A cerebellar-like terminal and postural tremor induced in normal man by transcranial magnetic stimulation.

Trains of repetitive transcranial magnetic stimulation (TMS) at 10-30 Hz and intensities of 90-120% motor threshold were delivered through a figure of eight coil over the motor cortex while normal subjects made either rapid, self-terminated (ballistic) wrist movements or maintained the position of their wrist at a fixed angle. Movement kinematics and EMG activity in antagonistic forearm muscles were analysed. In the ballistic task, repetitive TMS had little effect on the velocity or acceleration of the initial segment of the movement, although it induced large terminal oscillations (tremor) around the target position at frequencies between 4.4 and 7.2 Hz. The likelihood that tremor would occur increased with increasing stimulus intensities or frequencies. It was maximal with stimulation over the forearm area, and decreased with stimulation over the leg area, or over parietal sites; there was no tremor during stimulation of cervical nerve roots. The frequency of the induced tremor was independent of the rate of stimulation and did not depend on the presence of excitatory and inhibitory motor responses to the stimulus. Stimulation could also induce tremor of the same frequency in the fixed task, but only during co-contraction of forearm muscles. The amplitude of tremor was proportional to the level of co-contraction. Clinically, the tremor induced by repetitive TMS appeared very similar to cerebellar tremors. In order to confirm this we investigated two cerebellar patients, one with autosomal dominant cerebellar ataxia and the other with multiple sclerosis. Both of them had a terminal tremor of 6-7 Hz in the wrist movement task. In the holding task, the amplitude of their postural tremor increased with the level of co-contraction in forearm muscles. Since the frequency of repetitive TMS-induced tremor was independent of stimulus parameters, we conclude that it represents some intrinsic property of the CNS. We suggest that the tremor is caused by disruption of cortical processes involved in terminating a voluntary movement or maintaining a posture. Similarities to cerebellar patients suggest that repetitive TMS may cause tremor by interfering with adaptive cerebellar afferent inflow to motor cortex. Repetitive TMS-induced tremor, therefore, may represent a model of some forms of cerebellar tremor in man.

Cerebellar ataxia with glutamic acid decarboxylase autoantibodies.

Degenerative cerebellar ataxia with autoantibodies against glutamic acid decarboxylase (GAD) is a rare disorder and may represent a subset of ataxias previously classified as idiopathic. The authors report a patient with progressive cerebellar ataxia, insulin-dependent diabetes mellitus, and GAD antibodies who responded to i.v. immunoglobulins.

The effect of head-to-trunk position on the direction of arm movements before, during, and after space flight.

This contribution deals with the examination of the consequences of different head-to-trunk positions on arm movements under normal gravity and during prolonged space flight. One of the objectives of this study was to investigate the influence of weightlessness on the condition of the spatial analysis system. Aimed arm movements in the horizontal plane (pointings towards two visual targets) were recorded, first with eyes open, head straight (learning part), then with eyes closed, head straight and during yaw or roll position of the head (performance part). Measurements related to these different head-to-trunk-positions were taken in one short-term and nine long-term cosmonauts preflight, inflight, and postflight. Terrestrial control experiments were carried out with an extended experimental design in 14 healthy volunteers. The analysis of these experiments revealed that, with eyes closed and the head in yaw position, cosmonauts before flight and control subjects exhibit significant slants of the movement plane of the arm. Contrary to terrestrial measurements, in space experiments roll tilt of the head to the right is correlated with considerable counterclockwise slant of the movement plane. This slant of the movement plane of the arm was interpreted as tilt of the internal representation of the horizontal coordinate. The effect is larger with greater distortion induced by the changed head position and with larger muscular involvement to keep this position. This effect is also increased by the reduction of information (for example, in microgravity). The amount and the direction of the horizontal offset of the arm movements are shown to be dependent on the head-to-trunk position, too. Additionally, we have found changes in the amplitude and in the duration of the arm movement, in the vertical offset, and in the curvature of the movement paths, depending on the experimental conditions.

Pointing arm movements in short- and long-term spaceflights.

Accuracy and kinematics of horizontal arm pointing movements to visual targets were studied on three cosmonauts in 10-, 140-, and 172-d spaceflights in order to investigate mechanisms of the sensorimotor adaptation to microgravity. The Austrian equipment MONIMIR was mounted on board the Russian space station MIR and used for three-dimensional recording of the arm position and presentation of the targets. It was found that movement accuracy remained constant whereby movement durations significantly increased in all inflight sessions compared to the preflight baseline values. Inflight, movement peak velocities as well as acceleration and deceleration peak values decreased significantly. Analysis of the velocity-time profiles showed that the ratio between acceleration and deceleration phases decreased slightly for one cosmonaut and increased insignificantly for the other two cosmonauts. All phases of the acceleration-time profiles increased inflight by the same factor. These data fail to support the assumption of an increased role of the direct visual guidance in movement execution in microgravity. This suggests that the movement slowing in microgravity may be caused by a control strategy employed by the CNS to avoid the specific disadvantage of the absence of gravity. It is hypothesized that intra-movement control mechanisms play an important role in the movement coordination in the altered gravity environment.

