Movement precues in planning and execution of aiming movements in Parkinson's disease.

Two experiments tested how changing a planned movement affects movement initiation and execution in idiopathic Parkinson's disease (PD) patients. In Experiment 1, PD patients, elderly controls, and young adults performed discrete aiming movements to one of two targets on a digitizer. A precue (80% valid cue and 20% invalid cue of all trials) reflecting the subsequent movement direction was presented prior to the imperative stimulus. All groups produced slower reaction times (RTs) to the invalid precue condition. Only the subgroup of patients with slowest movement time showed a significant prolongation of movement for the invalid condition. This suggests that, in the most impaired patients, modifying a planned action also affects movement execution. In Experiment 2, two-segment aiming movements were used to increase the demand on movement planning. PD patients and elderly controls underwent the two precue conditions (80% valid, 20% invalid). Patients exhibited longer RTs than the controls. RT was similarly increased for the invalid condition in both groups. The patients, however, exhibited longer movement times, lower peak velocities, and higher normalized jerk scores of the first segment in the invalid condition compared to the valid condition. Conversely, the controls showed no difference between the valid and invalid cue conditions. Thus, PD patients demonstrated substantially pronounced movement slowness and variability when required to change a planned action. The results from both experiments suggest that modifying a planned action may continue beyond the initiation phase into the execution phase in PD patients.

A quantitative evaluation of the AVITEWRITE model of handwriting learning.

Much sensory-motor behavior develops through imitation, as during the learning of handwriting by children. Such complex sequential acts are broken down into distinct motor control synergies, or muscle groups, whose activities overlap in time to generate continuous, curved movements that obey an inverse relation between curvature and speed. The adaptive vector integration to endpoint handwriting (AVITEWRITE) model of Grossberg and Paine (2000) [A neural model of corticocerebellar interactions during attentive imitation and predictive learning of sequential handwriting movements. Neural Networks, 13, 999-1046] addressed how such complex movements may be learned through attentive imitation. The model suggested how parietal and motor cortical mechanisms, such as difference vector encoding, interact with adaptively-timed, predictive cerebellar learning during movement imitation and predictive performance. Key psychophysical and neural data about learning to make curved movements were simulated, including a decrease in writing time as learning progresses; generation of unimodal, bell-shaped velocity profiles for each movement synergy; size scaling with isochrony, and speed scaling with preservation of the letter shape and the shapes of the velocity profiles; an inverse relation between curvature and tangential velocity; and a two-thirds power law relation between angular velocity and curvature. However, the model learned from letter trajectories of only one subject, and only qualitative kinematic comparisons were made with previously published human data. The present work describes a quantitative test of AVITEWRITE through direct comparison of a corpus of human handwriting data with the model's performance when it learns by tracing the human trajectories. The results show that model performance was variable across the subjects, with an average correlation between the model and human data of 0.89+/-0.10. The present data from simulations using the AVITEWRITE model highlight some of its strengths while focusing attention on areas, such as novel shape learning in children, where all models of handwriting and the learning of other complex sensory-motor skills would benefit from further research.

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.

Discrete and dynamic scaling of the size of continuous graphic movements of parkinsonian patients and elderly controls.

OBJECTIVES: To systematically investigate the ability of Parkinson's disease patients to discretely and dynamically scale the size of continuous movements and to assess the impact of movement size on outcome variability. METHODS: Ten patients with Parkinson's disease (mean age 72 years) were compared with 12 healthy elderly controls (mean age 70 years). The subjects wrote with a stylus on a graphics tablet. In experiment 1 they drew circles, matching the size of five target circles ranging in magnitude from a radius of 0.5 cm up to 2.5 cm. In experiment 2 they drew spirals with a radius of at least 2 cm. In both experiments the drawings were initially performed as accurately as possible then as fast and accurately as possible. RESULTS: In both experiments the patients and controls drew at a similar speed. The within trial variability of the pen trajectory was greater for patients than controls, and increased disproportionately with the size of the movement. When the emphasis was on size rather than variability (circles), the patients' drawing movements were the same size as controls. When the emphasis was on accuracy of pen trajectory (that is, minimum variability) rather than size (spirals), the patients' drawing movements were smaller than controls. CONCLUSIONS: The movements made by Parkinson's disease patients are hypometric partly as an adaptive strategy used to reduce movement variability. This strategy is used primarily when the requirement to make accurate movements outweighs the need to make large movements.

