Fractality of tics as a quantitative assessment tool for Tourette syndrome

Tics manifest as brief, purposeless, and unintentional movements or noises that, for many individuals, can be suppressed temporarily with effort. Peterson and Leckman (1998) hypothesized that the chaotic temporal nature of tics could possess an inherent fractality, that is, have neighbor-to-neighbor correlation at all levels of time scale. However, demonstrating this phenomenon has eluded researchers for more than two decades, primarily because of the challenges associated with estimating the scale-invariant, power law exponent – called the fractal dimension D f – from fractional Brownian noise. Here, we confirm this hypothesis and establish the fractality of tics by examining two tic time series datasets collected 6-12 months apart in children with tics, using one-dimensional random walk models. We find that D f increases from ∼1.4 to 1.75 in order of decreasing tic severity, and is correlated with tic severity as measured by the YGTTS total tic score. We demonstrate D f to be a sensitive parameter in examining the effect of several tic suppression conditions on the tic time series. We confirm the fractal nature of tics in Tourette syndrome (TS) and extend the finding to Provisional Tic Disorder. Our findings pave the way for utilizing the fractal nature of tics as a robust quantitative tool for estimating tic severity and treatment effectiveness, as well as a marker for differentiating typical from functional tics.