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Acquisition of anticipatory postural adjustments in a bimanual load-lifting task: normal and pathological aspects.

Abstract Anticipatory adjustments of forearm posture are associated with a voluntary load-lifting movement in bimanual load-lifting tasks. Three aspects of these adjustments are analyzed: their goal, their central organization, and their acquisition. The goal of the anticipatory adjustment in this task is to minimize the perturbation of forearm posture that occurs during unloading. The central organization is based on two parallel controls responsible, respectively, for the lifting movement of the moving forearm and the anticipatory postural adjustment of the postural forearm, their coordination depending on a central timing signal. The acquisition of the anticipatory postural adjustment was tested using a paradigm where the voluntary movement performed by one hand triggered, via an electronic switch, the load release of the postural forearm. It was achieved after 40–60 trials and was not graded as a function of the voluntary movement parameters, but of the disturbance of the postural arm about to occur. The learned anticipation was not transferred when, after a first acquisition session with one forearm as the postural forearm, a second learning session was performed with the other forearm as the postural forearm. The acquisition was tested in Parkinsonian and in hemiparetic patients with capsular lesion. The highest acquisition deficit was observed in hemiparetic patients, when the contralateral forearm was the postural forearm; the deficit was less important when the ipsilateral arm was postural. Surprisingly, the anticipatory postural adjustments in hemiparetic patients were rather well preserved when the natural load-lifting task was tested. These results suggest that the basal-ganglia SMA circuit and M1 premotor areas are important in the acquisition process.

Pyramidal influences in establishment of new motor coordinations in dogs

Abstract Ablation of the motor cortex or section of the pyramidal tracts in dogs were studied for their effects on the performance of an alimentary instrumental reaction. The animal was required to keep its foreleg raised during feeding in order to gain access to food. The operated animals were capable of raising the leg in response to a signal and of holding it, but when snatching the food they immediately lower it. These dogs can, however, accomplish such a conditioned reflex if they do not have to lower the head during feeding. There may exist inborn reflexes from the cervical muscles or the vestibular apparatus ensuring the support on the forelegs when the head is lowered for feeding. When a new coordination including feeding with the leg raised, is elaborated, these inborn coordinations are inhibited. Evidently, inhibition of such inborn coordinations is effected through the pyramidal tract. The pyramidotomized animal is incapable of inhibiting the reation of lowering its leg when feeding with head down and cannot therefore keep its leg raised when feeding. However, there is no need of inhibiting inborn coordiantions when feeding with the head raised. For this reason pyramidotomized dogs are capable of keeping the leg raised in the latter condition.

Role of cerebellum in learning postural tasks.

For a long time, the cerebellum has been known to be a structure related to posture and equilibrium control. According to the anatomic structure of inputs and internal structure of the cerebellum, its role in learning was theoretically reasoned and experimentally proved. The hypothesis of an inverse internal model based on feedback-error learning mechanism combines feedforward control by the cerebellum and feedback control by the cerebral motor cortex. The cerebellar cortex is suggested to acquire internal models of the body and objects in the external world. During learning of a new tool the motor cortex receives feedback from the realized movement while the cerebellum produces only feedforward command. To realize a desired movement without feedback of the realized movement, the cerebellum needs to form an inverse model of the hand/ arm system. This suggestion was supported by FMRi data. The role of cerebellum in learning new postural tasks mainly concerns reorganization of natural synergies. A learned postural pattern in dogs has been shown to be disturbed after lesions of the cerebral motor cortex or cerebellar nuclei. In humans, learning voluntary control of center of pressure position is greatly disturbed after cerebellar lesions. However, motor cortex and basal ganglia are also involved in the feedback learning postural tasks.

Forearm postural control during unloading: anticipatory changes in elbow stiffness

Abstract In this study, the equilibrium-point hypothesis of muscle-torque generation is used to evaluate the changes in central control parameters in the process of postural-maintenance learning. Muscle torque is described by a linear spring equation with modifiable stiffness, viscosity, and equilibrium angle. The stiffness is considered to be the estimation of the central command for antagonist-muscle coactivation and the equilibrium angle to be the estimation of the reciprocal command for a shift of invariant characteristics of the joint. In the experiments, a load applied to the forearm was released. The subjects were instructed to maintain their forearm in the initial horizontal position. Five sessions of approximately twenty trials each were carried out by eight subjects. During two ”control” series, the load release was triggered by the experimenter. During three ”learning” series, the load supported by one forearm was released by the subject’s other hand. The elbow-joint angle, the angular acceleration, and the external load on the postural forearm were recorded. These recordings as well as anthropometric forearm characteristics were used to calculate the elbow-joint torque (which we called ”experimental”). Linear regression analysis was performed to evaluate the equilibrium angle, joint stiffness, and viscosity at each trial. The ”theoretical” torque was calculated using a linear spring equation with the found parameters. The good agreement observed between experimental and theoretical joint-torque time courses, apart from the very early period following unloading, argues in favor of the idea that the movement was mainly performed under a constant central command presetting the joint stiffness and the equilibrium angle. An overall increase in the stiffness occurred simultaneously with a decrease in the equilibrium angle during the ”learning” series in all the subjects. This suggests that subjects learn to compensate for the disturbing effects of unloading by increasing the joint stiffness. The mechanism possibly responsible for the presetting of the central control parameters is discussed.

