets falling under gravity, as this topic has received special attention in recent years. We show that visual cues alone are insufficient to predict the time and place of interception or avoidance, and they need to be supplemented by prior knowledge (or internal models) about several features of the dynamic interaction with the moving object.

Force requirements of observed object lifting are encoded by the observer’s motor system: a TMS study

Several transcranial magnetic stimulation (TMS) studies have reported facilitation of the primary motor cortex (M1) during the mere observation of actions. This facilitation was shown to be highly congruent, in terms of somatotopy, with the observed action, even at the level of single muscles. With the present study, we investigated whether this muscle‐specific facilitation of the observer’s motor system reflects the degree of muscular force that is exerted in an observed action. Two separate TMS experiments are reported in which corticospinal excitability was measured in the hand area of M1 while subjects observed the lifting of objects of different weights. The type of action ‘grasping‐and‐lifting‐the‐object’ was always identical, but the grip force varied according to the object’s weight. In accordance to previous findings, excitability of M1 was shown to modulate in a muscle‐specific way, such that only the cortical representation areas in M1 that control the specific muscles used in the observed lifting action became increasingly facilitated. Moreover, muscle‐specific M1 facilitation was shown to modulate to the force requirements of the observed actions, such that M1 excitability was considerably higher when observing heavy object lifting compared with light object lifting. Overall, these results indicate that different levels of observed grip force are mirrored onto the observer’s motor system in a highly muscle‐specific manner. The measured force‐dependent modulations of corticospinal excitability in M1 are hypothesized to be functionally relevant for scaling the observed grip force in the observer’s own motor system. In turn, this mechanism may contribute, at least partly, to the observer’s ability to infer the weight of the lifted object.

Effect of weight-related labels on corticospinal excitability during observation of grasping: a TMS study

Recent studies of corticospinal excitability during observation of grasping and lifting of objects of different weight have highlighted the role of agent’s kinematics in modulating observer’s motor excitability. Here, we investigate whether explicit weight-related information, provided by written labels on the objects, modulate the excitability of the observer’s motor system and how this modulation is affected when there is a conflict between label and object’s weight. We measured TMS-evoked motor potentials (MEPs) from right hand intrinsic muscles, while subjects were observing an actor lifting objects of different weights, in some trials labeled (heavy/light) in congruent or incongruent way. Results confirmed a weight-related modulation of MEPs based on kinematic cues. Interestingly, any conflict between the labels and the actual weight (i.e., explicit versus implicit information), although never consciously noticed by the observer, deeply affected the mirroring of others’ actions. Our findings stress the automatic involvement of the mirror-neuron system.

When Up Is Down in 0g: How Gravity Sensing Affects the Timing of Interceptive Actions

Humans are known to regulate the timing of interceptive actions by modeling, in a simplified way, Newtonian mechanics. Specifically, when intercepting an approaching ball, humans trigger their movements a bit earlier when the target arrives from above than from below. This bias occurs regardless of the balls true kinetics, and thus appears to reflect an a priori expectation that a downward moving object will accelerate. We postulate that gravito-inertial information is used to tune visuomotor responses to match the targets most likely acceleration. Here we used the peculiar conditions of parabolic flight—where gravitys effects change every 20 s—to test this hypothesis. We found a striking reversal in the timing of interceptive responses performed in weightlessness compared with trials performed on ground, indicating a role of gravity sensing in the tuning of this response. Parallels between these observations and the properties of otolith receptors suggest that vestibular signals themselves might plausibly provide the critical input. Thus, in addition to its acknowledged importance for postural control, gaze stabilization, and spatial navigation, we propose that detecting the direction of gravitys pull plays a role in coordinating quick reactions intended to intercept a fast-moving visual target.

Estimating time to contact and impact velocity when catching an accelerating object with the hand.

To catch a moving object with the hand requires precise coordination between visual information about the targets motion and the muscle activity necessary to prepare for the impact. A key question remains open as to if and how a human observer uses velocity and acceleration information when controlling muscles in anticipation of impact. Participants were asked to catch the moving end of a swinging counterweighted pendulum, and resulting muscle activities in the arm were measured. The authors also simulated muscle activities that would be produced according to different tuning strategies. By comparing data with simulations, the authors provide evidence that human observers use online information about velocity but not acceleration when preparing for impact.

Are There Any Left-Right Asymmetries in Saccade Parameters? Examination of Latency, Gain, and Peak Velocity

PURPOSE Hemispheric specialization in saccadic control is still under debate. Here we examine the latency, gain, and peak velocity of reactive and voluntary leftward and rightward saccades to assess the respective roles of eye and hand dominance. METHODS Participants with contrasting hand and eye dominance were asked to make saccades toward a target displayed at 5°, 10°, or 15° left or right of the central fixation point. In separate sessions, reactive and voluntary saccades were elicited by Gap-200, Gap-0, Overlap-600, and Antisaccade procedures. RESULTS Left-right asymmetries were not found in saccade latencies but appeared in saccade gain and peak velocity. Regardless of the dominant hand, saccades directed to the ipsilateral side relative to the dominant eye had larger amplitudes and faster peak velocities. CONCLUSIONS Left-right asymmetries can be explained by naso-temporal differences for some subjects and by eye dominance for others. Further investigations are needed to examine saccadic parameters more systematically in relation to eye dominance. Indeed, any method that allows one to determine ocular dominance from objective measures based on saccade parameters should greatly benefit clinical applications, such as monovision surgery.

