Jürgen Konczak

It has been suggested that the movement impairments
experienced by patients with neglect are not
restricted to spatial disorders, but also affect higher-order
kinematics (velocity and acceleration) to the extent that
movements towards the neglected side are slower than
movements away from it. In a recent study, we could not
confirm this hypothesis, but found that patients with unilateral
neglect exhibited no distinct direction-specific
deficits in hand velocity when performing goal-directed
reaching movements. Here we investigated whether neglect
patients might reveal direction-specific deficits
during exploratory hand movements. Six patients with
left-sided neglect and six age-matched healthy control
subjects scanned with their right hands the surface of a
large table searching for a (non-existent) tactile target.
Movements were performed in darkness. Time-position
data of the hand were recorded with an optoelectronic
camera system. Median activity of the patients’ exploratory
hand movements was shifted to the right (Karnath
and Perenin 1998). Hand trajectories were partitioned into
sections of leftward/rightward or, along the sagittal
plane, into sections of near/far movements. For each
movement section average and peak velocities were
computed. The patients’ hand movements were bradykinetic
when compared with the control group. However,
we found no evidence that average or peak velocities of
leftward intervals were systematically lower than during
rightward motion. Direction-specific deficits in velocity
were also not observed for movements to and away from
the body (sagittal plane). In conclusion, we found evidence
for general bradykinesia in neglect patients but not
for a direction-specific deficit in the control of hand velocity
during exploratory hand movements.

In cerebellar ataxia, kinematic aberrations of multijoint movements are thought to originate from deficiencies in generating muscular torques that are adequate to control the mechanical consequences of dynamic inter- action forces. At this point the exact mechanisms that lead to an abnormal control of interaction torques are not known. In principle, the generation of inadequate muscular torques may result from an impairment in generating sufficient levels of torques or from an inaccurate assessment and prediction of the mechanical consequences of movements of one limb segment on adjacent joints. We sought to differentiate the relative contribution of these two mechanisms and, therefore, analyzed intersegmental dynamics of multijoint pointing movements in healthy subjects and in patients with cerebellar degeneration. Un- restrained vertical arm movements were performed at three different target movement velocities and recorded using an optoelectronic tracking system. An inverse dynamics approach was employed to compute net joint torques, muscular torques, dynamic interaction torques and gravitational torques acting at the elbow and shoulder joint. In both groups, peak dynamic interaction forces and peak muscular forces were largest during fast movements. In contrast to normal subjects, patients produced hypermetric movements when executing fast movements. Hypermetric movements were associated with smaller peak muscular torques and smaller rates of torque change at elbow and shoulder joints. The patients􏰅 deficit in generating appropriate levels of muscular force were prominent during two different phases of the pointing movement. Peak muscular forces at the elbow were reduced during the initial phase of the movement when simultaneous shoulder joint flexion generated an extensor influence upon the elbow joint. When attempting to terminate the movement, gravitational and dynamic interaction forces caused overshooting extension at the elbow joint. In normal subjects, muscular torque patterns at shoulder and elbow joint were synchronized in that peak flexor and extensor muscular torques occurred simultaneously at both joints. This temporal pattern of muscular torque generation at shoulder and elbow joint was preserved in patients. Our data suggest that an impairment in generating sufficient levels of phasic muscular torques significantly contributes to the patients􏰅 difficulties in controlling the mechanical consequences of dynamic interaction forces during multijoint movements.

Kinematic abnormalities of fast multijoint movements in cerebellar ataxia include abnormally increased curvature of hand trajectories and an increased hand path and are thought to originate from an impairment in generating appropriate levels of muscle torques to support normal coordination between shoulder and elbow joints. Such a mechanism predicts that kinematic abnormalities are pronounced when fast movements are performed and large muscular torques are required. Experimental evidence that systematically explores the effects of increasing movement velocities on movement kinematics in cerebellar multijoint movements is limited and to some extent contradictory. We, therefore, investigated angular and hand kinematics of natural multijoint pointing movements in patients with cerebellar degenerative disorders and healthy controls. Subjects performed self-paced vertical pointing movements with their right arms at three different target velocities. Limb movements were recorded in three-dimensional space using a two-camera infra-
red tracking system. Differences between patients and healthy subjects were most prominent when the subjects performed fast movements. Peak hand acceleration and deceleration were similar to normals during slow and moderate velocity movements but were smaller for fast
movements. While altering movement velocities had little or no effect on the length of the hand path and angular motion of elbow and shoulder joints in normal subjects, the patients exhibited overshooting motions (hypermetria) of the hand and at both joints as movement velocity increased. Hypermetria at one joint always accompanied hypermetria at the neighboring joint. Peak elbow angular deceleration was markedly delayed in patients compared with normals. Other temporal movement variables such as the relative timing of shoulder and elbow joint motion onsets were normal in patients. Kinematic abnormalities of multijoint arm movements in cerebellar ataxia include hypermetria at both the elbow and the shoulder joint and, as a consequence, irregular and enlarged paths of the hand, and they are marked with fast but not with slow movements. Our findings suggest that kinematic movement abnormalities that characterize cerebellar limb ataxia are related to an impairment in scaling movement variables such as joint acceleration and deceleration normally with movement speed. Most likely, increased hand paths and decomposition of movement during slow movements, as described earlier, result from compensatory mechanisms the patients may employ if maximum movement accuracy is required.

