The issue of inhibitory control in TS CTD

The issue of inhibitory control in TS/CTD is actually quite complex, as there is debate over whether certain domains of inhibitory control are in fact affected. For example, some electrophysiological studies (using transcranial magnetic stimulation) have found reduced cortical inhibition in the primary motor cortex in TS/CTD (Ziemann et al., 1997; Gilbert et al., 2004), suggesting altered motor inhibition. However, other studies have shown that differences in cortical excitability may be more related to comorbid attention deficit hyperactivity disorder (ADHD) symptoms than to tics (Gilbert et al., 2005; Orth and Rothwell, 2009). Similarly, although behavioral studies in children and adults with TS/CTD have demonstrated impairments in response inhibition, selective attention, and cognitive flexibility (Bornstein et al., 1991; Channon et al., 2003, 2009; Watkins et al., 2005), some have argued that these impairments are driven by comorbid conditions, including ADHD and obsessive-compulsive disorder (OCD) (Ozonoff et al., 1998; Denckla, 2006). Further, some studies have even shown evidence for enhanced executive function in TS/CTD (Mueller et al., 2006; Jackson et al., 2007). Neuroimaging data are also somewhat inconsistent (for a review, see Greene et al., 2013). There is some evidence for atypical and immature task control systems in the ldv in TS/CTD (Church et al., 2009a, 2009b; Wang et al., 2011) as well as atypical activation of frontostriatal regions posited to support inhibitory control (Aron et al., 2014) in TS/CTD (Hershey et al., 2004a; Marsh et al., 2007; Baym et al., 2008; Raz et al., 2009). However, the directions of specific effects were inconsistent among the fMRI studies, and others have not been able to replicate differences between TS/CTD and controls with similar study designs (Hershey et al., 2004b; Debes et al., 2011). EEG and fMRI studies specifically investigating tic suppression have shown increased activation in frontostriatal regions that support inhibitory control during active tic suppression in children and adults with TS/CTD (Peterson et al., 1998; Hong et al., 2013). Thus, while studies of inhibitory control per se in TS/CTD may be inconsistent, a relationship between tic suppression and inhibitory control mechanisms likely exists. The ability to suppress tics has been measured using a standardized tic suppression paradigm (Woods and Himle, 2004). In this task, children are seated in front of a “tic detector” (described below) and asked to suppress or not to suppress their tics under varying conditions. Studies using this task have demonstrated that children with TS/CTD can suppress tics in response to a simple verbal request, though suppression is inconsistent and varies among individuals (Meidinger et al., 2005; Conelea and Woods, 2008). By contrast, when immediate rewards are given for brief periods of successful tic suppression, children with TS/CTD can robustly and reliably reduce tic rate (Woods and Himle, 2004; Himle and Woods, 2005; Himle et al., 2007, 2008; Woods et al., 2008; Specht et al., 2013). Further, rewards delivered specifically when tics were suppressed led to better tic suppression than rewards given without a temporal link to tic behavior (Himle et al., 2008). In other words, contextual variables that were not immediately linked to tic behavior (i.e., a verbal request to suppress tics, or non-contingent reward) had less impact on tics than contingent rewards. Thus, the presence of a reward that is specifically contingent upon tic behavior may create an environmental context for more consistent tic suppression in TS/CTD. Since a diagnosis of TS/CTD requires tics to be present for a minimum of one year, the children in the cited tic suppression studies had been living with tics for at least a year and usually much longer. Years of experience with tics usually brings years of experience attempting to suppress tics, and some argue that this experience enhances inhibitory control in TS/CTD (Jackson et al., 2011). Thus, exine is unclear whether inhibitory control over tics is present at the onset of the tic disorder or develops with experience. In the present study, we investigated children whose tics began within the previous six months. Surprisingly little is known from controlled studies about children with recent-onset tics. Anecdotally, many of these children display little awareness of their tics, and few of them have experienced the social pressure to inhibit their tics that children with TS/CTD have experienced. Children with recent-onset tics have, at most, a few months’ experience suppressing tics, and the extent to which they can suppress tics has not been reported.