Changes in conditioned postural responses. Comparison between cerebellar patients and healthy subjects.
Postural responses elicited by external perturbation change characteristically during classical conditioning. This is assumed to be controlled by the cerebellum. In this study conditioning of postural responses in cerebellar patients was compared with that of healthy subjects. Subjects were tested when standing on a platform. Perturbations consisted of platform tilts (unconditioned stimulus, US), preceded by an auditory signal (conditioned stimulus, CS). The recording session consisted of US-alone and paired CS-US trials. In healthy subjects, unconditioned response (UR) amplitude decayed significantly with time in the recording session, especially strongly during paired trials. Amplitudes of cerebellar patients, however, decayed modestly and continuously, independently of the presence (paired trials) or otherwise of a CS. In addition, only healthy subjects established conditioned responses. Our data suggest that the prior auditory information is used to prepare postural responses. Deficits in cerebellar patients suggest a possible role of the cerebellum in controlling this plastic motor-related process.
Available from: Fay Horak
- "This integration and the scaling of gains appear to be mediated by the cerebellum (Dietz 1993). Indeed, cerebellumimpaired patients have difficulty scaling postural (Nashner 1976; Horak et al. 1989; Kolb et al. 2001) and stepping (Timmann and Horak 1998) responses according to context. The basal ganglia also appear to contribute to scaling of posture, as evidenced by poor postural responses in patients suffering from Parkinson's disease (Horak et al. 1992, 1996). "
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ABSTRACT: We tested whether human postural responses can be described in terms of feedback control gains, and whether these gains are scaled by the central nervous system to accommodate biomechanical constraints. A feedback control model can describe postural responses for a wide range of perturbations, but biomechanical constraints—such as on the torque that can be exerted on the ground—make a single set of feedback gains inappropriate for all perturbations. To observe how postural responses change with perturbation magnitude, we applied fast, backward perturbations of magnitudes 3–15 cm to 13 healthy young volunteers (4 men, 9 women, aged 20–32 years). We used a 3-segment, sagittal-plane biomechanical model and a linear state feedback controller to reproduce the observed postural responses. Optimization was used to identify the best-fit feedback control gains for each trial. Results showed that trajectories of joint angles and joint torques were scaled with perturbation magnitude. This scaling occurred gradually, rather than abruptly changing at magnitudes where biomechanical constraints became active. Feedback gains were found to fit reasonably well with data (R
2=0.92) and to be multivariate and heterogenic in character, meaning that the torque produced at any joint is generally a function of motions not only at the same joint, but other joints as well. Hip gains increased and ankle gains decreased nearly linearly with perturbation magnitude, in accordance with biomechanical limitations on ground reaction torque. These results indicate that postural adjustments can be described as a single feedback control scheme, with scalable heterogenic gains that are adjusted according to biomechanical constraints.
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ABSTRACT: Postural equilibrium is known to be controlled by sensorimotor reflexes and automatic control loops but also depends on high-level body representation in space, probably implicating the right temporoparietal cortex. Indeed, short-term prism adaptation to a 10 degrees rightward visual shift has been shown to reduce predominant postural imbalance in patients with right hemisphere damage, as it did for neglect symptoms. These effects are likely to be explained by a high level effect of prism adaptation on body and space representation, rather than by a sensorimotor effect. Cognitive after-effects of prism adaptation to a leftward visual shift, suggesting neglect-like symptoms, have also recently been shown in normal subjects on line bisection tasks. In the present study, we investigated the effect of wedge prism adaptation on postural control in normal subjects. Two groups of seven healthy subjects were either adapted to a leftward or a rightward visual shift. Results showed that our procedure induced changes in lateral postural control in normal subjects. Furthermore, this lateral postural after-effect was dependent on direction of prism adaptation. Indeed, only adaptation to a leftward visual shift induced significant rightward postural bias in normal subjects. The rightward postural lateral displacement was negatively correlated with the visual vertical. Both transfer and direction specific effect of visuo-manual adaptation to prisms on postural control suggest that effects of adaptation act more on high-level postural control linked to body representation in space or at least reveal close interaction between sensorimotor plasticity and body representation.
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ABSTRACT: The aim of the present study was to investigate the role of the human cerebellum in short-term (STH) and long-term habituation (LTH) of postural responses to repeated platform perturbations. Ten cerebellar patients and ten age- and sex-matched healthy controls participated. Twenty backward platform translations were applied on each of 5 consecutive days. Changes of postural response size within each day were assessed to determine STH and changes across days to determine LTH. Both controls and cerebellar patients showed a significant reduction of postural response size within each day (i.e. STH). No significant reduction of postural response size was observed across days (i.e. no LTH). Both controls and cerebellar patients, however, showed a tendency of response size to increase across days suggesting long-term sensitization. The amount of changes within and across days did not significantly differ between groups. The present findings suggest that changes of postural response size to repeated perturbations do not depend upon the integrity of the cerebellum.
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