Do patients with unilateral neglect exhibit direction-specific deficits in the control of movement velocity when performing goal-directed arm movements? Five patients with left-sided neglect performed unrestrained three-dimensional pointing movements to visual targets presented at body midline, the left and right hemispace. A group of healthy adults and a group of patients with right-hemispheric brain damage but no neglect served as controls. Pointing was performed under normal room light or in darkness. Time-position data of the hand were recorded with an opto-electronic camera system. We found that compared to healthy controls, movement times were longer in both patient groups due to prolonged acceleration and deceleration phases. Tangential peak hand velocity was lower in both patient groups, but not significantly different from controls. Single peak, bell-shaped velocity profiles of the hand were preserved in all right hemispheric patients and in three out of five neglect patients. Most important, the velocity profiles of neglect patients to leftward targets did not differ significantly from those to targets in the right hemispace. In summary, we found evidence for general bradykinesia in neglect patients, but not for a direction-specific deficit in the control of hand velocity. We conclude that visual neglect induces characteristic changes in exploratory behavior, but not in the kinematics of goal-directed movements to objects in peripersonal space.

Neuromagnetic responses to separate tactile stimulation of digits I, II and V and simultaneous stimulation of digit pairs II and I, and II and V, were recorded in six healhty adult subjects using a 122-channel whole-head neuromagnetometer in order to investigate functional overlap of finger representations in primary somatosensory cortex (SI). Evoked responses to single digit stimulation were explained by time-varying equivalent current dipoles (ECDs) located in SI. These ECDs were then used to explain responses to stimulation of digit pairs. A cortical interaction ratio (IR( was defined as the vector sum of peak source amplitudes to separate stimulation of two fingers divided by the vector sum of source amplitudes to simultaneous stimulation of the two digits. Mean IR was significantly higher (P<0.05; Wilcoxon test) for digital pair II + I (1.69 +/- 0.15) compared to digit pair II + V ((1.14 +/- 0.12). These results indicate that there is an overlap of finger representations in human SI which differs between anatomically adjacent and non-adjacent digit pairs.

The aim of the present paper is to propose that the adoption of a framework of biological development is suitable for the construction of artificial systems. We will argue that a developmental approach does provide unique insights on how to build highly complex and adaptable artificial systems. To illustrate our point, we will use as an example the acquisition of goal-directed reaching. In the initial part of the paper we will outline (a) how mechanisms of biological development can be adapted to the artificial world, and (b) how this artificial development differs from traditional engineering approaches to robotics. An experiment performed on an artificial system initially controlled by motor reflexes is presented, showing the acquisition of visuomotor maps for ballistic control of reaching without explicit knowledge of the system’s kinematic parameters.

Knowledge of how stiffness, damping and the equilibrium position of specific limbs change during voluntary motion is important for understanding basic strategies of neuromotor control. Presented here is an algorithm for identifying time-dependent changes in joint stiffness, damping, and equilibrium position of the human forearm. The procedure requires data from only a single trial. The method relies neither on an analysis of the resonant frequency of the arm nor on the presence of an external bias force. Its validity was tested with a stimulated forward model of the human forearm. Using the parameter estimations as forward model input, the angular kinematics (model output) were reconstructed and compared to the empirically measure data. Identification of mechanical impedance is based on a least-squares solution of the model equation. As a regularization technique and to improve the temporal resolution of the identification process, a moving temporal window with a variable width was imposed. The method's performance was tested by (a) identifying a priori known hypothetical time-series of stiffness, damping and equilibrium position, and (b) determining impedance parameters from recorded single joint forearm movements during a hold and a goal-directed movement task. The method reliably reconstructed the original angular kinematics of the artificial and human data with an average positional error of less than .05 rad for movement amplitudes of up to 0.9 rad, and did not yield hypermetric trajectories like previous procedures not accounting for damping.

It has been suggested that the movement impairments experienced by patients with neglect are not restricted to spatial disorders, but also affect higher order kinematics (velocity and acceleration) to the extent that movements towards the neglected side are slower than movements away from it. In a recent study, we could not confirm this hypothesis, but found that patients with unilateral neglect exhibited no distinct direction-specific deficits in hand velocity when performing goal-directed reaching movements. Here we investigated whether neglect patients might reveal direction-specific deficits during exploratory hand movements. Six patients with left-sided neglect and six age-matched healthy control subjects scanned with their right hands the surface of a large table searching for a (non-existent) tactile target. Movements were performed in darkness. Time-position data of the hand were recorded with an optoelectronic camera system. Median activity of the patients’ exploratory hand movements was shifted to the right (Karnath and Perenin 1998). Hand trajectories were partitioned into sections of leftward/rightward or, along the sagittal plane, into sections of near/far movements. For each movement section average and peak velocities were computed. The patients’ hand movements were bradykinetic when compared with the control group. However, we found no evidence that average or peak velocities of leftward intervals were systematically lower than during rightward motion. Direction-specific deficits in velocity were also not observed for movements to and away from the body (sagittal plane). In conclusion, we found evidence for general bradykinesia in neglect patients but not for a direction-specific deficit in the control of hand velocity during exploratory hand movements.

