Pointing in cervical dystonia patients

Abstract

Introduction: The normal hemispheric balance can be altered by the asymmetric sensorimotor signal elicited by Cervical Dystonia (CD), leading to motor and cognitive deficits.

Methods: Directional errors, peak velocities, movement and reaction times of pointing towards out-of-reach targets in the horizontal plane were analysed in 18 CD patients and in 11 aged-matched healthy controls.

Results: CD patients displayed a larger scatter of individual trials around the average pointing direction (variable error) than normal subjects, whatever the arm used, and the target pointed. When pointing in the left hemispace, all subjects showed a left deviation (constant error) with respect to the target position, which was significantly larger in CD patients than controls, whatever the direction of the abnormal neck torsion could be. Reaction times were larger and peak velocities lower in CD patients than controls.

Discussion: Deficits in the pointing precision of CD patients may arise from a disruption of motor commands related to the sensorimotor imbalance, from a subtle increase in shoulder rigidity or from a reduced agonists activation. Their larger left bias in pointing to left targets could be due to an increased right parietal dominance, independently upon the direction of head roll/jaw rotation which expands the left space representation and/or increases left spatial attention. These deficits may potentially extend to tracking and gazing objects in the left hemispace, leading to reduced skills in spatial-dependent motor and cognitive performance.

Keywords: asymmetry; cervical dystonia; neck input; pointing errors; space representation.

Figure 1

Experimental set-up. (A) Resting position: the subject keeps his/her index finger tip at a grey disk located 5 cm in front of the sternum and aligned with the central target. (B) Example of a pointing movement to the left target. The angle between the two dashed lines corresponds to the Mean Directional Error (MDE).

Figure 2

Mean Directional Error (MDE) at different target positions. The bar height represents the average MDE values, evaluated in each subject as the mean value of left and right arm. The dispersion bars correspond to the SE. Data relative to CD patients and controls have been separately represented by black and white columns, respectively.

Figure 3

Scatterplots of Direction Error Variability (DEV) versus Mean Directional Error (MDE). (A) Control subjects. (B) CD patients. Right and left arm data are indicated by square and circles, respectively. Black, grey and white filling of the symbols indicates −20°, 0° and 20° targets, respectively.

Figure 4

Peak of Tangential Velocity (PTV) values observed during pointing to the different targets. (A) PTV, left arm pointing. (B) PTV, right arm pointing. The dispersion bars correspond to the SE.

Figure 5

Relation between Direction Error Variability (DEV) and Movement Time (MT). (A) Control subjects. (B) CD patients. Right and left arm data are indicated by square and circles, respectively. Black, grey and white filling of the symbols indicates −20°, 0° and 20° targets, respectively. The regression line and the relative data refer to all the plotted points.

Figure 6

Relation between Movement Time (MT) and Reaction Time (RT). (A) Control subjects. (B) CD patients. Right and left arm data are indicated by square and circles, respectively. Black, grey, and white filling of the symbols indicates −20°, 0° and 20° targets, respectively. The regression line and the relative data refer to all the plotted points.