Programming of a movement sequence in Parkinson's disease

How laminar frontal cortex and basal ganglia circuits interact to control planned and reactive saccades

How does the brain learn to balance between reactive and planned behaviors? The basal ganglia (BG) and frontal cortex together allow animals to learn planned behaviors that acquire rewards when prepotent reactive behaviors are insufficient. This paper proposes a new model, called TELOS, to explain how laminar circuitry of the frontal cortex, exemplified by the frontal eye fields, interacts with the BG, thalamus, superior colliculus, and inferotemporal and parietal cortices to learn and perform reactive and planned eye movements. The model is formulated as fourteen computational hypotheses. These specify how strategy priming and action planning (in cortical layers III, Va and VI) are dissociated from movement execution (in layer Vb), how the BG help to choose among and gate competing plans, and how a visual stimulus may serve either as a movement target or as a discriminative cue to move elsewhere. The direct, indirect and hyperdirect pathways through the BG are shown to enable complex gating functions, including deferred execution of selected plans, and switching among alternative sensory-motor mappings. Notably, the model can learn and gate the use of a What-to-Where transformation that enables spatially invariant object representations to selectively excite spatially coded movement plans. Model simulations show how dopaminergic reward and non-reward signals guide monkeys to learn and perform saccadic eye movements in the fixation, single saccade, overlap, gap, and delay (memory-guided) saccade tasks. Model cell activation dynamics quantitatively simulate seventeen established types of dynamics exhibited by corresponding real cells during performance of these tasks.

Disruption of self-organized actions in monkeys with progressive MPTP-induced parkinsonism. I. Effects of task complexity

Parkinson's disease (PD) is characterized by motor symptoms, usually accompanied by cognitive deficits. The question addressed in this study is whether complexity of routine actions can exacerbate parkinsonian disorders that are often considered to be motor symptoms. To examine this question, we trained four vervet monkeys (Cercopithecus aethiops) to perform three multiple-choice retrieval tasks. In order of ascending complexity, rewards were freely available (task 1), covered with transparent sliding plaques (task 2), and covered with opaque sliding plaques cued by symbols (task 3). Thus, from task 1 to task 2 we added a motor difficulty--the recall of context-adapted movement; and from task 2 to task 3 we added a cognitive difficulty: the recall of symbol-reward associations. The more complex the task, the longer it took to learn, but after extensive training the performance was stable in all tasks, with similar retrieval durations. The monkeys then received systemic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) injections (0.3-0.4 mg/kg) every 4-7 days, until the first motor symptoms appeared. In the course of MPTP intoxication, the behavioural performance declined while the motor symptoms were absent or mild--the retrieval duration increased, and non-initiated choices and hesitations between choices became frequent. Interestingly, this decline was in proportion to task complexity, and was particularly pronounced with the cognitive difficulty. Furthermore, freezing appeared only with the cognitive difficulty. We therefore suggest that everyday cognitive difficulties may exacerbate hypokinesia (lack of initiation, abnormal slowness) and executive disorders (hesitations, freezing) in the early stages of human PD.

Deficits in the evolution of hand preshaping in Parkinson's disease

Parkinson's disease (PD) results in various types of motor impairments including bradykinesia, tremor and rigidity. Recent research has implicated more fundamental processes at the source of the observed motor deficits. Among these, problems in the sequencing and/or timing of complex movements and in the execution of internally-guided tasks. Furthermore, PD patients exhibit procedural learning deficits which may complicate the interpretation of experimental results of studies involving novel sensorimotor tasks. The reach-to-grasp movement is a complex, overlearned sensorimotor task consisting of two semi-independent components, a relatively simple reach or transport phase and a more complex manipulation or prehension phase. In the present study, we used a novel technique in order to study the evolution of hand preshaping during the reach-to-grasp movement of PD patients and age-matched controls to objects of different shapes in three different spatial locations. Our results indicate that while PD patients are able to specify movement direction as well as controls, their hand preshaping exhibits substantial impairments. Other prehension measures, such as the time to peak aperture (TPA), indicate that PD patients delayed execution of the grasp until visual feedback of their hand was available. Overall, our results suggest that PD patients' internal guidance processes are severely disrupted, having to rely on visual feedback in order to modulate their hand shape to fit the contours of the target objects during a reach-to-grasp movement.