Short term vibration of the dorsal neck muscles (10–35 s) is known to induce involuntary movements of the head in patients with spasmodic torticollis. To investigate whether neck muscle vibration might serve as a therapeutic tool when applyed for a longer time interval, we compared a vibration interval of 5 seconds with a 15 minute interval in a patient with spasmodic torticollis with an extreme head tilt to the right shoulder. Head position was recorded with a two camera optoelectronic motion analyzer in six diVerent test conditions. Vibration regularly induced a rapid change of head position that was markedly closer to a normal, upright posture. After 5 seconds of vibration, head position very quickly returned to the initial position within seconds. During the 15 minute interval, head position remained elevated. After terminating vibration in this condition, the corrected head position remained stable at first and then decreased slowly within minutes to the initial tilted position. Conclusions—(1) In this patient, muscle vibration was the specific sensory input that induced lengthening of the dystonic neck muscles. Neither haptic stimulation nor transcutaneous electrical stimulation had more than a marginal eVect. (2) The marked diVerence in the change of head position after short and prolonged stimulation supports the hypothesis that spasmodic torticollis might result from a disturbance of the central processing of the aVerent input conveying head position information—at least in those patients who are sensitive to sensory stimulation in the neck region. (3) Long term neck muscle vibration may provide a convenient non-invasive method for treating spasmodic torticollis at the central level by influencing the neural control of head on trunk position.

Within the context of the Ebbinghaus illusion, adults regularly misjudge the physical size of a centre disc, yet scale their hand aperture according to its actual size. Separate visual pathways for perception and action are assumed to account for this finding. The dorsal visual stream is said to elaborate on egocentric (visuomotor), while the ventral stream is involved in allocentric transformations (object recognition). This study examines the ontogenetic development of this dissociation between perception and action in 35 children between the ages of 5 and 12 years. We report four major results. First, when children judged object size without grasping the disc, their judgements were deceived by the illusion to the same extent as adults. However, when asked to estimate size and then to grasp the disc, young children's (5–7 years) perceptual judgements became unreliable, while adults were still reliably deceived by the illusion in 80% of their trials. Second, the younger the children, the more their aperture was affected by the illusional surround. Discs of the same size were grasped with a smaller aperture when surrounded by a small annulus, although they were perceived as being larger. Third, young children used the largest safety margin during grasping. Fourth, the reliance on visual feedback decreased with increasing age, which was documented by shorter movement times and earlier maximum hand opening during grasping in the older children (feedforward control). Our results indicate that grasping behaviour in children is subject to an interaction between ventral and dorsal processes. Both pathways seem not to be functionally segregated in early and middle childhood. The data are inconclusive about whether young children predominantly use a specific visual stream for either a perceptual or motor task. However, our data demonstrate that children were relying on both visual processing streams during perceptual as well as visuomotor tasks. We found that children used egocentric cues to make perceptual judgements, while their grasping gestures were not exclusively shaped by viewer-centred but also by object-centred information .

Neurobiological evidence reveals that neurally coded inverse models of limb dynamics form the basis for feed-forward motor control in humans. This study investigates the role of visual feedback for the acquisition of inverse motor models in children and adults. Eight 9-year-old and 5-year-old children and eight adults performed goal-directed horizontal forearm movements using a single-joint arm manipulandum. When visual feedback was not available before and after movement execution (partial feedback), spatial error increased in adults and children. however, the lack of visual information during the movement execution did not affect adult motor performance. In contrast, spatial error increased in both children groups when visual feedback was removed. Spatial accuracy was improved during the partial feedback condition, if children had prior practice under full visual feedback. The increased dependence on visual feedback, especially in the younger children, is a sign that children relied predominantly on central feedback mechanisms for motor control, because their feed--forward control was not yet functional. The reasons for the lack of feed-forward control are twofold: First, there are problems in motor planning, specifically with the inverse kinematics transformation (from hand position to join angles). Second, there are deficits in the neural controller, specifically due to imprecise neural estimations of the true limb dynamics.

When humans perform goal-directed arm movements under the influence of an external damping force, they learn to adapt to these external dynamics. After removal of the external force field, they reveal kinematic aftereffects that are indicative of a neural controller that still compensates the no longer existing force. Such behavior suggests that the adult human nervous system uses a neural representation of inverse arm dynamics to control upper-extremity motion. Central to the notion of an inverse dynamic model (IDM) is that learning generalizes. Consequently, aftereffects should be observable even in untrained workspace regions. Adults have shown such behavior, but the ontogenetic development of this process remains unclear. This study examines the adap-tive behavior of children and investigates whether learning a force field in one hemifield of the right arm work-space has an effect on force adaptation in the other hemi-field. Thirty children (aged 6–10 years) and ten adults performed 30° elbow flexion movements under two conditions of external damping (negative and null). We found that learning to compensate an external damping force transferred to the opposite hemifield, which indicates that a model of the limb dynamics rather than an association of visited space and experienced force was acquired. Aftereffects were more pronounced in the younger children and readaptation to a null-force condition was prolonged. This finding is consistent with the view that IDMs in children are imprecise neural representations of the actual arm dynamics. It indicates that the acquisition of IDMs is a developmental achievement and that the human motor system is inherently flexible enough to adapt to any novel force within the limits of the organism's biomechanics.