Impairment of Learning the Voluntary Control of Posture in Patients with Cortical Lesions of Different Locations: the Cortical Mechanisms of Pose Regulation

The process of learning to produce voluntary changes in the position of the center of pressures using biological feedback was studied by stabilography in patients with hemipareses due to cerebrovascular lesions in the zone supplied by the middle cerebral artery. There were significant impairments to learning in all groups of patients, who had lesions in different sites, demonstrating that cortical mechanisms are involved in learning to control posture voluntarily. These studies showed that patients with lesions in the right hemisphere had rather greater deficits in performing the task than those with lesions in the left hemisphere. There were significant differences in the initial deficit in performing the task on the first day of training depending on the side of the lesion. All groups of patients differed from healthy subjects in that significant learning occurred only at the initial stages of training (the first five days). Learning at the initial stage in patients with concomitant lesions of the parietal-temporal area and with combined lesions with motor, premotor, and parietal-temporal involvement was significantly worse and the level of task performance at the end of the initial stage was significantly worse than in patient with local lesions of the motor cortex. The level of learning was independent of the severity of the motor deficit (paresis, spasticity), but was associated with the severity of impairment of the proprioceptive sense and the severity of disruption to the upright posture (asymmetry in the distribution of support pressures, amplitude of variation in the position of the center of pressures). The learning process had positive effects on the severity of motor impairment and on the asymmetry of the distribution of support pressures in the standing posture. Reorganization of posture during bodily movements occurred mainly because of impairment to the developed “non-use” stereotype of the paralyzed lower limb.

The ground reaction forces of postural adjustments during skilled reaching in unilateral dopamine-depleted hemiparkinson rats

Rats with unilateral dopamine (DA) depletions (hemiParkinson analogue rats) produced by intracerebral 6-hydroxydopamine injection are impaired in using the contralateral (bad) limbs for postural adjustments. This article examines whether the bad limbs are impaired in applying the forces required to initiate postural adjustments that anticipate and accompany voluntary movements. The rats were trained to reach for food using their good paw while standing on small platforms, each of which measured force changes produced by an individual limb. In one condition the force platforms were aligned to support the limb placement of normal rats and in the second they were aligned to permit the DA-depleted rats to use a compensatory reaching stance. It was found that the bad limbs of the DA-depleted rats produced normal supporting reactions but did not initiate adjustments in posture. Postural adjustments were initiated with the good limbs and preceded rather than accompanied the reaching movements. When constrained to use the posture of normal rats, the DA-deplete rats could not reach successfully, but when allowed to adjust their stance to increase reliance on the good limbs, reaching performance improved. Measures of ground reaction forces confirm that DA-depleted rats can support posture but cannot initiate postural adjustments with their impaired limbs.

Role of the sensorimotor cortex in postural adjustments accompanying a conditioned paw lift in the standing cat

SummaryThe role of the sensorimotor cortex in the postural adjustments associated with conditioned paw lifting movements was investigated in the cat. Cats were trained to stand quietly on four strain gauge equipped platforms and to perform a lift-off movement with one forelimb when a conditioned tone was presented. The parameters recorded were the vertical forces exerted by the paws on each platform, the lateral and antero-posterior displacements of rods implanted on the T2, T12, L5 vertebrae as well as their rotation, and the EMG of triceps and biceps of both forelimbs. Before lesion, the postural adjustment consisted of a “nondiagonal” pattern where the CG was displaced laterally inside the triangle formed by the three remaining supporting limbs. Here a lateral bending of the thoracic column toward the supporting forelimb could be observed. The associated EMG pattern consisted of an early activation of the triceps lateral head in the moving limb which was probably responsible for the body displacement toward the opposite side, and a late biceps activation associated with the lift. In the supporting forelimb, a coactivation of the biceps and triceps was usually present. After contralateral sensorimotor lesion, the conditioned lifting movements were lost for 4–15 days after the lesion, before being subsequently recovered. The same lateral CG displacement and bending of the back was seen after lesion as before, which indicates that the goal of postural adjustment was preserved. However, the means of reaching it were modified. In most of the intact animals, the CG displacement was achieved in one step, whereas in the animals with lesions, the displacement was made either according to a slow ramp mode or in a discontinuous manner involving several steps. The mechanisms responsible for this disturbance are discussed.

