Théophile Ohlmann

Selection of spatial frame of reference and postural control variability

Abstractu2002The present paper addresses the question of the possible links between perceptive visual field dependence-independence and the visual contribution to postural control. In our differential approach, visual field dependent (FD) and independent (FI) subjects were selected on the basis of their score in the Rod and Frame Test (subjective vertical). The hypothesis that we have tested is that the FD subjects use mainly visual cues for estimating not only their subjective vertical but also their body orientation and stability. Moreover, we have postulated that these subjects use mainly dynamic visual cues to control their postural stability. In the postural test, the selected subjects were instructed to stand in the sharpened Romberg position in darkness and under normal or stroboscopic illumination, in front of either a vertical or a tilted frame. Lateral head and body orientation and stability were measured. We found that: (1) all subjects leaned slightly towards the tilted frame (postural frame effect), and this was obtained on the basis of the static visual cues alone; (2) FD subjects were less stable than FI subjects, and their stability required the use of dynamic visual cues, mainly extracted from the vertical frame. In FI subjects, static visual cues may act as a complementary regulation, enhancing stability even with a strobe tilted frame. We thus demonstrate that visual field dependence interacts with the visual contribution to postural control.

Visual contribution to self-induced body sway frequencies and visual perception of male professional dancers

We studied the degree of dependence on vision, for postural control and for perception, among male adult dancers and untrained subjects. First, body sways were analyzed on a free seesaw platform. Fast Fourier transform processing allowed spectral frequency analysis of the platform sways recorded by an accelerometer. Secondly, a visual dependence test, the rod and frame test (RFT) was used. Professional dancers were significantly more stable and less dependent on vision for postural control and for perception than untrained subjects. Presumably, professional dance training strengthens the accuracy of proprioceptive inputs and shifts sensorimotor dominance from vision to proprioception. For the dancers, there was interaction between the RFT visual dependence and the visual control of posture: the less visual-dependent they were for the RFT, the more stable they were in dynamic balance conditions.

How dynamic visual field dependence–independence interacts with the visual contribution to postural control

Abstract The present study addresses the question of the links between visual perceptual dependence–independence and the visual contribution to postural control. In our differential approach, the effects of both static and dynamic visual cues on body posture (orientation and stabilisation) and the cognitive spatial orientation (subjective visual vertical) were considered and compared. In this view, nine dependent (DFD) and nine independent (DFI) male subjects with respect to dynamic visual field were selected on the basis of their score in the so-called rod and disc test (RDT). These subjects have also been subjected to the rod and frame Test (RFT), in order to establish their static visual field dependence–independence. In the postural test, the subjects were instructed to stand in the sharpened Romberg position in darkness and under normal or stroboscopic illumination, in front of either a vertical or a tilted frame. Lateral head and body orientation and stability were measured. The hypothesis is that the DFD subjects use mainly visual cues for estimating not only their subjective vertical but also their body orientation and stability. Moreover, we have postulated that these subjects use mainly dynamic visual cues to control balance. The main results are: (1) A strong correlation ( r =0.64) between the dependence–independence with respect to static and dynamic visual field. (2) A slight body lean towards the tilted frame (postural frame effect) in both DFD and DFI subjects, mainly induced by static visual cues. This effect, however, was greater in the DFD subjects. Moreover, dynamic field dependence was correlated with the postural frame effect in continuous vision. (3) DFD subjects were generally much less stable than DFI subjects. (4) In DFD subjects, the improvement of postural stability required mainly the use of dynamic visual cues, for the most part extracted from the vertical frame. DFI subjects were the only ones to be able to improve their stability by means of static visual cues. Moreover, postural performances were correlated with both dynamic and static field dependence. These results clearly show complex interactions between the perceptive styles and the visual contribution to postural control, which largely contribute to the between-subjects variability in balance skills.

Uses of Virtual Reality for Diagnosis, Rehabilitation and Study of Unilateral Spatial Neglect: Review and Analysis

Unilateral spatial neglect is a disabling condition frequently occurring after stroke. People with neglect suffer from various spatial deficits in several modalities, which in many cases impair everyday functioning. A successful treatment is yet to be found. Several techniques have been proposed in the last decades, but only a few showed long-lasting effects and none could completely rehabilitate the condition. Diagnostic methods of neglect could be improved as well. The disorder is normally diagnosed with pen-and-paper methods, which generally do not assess patients in everyday tasks and do not address some forms of the disorder. Recently, promising new methods based on virtual reality have emerged. Virtual reality technologies hold great opportunities for the development of effective assessment and treatment techniques for neglect because they provide rich, multimodal, and highly controllable environments. In order to stimulate advancements in this domain, we present a review and an analysis of the current work. We describe past and ongoing research of virtual reality applications for unilateral neglect and discuss the existing problems and new directions for development.