The independence of response structure and element production in timing sequences

In three experiments, participants were asked to produce a prescribed temporal sequence of key presses. The number of elements in a key press sequence, the movement time of the elements, and uniformity of the timing elements comprising the sequence were manipulated. If the processing of the sequence structure was independent of the processing of elements comprising the sequence, increasing the number of the elements in the sequence should affect the production of the sequence but not the proficiency with which the individual elements are, produced. Increasing the movement time of the elements, however, should affect the production of the elements but have little, if any, effect on the integrity of the sequence. The findings indicated that increasing the number of elements negatively affected sequence production when the elements were nonuniform but had little or no effect when elements were uniform. Alternatively, element production was affected by movement time but not number of elements. The results of these experiments appear to confirm the independence of sequence and element production. However, Experiment 3, in which the uniformity of the elements was directly contrasted, found strong evidence for coarticulatory influences in the learning processes, whereby the longer and shorter than average elements in the sequence negatively affected the sequence integrity and the production of the individual elements. When faced with a nonuniformn timing pattern, participants appear to elongate or shrink individual elements of a uniform timing pattern to fit the desired sequence requirements. The result is that the longer or shorter than average elements require more practice to produce correctly, tend to regress across retention intervals, and are generally less stable than the elements nearer the average.

Modeling facilitation and inhibition of competing motor programs in basal ganglia subthalamic nucleus-pallidal circuits

The motor symptoms of Parkinson's disease (PD) implicate the basal ganglia (BG) in some aspect of motor control, although the role the BG play in regulation of motor behavior is not completely understood. The modeling study presented here takes advantage of available cellular, systems, and clinical data on BG and PD to begin to build a biophysically based network model of pallidosubthalamic circuits of BG, to integrate this information and better understand the physiology of the normal BG and PD pathophysiology. The model reflects the experimentally supported hypothesis that the BG are involved in facilitation of the desired motor program and inhibition of competing motor programs that interfere with the desired movement. Our model network consists of subthalamic and pallidal (both external and internal segments) neural assemblies, with inputs from cortex and striatum. Functional subsets within each of the BG nuclei correspond to the desired motor program and the unwanted motor programs. A single compartment conductance-based model represents each subset. This network can discriminate between competing signals for motor program initiation, thus facilitating a single motor program. This ability depends on metabotropic gamma-aminobutyric acid B projections from the external pallidum to subthalamic nucleus and rebound properties of subthalamic cells, as well as on the structure of projections between pallidum and subthalamus. The loss of this ability leads to hypokinesia, known PD motor deficits characterized by a slowness or inability to switch between motor programs.

Parkinson's disease patients undershoot target size in handwriting and similar tasks

OBJECTIVES

Previous research suggested that people with Parkinson's disease are able to increase handwriting stroke size up to 1.5 cm without an increase of stroke duration; whereas age matched individuals in normal health are able to modulate stroke size without changes in stroke duration for sizes up to 2 cm. This study was designed to test this finding by examining whether sizes larger than 1.5 cm show different relationships with stroke duration for patients with Parkinson's disease as compared with age matched controls.

METHODS

The study included 13 subjects with Parkinson's disease and 13 age matched controls. Participants were required to write a cursive "llllllll" pattern, or a cursive "lililili" pattern without the dots, at a comfortable speed and also as fast as possible, in five different sizes (1.0, 1.5, 2.0, 3.0, and 5.0 cm). The participants wrote with a ballpoint pen on a digitiser tablet. The target pattern was displayed at its required size on a screen, but disappeared as soon as the pen touched the surface of the digitiser tablet. Online visual monitoring of the hand was prevented by a cover over the digitiser. After each trial, the recorded movement of the tip of the pen was displayed with two lines to indicate whether the size requirement had been met. The writing conditions were presented in random order and consisted of 12 trials for each participant.

