Visuo-motor coordination deficits and motor impairments in Parkinson's disease

Abstract

Background: Visuo-motor coordination (VMC) requires normal cognitive executive functionality, an ability to transform visual inputs into movement plans and motor-execution skills, all of which are known to be impaired in Parkinson's disease (PD). Not surprisingly, a VMC deficit in PD is well documented. Still, it is not known how this deficit relates to motor symptoms that are assessed routinely in the neurological clinic. Such relationship should reveal how particular motor dysfunctions combine with cognitive and sensory-motor impairments to produce a complex behavioral disability.

Methods and findings: Thirty nine early/moderate PD patients were routinely evaluated, including motor Unified Parkinson's Disease Rating Scale (UPDRS) based assessment, A VMC testing battery in which the subjects had to track a target moving on screen along 3 different paths, and to freely trace these paths followed. Detailed kinematic analysis of tracking/tracing performance was done. Statistical analysis of the correlations between measures depicting various aspects of VMC control and UPDRS items was performed. The VMC measures which correlated most strongly with clinical symptoms represent the ability to organize tracking movements and program their direction, rather than measures representing motor-execution skills of the hand. The strong correlations of these VMC measures with total UPDRS score were weakened when the UPDRS hand-motor part was considered specifically, and were insignificant in relation to tremor of the hand. In contrast, all correlations of VMC measures with the gait/posture part of the UPDRS were found to be strongest.

Conclusions: Our apparently counterintuitive findings suggest that the VMC deficit pertains more strongly to a PD related change in cognitive-executive control, than to a reduction in motor capabilities. The recently demonstrated relationship between gait/posture impairment and a cognitive decline, as found in PD, concords with this suggestion and may explain the strong correlation between VMC dysfunction and gait/posture impairment. Accordingly, we propose that what appears to reflect a motor deficit in fact represents a multisystem failure, dominated by a cognitive decline.

Figure 1. Apparatus for VMC testing.

A digitizing tablet is enclosed in the bottom part, with a dome-shaped manipulandum resting over it. A sinusoidal path with a target at its left end and a subject-controlled cursor are shown on screen.

Figure 2. Tracking (left) and tracing (right) along a circular path, as performed by a single moderate PD patient.

Each test started from the white dot and continued counter clockwise (indicated by the inserted arrow). The number of tracking interruptions, their rate and the mean±SD of the directional error for the above tests are shown. In elderly control subjects (n = 57) the mean values of these measures are: 7.0±3.7 (nm. of interruptions), 0.32±0.17 (Ints/Sec) and 24.8±1.3 (Dir_Er in tracing).

Figure 3. The relationship between all VMC measures (abscissa) and the various UPDRS items groups.

Ordinate: Spearman correlation coefficient value. Insert: Designation of the different UPDRS items groups.

Figure 4. Relationship between the frequency of tracking interruptions and the directional error in tracing.

The results of each patient are shown for the right hand (blue circles) and for the left hand (red triangles) performance of the VMC test battery. Abscissa: mean directional error in tracing. Ordinate: mean frequency of tracking interruptions. Abbreviations: NinFreqR – Frequency of tracking interruptions with the right hand. C_DER – Directional error in tracing with the right hand. Same abbreviations when terminated with “L” apply to performance with the left hand.