Differential approach to strategies of segmental stabilisation in postural control

The present paper attempts to clarify the between-subjects variability exhibited in both segmental stabilisation strategies and their subordinated or associated sensory contribution. Previous data have emphasised close relationships between the interindividual variability in both the visual control of posture and the spatial visual perception. In this study, we focused on the possible relationships that might link perceptual visual field dependence–independence and the visual contribution to segmental stabilisation strategies. Visual field dependent (FD) and field independent (FI) subjects were selected on the basis of their extreme score in a static rod and frame test where an estimation of the subjective vertical was required. In the postural test, the subjects stood in the sharpened Romberg position in darkness or under normal or stroboscopic illumination, in front of either a vertical or a tilted frame. Strategies of segmental stabilisation of the head, shoulders and hip in the roll plane were analysed by means of their anchoring index (AI). Our hypothesis was that FD subjects might use mainly visual cues for calibrating not only their spatial perception but also their strategies of segmental stabilisation. In the case of visual cue disturbances, a greater visual dependency to the strategies of segmental stabilisation in FD subjects should be validated by observing more systematic en bloc functioning (i.e. negative AI) between two adjacent segments. The main results are the following:1.Strategies of segmental stabilisation differed between both groups and differences were amplified with the deprivation of either total vision and/or static visual cues.2.In the absence of total vision and/or static visual cues, FD subjects have shown an increased efficiency of the hip stabilisation in space strategy and an en bloc operation of the shoulder–hip unit (whole trunk). The last en bloc operation was extended to the whole head–trunk unit in darkness, associated with a hip stabilisation in space.3.The FI subjects have adopted neither a strategy of segmental stabilisation in space nor on the underlying segment, whatever the body segment considered and the visual condition. Thus, in this group, head, shoulder and hip moved independently from each other during stance control, roughly without taking into account the visual condition.The results, emphasising a differential weighting of sensory input involved in both perceptual and postural control, are discussed in terms of the differential choice and/or ability to select the adequate frame of reference common to both cognitive and motor spatial activities. We assumed that a motor-somesthetics neglect or a lack of mastering of these inputs/outputs rather than a mere visual dependence in FD subjects would generate these interindividual differences in both spatial perception and postural balance. This proprioceptive neglect is assumed to lead FD subjects to sensory reweighting, whereas proprioceptive dominance would lead FI subjects to a greater ability in selecting the adequate frame of reference in the case of intersensory disturbances. Finally, this study also provides evidence for a new interpretation of the visual field dependence–independence dimension in both spatial perception and postural control.

Subjective vertical and postural activity.

Three experiments were conducted to test the effect of postural information, resulting from the active control of balance, on the perception of the vertical. Subjects were required to adjust a luminous rod in two different visual contexts: in the dark or within a tilted visual frame. In these experiments, postural activity was manipulated by placing observers either in a situation of slight postural imbalance (Experiment 1) or in a situation of postural immobilization (Experiment 2). In both situations performance was compared with a control condition in which subjects were standing upright freely (Experiment 1) or sitting unconstrained (Experiment 2). Results showed no main effect of active posture or of immobilization on the visual perception of the vertical. In the third experiment, subjects were supine with their Z body axis perpendicular to the plane of the luminous rod. Thus, body orientation relative to gravity was modified and motor activity reduced. In this position, the physical vertical was perceived quite accurately in a dark environment. Moreover, in the titled frame condition, the supine body position clearly improved vertical judgements. These results are discussed in relation to the ecological theory of orientation.

The role of head-centric spatial reference with a static and kinetic visual disturbance

A static or kinetic visual disturbance affects subjects’ ability to estimate the direction of the gravitational vertical. This kind of error is increased by a head roll inclination. In two experiments, we combined head orientation with a static (Experiment 1: tilted frame) versus kinetic (Experiment 2: rotating disk) visual disturbance. The results showed that with a static visual disturbance, the increase of errors in the inclined head condition was mainly the consequence of a postural head effect like an Aubert effect. On the contrary, with a kinetic visual disturbance, it appears that the disk effect increases with head inclination. However, individual errors observed with the head inclined in front of a stationary disk were systematically correlated with the errors triggered by the same head inclination in front of a rotating disk. These observations confirm that the head axis spatial reference plays an important role in orientation perception, whatever the head position and the kind of visual display.

