The neural foundations of handedness: insights from a rare case of deafferentation.

The role of proprioceptive feedback on motor lateralization remains unclear. We asked whether motor lateralization is dependent on proprioceptive feedback by examining a rare case of proprioceptive deafferentation (GL). Motor lateralization is thought to arise from asymmetries in neural organization, particularly at the cortical level. For example, we have previously provided evidence that the left hemisphere mediates optimal motor control that allows execution of smooth and efficient arm trajectories, while the right hemisphere mediates impedance control that can achieve stable and accurate final arm postures. The role of proprioception in both of these processes has previously been demonstrated empirically, bringing into question whether loss of proprioception will disrupt lateralization of motor performance. In this study, we assessed whether the loss of online sensory information produces deficits in integrating specific control contributions from each hemisphere by using a reaching task to examine upper limb kinematics in GL and five age-matched controls. Behavioral findings revealed differential deficits in the control of the left and right hands in GL and performance deficits in each of GL's hands compared with controls. Computational simulations can explain the behavioral results as a disruption in the integration of postural and trajectory control mechanisms when no somatosensory information is available. This rare case of proprioceptive deafferentation provides insights into developing a more accurate understanding of handedness that emphasizes the role of proprioception in both predictive and feedback control mechanisms.NEW & NOTEWORTHY The role of proprioceptive feedback on the lateralization of motor control mechanisms is unclear. We examined upper limb kinematics in a rare case of peripheral deafferentation to determine the role of sensory information in integrating motor control mechanisms from each hemisphere. Our empirical findings and computational simulations showed that the loss of somatosensory information results in an impaired integration of control mechanisms, thus providing support for a complementary dominance hypothesis of handedness.

Kinematic features of whole-body reaching movements underwater: Neutral buoyancy effects.

Astronauts' training is conventionally performed in a pool to reproduce weightlessness by exploiting buoyancy which is supposed to reduce the impact of gravity on the body. However, this training method has not been scientifically validated yet, and requires first to study the effects of underwater exposure on motor behavior. We examined the influence of neutral buoyancy on kinematic features of whole-body reaching underwater and compared them with those produced on land. Eight professional divers were asked to perform arm reaching movements toward visual targets while standing. Targets were presented either close or far from the subjects (requiring in the latter case an additional whole-body displacement). Reaching movements were performed on land or underwater in two different contexts of buoyancy. The divers either wore a diving suit only with neutral buoyancy applied to their center of mass or were additionally equipped with a submersible simulated space suit with neutral buoyancy applied to their body limbs. Results showed that underwater exposure impacted basic movement features, especially movement speed which was reduced. However, movement kinematics also differed according to the way buoyancy was exerted on the whole-body. When neutral buoyancy was applied to the center of mass only, some focal and postural components of whole-body reaching remained close to land observations, notably when considering the relative deceleration duration of arm elevation and concomitant forward trunk bending when reaching the far target. On the contrary, when neutral buoyancy was exerted on body segments, movement kinematics were close to those reported in weightlessness, as reflected by the arm deceleration phase and the whole-body forward displacement when reaching the far target. These results suggest that astronauts could benefit from the application of neutral buoyancy across the whole-body segments to optimize underwater training and acquire specific motor skills which will be used in space.

Spatial localization investigated by continuous pointing during visual and gravitoinertial changes.

In order to accurately localize an object, human observers must integrate multiple sensory cues related to the environment and/or to the body. Such multisensory integration must be repeated over time, so that spatial localization is constantly updated according to environmental changes. In the present experimental study, we examined the multisensory integration processes underlying spatial updating by investigating how gradual modifications of gravitoinertial cues (i.e., somatosensory and vestibular cues) and visual cues affect target localization skills. These were assessed by using a continuous pointing task toward a body-fixed visual target. The "single" rotation of the gravitoinertial vector (produced by off-axis centrifugation) resulted in downward pointing errors, which likely were related to a combination of oculogravic and somatogravic illusions. The "single" downward pitch rotation of the visual background produced an elevation of the arm relative to the visual target, suggesting that the rotation of the visual background caused an illusory target elevation (induced-motion phenomenon). Strikingly, the errors observed during the "combined" rotation of the visual background and of the gravitoinertial vector appeared as a linear combination of the errors independently observed during "single" rotations. In other words, the centrifugation effect on target localization was reduced by the visual background rotation. The observed linear combination indicates that the weights of visual and gravitoinertial cues were similar and remained constant throughout the stimulation.

The influence of target sensory modality on motor planning may reflect errors in sensori-motor transformations.

Multi-sensory integration studies have shown that combining heterogeneous signals can optimize motor performance by reducing errors inherent to any single modality. However, it has also been suggested that errors could arise from erroneous transformations between heterogeneous coordinate systems. Here we investigated the effect of visuo-proprioceptive integration on the control of multi-joint arm movements by manipulating target modality. When the target was visual, movement control required the integration of visual target signals with proprioceptive signals about limb configuration. In contrast, when the target was the unseen fingertip, movement control relied solely on proprioceptive signals since visual feedback of hand position was precluded. We hypothesized that a faulty integration of visual target signals with proprioceptive arm signals would result in a less accurate planning of visually-targeted movements with respect to proprioceptively-targeted movements. Different inter-joint coordinations patterns were tested by varying starting hand position. Results showed larger initial trajectory deviations from target direction for visually-targeted movements involving substantial shoulder and elbow motions. Inverse dynamic analysis revealed that these deviations were associated with less efficient intersegmental coordination. The control of visually-targeted movements thus appeared sub-optimal compared to proprioceptively-targeted movements when considering theoretical models of motor planning assuming kinematic or dynamic optimizations. Additional experiments further highlighted the effect of target position, and visual feedback of starting hand position, on motor planning for proprioceptively- and visually-targeted movements. Our findings suggest that the integration of heterogeneous sensory signals related to hand and target positions introduces errors in motor planning.