Microsaccades are modulated by both attentional demands of a visual discrimination task and background noise.

Microsaccades are miniature saccades occurring once or twice per second during visual fixation. While microsaccades and saccades share similarities at the oculomotor level, the functional roles of microsaccades are still debated. In this study, we examined the hypothesis that the microsaccadic activity is affected by the type of noisy background during the execution of a particular discrimination task. Human subjects had to judge the orientation of a tilted stimulus embedded in static or dynamic backgrounds in a forced choice-task paradigm, as adapted from Rucci, Iovin, Poletti, and Santini (2007). Static backgrounds induced more microsaccades than dynamic ones only during the execution of the discrimination task. A directional bias of microsaccades, dictated by the stimulus orientation, was temporally coupled with this period of increased activity. Both microsaccade rates and orientations were comparable across background types after the response time although subjects maintained fixation until the end of the trial. This represents a background-specific modulation of the microsaccadic activity driven by attentional demands. The visual influence of microsaccades on discrimination performances was modeled at the retinal level for both types of backgrounds. A higher simulated microsaccadic activity was necessary for static backgrounds in order to achieve discrimination performance scores comparable to that of dynamic ones. Taken together, our experimental and theoretical findings further support the idea that microsaccades are under attentional control and represent an efficient sampling strategy allowing spatial information acquisition.

Invariance of locomotor trajectories across visual and gait direction conditions.

We studied the influence of vision (walking with or without vision) and of gait direction (walking forward or backward) on goal-oriented locomotion in humans. Subjects had to walk, in a free environment, from a given position and orientation towards a distant arrow which constrained their final position and orientation. We found that the average trajectories were mostly similar across the tested conditions, which suggests that locomotor trajectories are generated at a high cognitive level and, to some extent, independently of the detailed sensory and motor implementation levels. The variability profiles around the average trajectories were similar across the gait direction conditions but differed greatly across the visual conditions, indicating the existence of motor-independent and vision-dependent control mechanisms. Taken together, our observations argue further in favour of a top-down implementation of goal-oriented locomotion, where the control of locomotion is specified at the level of whole-body trajectories and then implemented through specific motor strategies.

Augmented-Feedback Training Improves Cognitive Motor Performance of Soccer Players.

PURPOSE: In this study, we tested the hypothesis that augmented feedback (AF) training can improve both perceptual-cognitive and/or motor skills specific to soccer. METHODS: Three groups of young elite players (U14-U15 categories) performed a test consisting in passing the ball as accurately and as quickly as possible toward a visual target moving briefly across a large screen located at 6 m from the player. The performed task required players to correctly perceive the target, anticipate its future location, and to adequately adjust the pass direction and power. The control group (CON) performed normal soccer training and was compared with two visuomotor training groups (AF and no-feedback [NF]) that followed the same training regime but integrated series of 32 passes three times per week over a 17-d period into their normal soccer training. Objective measurements of the passing performance were provided using a high-technology system (COGNIFOOT) before, during, and after training. During training, only players of the AF group received visuoauditory feedback immediately after each trial informing them about the accuracy of their passes. RESULTS: The results show that only players of the AF group significantly improved passing accuracy, reactiveness, and global passing performance (+22%), whereas the NF group only improved passing accuracy. None of these parameters was improved in the CON group. The objectively measured changes in passing performance were compared with the more subjectively judged passing performance provided by coaches and players. Coaches' judgments were more reliable than players' judgments and exhibited a training group effect comparable to the ones objectively measured by COGNIFOOT. CONCLUSIONS: This study provides evidence that the training of cognitive motor performance in soccer players highly benefits from the use of augmented feedback.

On the open-loop and feedback processes that underlie the formation of trajectories during visual and nonvisual locomotion in humans.

