Jean Blouin

Reference systems for coding spatial information in normal subjects and a deafferented patient.

To produce accurate goal-directed arm movements, subjects must determine the precise location of target object. Position of extracorporeal objects can be determined using: (a) an egocentric frame of reference, in which the target is localized in relation to the position of the body; and/or (b) an allocentric system, in which target position is determined in relation to stable visual landmarks surrounding the target (Bridgeman 1989; Paillard 1991). The present experiment was based on the premise that (a) the presence of a structured visual environment enables the use of an allocentric frame of reference, and (b) the sole presence of a visual target within a homogeneous background forces the registration of the target location by an egocentric system. Normal subjects and a deafferented patient (i.e., with an impaired egocentric system) pointed to visual targets presented in both visual environments to evaluate the efficiency of the two reference systems. For normals, the visual environment conditions did not affect pointing accuracy. However, kinematic parameters were affected by the presence or absence of a structured visual surrounding. For the deafferented patient, the presence of a structured visual environment permitted a decrease in spatial errors when compared with the unstructured surrounding condition (for movements with or without visual feedback of the trajectory). Overall, results support the existence of an egocentric and an allocentric reference system capable of organizing extracorporeal space during arm movements directed toward visual targets.

Altered sensory-weighting mechanisms is observed in adolescents with idiopathic scoliosis

BackgroundScoliosis is the most common type of spinal deformity. In North American children, adolescent idiopathic scoliosis (AIS) makes up about 90% of all cases of scoliosis. While its prevalence is about 2% to 3% in children aged between 10 to 16 years, girls are more at risk than boys for severe progression with a ratio of 3.6 to 1. The aim of the present study was to test the hypothesis that idiopathic scoliosis interferes with the mechanisms responsible for sensory-reweighting during balance control.MethodsEight scoliosis patients (seven female and one male; mean age: 16.4 years) and nine healthy adolescents (average age 16.5 years) participated in the experiment. Visual and ankle proprioceptive information was perturbed (eyes closed and/or tendon vibration) suddenly and then returned to normal (eyes open and/or no tendon vibration). An AMTI force platform was used to compute centre of pressure root mean squared velocity and sway density curve.ResultsFor the control condition (eyes open and no tendon vibration), adolescent idiopathic scoliosis patients had a greater centre of pressure root mean squared velocity (variability) than control participants. Reintegration of ankle proprioception, when vision was either available or removed, led to an increased centre of pressure velocity variability for the adolescent idiopathic scoliosis patients whereas the control participants reduced their centre of pressure velocity variability. Moreover, in the absence of vision, adolescent idiopathic scoliosis exhibited an increased centre of pressure velocity variability when ankle proprioception was returned to normal (i.e. tendon vibration stopped). The analysis of the sway density plot suggests that adolescent idiopathic scoliosis patients, during sensory reintegration, do not scale appropriately their balance control commands.ConclusionAltogether, the present results demonstrate that idiopathic scoliosis adolescents have difficulty in reweighting sensory inputs following a brief period of sensory deprivation.

Galvanic vestibular stimulation in humans produces online arm movement deviations when reaching towards memorized visual targets.

Using galvanic vestibular stimulation (GVS), we tested whether a change in vestibular input at the onset of goal-directed arm movements induces deviations in arm trajectory. Eight head-fixed standing subjects were instructed to reach for memorized visual targets in complete darkness. In half of the trials, randomly-selected, a 3 mA bipolar binaural galvanic stimulation of randomly alternating polarity was triggered by the movement onset. Results revealed significant GVS-induced directional shifts of reaching movements towards the anode side. The earliest significant deviations of hand path occurred 240 ms after stimulation onset. The likely goal of these online deviations of arm trajectory was to compensate for a vestibular-evoked apparent change in the spatial relationship between the target and the hand.