Parkinsonian patients reduce their stroke size with increased processing demands.

Parkinson's disease (PD) patients often show reductions in writing size (micrographia) as the length of the text they produce increases. The cause for these reductions in stroke size are not well understood. Reductions in stroke size could be associated with either concurrent processing demands that result from the coordination and control of fingers, wrist, and arm during writing and the processing of future words or increased extension of the wrist joint as the execution of the writing progresses to the right across the page, resulting in increased stiffness in the pen-limb system. Parkinson's patients and elderly controls wrote phrases of different lengths with target patterns in various serial positions. When the number of words to be written increased, PD patients reduced their stroke size of the initial target pattern, while the elderly controls did not reduce their stroke size. There was no systematic change in stroke size of the second pattern as function of serial position. This result suggests that PD patients reduce the size of their handwriting strokes when concurrent processing load increases.

Interjoint coordination during handwriting-like movements.

The present study investigates intrinsic preferences and tendencies in coordination of the wrist and finger movements during handwriting-like tasks. Movement of the inkless pen tip in nine right-handed subjects was registered with a digitizer. One circle-drawing task and four line-drawing tasks were included in the experiment. The line-drawing task included: (1) drawing with the wrist only, (2) drawing with the fingers only, (3) an equivalent pattern consisting of the simultaneous flexion/extension of the wrist and fingers, and (4) a nonequivalent pattern in which wrist flexion was accompanied by finger extension and wrist extension was accompanied by finger flexion. Both the line and circle drawing were performed repetitively at four speed levels, ranging from slow to "as fast as possible" movements. The analysis of the line drawing revealed differential variability and temporal characteristics across the four movement patterns. While the equivalent pattern had characteristics of performance similar to those observed in the wrist-only and fingers-only pattern, the nonequivalent pattern was more variable and was executed slower when as fast as possible movement was required, compared to the other three patterns. The circle-drawing task also revealed intrinsic tendencies in coordination of the wrist and fingers. These tendencies were manifested by a spontaneous transition of the circular path of the pen tip to a tilted oval with increases in movement speed. The transition to the oval shape was accompanied by decreases in relative phase between the wrist and finger movements, whereas amplitudes of these movements were not affected by movement speed manipulations. The results suggest that subjects did not display a tendency to decrease the number of joints involved when executing the patterns that required simultaneous wrist and finger movements. Instead, there were preferences during these patterns to integrate wrist and finger movements with low relative phase. The findings are interpreted in terms of biomechanical constraints imposed on the wrist-finger linkage. This interpretation was further examined by testing two left-handed subjects. The data obtained showed symmetrical preferences in joint coordination. Collectively, the findings support a supposition that the shape of cursive letters may have been adjusted to the biomechanical structure of the hand to facilitate the motor act of handwriting.

Parkinson's disease and the control of size and speed in handwriting.

This experiment investigated whether Parkinson's disease (PD) patients experience problems in producing stroke size, stroke duration or both, in a handwriting task. Thirteen PD patients and 15 elderly controls wrote four patterns of varying complexity on a digitizer tablet. The participants were instructed to execute the writing movements: at a normal size and speed; as fast as possible; two times larger than normal; and two times larger and as fast as possible. PD patients had no difficulty increasing speed while maintaining size and had no difficulty increasing size while maintaining speed. However, they showed significantly smaller size increases in the two times larger condition as compared to the elderly controls. The conditions were also simulated by a neural network model of normal and PD movement control that produced a stroke pattern that approximated the experimental data. For the instructions used, the results suggest that when patients scale speed, they have no difficulty controlling force amplitude, but when they scale stroke size, they have a problem controlling force amplitude. Thus, PD patients may have reduced capability to maintain a given force level for the stroke time periods tested with the instructions.