We investigated the nature of the control mechanisms at work during goal-oriented locomotion. In particular, we tested the effects of vision, locomotor speed, and the presence of via points on the geometric and kinematic properties of locomotor trajectories. We first observed that the average trajectories recorded in visual and nonvisual locomotion were highly comparable, suggesting the existence of vision-independent processes underlying the formation of locomotor trajectories. Then by analyzing and comparing the variability around the average trajectories across different experimental conditions, we were able to demonstrate the existence of on-line feedback control in both visual and nonvisual locomotion and to clarify the relations between visual and nonvisual control strategies. Based on these insights, we designed a model in which maximum-smoothness and optimal feedback control principles account, respectively, for the open-loop and feedback processes. Taken together, the experimental and modeling findings provide a novel understanding of the nature of the motor, sensory, and "navigational" processes underlying goal-oriented locomotion.

Effects of age on the soccer-specific cognitive-motor performance of elite young soccer players: Comparison between objective measurements and coaches' evaluation.

The cognitive-motor performance (CMP), defined here as the capacity to rapidly use sensory information and transfer it into efficient motor output, represents a major contributor to performance in almost all sports, including soccer. Here, we used a high-technology system (COGNIFOOT) which combines a visual environment simulator fully synchronized with a motion capture system. This system allowed us to measure objective real-time CMP parameters (passing accuracy/speed and response times) in a large turf-artificial grass playfield. Forty-six (46) young elite soccer players (including 2 female players) aged between 11 and 16 years who belonged to the same youth soccer academy were tested. Each player had to pass the ball as fast and as accurately as possible towards visual targets projected onto a large screen located 5.32 meters in front of him (a short pass situation). We observed a linear age-related increase in the CMP: the passing accuracy, speed and reactiveness of players improved by 4 centimeters, 2.3 km/h and 30 milliseconds per year of age, respectively. These data were converted into 5 point-scales and compared to the judgement of expert coaches, who also used a 5 point-scale to evaluate the same CMP parameters but based on their experience with the players during games and training. The objectively-measured age-related CMP changes were also observed in expert coaches' judgments although these were more variable across coaches and age categories. This demonstrates that high-technology systems like COGNIFOOT can be used in complement to traditional approaches of talent identification and to objectively monitor the progress of soccer players throughout a cognitive-motor training cycle.

Planning of spatially-oriented locomotion following focal brain damage in humans: A pilot study.

Motor impairments in human gait following stroke or focal brain damage are well documented. Here, we investigated whether stroke and/or focal brain damage also affect the navigational component of spatially oriented locomotion. Ten healthy adult participants and ten adult brain-damaged patients had to walk towards distant targets from different starting positions (with vision or blindfolded). No instructions as to which the path to follow were provided to them. We observed very similar geometrical forms of paths across the two groups of participants and across visual conditions. This spatial stereotypy of whole-body displacements was observed following brain damage, even in the most severely impaired (hemiparetic) patients. This contrasted with much more variability at the temporal level. In particular, healthy participants and non-hemiparetic patients varied their walking speed according to curvature changes along the path. On the contrary, the walking speed profiles were not stereotypical and were not systematically constrained by path geometry in hemiparetic patients where it was associated with different stepping behaviors. These observations confirm the dissociation between cognitive and motor aspects of gait recovery post-stroke. The impact of these findings on the understanding of the functional and anatomical organization of spatially-oriented locomotion and for rehabilitation purposes is discussed and contextualized in the light of recent advances in electrophysiological studies.

Head motion in humans alternating between straight and curved walking path: combination of stabilizing and anticipatory orienting mechanisms.

