Adaptation of handwriting size under distorted visual feedback in patients with Parkinson's disease and elderly and young controls.

OBJECTIVE: The ability to use visual feedback to control handwriting size was compared in patients with Parkinson's disease (PD), elderly people, and young adults to better understand factors playing a part in parkinsonian micrographia. METHODS: The participants wrote sequences of eight cursive l loops with visual target sizes of 0.5 and 2 cm on a flat panel display digitiser which both recorded and displayed the pen movements. In the pre-exposure and postexposure conditions, the display digitiser showed the actual pen trace in real time and real size. In the distortion exposure conditions, the gain of the vertical dimension of the visual feedback was either reduced to 70% or enlarged to 140%. RESULTS: The young controls showed a gradual visuomotor adaptation that compensated for the visual feedback distortions during the exposure conditions. They also showed significant after effects during the postexposure conditions. The elderly controls marginally corrected for the size distortions and showed small after effects. The patients with PD, however, showed no trial by trial adaptations or after effects but instead, a progressive amplification of the distortion effect in each individual trial. CONCLUSION: The young controls used visual feedback to update their visuomotor map. The elderly controls seemed to make little use of visual feedback. The patients with Parkinson's disease rely on the visual feedback of previous or of ongoing strokes to programme subsequent strokes. This recursive feedback may play a part in the progressive reductions in handwriting size found in parkinsonian micrographia.

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.

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.

Visuo-motor adaptation in smokeless tobacco users.

Ten smokeless tobacco (ST) users and 11 non-smokers participated in a visuo-motor adaptation experiment in which the visual feedback of point-to-point horizontal arm movements, displayed in real-time on a computer screen, was rotated by 45 degrees counterclockwise for some trials. Visuo-motor performance between smokers and non-smokers was compared on three occasions, once after at least 8 h of tobacco abstinence (Session 1), a second time following ST intake (Session 2), and a third time 45 min after the original ST intake (Session 3). Non-smokers were tested at the same relative times as the smokers in the absence of any tobacco. Both groups performed the three conditions during each session: baseline (normal visual feedback), rotated visual feedback (45 degrees visual feedback rotation), and post-adaptation (normal visual feedback immediately following feedback rotation). Compared with non-smokers, ST users had significantly larger normalized jerk scores (a measure of movement smoothness) after ST intake during the adaptation and post-adaptation conditions in Sessions 2 and 3, but not during the baseline conditions, implying a differential effect of ST use specific to rotated visual feedback. Movement duration was also longer for smokers than for non-smokers after ST intake, but only in the post-adaptation condition. Overall the results suggest that ST use, and hence nicotine, has a detrimental effect on visuo-motor performance, particularly on movement smoothness.

Elderly subjects are impaired in spatial coordination in fine motor control.

Elderly and young control subjects performed back-and-forth handwriting movements in various orientations, therefore varying the coordination demands. Elderly subjects showed higher normalized jerk and straightness scores than the young subjects. However, jerk scores were independent of the coordination demands in either group. In contrast, the straightness scores were highly dependent on stroke orientation for the elderly, but they remained constant across orientations for the young controls. Moreover, group differences in stroke size and stroke duration were not significant, and orientation effects were unrelated. It is suggested that the orientation-dependent straightness scores in the elderly may result from unequal timing or improper scaling of muscle forces. These data suggest that aging deteriorates the spatial coordination of finger and wrist movements, but not accelerative force control.

Handwriting and speech changes across the levodopa cycle in Parkinson's disease.

Levodopa treatment is used to reduce rigidity and bradykinesia in Parkinson's disease (PD). This study tracked the handwriting and speech performance of 10 PD patients at 30-minute intervals across one levodopa drug cycle to evaluate levodopa-related changes in temporal and spatial measures that are assumed to correspond to changes in rigidity and bradykinesia. The handwriting measures included l and e upstroke duration and size. The speech measures included duration of the vowels /i/, /u/, /ae/, and /[symbol: see text]/ and the quadrilateral area produced by these vowels, and the slope of the diphthong /aI/. Levodopa significantly improved handwriting upstroke duration but not upstroke size. Speech measures did not show a significant trend across the levodopa cycle. The results suggest that upstroke duration is sensitive to the presumed effects of levodopa and that handwriting analysis may hold promise in helping to estimate an optimum levodopa regimen for PD patients.

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.

Neural dynamics of short and medium-term motor control effects of levodopa therapy in Parkinson's disease.

A neural network model of movement control in normal and Parkinson's disease (PD) conditions is proposed to simulate the time-varying dose-response relationship underlying the effects of levodopa on movement amplitude and movement duration in PD patients. Short and long-term dynamics of cell activations and neurotransmitter mechanisms underlying the differential expression of neuropeptide messenger RNA within the basal ganglia striatum are modeled to provide a mechanistic account for the effects of levodopa medication on motor performance (e.g. the pharmacodynamics). Experimental and neural network simulation data suggest that levodopa therapy in Parkinson's disease has differential effects on cell activities, striatal neuropeptides, and motor behavior. In particular, it is shown how dopamine depletion in the striatum may modulate differentially the level of substance P and enkephalin messenger RNA in the direct and indirect basal ganglia pathways. This dissociation in the magnitude and timing of peptide expression causes an imbalance in the opponently organized basal ganglia pathways which results in Parkinsonian motor deficits. The model is validated with experimental data obtained from handwriting movements performed by PD subjects before and after medication intake. The results suggest that fine motor control analysis and network modeling of the effects of dopamine in motor control are useful tools in drug development and in the optimization of pharmacological therapy in PD patients.