Individual differences in the ability to identify, select and use appropriate frames of reference for perceptuo-motor control

The causes of the interindividual differences (IDs) in how we perceive and control spatial orientation are poorly understood. Here, we propose that IDs partly reflect preferred modes of spatial referencing and that these preferences or styles are maintained from the level of spatial perception to that of motor control. Two groups of experimental subjects, one with high visual field dependency (FD) and one with marked visual field independency (FI) were identified by the Rod and Frame Test, which identifies relative dependency on a visual frame of reference (VFoR). FD and FI subjects were tasked with standing still in conditions of increasing postural difficulty while visual cues of self-orientation (a visual frame tilted in roll) and self-motion (in stroboscopic illumination) were varied and in darkness to assess visual dependency. Postural stability, overall body orientation and modes of segmental stabilization relative to either external (space) or egocentric (adjacent segments) frames of reference in the roll plane were analysed. We hypothesized that a moderate challenge to balance should enhance subjects reliance on VFoR, particularly in FD subjects, whereas a substantial challenge should constrain subjects to use a somatic-vestibular based FoR to prevent falling in which case IDs would vanish. The results showed that with increasing difficulty, FD subjects became more unstable and more disoriented shown by larger effects of the tilted visual frame on posture. Furthermore, their preference to coalign body/VFoR coordinate systems lead to greater fixation of the head-trunk articulation and stabilization of the hip in space, whereas the head and trunk remained more stabilized in space with the hip fixed on the leg in FI subjects. These results show that FD subjects have difficulties at identifying and/or adopting a more appropriate FoR based on proprioceptive and vestibular cues to regulate the coalignment of posturo/exocentric FoRs. The FI subjects resistance in the face of altered VFoR and balance challenge resides in their greater ability to coordinate movement by coaligning body axes with more appropriate FoRs (provided by proprioceptive and vestibular co-variance).

Perception of longitudinal body axis in patients with stroke: a pilot study

Background and aims: To investigate the hypothesis that patients with a hemisphere stroke may perceive their longitudinal body axis (LBA) rotated in the frontal plane. This error in an egocentric frame of reference could be detrimental to posture, as tilted LBA would imply an unequal distribution of body mass about the true vertical. Method: 26 healthy subjects matched in age with 18 patients living with stroke participated in the study. The 18 patients were tested on average 80 days after a first left (nu200a=u200a8) or right (nu200a=u200a10) hemisphere stroke. Participants perceived their LBA by adjustments of the orientation of a luminous rod pivoting around a dorsonavel axis to the subjective direction of LBA. Participants were studied in the supine position to dissociate somaesthetic cues from graviceptive cues. Results: Patients with stroke perceived their LBA rotated to the contralesional side in comparison with controls (pu200a=u200a0.004). For all controls and 10 patients with stroke, the perceived LBA was very close to true LBA (mean (SD) 0.24° (1.31°)). For eight patients with stroke (six right stroke, two left stroke), the perceived LBA was rotated from true body orientation in the direction opposite to the lesioned side (range 3–9.5°, mean 5.2°). These eight patients provided similar estimates by tactile manipulation of the rod (without vision). The rotation of perceived LBA was more pronounced for right-hemisphere strokes. The magnitudes of perceptual rotations correlated with sensory loss, signs of spatial neglect and the degree of postural and gait disability. Conclusion: This is the first study showing that certain patients with a hemisphere stroke perceive their LBA rotated to the contralesional side. The consequences for perceptuomotor coordination have implications for their postural disorders.

Visual, proprioceptive and tactile performance in left neglect patients.

Patients with unilateral spatial neglect due to right-hemisphere lesions typically fail to attend to and explore left-sided stimulus objects. It has been postulated that in right-brain damaged (RBD) patients an ipsilesional displacement of the egocentric frame of reference (ER), whether visual or tactile, may be responsible for a contralesional supramodal spatial bias causing their left neglect behavior. However, this hypothesis had been proposed without testing, in the same patients, the position of the ER or their performance in the visual and tactile modalities. Thus, the aim of the present study was to test the hypothesis that an ipsilateral shift of the ER is responsible for a supramodal spatial bias in neglect. For this purpose, a within-subject design is required. Consequently, 12 left neglect patients and 12 control subjects were asked to perform a proprioceptive straight-ahead pointing task while blindfolded, as well as visual and tactile bisection tasks. In the left neglect patients, we found:no systematic deviation of the ER on the ipsilesional right side;a significant rightward bias in visual bisection, and normal performance in tactile bisection;no correlation among the three tasks;that only visual bisection correlated with the severity of neglect. These results are discussed with regard to the egocentric and attentional hypothesis of neglect.