RESULTS

The results demonstrated that stroke size and duration produced by the participants with Parkinson's disease were independently modulated up to 1.5 cm; sizes over 1.5 cm resulted in progressive undershooting by patients with Parkinson's disease (PD). It was also shown that these participants modulated acceleration measures inefficiently as compared with controls.

CONCLUSIONS

The findings suggest that individuals with Parkinson's disease writing at speed produce inadequate stroke sizes when these should equal or exceed 1.5 cm.

Classifying lower limb dynamics in Parkinson's disease

To classify lower limb dynamics in patients with Parkinson's disease (PD), we conducted a clinical study by using pedaling exercise.Twenty-seven patients with idiopathic PD were included in this study. We measured rotational velocities of pedals during pedaling movements with a newly developed ergometer. The velocity waveforms exhibited different characteristics among patients, which could be categorized into four different clusters. In cluster 1, the amplitude on each side was constant and the relative phase was locked at 180 degrees. The pattern was the same as seen in normal subjects. In cluster 2, the amplitude on each side was constant, but the relative phase was locked at 90 degrees. In cluster 3, the amplitude on each side was modulated, and the relative phase drifted monotonously from 0 to 360 degrees during pedaling cycles. In cluster 4, the amplitude on each side was synchronously and irregularly modulated, and the relative phase fluctuated with intermittent spike-like decrement. In order to evaluate, the correlation between pattern and severity of PD, we divided 13 patients, who underwent measurement of pedaling patterns more than three times, into three groups, and found that the abnormal coordination pattern correlated with the presence of freezing phenomenon in patients with PD. Our clinical analysis may contribute in analyzing and classifying the dynamics of PD.

Switching of movement direction is central to parkinsonian bradykinesia in sit-to-stand

Patients with Parkinson's disease (PD) are known to manifest slowness in movements. We sought to identify the particular kinematic and kinetic disorders that contribute to the slowness in performing sit-to-stand in these patients. Two inter-related studies were carried out. In the first study, 20 patients with PD and 20 control subjects were instructed to perform sit-to-stand at a natural speed. In the second study, 15 control subjects were instructed to simulate the slower speed of sit-to-stand of the patients identified in the first study. Kinematic and kinetic data were recorded by a PEAK motion analysis system and two force platforms. The results showed that patients with PD generated smaller peak horizontal and vertical velocities during the task. They took a longer time to complete each individual phase as well as the whole movement of sit-to-stand. Patients also produced smaller peak hip flexion and ankle dorsiflexion joint torques and had prolonged time-to-peak torques from sit-to-stand onset. When control subjects simulated the patients' speed of sit-to-stand, there was no difference in all the kinematic and kinetic data between groups. The only exception was that they exhibited a shorter transition time between peak horizontal velocity (flexion phase) and seat-off (extension phase) than the patients. This study demonstrated that the slowness of PD patients during sit-to-stand at a natural speed could be attributed to inadequate peak hip flexion and ankle dorsiflexion torques, a prolonged torque production, as well as a difficulty in switching from the flexion to extension direction during sit-to-stand. As the latter difficulty persisted when the control subjects performed the task at a speed similar to that of the patients, our findings suggest that a fundamental problem of patients with Parkinson's disease could be a switch between movement directions.