Prehension with trunk assisted reaching.

For prehensile tasks, where objects are located beyond the normal reaching space, the trunk is bent forward to assist in the transport of the wrist to the object. Such task behaviors raise complex motor control issues such as how is the trunk movement incorporated into the motor plan. In this experiment, seated subjects were asked to reach and grasp a small and a large object placed on a table located beyond their maximal reach. Forward trunk bending was required to extend the reach distance. For such reaching movements, the wrist velocity consisted of a bell shape profile similar to those seen when the arm is the sole transport agent. In most trials, the trunk was the first to initiate movement, although there was no strict pattern of initiation order. The transport data showed that trunk and arm movement components were decoupled at the end of the reach. While the object was being grasped and lifted, the trunk continued moving for approximately 180 ms after the grasp. Wrist deceleration time expressed in absolute and relative values was sensitive to object size. The time from maximum peak aperture to the end of wrist movement also was significantly longer for grasping the small compared to the large object. No such relationships were observed for the trunk. Temporal coupling was only observed between the grip and wrist transport component. Time to maximum aperture was significantly correlated with time to peak wrist deceleration and only rarely with time to trunk deceleration peak. When the trunk participates in the transport of the wrist to an object, these findings suggest that only the wrist component is directly related to the achievement of the grasp. While the trunk assisted the arm to reach the object, the kinematic parameter recorded did not reveal any evidence of direct coupling. The presented data suggests that the planning takes place at the level of the hand and that endpoint is the primary variable controlled.

Grip reorganization during wrist transport: the influence of an altered aperture.

Past studies have examined the coupling of reach and grasp components during prehensile movements. Many of these studies have supported the view that these components reflect the output of two parallel, though temporally coupled, motor programs. When the grip aperture is Altered prior to the onset of prehension from its usual, normally flexed position to one of maximal finger extension, our previous work has shown that the grasp component appears to reorganize itself during the reach. This reorganization, consisting of a brief closing and reopening of the grip aperture, only slightly influenced the temporal components of the wrist transport. The present experiment continues this research theme by examining the characteristics of grip aperture reorganization through the comparison of the kinematics of prehension components during movements to two different size objects under normal and Altered grip aperture conditions. It was hypothesized that if the grip reorganization is task dependent it should be related to object size. The experiment found that in the Altered grip condition reorganization did occur, as indicated by a slight closing and reopening of the aperture without influencing the transport of the wrist. The amplitude of and the time to the observed inflection point in the aperture time course were related to object size. The velocity of grip closing for the large object showed double peaks, with the first substantially smaller than the second. Moreover, for the small object, the velocity of grip aperture closing also was double peaked, but the difference between peaks was less pronounced. These changes in grip velocity suggest that the grip reorganization is related to object size. No effect of Altered aperture was observed on the transport component. For both object sizes in the Altered condition, the final peak velocity of grip aperture was statistically significantly correlated with transport time and time to peak deceleration. In contrast, such correlations were not observed for the initial peak velocity of the grip aperture. Furthermore, time to maximum grip aperture was correlated with both time to peak wrist velocity and time peak to wrist deceleration. Thus, as the reach progressed toward the object, the grip and transport components became more interdependent. The results are consistent with the notion that, when a well-practiced, coordinated act such as prehension is confronted with an Altered grip posture at the onset of the reach, the grip can be reorganized during the transport to preserve the relative timing between them. Thus these data add to the growing awareness that not only is there temporal coupling between the reach and grasp components but that these components may be integrated by higher-order control mechanism.

A new method of fitting and analysis of simple uni-joint arm movements.

A new method for fitting and analysis of simple uni-joint arm movements is proposed. The method is based on a model which postulates that the acceleration-time profile of the movement can be described by a linear combination of two Gaussian functions (positive for acceleration and negative for deceleration). The method was tested on more than 21000 arm movements performed under different control conditions and showed high fitting precision. It allows to completely describe a movement using only five parameters of two Gaussian functions. The method is sensitive to differences between the acceleration and deceleration phases of movement as well as between subsequent movements because of the independent calculation of the Gaussian functions for accelerative and decelerative movement parts. Relationships between conventional kinematic and model parameters as well as areas of application of the method are discussed.