Cortical dynamics of anticipatory mechanisms in interception: A neuromagnetic study

Humans demonstrate an amazing ability for intercepting and catching moving targets, most noticeably in fast-speed ball games. However, the few studies exploring the neural bases of interception in humans and the classical studies on visual motion processing and visuomotor interactions have reported rather long latencies of cortical activations that cannot explain the performances observed in most natural interceptive actions. The aim of our experiment was twofold: (1) describe the spatio-temporal unfolding of cortical activations involved in catching a moving target and (2) provide evidence that fast cortical responses can be elicited by a visuomotor task with high temporal constraints and decide if these responses are task or stimulus dependent. Neuromagnetic brain activity was recorded with whole-head coverage while subjects were asked to catch a free-falling ball or simply pay attention to the ball trajectory. A fast, likely stimulus-dependent, propagation of neural activity was observed along the dorsal visual pathway in both tasks. Evaluation of latencies of activations in the main cortical regions involved in the tasks revealed that this entire network of regions was activated within 40 msec. Moreover, comparison of experimental conditions revealed similar patterns of activation except in contralateral sensorimotor regions where common and catch-specific activations were differentiated.

Egocentric and allocentric reference frames for catching a falling object

When programming movement, one must account for gravitational acceleration. This is particularly important when catching a falling object because the task requires a precise estimate of time-to-contact. Knowledge of gravity’s effects is intimately linked to our definition of ‘up’ and ‘down’. Both directions can be described in an allocentric reference frame, based on visual and/or gravitational cues, or in an egocentric reference frame in which the body axis is taken as vertical. To test which frame humans use to predict gravity’s effect, we asked participants to intercept virtual balls approaching from above or below with artificially controlled acceleration that could be congruent or not with gravity. To dissociate between these frames, subjects were seated upright (trunk parallel to gravity) or lying down (body axis orthogonal to the gravitational axis). We report data in line with the use of an allocentric reference frame and discuss its relevance depending on available gravity-related cues.

Vers une quantification de la dominance oculaire pour une meilleure prise en charge des pathologies de l’œil☆

INTRODUCTION The dominant eye is defined as the one we unconsciously choose when we have to perform monocular tasks. In the field of clinical neuro-ophthalmology, it is well-established that ocular dominance plays a key role in several eye diseases. Furthermore, the accurate quantification of ocular dominance is crucial with regard to certain surgical techniques. However, classical preoperative tests cannot determine the amount of ocular dominance. MATERIALS AND METHODS In order to obtain further insight into the phenomenon of ocular dominance, we study its influence at behavioral and neurophysiological levels (experiments 1 and 2). Based on these new data, we suggest a method to improve quantification of ocular dominance (experiment 3). RESULTS We demonstrate that ocular dominance has an influence on hand movements and on interhemispheric transfer time. Moreover, we show that an analysis of the dynamics of saccades allows us to sort out participants with strong or weak ocular dominance. CONCLUSION In conclusion, this better understanding of the phenomenon of ocular dominance, coupled with the analysis of saccadic dynamics, might, in the short or medium term, lead to the establishment of a quick and straightforward battery of tests allowing determination of the amount of ocular dominance for each patient.

Asymmetry in visual information processing depends on the strength of eye dominance

ABSTRACT Unlike handedness, sighting eye dominance, defined as the eye unconsciously chosen when performing monocular tasks, is very rarely considered in studies investigating cerebral asymmetries. We previously showed that sighting eye dominance has an influence on visually triggered manual action with shorter reaction time (RT) when the stimulus appears in the contralateral visual hemifield with respect to the dominant eye (Chaumillon et al. 2014). We also suggested that eye dominance may be more or less pronounced depending on individuals and that this eye dominance strength could be evaluated through saccadic peak velocity analysis in binocular recordings (Vergilino‐Perez et al. 2012). Based on these two previous studies, we further examine here whether the strength of the eye dominance can modulate the influence of this lateralization on manual reaction time. Results revealed that participants categorized as having a strong eye dominance, but not those categorized as having a weak eye dominance, exhibited the difference in RT between the two visual hemifields. This present study reinforces that the analysis of saccade peak velocity in binocular recordings provides an effective tool to better categorize the eye dominance. It also shows that the influence of eye dominance in visuo‐motor tasks depends on its strength. Our study also highlights the importance of considering the strength of eye dominance in future studies dealing with brain lateralization. HIGHLIGHTSInfluence of eye dominance and its strength were examined in a visuo‐motor task.Strong‐eye dominant people showed a speed advantage for the contralateral hemifield.Weak‐eye dominant people showed no difference between the two hemifields.Eye dominance and its strength are sources of variation in visuo‐motor performance.Saccades recording is an effective tool to characterize the eye dominance strength.