Practice effects in three-dimensional sequential rapid aiming in Parkinson's disease

One hypothesized role of the basal ganglia, based largely on findings in Parkinson's disease (PD) patients, is the control of movement sequences. We examined changes in performance with practice of a movement sequence in PD patients and age-matched controls (n = 8 per group). Subjects practiced a complex three-dimensional sequential aiming task over 2 days, completing 180 trials, with the goal to minimize response time within specified accuracy limits. The results indicated that both groups became faster in planning the movement, and both groups moved more quickly through the sequence with practice. The PD group's decrease in movement time occurred primarily within the first 45 trials, whereas the control group continued to improve through the first 150 trials. Flight time (time between targets) to a small target decreased with practice in both groups, but flight time to a large target decreased only in the control group. This finding indicates that error corrective processes are more amenable to practice than ballistic processes in people with PD. There was little evidence that either group improved their performance by planning shorter (lower) trajectories, but rather increased mean velocity. Contact time (time on target) decreased with practice in both groups, and there were no group differences. These data indicate that practice can improve performance, both in planning and in specific execution processes.

Impairment of pronation, supination, and body co-ordination in reach-to-grasp tasks in human Parkinson's disease (PD) reveals homology to deficits in animal models

Animal (monkey, rat, mouse) models are widely used to investigate degenerative processes and potential therapeutic treatments for human Parkinson's disease (PD). One task that has proved useful in these investigations is a reach-to-grasp task (skilled reaching) in which an animal reaches for a piece of food that it then consumes. Rats with extensive unilateral Dopamine depletions are impaired in using the contralateral limb. The qualitative features of posture, lifting and advancing the limb, pronating the paw to grasp food, and in withdrawing and supinating the paw to place the food in the mouth are impaired, as is reaching success. Humans with PD are often described as having poor manual dexterity that worsens as the disease progresses. As there have been no detailed comparisons of reaching movements in the animal models and in PD subjects, the following descriptive analysis was performed. Ten subjects with PD, eight age matched controls and 14 young normal subjects were studied as they used a natural movement of reaching for a small piece of food that they then placed in the mouth to eat. The reaching movements were described using Eshkol-Wachman Movement Notation (EWMN), supplemented with kinematic analyses. From this description, a 21-point rating scale was devised to describe the component movements of the reach. Movements included: orienting the head and eyes to the target, adjusting posture, lifting the hand, shaping and aiming the digits to the target, pronating the hand to grasping the food with a pincer grip, lifting and supinating the hand to transporting the food to the mouth, and further supinating the hand and opening the digits to place food in the mouth, and finally returning the hand to the starting position. Analysis indicated that most aspects of the reaching movements of the PD subjects were significantly different relative to both young control subjects and old control subjects. As compared to the control groups, postural and reaching components of the movements were fragmented, movements were achieved using more proximal segments of the body, and rotatory movements of the hand were limited. The PD subjects did use a pincer grasp to obtain the food, but the grasp was less independent of other digit movements than was observed in the control subjects. These results are discussed in terms of a homology to impairments displayed animal models of PD.

Effect of speech task on intelligibility in dysarthria: a case study of Parkinson's disease

This study assessed intelligibility in a dysarthric patient with Parkinson's disease (PD) across five speech production tasks: spontaneous speech, repetition, reading, repeated singing, and spontaneous singing, using the same phrases for all but spontaneous singing. The results show that this speaker was significantly less intelligible when speaking spontaneously than in the other tasks. Acoustic analysis suggested that relative intensity and word duration were not independently linked to intelligibility, but dysfluencies (from perceptual analysis) and articulatory/resonance patterns (from acoustic records) were related to intelligibility in predictable ways. These data indicate that speech production task may be an important variable to consider during the evaluation of dysarthria. As speech production efficiency was found to vary with task in a patient with Parkinson's disease, these results can be related to recent models of basal ganglia function in motor performance.