Axial pen force increases with processing demands in handwriting.

In two experiments, during handwriting movements, the on-line visual feedback of either slant (Experiment 1) or size (Experiment 2) was transformed to study the time course and biomechanics of the participants' compensations for these distortions. Fluency, movement time, and axial pen force were measured. According to our theory, changing the scaling factor of slant or size is equivalent to a processing demand that is reflected in deteriorated signal-to-noise ratios (SNRs) in the neuromotor system. At the behavioral level, deteriorated SNRs will result in less fluent writing, which can be compensated by applying a biomechanical noise-filtering strategy of increased limb stiffness. This strategy will lead to increased axial pen force, and, with higher degrees of difficulty, to a loss of movement speed. Results revealed decrements in writing fluency together with increments in axial pen force and increments in movement time when compensations to the feedback transformations coincided with the more difficult task conditions. These findings contrast with the traditional resource theory (Kahneman, 1973) in which chronometric measures alone indicate increased processing demands.

The influence of mental and motor load on handwriting movements in parkinsonian patients.

This experiment tested the hypothesis that Parkinson's disease (PD) patients are more vulnerable to a moderate level of secondary task load than elderly or young controls due to heightened variability in the motor system. PD patients, elderly, and young adults performed a handwriting task with different secondary tasks. The secondary task imposed motor load (i.e., speech) and/or a mental load (i.e., ignoring, repeating, or subtracting). The findings showed that, in contrast to young and elderly controls, PD patients tended to increase MT, accumulated pause time, and normalized jerk when the secondary task consisted primarily of motor load. Furthermore, it was shown that PD patients did not reduce writing sizes as result of a high level of mental load which finding suggests that writing in an automated fashion does not result in micrographia. The results are discussed in relation to strategies imposed to contend with reduced signal-to-noise levels in the motor system.

Auditory stress effects on preparation and execution of graphical aiming: a test of the neuromotor noise concept.

Effects of physical and mental stress, on the preparation and execution of a psychomotor task were studied to test the applicability of the neuromotor noise concept (Van Gemmert and Van Galen, 1997) as an explanation of stress effects. Central to this notion is that both physical stress and mental load raise neuromotor noise levels in the human information processing system. It is proposed that increased levels of neuromotor noise lead to decreased processing times during task preparation (activation effect), decreased or increased reaction times during task initiation, depending on task difficulty (impoverished signal-to-noise effect) and increased limb stiffness during task execution (biomechanical filtering effect). To test these predictions, an experiment was conducted in which two types of auditory stressors, physical stress and mental load, were manipulated across the stages of preparation, initiation, and execution of a graphical aiming task. The results confirmed the notion that the neuromotor noise concept is a tenable approach to explain the effects of stress on human performance.

Stress, neuromotor noise, and human performance: a theoretical perspective.

A new theory on stress and human performance is proposed in which physical and cognitive stressors enhance the level of neuromotor noise in the information-processing system. The neuromotor noise propagates in time and space. A 2nd assumption states that such noise facilitates easy tasks but disrupts complex tasks. In 4 experiments, 2 graphic tasks (number writing and graphic aiming) were crossed with 2 stressors (cognitive stress from a dual-task situation and physical stress in the form of loud auditory noise). Reaction time (RT), movement time (MT), and axial pen pressure were measured. In the RT phase, stress was predicted to lead to decreased RT with easy tasks and to increased RT with difficult tasks. In the execution phase, biomechanical adaptation to enhanced levels of noise was expected to manifest in higher levels of limb stiffness. In all 4 experiments, an increase of axial pen pressure with higher levels of stress evidenced the generality of biomechanical adaptation as a response to stress. RT and MT showed differential effects among the 4 experiments.