Brain Mechanisms for the Formation of New Movements during Learning: The Evolution of Classical Concepts

Current concepts hold that the role of the motor cortex is limited to the control of the appropriate motoneurons on the “point-to-point” principle during the performance of specialized movements of the distal parts of the limbs. However, the last decade has seen the appearance of many data on the plasticity of the motor cortex and its active participation in the process of motor learning. Expression of fos genes has been observed in the motor cortex during the formation of specialized movements. Increases in intracortical horizontal connections in layers II–III during learning fine movements has been seen. The cholinergic input to layers II–III of the motor cortex plays a significant role in this. At the same time, data obtained by functional brain mapping have provided evidence that the activity of the motor cortex also increases during the practice of previously learned movements. This raises the question of the specific function of the motor cortex in the process of motor learning. During the formation of new movements during motor training, a number of previously used synergies interfere with the performance of newly formed coordinations and must be inhibited. The central mechanisms of interference of coordinations in humans have only just started to receive study. At the same time, there is an experimental model for the reorganization and inhibition of interfering synergies in animals. Reorganization of coordinations and inhibition of synergies interfering with the performance of a new movement have been shown to be a specific function of the motor area of the cortex. Cortical control persists during the automation of these synergies, which is not the case in other types of learned movements, though this in itself does not mean that conscious control of their performance also persists.Current concepts hold that the role of the motor cortex is limited to the control of the appropriate motoneurons on the “point-to-point” principle during the performance of specialized movements of the distal parts of the limbs. However, the last decade has seen the appearance of many data on the plasticity of the motor cortex and its active participation in the process of motor learning. Expression of fos genes has been observed in the motor cortex during the formation of specialized movements. Increases in intracortical horizontal connections in layers II–III during learning fine movements has been seen. The cholinergic input to layers II–III of the motor cortex plays a significant role in this. At the same time, data obtained by functional brain mapping have provided evidence that the activity of the motor cortex also increases during the practice of previously learned movements. This raises the question of the specific function of the motor cortex in the process of motor learning. During the formation of new movements during motor training, a number of previously used synergies interfere with the performance of newly formed coordinations and must be inhibited. The central mechanisms of interference of coordinations in humans have only just started to receive study. At the same time, there is an experimental model for the reorganization and inhibition of interfering synergies in animals. Reorganization of coordinations and inhibition of synergies interfering with the performance of a new movement have been shown to be a specific function of the motor area of the cortex. Cortical control persists during the automation of these synergies, which is not the case in other types of learned movements, though this in itself does not mean that conscious control of their performance also persists.

Coordination between posture and movement in a bimanual load-lifting task: is there a transfer?

The present experimental series was designed to test the possibility that an anticipatory postural adjustment learned during the performance of a bimanual load lifting task may be transferred between the upper extremities. Eight seated subjects were asked to maintain horizontally one forearm (postural arm) loaded with a 1kg load, which was fixed to the arm by means of an electromagnet. The unloading was triggered either by the experimenter pressing a switch (control) or by the subjects making a voluntary movement with their other arm (moving arm). In the latter case, the subject lifted a 1-kg load resting on a force platform with the moving hand, and the switching off was triggered when the force level reached a threshold of 0.5 kg. The maximum amplitude (MA) and the maximum velocity (MV) of the postural forearm elbow joint rotation occuring after the unloading were measured at each trial. The learning process was estimated by performing a regression analysis on each series of trials, using an exponential model, and from the intercept of the regression curve with the ordinate. 1. During the original learning session (three series of 20 trials), a decrease in MA and MV was found to occur both within the series and between the series during a session. 2. After the initial learning session, the sides of the postural and moving arm were interchanged to test whether any transfer had occurred. The first series of trials in the second session (transfer) and the last series of trials in the original learning session were compared and found to be significantly different in terms of the intercept (seven subjects in the case of MA, five subjects in the case of MV) and the slope (five subjects), indicating a lack of transfer. 3. The data recorded during the second transfer learning session indicated that learning occurred in all eight subjects in the case of MA and in six subjects in the case of MV. It was observed that the original learning session did not facilitate the second one. 4. The lack of transfer of the anticipatory postural adjustment observed in this task is discussed with reference to the data in the literature.

On The Role of Motor Cortex in the Learned Rearrangement of Postural Coordinations

Two kinds of postural patterns opposite to inborn ones were elaborated in dogs. Some dogs were trained to avoid electrical stimulation of left forelimb by lifting the limb and keeping it lifted for 5s. This reaction had to be accompanied by decreasing the pressure on the support of the ipsilateral hindlimb, so the inborn “diagonal” postural pattern (unloading the limb diagonally opposite to the lifted one and loading the other pair of limbs) was rearranged into the “ipsilateral” one. The other dogs were trained to escape electrical stimulation of one forelimb by increasing the support pressure of the limb (this reaction is opposite to the inborn flexor reflex). Ablation of the motor cortex contralateral to the performing limb resulted in temporary disturbances of the rearranged postural patterns. After bilateral ablation of the motor cortex the elaborated postural patterns disappeared and mainly inborn postural coordinations were performed. The program of the elaborated reaction was maintained and mainly the executive mechanisms were disturbed (inhibition of the inborn reactions). The motor cortex probably controls the rearrangement of postural coordinations modulating functions of the appropriate brain stem structures.