Bimanual coordination deficits with Parkinson's disease: the influence of movement speed and external cueing

Continuous coordinated movement of the upper limbs performed at different movement frequencies, and with different external timing conditions, was examined in individuals with Parkinson's disease (PD) and healthy, age- and gender-matched controls. Participants performed symmetric in-phase movements toward and away from the midline of the body, and isodirectional anti-phase movements at one of three metronome speeds (0.75, 1.25, and 1.75 Hz), and in two different auditory cueing conditions (cue present, cue absent). Measures of relative phase accuracy (absolute mean error) and stability (standard deviation) revealed that individuals with PD performed in-phase movements as well as the controls, while anti-phase movements were performed with greater mean error and variability. The adverse effects of PD on the anti-phase task were also reflected by freezing (8.1% of anti-phase trials) and hypometric deficits (5.1% of anti-phase trials) during movement. None of these PD-related impairments occurred during in-phase trials. The overall accuracy or stability of movement coordination was not improved with the presence of external pacing cues, suggesting that although execution impairments of individuals with PD may be dramatically influenced by attention, external cueing does not necessarily improve movement performance.

Mental and motor switching in Parkinson's disease

Switching difficulties in Parkinson's disease (PD) are expressed in both mental and motor tasks. The authors of the present study investigated whether those deficits coexist in the same patient and are positively correlated. They tested 8 nondemented PD patients and 6 age-matched control participants by using the modified Wisconsin Card Sorting Test and a motor switching paradigm that is based on the task of reaching toward visual targets, the location of which could unexpectedly be altered within the reaction time. In both mental and motor tasks, patients performed significantly worse than controls. There were no significant correlations between the two types of pathology in individual patients. Mental and motor switching deteriorate in PD patients, but the deficits are not necessarily of parallel severity.

The preparation and execution of saccadic eye and goal-directed hand movements in patients with Parkinson's disease

The oculomotor and manual motor systems were studied in a two-segment movement task in a group of patients with Parkinson's disease and in age matched normal controls. In order to avoid reflexive motor movements the selection of the correct motor sequence was derived from the interpretation of symbolic (coloured) cues. The latencies and dynamics of eye and hand (pointing) movements performed during the first (fixed) movement segment were measured and the planning and execution processes were manipulated by varying the complexity of the second movement segment relative to the first (with regard to direction and amplitude). The results showed that the eye and hand movements made by patients with Parkinson's disease were not impaired in the initiation of the first movement segment. Interestingly, both Parkinson's patients and controls showed increased eye and hand reaction time latencies for the first movement when the second movement was in the direction opposite to the first. This indicates that the complexity of the second movement influences the execution of the first movement, and importantly that complexity affects motor initiation and execution processes in both normal subjects and in patients with Parkinson's disease. The execution of hand movements was found to be impaired in patients with Parkinson's disease as indicated by a reduced peak velocity of manual pointing responses when compared to age matched controls. By contrast, no differences were found in the dynamics of saccadic eye movements. This dissociation is consistent with the notion that the skeletomotor loop passes through the functionally corresponding portions of the basal ganglia independently of the oculomotor loop. Together, these results demonstrate that Parkinson's patients are able to generate multiple non-reflexive eye and hand movements and that the observed (manual) motor deficits are specific to the processes of motor execution.

Distinct contribution of the striatum and cerebellum to motor learning

The striatum and cerebellum have been shown to be key structures of a distributed system for the control of skilled movements. However, the mechanisms under which they operate remain unclear. This study compared the performance of patients with Parkinson's disease (PD) or with cerebellar damage (CE) to that of age-matched controls. Each group performed two visuomotor paradigms: a random variant of the serial reaction time (SRT) task that tested the subject's ability to make efficient stimulus-response associations and an adapted version of the mirror-tracing task that measured their capacity to combine simple movements into complex ones. PD patients with bilateral striatal damage showed an impaired learning profile on the SRT task and a normal facilitation effect in the tracing task, while CE patients showed the reverse pattern. Although further research is needed, the present findings suggest that the striatum and cerebellum are involved in distinct learning mechanisms.