Neurobiological evidence reveals that neurally coded inverse models of limb dynamics form the basis for feed-forward motor control in humans. This study investigates the role of visual feedback for the acquisition of inverse motor models in children and adults. Eight 9-year-old and eight 5-year-old children and eight adults performed goal-directed horizontal forearm movements using a single-joint arm manipulandum. When visual feedback was not available before and after movement execution did not affect adult motor performance. In contrast, spatial error increased in both children groups when visual feedback was removed. Spatial accuracy was improved during the partial feedback condition, if children had prior practice under full visual feedback. The increased dependence on visual feedback, especially in the younger children, is a sign that children relied predominantly on central feedback mechanisms for motor control, because their feed-forward control was not yet functional. The reasons for the lack of feed-forward control are twofold: first, there are problems in motor planning, specifically with the inverse kinematic transformation (from hand position to joint angles). Second, there are deficits in the neural controller, specifically due to imprecise neural estimations of the true limb dynamics.

this study addresses the question of whether external timing signals and/or simultaneous rhythmic movements of other limbs can alleviate sequencing motor deficits associated with Parkinson's disease (PD). Subjects performed rhythmic lip and finger movements simultaneously or in isolation. In addition, they had to self-pace their movements or match them to an external signal. Our results are summarized as follows: (a) Seven of 12 patients had adequate mean repetition rates; that is, they fulfilled the task requirements on a global scale. the remaining five patients had various degrees of hastened responses and were not fully able to synchronize their movements to an external pacing signal. (b) PD patients exhibited hypometria in their finger tapping, but not in their lip movements. their movements were not abnormally slowed, but peak velocity was appropriately scaled, even reduced to movement amplitudes. (c) Mean repetition rates, stability of frequency response, and kinematics did not differ between conditions of external and internal stimulation within the Pd group, but were different from the control group performance. (d) Kinematic measures were not improved ruing dual-task execution. PD patients were not able to maintain a 1:1 rhythm between effectors. The incidence of hastening increased during simultaneous motor execution. We conclude that the use of external pacing signals might aid movement initiation of PD patients, but does not improve their temporal or spatial coordination when generating repetitive movements. Simultaneous execution does not necessarily enhance motor performance, but might actually have detrimental effects in patients prone to hastening.

Nine young infants were followed longitudinally from 4 to 15 months of age. They performed multi joint reaching movements to a stationary target presented at shoulder height. Time-position data of the hand, shoulder, and elbow were collected using an optoelectronic measurement system. In addition, we recorded electromyographic activity (EMG) from arm extensors and flexors. This paper documents how control problems of proximal torque generation may account for the segmented hand paths seen during early reaching. Our analysis revealed the following results: first, muscular impulse (integral of torque) increased significantly between the ages of 20 (reaching onset) and 64 weeks. That is, as infants got older they produced higher levels of mean muscular flexor torque during reaching. Data were normalized by body weight and movement time, so differences are not explained by anthropometric changes or systematic variations in movement time. second, while adults produced solely flexor muscle torque to accomplish the task, infants generated flexor and extensor muscle torque at shoulder and elbow throughout a reach. At reaching onset more than half of the trials revealed this latter kinetic profile. Its frequency declined systematically as infants got older. Third, we examined the pattern of muscle coordination in those trials that exhibited elbow extensor muscle torque. We found that during elbow extension co-activation of flexor and extensor muscles was the predominant pattern in 67% of the trials. This pattern was notably absent in comparable adult reaching movements. Fourth, fluctuations in force generation, as measured by the rate of change of total torque (NET) and muscular torque (MUS), were more frequent in early reaching (20-28 weeks) than in the older cohort (52-64 weeks), indicating that muscular torque production became increasingly smoother and task-efficient. Our data demonstrate that young infants have problems in generating smooth profiles of proximal joint torques. One possible reason for this imprecision in infant force control is their inexperience in predicting the magnitude and direction of external forces. That infants learned to consider external forces is documented by their increasing reliance on these forces when performing voluntary elbow extensions. The patterns of muscle coordination underlying active elbow extensions were basically the same as during the prereaching phase, indicating that the formation of functional synergies is based on basal repertoire of innervation patterns already observable in very early, spontaneous movements.

Recent clinical data indicate that internal cueing mechanisms required for the triggering of movement sequences are impaired in Parkinson's disease (PD). Nevertheless , most PD subjects produce maximal syllable repetition rates similar to those observed in healthy control individuals during oral diadochokinesis tasks. There is some evidence that tremor oscillations may pace repetitive movements in Parkinso-nians giving rise to hastening phenomena. Conceivably, the performance of PD patients in syllable repetition tasks thus reflects a specific timing deficit, i.e., articula-tory hastening. It is the aim of the present study to investigate the contribution of speech hastening to oral diadochokinesis in the presence of internal and external cues. By means of an optoelectric movement analysis system, the displacements of the lips during repetitions of the syllable /pa/ were recorded in two akinetic-rigid PD individuals. Subjects were asked to synchronize labial diadochokinesis to sequences of periodic acoustic stimuli (2.5–6 Hz). One of the PD patients showed speech hastening, i.e., he produced repetitions of 8 to 9 Hz whenever stimulus frequencies exceeded 4 Hz. The other Parkinsonian adequately matched the stimulus frequencies required. However, she achieved a higher diadochokinesis rate in the matching task than under the instruction to repeat ''as fast as possible.'' Thus, the presence of an external cue improved performance. In conclusion, our data indicate two deficits of the temporal control of repetitive articulatory gestures in PD: speech hastening and impaired self-paced sequencing. These two pathomechanisms may allow to reconcile the controversial findings on oral diadochokinesis in PD reported so far.

The present study investigated unrestrained, three-dimensional arm movements during goal-directed pointing in five patients with clinically manifest neglect to targets positioned either in the center or the left and right hemispace. Five patients with unilateral right hemispheric lesions without neglect and six healthy subjects served as controls. All subjects were able to point to these targets. Terminal accuracy of pointing did not differ between the three groups along the horizontal, vertical and anterior posterior axis. Subjects' hand trajectories did not reveal direction-specefic deviations from a straight-line hand path. Our data show that deviations in the trajectories toward the ipsilesional side are not characteristic for patients with spatial neglect. We argue that exploratory and goal-directed behavior might not share the same egocentric, body-centered reference frame. A spatial reference by eye or limb movements. Its failure does not induce a spatial bias in hand trajectory formation during goal-directed arm movements in peripersonal space. Such deviations of reaching or pointing rather seem to be characteristic for patients with optic ataxia.