Anticipatory head orientation relative to walking direction was investigated in humans. Subjects were asked to walk along a 20 m perimeter, figure of eight. The geometry of this path required subjects to steer their body according to both curvature variations (alternate straight with curved walking) and walking direction (clock wise and counter clock wise). In agreement with previous results obtained during different locomotor tasks [R. Grasso, S. Glasauer, Y. Takei, A. Berthoz, The predictive brain: anticipatory control of head direction for the steering of locomotion, NeuroReport 7 (1996) 1170-1174; R. Grasso, P. Prevost, Y.P. Ivanenko, A. Berthoz, Eye-head coordination for the steering of locomotion in humans: an anticipatory synergy, Neurosci. Lett. 253 (2) (1998) 115-118; T. Imai, S.T. Moore, T. Raphan, B. Cohen, Interaction of body, head, and eyes during walking and turning, Exp. Brain Res. 136 (2001) 1-18; P. Prevost, Y. Ivanenko, R. Grasso, A. Berthoz, Spatial invariance in anticipatory orienting behaviour during human navigation, Neurosci. Lett. 339 (2002) 243-247; G. Courtine, M. Schieppati, Human walking along a curved path. I. Body trajectory, segment orientation and the effect of vision, Eur. J. Neurosci. 18 (2003) 177-190], the head turned toward the future walking direction. This anticipatory head behaviour was continuously modulated by the geometrical variations of the path. Two main components were observed in the anticipatory head behaviour. One was related to the geometrical form of the path, the other to the transfer of body mass from one foot to the other during stepping. A clear modulation of the head deviation pattern was observed between walking on curved versus straight parts of the path: head orientation was influenced to a lesser extent by step alternation for curved path where a transient head fixation was observed. We also observed good symmetry in the head deviation profile, i.e. the head tended to anticipate the future walking direction with the same amplitude when turning to the left (29.75 +/- 7.41 degrees of maximum head deviation) or to the right (30.86 +/- 9.92 degrees ). These findings suggest a combination of motor strategies underlying head stabilization in space and more global orienting mechanisms for steering the whole body in the desired direction.

The formation of trajectories during goal-oriented locomotion in humans. I. A stereotyped behaviour.

Human locomotion was investigated in a goal-oriented task where subjects had to walk to and through a doorway starting from a fixed position and orientation in space. The door was located at different positions and orientations in space, resulting in a total of 40 targets. While no specific constraint was provided to subjects in terms of the path they were to follow or the expected walking speeds, all of them generated very similar trajectories in terms of both path geometry and velocity profiles. These results are reminiscent of the stereotyped properties of the hand trajectories observed in arm reaching movements in studies over the last 20 years. This observation supports the hypothesis that common constraining mechanisms govern the generation of segmental and whole-body trajectories. In contrast, we observed that the subjects placed their feet at different spatial positions across repetitions, making unlikely the hypothesis that goal-oriented locomotion is planned as a succession of steps. Rather, our results suggest that common planning and/or control strategies underlie the formation of the whole locomotor trajectory during a spatially oriented task.

The formation of trajectories during goal-oriented locomotion in humans. II. A maximum smoothness model.

Despite the theoretically infinite number of possible trajectories a human may take to reach a distant doorway, we observed that locomotor trajectories corresponding to this task were actually stereotyped, both at the geometric and the kinematic levels. In this paper, we propose a computational model for the formation of human locomotor trajectories. Our model is adapted from smoothness maximization models that have been studied in the context of hand trajectory generation. The trajectories predicted by our model are very similar to the experimentally recorded ones. We discuss the theoretical implications of this result in the context of movement planning and control in humans. In particular, this result supports the hypothesis that common principles, such as smoothness maximization, may govern the generation of very different types of movements (in this case, hand movements and whole-body movements).

Intersegmental coordination during human locomotion: does planar covariation of elevation angles reflect central constraints?

To study intersegmental coordination in humans performing different locomotor tasks (backward, normal, fast walking, and running), we analyzed the spatiotemporal patterns of both elevation and joint angles bilaterally in the sagittal plane. In particular, we determined the origins of the planar covariation of foot, shank, and thigh elevation angles. This planar constraint is observable in the three-dimensional space defined by these three angles and corresponds to the plane described by the three time-varying elevation angle variables over each step cycle. Previous studies showed that this relation between elevation angles constrains lower limb coordination in various experimental situations. We demonstrate here that this planar covariation mainly arises from the strong correlation between foot and shank elevation angles, with thigh angle independently contributing to the pattern of intersegmental covariation. We conclude that the planar covariation of elevation angles does not reflect central constraints, as previously suggested. An alternative approach for analyzing the patterns of coordination of both elevation and joint (hip, knee, and ankle) angles is used, based on temporal cross-correlation and phase relationships between pairs of kinematic variables. We describe the changes in the pattern of intersegmental coordination that are associated with the changes of locomotor modes and locomotor speeds. We provide some evidence for a distinct control of thigh motion and discuss the respective contributions of passive mechanical factors and of active (arising from neural control) factors to the formation and the regulation of the locomotor pattern throughout the gait cycle.