Bimanual coordination and limb-specific parameterization in patients with Parkinson's disease

Bimanual coordination and the capability to parameterize the individual limb movements were examined in patients with Parkinson's disease (PD) as compared to healthy control subjects. In-phase and anti-phase patterns were performed while the individual limb movements were subjected to amplitude and loading manipulations. Findings showed that PD patients produced the bimanual configurations with lower degrees of phasing accuracy and consistency than control subjects, indicating an impairment at the global (coordinative) level of simultaneously produced movements. At the local (limb-specific) level, the imposed distances with and without loading were unaffected in PD patients as compared to control subjects, whereas cycle times were prolonged and depended on the task requirements. This illustrates a disturbance at the limb-specific level in complying with the execution of the submovements. The finding that movement slowness only became evident in the more complex conditions, suggests that it did not mainly represent a deficit in the execution of coordinated movements, but rather an inability to accommodate the motor output during stringent spatiotemporal task constraints.

Function of the 'direct' and 'indirect' pathways of the basal ganglia motor loop: evidence from reciprocal aiming movements in Parkinson's disease

The purpose of this study was to test the validity of a neural-network model of the basal ganglia developed by Bischoff and colleagues (A. Bischoff, Modeling the basal ganglia in the control of arm movements (Doctoral dissertation, University of Southern California, 1998). Dissertation Abstr. Int. 59-08B (1998) 3924, 0208; A. Bischoff, M.A. Arbib, Modeling the role of basal ganglia and supplementary motor areas in sequential arm movements, Abstr. Soc. Neurosci. 23 (1997) 466; A. Bischoff, M.A. Arbib, C.J. Winstein, Modeling the role of the basal ganglia in reciprocal aiming task, Proceedings of the Fourth Annual Joint Symposium on Neural Computation, University of Southern California, Los Angeles, 7, 1997, pp. 20-27), and to examine the effects of levodopa on aiming movement performance. Findings confirm the model predictions for repetitive aiming movements. Individuals with late stage Parkinson's disease demonstrated longer movement times and longer pauses between aiming sequences compared to controls. Levodopa only slightly improved bradykinesia but not akinesia in these patients.

The relation between EMG activity and kinematic parameters strongly supports a role of the action tremor in parkinsonian bradykinesia

The kinematics characteristics of an upper arm extension of large amplitude (90 degrees) performed in the horizontal plane and the simultaneous activity of the shoulder muscles were recorded in 12 parkinsonian patients and in six normal control subjects. The movement, triggered by an acoustic "go" signal, was preceded by an isometric adduction. Within the whole population of individuals (n = 18) a strong, positive correlation was observed between the root mean square value of agonist EMG activity, evaluated during the acceleration phase of the movement, and both peak velocity and acceleration. In six patients tremor bursts at the frequency of 8-14 Hz (action tremor) were observed during the movement phase in the anterior, middle, and posterior deltoid: all these patients showed low root mean square values and were bradykinetic with respect to the control subjects. The remaining six patients did not show this action tremor during the movement phase. All but one had an agonist activation of normal duration and amplitude, showed high root mean square values, and performed well in the range of control subjects. We conclude that the inability to suppress the activity of pathological oscillator(s) responsible for the action tremor plays a fundamental role in the bradykinesia associated with Parkinson's disease.

Role of the basal ganglia in the control of purposive saccadic eye movements

In addition to their well-known role in skeletal movements, the basal ganglia control saccadic eye movements (saccades) by means of their connection to the superior colliculus (SC). The SC receives convergent inputs from cerebral cortical areas and the basal ganglia. To make a saccade to an object purposefully, appropriate signals must be selected out of the cortical inputs, in which the basal ganglia play a crucial role. This is done by the sustained inhibitory input from the substantia nigra pars reticulata (SNr) to the SC. This inhibition can be removed by another inhibition from the caudate nucleus (CD) to the SNr, which results in a disinhibition of the SC. The basal ganglia have another mechanism, involving the external segment of the globus pallidus and the subthalamic nucleus, with which the SNr-SC inhibition can further be enhanced. The sensorimotor signals carried by the basal ganglia neurons are strongly modulated depending on the behavioral context, which reflects working memory, expectation, and attention. Expectation of reward is a critical determinant in that the saccade that has been rewarded is facilitated subsequently. The interaction between cortical and dopaminergic inputs to CD neurons may underlie the behavioral adaptation toward purposeful saccades.