We recorded reaching movements from nine infants longitudinally from the onset of reaching (5th postnatal month) up to the age of 3 years. Here we analyze hand and proximal joint trajectories and examine the emerging temporal coordination between arm segments. The present investigation seeks (a) to determine when infants acquire consistent, adult-like patterns of multijoint coordination within that 3-year period, and (b) to relate their hand trajectory formation to underlying patterns of proximal joint motion (shoulder, elbow). Our results show: First, most kinematic parameters do not assume adult-like levels before the age of 2 years. At this time, 75% of the trials reveal a single peaked velocity profile of the hand. Between the 2nd and 3rd year of life, ªimprovementsº of hand-or joint-related movement units are only marginal. Second, infant motor systems strive to obtain velocity patterns with as few force reversals as possible (uni-or bimodal) at all three limb segments. Third, the formation of a consistent interjoint synergy between shoulder and elbow motion is not achieved within the 1st year of life. Stable patterns of temporal coordination across arm segments begin to emerge at 12±15 months of age and continue to develop up to the 3rd year. In summary , the development toward adult forms of multijoint coordination in goal-directed reaching requires more time than previously assumed. Although infants reliably grasp for objects within their workspace 3±4 months after the onset of reaching, stereotypic kinematic motor patterns are not expressed before the 2nd year of life.

Nine young infants were followed longitudinally from 4 to 15 months of age. We recorded early spontaneous movements and reaching movements to a stationary target. Time-position data of the hand (end-point), shoulder, and elbow were collected using an opto-electronic measurement system (ELITE). We analyzed the endpoint kinematics and the intersegmental dynamics of the shoulder and elbow joint to investigate how changes in proximal torque control determined the development of hand trajectory formation. Two developmental phases of hand trajectory formation were identified: a first phase of rapid improvements between 16 and 24 weeks of age, the time of reaching onset for all infants. During that time period the number of movement units per reach and movement time decreased dramatically. In a second phase (28-64 weeks), a period of "fine-tuning" of the sensorimotor system, we saw slower, more gradual changes in the endpoint kinematics. The analysis of the underlying intersegmental joint torques revealed the following results: first, the range of muscular and motion-dependent torques (relative to body weight) did not change significantly with age. That is, early reaching was not confined by limitations in producing task-adequate levels of muscular torque. Second, improvements in the endpoint kinematics were not accomplished by minimizing amplitude of muscle and reactive torques. Third, the relative timing of muscular and motion-dependent torque peaks showed a systematic development toward an adult timing profile with increasing age. In conclusion , the development toward invariant characteristics of the hand trajectory is mirrored by concurrent changes in the control of joint forces. The acquisition of stable patterns of intersegmental coordination is not achieved by simply regulating force amplitude, but more so by modulating the correct timing of joint force production and by the system's use of reactive forces. Our findings J. Konczak (~) 9 M. Borutta 9 H. support the view that development of reaching is a process of unsupervised learning with no external or innate teacher prescribing the desired kinematics or kinetics of the movement.

This experiment studied the effect of imposed optic flow on human locomotion. Six young and 6 older adults were exposed to various patterns of optic flow while walking in a moving hallway. Results showed few cases of impaired postural control (staggers, parachute reactions). No falls were recorded. Kinematic  patterns of gait were altered when vision was absent or inconsistent  optic flow was presented: Ninety two percent of the subjects mean step velocity differed from their step velocities under normal vision. Compared with imposed central flow, peripheral optic flow  was not dominant in inducing kinematic changes. Characteristic gait profiles were obtained, depending on flow direction. Global backward flow tended to slow down step velocity, whereas subjects’ step velocity increased during conditions of forward flow.  The results suggest that subjects attempted to match their own  walking speed to the velocity of the moving visual scenes. It is  concluded that in an uncluttered environment, imposed optic flow has a modulating rather than a destabilizing effect on human locomotion.

Hereditary cerebellar ataxia progressively impairs force adaptation during goal-directed arm movements. J Neurophysiol 91: 230–238, 2004. First published September 17, 2003; 10.1152/jn.00557.2003. We investigated how humans with hereditary cerebellar degeneration [spinocerebellar ataxia (SCA) type 6 and 8, n
9] and age- and sex-matched healthy controls (n
9) adapted goal-directed arm movements to an unknown external force field. We tested whether learning could be generalized to untrained regions in the workspace, an aspect central to the idea of an internal model, and if any learning could be retained. After removal of the force field, SCA patients showed little or no learning-related aftereffects indicating that repeated force-field exposure never led to successful force compensation. In contrast, healthy control subjects quickly adapted their movements to the new force field. The difference in force adaptation was significant for movements to targets that required both the shoulder and elbow joint (P  0.001). Moreover, the generalization of learned movements to targets outside the learned workspace was prevented by the cerebellar degeneration (P  0.01). Retention of force adaptation was significantly lower in SCA patients (P
0.003). The severity of ataxia in SCA patients correlated negatively with the extent of learning (r
0.84, P
0.004). Our findings imply that progressive loss of cerebellar function gradually impairs force adaptation. The failure to generalize learning suggests that cerebellar degeneration prevents the formation of an internal representation of the limb dynamics.