Role of sensory information in updating internal models of the effector during arm tracking

This chapter is divided into three main parts. Firstly, on the basis of the literature, we will shortly discuss how the recent introduction of the concept of internal models by Daniel Wolpert and Mitsuo Kawato contributes to a better understanding of what is motor learning and what is motor adaptation. Then, we will present a model of eye-hand co-ordination during self-moved target tracking, which we used as a way to specifically address these topics. Finally, we will show some evidence about the use of proprioceptive information for updating the internal models, in the context of eye-hand co-ordination. Motor and afferent information appears to contribute to the parametric adjustment (adaptation) between arm motor command and visual information about arm motion. The study reported here was aimed at assessing the contribution of arm proprioception in building (learning) and updating (adaptation) these representations. The subjects (including a deafferented subject) had to make back and forth movements with their forearm in the horizontal plane, over learned amplitude and at constant frequency, and to track an arm-driven target with their eyes. The dynamical conditions of arm movement were altered (unexpectedly or systematically) during the movement by changing the mechanical properties of the manipulandum. The results showed a significant change of the latency and the gain of the smooth pursuit system, before and after the perturbation for the control subjects, but not for the deafferented subject. Moreover, in control subjects, vibrations of the arm muscles prevented adaptation to the mechanical perturbation. These results suggest that in a self-moved target tracking task, the arm motor system shares with the smooth pursuit system an internal representation of the arm dynamical properties, and that arm proprioception is necessary to build this internal model. As suggested by Ghez et al. (1990) (Cold Spring Harbor Symp. Quant. Biol., 55: 837-8471), proprioception would allow control subjects to learn the inertial properties of the limb.

Evidence for cognitive vestibular integration impairment in idiopathic scoliosis patients

BackgroundAdolescent idiopathic scoliosis is characterized by a three-dimensional deviation of the vertebral column and its etiopathogenesis is unknown. Various factors cause idiopathic scoliosis, and among these a prominent role has been attributed to the vestibular system. While the deficits in sensorimotor transformations have been documented in idiopathic scoliosis patients, little attention has been devoted to their capacity to integrate vestibular information for cognitive processing for space perception. Seated idiopathic scoliosis patients and control subjects experienced rotations of different directions and amplitudes in the dark and produced saccades that would reproduce their perceived spatial characteristics of the rotations (vestibular condition). We also controlled for possible alteration of the oculomotor and vestibular systems by measuring the subjects accuracy in producing saccades towards memorized peripheral targets in absence of body rotation and the gain of their vestibulo-ocular reflex.ResultsCompared to healthy controls, the idiopathic scoliosis patients underestimated the amplitude of their rotations. Moreover, the results revealed that idiopathic scoliosis patients produced accurate saccades to memorized peripheral targets in absence of body rotation and that their vestibulo-ocular reflex gain did not differ from that of control participants.ConclusionOverall, results of the present study demonstrate that idiopathic scoliosis patients have an alteration in cognitive integration of vestibular signals. It is possible that severe spine deformity developed partly due to impaired vestibular information travelling from the cerebellum to the vestibular cortical network or alteration in the cortical mechanisms processing the vestibular signals.

Updating visual space during passive and voluntary head-in-space movements

Abstract The accuracy of our spatially oriented behaviors largely depends on the precision of monitoring the change in body position with respect to space during self-motion. We investigated observers’ capacity to determine, before and after head rotations about the yaw axis, the position of a memorized earth-fixed visual target positioned 21° laterally. The subjects (n=6) showed small errors (mean=–0.6°) and little variability (mean=0.9°) in determining the position of an extinguished visual-target position when the head (and gaze) remained in a straight-ahead position. This accuracy was preserved when subjects voluntary rotated the head by various magnitudes in the direction of the memorized visual target (head rotations ranged between 5° and 60°). However, when the chair on which the subjects were seated was unexpectedly rotated about the yaw axis in the direction of the target (chair rotations ranged between 6° and 36°) during the head-on-trunk rotations, the performance was markedly decreased, both in terms of spatial precision (mean error=5.6°) and variability (mean=5.7°). A control experiment showed that the prior knowledge of chair rotation occurrence had no effect on the perceived target position after head-trunk movements. Updating an earth-fixed target position during head-on-trunk rotations could be achieved through both cervical and vestibular signals processing, but, in the present experiment, the vestibular output was the only signal that had the potentiality to contribute to accurate coding of the target position after simultaneous head and trunk movements. Our results therefore suggest that the vestibular output is a noisy signal for the central nervous signal to update the visual space during head-in-space motion.