Parkinsonism reduces coordination of fingers, wrist, and arm in fine motor control.

This experiment investigates movement coordination in Parkinson's disease (PD) subjects. Seventeen PD patients and 12 elderly control subjects performed several handwriting-like tasks on a digitizing writing tablet resting on top of a table in front of the subject. The writing patterns, in increasing order of coordination complexity, were repetitive back-and-forth movements in various orientations, circles and loops in clockwise and counterclockwise directions, and a complex writing pattern. The patterns were analyzed in terms of jerk normalized for duration and size per stroke. In the PD subjects, back-and-forth strokes, involving coordination of fingers and wrist, showed larger normalized jerk than strokes performed using either the wrist or the fingers alone. In the PD patients, wrist flexion (plus radial deviation) showed greater normalized jerk in comparison to wrist extension (plus ulnar deviation). The elderly control subjects showed no such effects as a function of coordination complexity. For both PD and elderly control subjects, looping patterns consisting of circles with a left-to-right forearm movement, did not show a systematic increase of normalized jerk. The same handwriting patterns were then simulated using a biologically inspired neural network model of the basal ganglia thalamocortical relations for a control and a mild PD subject. The network simulation was consistent with the observed experimental results, providing additional support that a reduced capability to coordinate wrist and finger movements may be caused by suboptimal functioning of the basal ganglia in PD. The results suggest that in PD patients fine motor control problems may be caused by a reduced capability to coordinate the fingers and wrist and by reduced control of wrist flexion.

Micrographia in Parkinson's disease.

A computational neural model of movement production in normal and Parkinson's disease (PD) is used to provide a neural account for the source of micrographia in PD handwriting. It is hypothesized that smaller than normal pallido-thalamic signals, due to dopamine depletion, are responsible for the observed overall smallness, slowness and variability in PD handwriting. Experimental data from PD patients that show micrographia support this hypothesis and imply the functional segregation of basal ganglia neural populations.

Invariant properties between stroke features in handwriting.

A handwriting pattern is considered as a sequence of ballistic strokes. Replications of a pattern may be generated from a single, higher-level memory representation, acting as a motor program. Therefore, those stroke features which show the most invariant pattern are probably related to the parameters of the higher-level representation, whereas the more noisy features are probably related to the parameters derived at the lower levels (top-down hierarchy). This hierarchy of invariances can be revealed by the signal-to-noise ratio (SNR), the between-parameter correlations, and the between-condition correlations. Similarly, at the higher level a sequence of strokes may act as a unit from which individual strokes are derived (sequence hierarchy). This hierarchy of invariances can be revealed by the between-stroke correlation, which forms a weaker criterion than rescalability, which has been rejected mostly. Previous research showed that vertical stroke size has higher SNRs and higher between-condition correlations than stroke duration or peak force, whereas the latter two features were also negatively correlated. This suggested that vertical stroke size is a higher-level parameter than the other two. The present research largely confirmed this top-down hierarchy and even for upstrokes and downstrokes separately. Downstrokes were more invariant than upstrokes in terms of vertical stroke size. However, contrary to the vertical stroke size, the horizontal stroke size was not invariant. Both vertical and horizontal sizes showed substantial between-stroke correlations. In contrast, the stroke durations did not show any between-stroke correlations. This suggests that stroke segmentation is reliable in spite of the discrete sampling of the handwriting movements.

The independence of horizontal and vertical dimensions in handwriting with and without vision.

The purpose of this study was to investigate the relationship between horizontal and vertical components of handwriting production when subjects were instructed to vary the size of these components separately or together. The effect of vision on these instructed size transformations also was examined. Eight female adults participated in the experiment. The basic task was to write the words 'poppy' and 'wood' cursively five times, the first time in their normal size and then with four size transformations. These transformations--one-fourth, one-half, double, and four-times their normal size--were made under three different sets of instructions (width only, height only, both) and in two visual conditions (normal, blindfolded), for a total of six sets of five repetitions. The individual slopes (changes in actual values across the transformation values) for width and height under instructions to change both parameters were almost identical to the width and height slopes under instructions to change only the single parameter, supporting the notion of the independence of the horizontal and vertical components. Further, an analysis of these individual slopes indicated that the size transformations were significantly greater (p less than 0.05) (and closer to the instructed values) with vision than without vision.

Temporal and spatial characteristics in repetitive movement.

The question addressed is whether temporal or spatial characteristics of a movement sequence are represented in a motor program. Subjects executed one to five straight-line strokes as fast as possible on a digitizing tablet. The duration and length of each stroke were computed and intercorrelated. Nineteen out of 20 possible correlations were significant for length per stroke data while only one of the 20 correlations was significant for duration. Further, performance data on the five stroke patterns were sorted to examine whether the first stroke determined the overall temporal/spatial pattern of the subsequent strokes executed. It was found that only sorting by length of the first stroke produced different subsets of movement patterns. These results are argued to be in support of the hypothesis that the spatial microstructure is controlled in a prepared movement sequence.