Precise knowledge about limb position and orientation is essential for the ability of the nervous system to plan and control voluntary movement. While it is well established that proprioceptive signals from peripheral receptors are necessary for sensing limb position and motion, it is less clear which supraspinal structures mediate the signals that ultimately lead to the conscious awareness of limb position (kinaesthesia). Recent functional imaging studies have revealed that the cerebellum, but not the basal ganglia, are involved in sensory processing of proprioceptive information induced by passive and active movements. Yet psychophysical studies have suggested a prominent role of the basal ganglia in kinaesthesia. This study addresses this apparent dichotomy by investigating the contributions of the cerebellum and the basal ganglia to the perception of limb position. Using a passive movement task, we examined the elbow position sense in patients with a dysfunction of the basal ganglia (Parkinson's disease, n = 9), patients with cerebellar degeneration [spinocerebellar ataxia (SCA) types 6 and 8, n = 6] and age-matched healthy control subjects (n = 11). In comparison with healthy control subjects, Parkinson's disease patients, but not SCA patients, were significantly impaired in the ability to detect displacements correctly. A 1° forearm displacement was correctly recognized in >75% of trials by control subjects and SCA patients, but only in 55% of Parkinson's disease patients. Only at 6° displacement did Parkinson's disease patients exhibit a response rate similar to those of the two other groups. Thresholds for 75% correct responses were 1.03° for controls, 1.15° for cerebellar patients and 2.10° for Parkinson's disease patients. This kinaesthetic impairment significantly correlated with the severity of disease in Parkinson's disease patients, as determined by the Unified Parkinson's Disease Rating Scale (r = ±0.7, P = 0.03) and duration of disease (r = ±0.7, P = 0.05). In contrast, there was no significant correlation between performance and the daily levodopa equivalent dose. These results imply that an intact cerebro-basal ganglia loop is essential for awareness of limb position and suggest a selective role of the basal ganglia but not the cerebellum in kinaesthesia.

Humans learn to make reaching movements 1n novel dynamic environments by acquiring an internal motor model of their limb dynamics. Here, the authors investigated how 4-to ll-year-old children (N = 39) and adults (N = 7) adapted to changes in arm dynamics, and they examined whether those data ruppö.t the view that the human brain acquires inverse dynamics möd"ls (IDM) during development. While external damping forces were applied, the children leamed to perform goal-directed forearm flexion movements. After changes in damping' all children showed kinematic aftereffects indicative of a neural controller that still attempted to compensate the no longer existing damping force. With increasing age, the number of trials toward co-plei" adaptation decreased. When damping was present' forearm paths were most perturbed and most variable in the youngest children but were improved in the older children. The findings indicate that the neural representations of limb dynamics are less precise in children and less stable in time than those of adults' Such controller instability might be a primary cause of the high kinematic variability observed in many motor tasks during childhood. Finally, the young children were not able to update those models at the same rate as the older children, who, in turn, adapted more slowly than adults. In conclusion, the ability to adapt to unknown forces is a developmental achievement. The present results are consistent with the view that the acquisition and modification of internal models of the limb dynamics form the basis of that adaptive process. A Outt humans learn to manipulate novel objects with Flrelative ease. As the result of practice, those objects are moved along desired, preplanned trajectories. The tra-jectories remain surprisingly stereotypic for a wide range of movement speeds and amplitudes despite the complexity of the underlying limb dynamics (Atkeson, 1989). The results of recent research on goal-directed action in adult humans suggest that neural representations of the limb dynamics or kinematics, so-called internal motor models, form the basis of that control process. In general, two types of internal

The authors investigated adaptation of goal-directed forearm movements to unknown external viscous force assisting forearm flexion in 6 patients with cerebellar dysfunction and in 6 control participants. Motor performance was generally degraded in cerebellar patients and was markedly reduced under the force condition in both groups. however, patients and controls were able to adapt to the novel force within 8 trials. Only the healthy controls were able to improve motor performance when readapting to a null-force condition. The results indicate that cerebellar patients' motor control system has imprecise estimations of actual limb dynamics at its disposal. Force adaptation may have been preserved because single-joint movements were performed, where as the negative viscous force alone and no interaction forces had to be compensated.

What is the relationship between development of the nervous system and the emergence of voluntary motor behavior? This is the central question of the nature-nurture discussion that has intrigued child psychologists and pediatric neurologist for decades. This paper attempts to revisit this issue. Recent empirical evidence on how infants acquire multi-joint coordination and how children learn to adapt to novel force environments will be discussed with reference to the underlying development of the nervous system. The claim will be made that the developing human nervous system by no means constitutes an ideal controller. However, its redundancy, its ability to integrate multi-modal sensory information and motor commands and its facility of time-critical neural plasticity are features that may prove to be useful for the design of adaptive robots.

We investigated how humans with hereditary cerebellar degeneration [spinocerebellar ataxia (SCA) type 6 and 8, n 9] and age-and sex-matched healthy controls (n 9) adapted goal-directed arm movements to an unknown external force field. We tested whether learning could be generalized to untrained regions in the workspace, an aspect central to the idea of an internal model, and if any learning could be retained. After removal of the force field, SCA patients showed little or no learning-related aftereffects indicating that repeated force-field exposure never led to successful force compensation. In contrast, healthy control subjects quickly adapted their movements to the new force field. The difference in force adaptation was significant for movements to targets that required both the shoulder and elbow joint (P 0.001). Moreover, the generalization of learned movements to targets outside the learned workspace was prevented by the cerebellar degeneration (P 0.01). Retention of force adaptation was significantly lower in SCA patients (P 0.003). The severity of ataxia in SCA patients correlated negatively with the extent of learning (r 0.84, P 0.004). Our findings imply that progressive loss of cerebellar function gradually impairs force adaptation. The failure to generalize learning suggests that cerebellar degeneration prevents the formation of an internal representation of the limb dynamics.

