Variability in interval production is due to timing-dependent deficits in Huntington's disease

In Huntington's disease (HD), increased variability is seen in performance of motor tasks that require implicit control of timing. We examined whether timing variability was also evident in an explicit interval-timing task. Sixty subjects (21 controls, 19 manifest HD, and 20 pre-manifest HD) performed a single-interval production task with three target intervals (1.1 s, 2.2 s, 3.3 s). We analyzed accuracy (proportional error) and precision (standard deviation) across groups and intervals. No differences were seen in accuracy across groups or intervals. Precision was significantly lower in manifest (P = 0.0001) and pre-manifest HD (P = 0.04) compared with controls. This was particularly true for pre-manifest subjects close to diagnosis (based on probability of diagnosis in 5 years). Precision was correlated with proximity to diagnosis (r2  = 0.3, P < 0.01). To examine the source of reduced precision, we conducted linear regression of standard deviation with interval duration. Slope of the regression was significantly higher in manifest HD (P = 0.02) and in pre-manifest HD close to diagnosis (P = 0.04) compared with controls and pre-manifest participants far from diagnosis. Timing precision is impaired before clinical diagnosis in Huntington's disease. Slope analysis suggests that timing variability (decreased precision) was attributable to deficits in timing-dependent processes. Our results provide additional support for the proposal that the basal ganglia are implicated in central timekeeping functions. Because the single interval production task was sensitive to deficits in pre-manifest HD, temporal precision may be a useful outcome measure in future clinical trials.