Motor learning and Parkinson's disease: refinement of within-limb and between-limb coordination as a result of practice

Even though the basal ganglia have been assigned a role in motor learning, few studies have addressed motor learning capabilities in Parkinson patients. In the present experiment, improvement of bimanual figure drawing across practice was compared between Parkinson patients and normal age-matched controls. At regular intervals during acquisition, performance was assessed under normal vision and blindfolded conditions. At initiation of practice, the typical signs associated with Parkinson's disease became evident, such as bradykinesia and hypometria. Moreover, reduced synchronization between the force-time specifications of both limbs was observed. When vision was withdrawn, Parkinson patients showed a larger drift of drawing performance across the workspace, indicative of a decline in proprioception. In spite of the aforementioned deficits, Parkinson patients made marked improvements in the speed of execution, the consistency of the spatial trajectories, and the synchronization between the limbs across practice, even though they never reached the performance levels obtained in elderly controls. The findings demonstrate that Parkinson patients do benefit from practice to refine their upper limb control and to alleviate their most basic motor deficits.

Changes in handwriting resulting from bilateral high-frequency stimulation of the subthalamic nucleus in Parkinson's disease

High-frequency stimulation of the subthalamic nucleus (STN) is a promising therapeutic approach in patients with severely disabling Parkinson's disease (PD). Whereas STN stimulation improves the cardinal signs of PD, little is known about the effects of STN stimulation on fine manual skills like handwriting. Therefore, the present study investigated the changes in handwriting during bilateral STN stimulation in 12 patients with advanced PD. Dopaminergic medication was discontinued at least 12 hours before the study. The patients were asked to write a standardized sentence repetitively. Five samples of the patient's script were recorded during effective bilateral STN stimulation and 1 hour after both stimulators had been switched off. The movements of the tip of the pencil were recorded using a digitizing tablet. Handwriting movements were segmented into subsequent up- and down-strokes, and a stroke-based kinematic analysis of handwriting was performed. During high-frequency STN stimulation, handwriting movements became faster and smoother indicating a partial restoration of an "open-loop" automatic performance. In addition, STN stimulation gave rise to a significant increase in the mean vertical stroke length demonstrating a stimulation-related reduction in micrographia. The present data underscores the importance of the STN in "open-loop" performance of highly skilled sequential hand movements.

Planning and executing an action in Parkinson's disease

We evaluated the possible impairment in planning and executing an action in patients with Parkinson's disease (PD). The action considered in the present study was formed by two successive motor acts: reaching-grasping an object (first target) and placing it on a second target of the same shape and size. We examined the effects of extrinsic properties of the second target (that is, distance) on the various kinematic phases of reaching-grasping movements. Distance, position, and size of both stimuli were randomly varied across the experimental session. Movements were executed with and without visual control of both targets and arm. The performance of six patients with PD was compared with an age-matched control group. The kinematics of the initial phase of reaching was influenced by position and size of the first target and by distance of the second target in both patients and control subjects. In particular, peak acceleration was higher for farther position of the second target. However, in the subsequent phase patients, differently from control subjects, removed the effects of the second target distance by modifying their reaching kinematics. This was obtained by varying the duration of the acceleration phase. In summary, the patients reprogrammed the reaching component by taking into account only the properties of the first target. The decreasing influence of second-target distance on reaching kinematics of patients was more evident during movements executed under visual control. Moreover, their movements executed without visual control were slowed down from the beginning. The second target affected the grasping kinematics only of the control subjects. Globally, these results indicate that PD patients are able to compute the general program of an action that takes into account extrinsic properties of the final target. However, the finding that PD patients reprogrammed the movement during its execution suggests a decay of the program during its time course, that is, basal ganglia can be involved in storing the plan of an action and in controlling its correct execution.