When humans are exposed to external forces while performing arm movements, they adapt by compensating for these novel forces. The basis of this learning process is thought to be a neural representation that models the relation between all forces acting upon the system and the kinematic effects they produce, called inverse dynamic model (IDM). The present study investigated whether and how the predictability of a given external force affects the selection of an appropriate motor response to compensate for such force. Adult human subjects (N=32) held a handle that could rotate around the elbow joint and learned to perform goal-directed forearm flexion movements, while an external velocity-dependent negative damping force was applied that assisted forearm movement. Subjects were randomly assigned to two groups. In the associative group, the applied damping force was always associated with a specific initial position. Thus, after initial learning , the force application became predictable. In the non-associative group, where the same movements were performed, the applied force was independent of the initial position, so that no association between force and location could be formed. We found that only the associative group significantly reduced target error when damping was present. That is, the location cue aided these subjects in generating dynamic responses in the appropriate limb. Our results indicate that motor adaptation to different dynamic environments can be facilitated by indicative stimuli.

The perception of limb motion is a kinaesthetic property that is essential for voluntary motor control. This study examined the ability of patients with Parkinson’s disease (PD) to detect the velocity of a passively moved limb. Eight patients with mild to moderate PD and eight age-matched healthy controls participated. They placed their forearm on a padded splint of a passive motion apparatus, which horizontally extended or flexed the elbow joint at velocities between 1.65 and 0.075/s (in steps of 0.15/s). Passive movement persisted until subjects detected arm motion and pressed a trigger held in the hand of their non-tested arm. Time until detection and associated arm displacement were recorded and subsequently adjusted for each subject’s reaction time. We found that PD patients needed significantly larger limb displacements before they could judge the presence of passive motion. With decreasing passive motion velocity the detection time increased exponentially in both groups. Yet, the mean detection times of the PD group were 92–166% higher than in the control group for each of the 12 tested velocity conditions. Five of the eight patients were on Parkinsonian medication when tested. Yet,the degree of impairment in the PD group did not correlate significantly with the patients’ levodopa equivalent dosage. Our results demonstrate that PD patients were impaired in the detection of passive fore arm movements. This study compliments a growing body of evidence indicating that various aspects of kinaesthesis (position sense,weight perception, passive motion sense) are affected even at early stages of PD. The impaired processing of proprioceptive signals likely contributes to motor symptoms in PD.

Parkinson’s disease (PD) and focal dystonia (FD) are both predominantly characterized by motor symptoms. Also, recent research has shown that sensory processing is impaired in both movement disorders. FD is characterized by involuntary movements and abnormal limb postures; thus, abnormal kinesthesia could be involved in the pathogenesis. We examined passive index finger movements in patients with FD (n  12) and PD (n  11) and in age-matched healthy controls (n  13). Compared to healthy controls, patients with PD and FD were significantly impaired in the correct detection of the movement direction. The perceptual thresholds for 75% correct responses of movement direction were 0.21 degrees for FD and 0.28 degrees for PD patients compared to 0.13 degrees in control subjects. Subjects with PD and FD were also significantly impaired when they had to judge consecutive amplitudes. Results of the present study point to impaired kinesthesia in FD. Defective sensory processing could be involved in the pathophysiology of the disease and may influence dystonic contractions.

The present study investigated whether a specific aspect of proprioception, the sense of heaviness or weight is affected in PD. We determined detection thresholds for the perception of a gravito-inertial load in 10 PD patients and 11 age-matched control subjects. A gradually increasing weight
was applied to the index finger by means of two slings of different width (low vs. high skin pressure). For the controls, mean detection thresholds were 31.3 g at skin high pressure and
33.0 g under low pressure. PD patients revealed significantly higher thresholds than the control group in both pressure conditions (mean high pressure,47.7 g; mean low pressure, 52.3 g; group effect, P  0.001). Thresholds of PD patients tended to increase with disease severity as measured by the Unified Parkinson’s Disease Rating Scale Motor score (r  0.55) but did not correlate significantly with levodopa equivalent dosage. The results demonstrate that the perception of heaviness or weight is already affected in the early stages of PD. These findings underline the growing evidence that proprioceptive and possibly haptic dysfunction is a common feature of PD.

There is increasing evidence that the cerebellum and the basal ganglia serve not only a role in motor control but also in visual perception. Patients with Parkinson’s disease (PD) as well as patients with cerebellar lesions exhibit impairments of vision that are not fully explained by ocular motor deficits. It is less clear to which extent these visual deficits contribute to an impaired control of visually guided movements. This study examined whether a dysfunction of the cerebellum or the basal ganglia induces impairments in depth perception, which affect action. We employed an illusionary display, the Ames trapezoidal window, to determine the ability of PD patients (n=10) and patients with spinocerebellar ataxia (SCA) (n=6) to process depth cues when estimating object slant. Participants either pointed to the edges of the window (motor judgement) or verbally indicated the perceived orientation of the display (verbal judgement). To control for ocular and limb motor deficits, participants judged the slant of a non-illusionary display in a second task. Slant estimation of the non-illusionary window was not impaired in either patient group when compared to control subjects (all P>0.2). In contrast, SCA as well as PD patients exhibited significantly greater slant estimation errors than controls when pointing to the illusionary window (P=0.005). In addition, both patient groups made larger errors than controls in their verbal judgements during binocular viewing of the illusion (P=0.005), but not during monocular viewing (P>0.2). In sum, the present findings point towards a role for both the basal ganglia and cerebellum for the processing of visual information about depth. Since the deficits were seen both in the context of action and perception and were only partially reconciled by the availability of binocular depth cues, we conclude that basal ganglia as well as cerebellar disease may affect the visual perception of depth.