Direct Evidence for Cortical Suppression of Somatosensory Afferents during Visuomotor Adaptation

Upon exposure to novel visuomotor relationships, the information carried by visual and proprioceptive signals becomes discrepant, often disrupting motor execution. It has been shown that degradation of the proprioceptive sense (arising either from disease or experimental manipulation) enhances performance when drawing with mirror-reversed vision. Given that the central nervous system can exert a dynamic control over the transmission of afferent signals, reducing proprioceptive inflow to cortical areas could be part of the normal adaptive mechanisms deployed in healthy humans upon exposure to novel visuomotor environments. Here we address this issue by probing the transmission of somatosensory afferents throughout the course of adaptation to a visuomotor conflict, by recording median nerve somatosensory evoked potentials. We show that early exposure to tracing with mirror-reversed vision is accompanied by substantial proprioceptive suppression occurring in the primary somatosensory cortex (S1). This proprioceptive gating is gradually alleviated as performance increases with adaptation, returning to baseline levels. Peripheral and spinal evoked potentials were not modulated throughout, suggesting that the gating acted to reduce cortico-cortico excitability directly within S1. These modulations provide neurophysiological evidence for flexibility in sensory integration during visuomotor adaptation, which may functionally serve to reduce the sensory conflict until the visuo-proprioceptive mapping is updated.

On the nature of the vestibular control of arm-reaching movements during whole-body rotations

Recent studies report efficient vestibular control of goal-directed arm movements during body motion. This contribution tested whether this control relies (a) on an updating process in which vestibular signals are used to update the perceived egocentric position of surrounding objects when body orientation changes, or (b) on a sensorimotor process, i.e. a transfer function between vestibular input and the arm motor output that preserves hand trajectory in space despite body rotation. Both processes were separately and specifically adapted. We then compared the respective influences of the adapted processes on the vestibular control of arm-reaching movements. The rationale was that if a given process underlies a given behavior, any adaptive modification of this process should give rise to observable modification of the behavior. The updating adaptation adapted the matching between vestibular input and perceived body displacement in the surrounding world. The sensorimotor adaptation adapted the matching between vestibular input and the arm motor output necessary to keep the hand fixed in space during body rotation. Only the sensorimotor adaptation significantly altered the vestibular control of arm-reaching movements. Our results therefore suggest that during passive self-motion, the vestibular control of arm-reaching movements essentially derives from a sensorimotor process by which arm motor output is modified on-line to preserve hand trajectory in space despite body displacement. In contrast, the updating process maintaining up-to-date the egocentric representation of visual space seems to contribute little to generating the required arm compensation during body rotations.

Encoding the position of a flashed visual target after passive body rotations.

&NA; The capacity of the central nervous system (CNS) for processing vestibular signals during passive whole‐body rotations to update the internal representation of a visual target position in relation to the body was assessed. Results showed that subjects mislocalized previously presented visual targets after body rotations in complete darkness. Detailed analysis of the results suggested that the large target mislocalization stemmed not only from a systematic underestimation of rotation magnitude but also from the incapacity of the CNS to use the vestibular signals to accurately update the internal representation of the target position in relation to the body after passive rotations.

The relative contribution of retinal and extraretinal signals in determining the accuracy of reaching movements in normal subjects and a deafferented patient

This experiment investigated the relative extent to which different signals from the visuo-oculomotor system are used to improve accuracy of arm movements. Different visuo-oculomotor conditions were used to produce various retinal and extraretinal signals leading to a similar target amplitude: (a) fixating a central target while pointing to a peripheral visual target, (b) tracking a target through smooth pursuit movement and then pointing to the target when its excursion ceased, and (c) pointing to a target reached previously by a saccadic eye movement. The experiment was performed with a deafferented subject and control subjects. For the deafferented patient, the absence of proprioception prevented any comparison between internal representations of target and limb (through proprioception) positions during the arm movement. The deafferented patients endpoint therefore provided a good estimate of the accuracy of the target coordinates used by the arm motor system. The deafferented subject showed relatively good accuracy by producing a saccade prior to the pointing, but large overshooting in the fixation condition and undershooting in the pursuit condition. The results suggest that the deafferented subject does use oculomotor signals to program arm movement and that signals associated with fast movements of the eyes are better for pointing accuracy than slow ramp movements. The inaccuracy of the deafferented subject when no eye movement is allowed (the condition in which the controls were the most accurate) suggests that, in this condition, a proprioceptive map is involved in which both the target and the arm are represented.