CLOCK and Parkinson disease: Earlier studies have provided evidence consistent with the slowed clock hypothesis, which states that dopamine deficiency slows down the pace of the internal clock, which is corrected by dopaminergic medication improving the estimation of duration in the time production task in PD patients (Pastor et al., 1992; Lange et al., 1995; Smith et al., 2007; Koch et al., 2008; Wild-Wall et al., 2008).