With head movement, suppression of vestibular inputs during visual exploration is necessary not only for reorienting gaze, but also to direct attention to new visual targets. People with progressive supranuclear palsy (PSP) have difficulty suppressing the vestibuloocular reflex (VOR) and it was hypothesized that the magnitude of VOR suppression deficit correlates with the degree of degradation of attention and visuospatial performance. We evaluated cognitive and visuo-motor function in 8 subjects with PSP (4 men and 4 women; ages 59 – 83 years). Gaze control was studied by measuring the accuracy of eye– head coordination during passive vertical and horizontal head-on-trunk movements. Fixation was assessed when subjects viewed either an earth-fixed or head-fixed target. A gaze fixation score (GFS) was calculated to represent the amount of error between eye and head movement in each plane (eye– head root mean square error normalized to the range of head rotation). The vertical but not horizontal GFS during attempted suppression of the VOR was significantly related to attention (r 0.70; P 0.05) and visuospatial ability (r 0.76; P 0.03). These findings suggest that the ability to suppress the VOR during vertical smooth movements of the head is associated with the magnitude of cognitive deficit in PSP.

Kinaesthesia is the conscious perception of limb and body position, orientation, and motion. Recent studies have shown that patients with Parkinson’s disease have higher thresholds for detecting passive elbow movements, reflecting kinaesthesic deficits.1 2 Patients with Parkinson’s disease show altered proprioception related EEG potentials during passive movements, which probably reflect changes in the cortical processing of kinaesthesic signals.3 Dopaminergic drugs appear to enhance this deficit.4 In contrast, we found no effect of levodopa on kinaesthesia.2 Deep brain stimulation of the subthalamic nucleus (STN-DBS) improves motor deficits in Parkinson’s disease5 but it may worsen cognitive functions such as working memory.6 However, the effects of STN-DBS on kinaesthesia are unknown. We therefore sought to determine how STN-DBS affects the perception of limb position.

This article reviews to reflexive motor patterns in humans: Primitive reflexes and motor primitives. Both terms coexist in the literature of motor development and motor control yet they are not synonyms. While primitive reflexes are a part of the temporary motor repertoire in early ontogeny, motor primitives refer to sets of motor patterns that are considered basic units of voluntary motor control thought to be present throughout the life-span. The article provides an overview of the anatomy and neurophysiology of human reflexive motor patterns to elucidate that both concepts are rooted in architecture of the spinal cord. I will advocate that an understanding of the human motor system that encompasses both primitive reflexes and motor primitives as well as the interaction with supraspinal motor centers will lead to an appreciation of the richness of the human motor repertoire, which in turn seems imperative for designing epigenetic robots and highly adaptable human machine interfaces.

This study examined whether lesions to the cerebellum obtained in early childhood are better compensated than lesions in middle childhood or adolescence. Since cerebellar lesions might affect motor as well a cognitive performance, posture, upper limb and working memory function were assessed in 22 patients after resection of a cerebellar tumour (age at surgery 1–17 years, minimum 3 years post-surgery). Working memory was only impaired in those patients who had received chemo- or radiation therapy. Postural sway was enhanced in 64% of the patients during dynamic posturography conditions, which relied heavily on vestibular input for equilibrium control. Upper limb function was generally less impaired, but 54% of the patients revealed prolonged deceleration times in an arm pointing task, which probably does not reflect a genuine cerebellar deficit but rather the patients’ adopted strategy to avoid overshooting. Age at surgery, time since surgery or lesion volume were poor predictors of motor or cognitive recovery. Brain imaging analysis revealed that lesions of all eight patients with abnormal posture who did not receive chemo- and/or radiation therapy included the fastigial and interposed nuclei (NF and NI). In patients with normal posture, NI and NF were spared. In 11 out of 12 patients with abnormal deceleration time, the region with the highest overlap included the NI and NF and dorsomedial portions of the dentate nuclei in 10 out of 12 patients. We conclude that cerebellar damage inflicted at a young age is not necessarily better compensated. The lesion site is critical for motor recovery, and lesions affecting the deep cerebellar nuclei are not fully compensated at any developmental age in humans.

This review provides a developmental perspective on our current understanding of the role of the cerebellum for sensorimotor and cognitive function. A synopsis on the contribution of the cerebellum on motor control, learning and cognition based on experiments in human adults and animals is presented. This knowledge is contrasted to the relevant literature on children and adolescents. Special attention is given to findings derived from lesion studies and clinical reports that examined the effect of cerebellar damage during development. In general, it is established that children may show the same sensorimotor deficits as adults as a result of cerebellar damage, while the findings of cognitive dysfunction in children are less clear and remain controversial. Younger children do not necessarily recover better than older children or adolescents. The sparing of the deep cerebellar nuclei and the extent of adjuvant chemo or radiation therapy are better predictors of later motor and cognitive function in children and adolescents.