Temporal processing of perceived body movement in cervical dystonia

Patients with idiopathic dystonia exhibit changes in the cognitive processing of movement. We showed that patients with writer’s cramp are less accurate than normal subjects in temporally predicting perceived handwriting. Whether this is selectively linked to the body area affected by dystonia or is a generalized cognitive feature of dystonia remains unclear. We addressed this issue by applying the same experimental paradigm to patients with focal cervical dystonia (CD). Fifteen patients with focal CD, aged 56.2 6 13.9 y and treatment-free for at least 6 mo, and 15 age-matched healthy subjects were recruited in the Department of Neuroscience, University of Genoa. Patients’ disease duration was 9 6 6.5 y, and mean 6 standard deviation score on the Toronto Western Spasmodic Torticollis Rating Scale was 14.3 6 4.8. The experimental paradigm, previously published in Avanzino et al., consisted of the perception on a screen of two videos, one showing a right hand writing a sentence (target task), and another showing a ball reaching a target (control task). After a variable interval from its onset (6, 9, and 12 seconds), videos were darkened. Subjects were asked to indicate when the perceived movement reached its end by clicking on the keyboard space-bar (Supplemental Data). The timing error (Reproduced Interval – Dark Interval), the normalized absolute timing error ([Timing Error/Dark Interval] 3 100), and the coefficient of variability (standard deviation/mean of Reproduced Intervals) were measured and analyzed with a repeated-measures analysis of variance with the factors GROUP, TASK, and DARK INTERVAL. Repeated-measures analysis of variance showed a significant GROUP*TASK interaction only for the normalized absolute timing error (F[1,28] 5 5.85; P 5 0.022; Fig. 1). On post hoc, this parameter was greater at all dark intervals only in CD patients (P 5 0.006) and exclusively for the target task (P 5 0.024). In both groups of subjects, consistently with what was observed in our previous work, the ability to temporally predict the end of the perceived movement was influenced by the duration of the target interval and the type of motion. Shorter dark intervals were associated with a tendency to overestimate the duration of movement (F[2,56] 5 136.61; P< 0.001), greater variability (F[2,56] 5 50.60, P< 0.001), and greater absolute timing error (F[2,56] 5 26.06; P< 0.001). Finally, a tendency to overestimate the duration of movement was observed for the target task compared with the control task (F[1,28] 5 6.38, P 5 0.017). Absolute timing error did not correlate with disease severity (Spearman’s rho 5 –0.161; P 5 0.58) or duration (Spearman’s rho 5 0.040; P 5 0.89). Our findings suggest that the abnormal timing of visually perceived human body motion is not exclusive to movements topographically related to dystonia. Brain regions relevant to the pathophysiology of dystonia, for example, sensorimotor regions of premotor and parietal cortices and cerebellum, modulate the spatiotemporal prediction of dynamic visual stimuli, and could be involved in the detected abnormality. Despite the relatively small sample size, the lack of correlation between timing performance and severity/duration of CD suggests that the observed abnormality may not be a direct expression of the dystonia. The selectivity of the timing abnormality might depend in part on the difference in complexity between handwriting and the inanimate object motion. However, if motion complexity is the main determinant of implicit timing performance, timing error should decrease at the increase of task complexity also in control subjects, but this was not observed. This notwithstanding, future studies should explore temporal processing of motion in dystonia, using