5 RESPONSE INHIBITION IN TOURETTE SYNDROME AND TIC
5.3.2 Experiment 1: Measuring proactive and reactive inhibition using the
5.3.2.1 Behavioural measures
Behavioural measurements are shown in table 5.2. As in previous stopping experiments, there was an expected go reaction time difference between critical and non-critical trials due to the anticipation to stopping in critical trials (right hand critical: t = 5.361, p < 0.001, d = 1.01, right hand non-critical: t = 2.097, p < 0.001, d = 0.407). This is indexed by the response delay effect. Participants unexpectedly achieved a greater than expected probability of successful inhibition. These results show that volitional inhibition is intact in patients with TS and tic disorders.
As OCD and ADHD can both modulate performance on tasks of stopping, we conducted a mixed ANOVA with each of the parameters as dependent variables and OCD and ADHD status as main factors. This mixed ANOVA found a significant effect of OCD status (F (1, 15) = 5.745; p = 0.030, η2 = 0.277) but not ADHD status (F (1, 15) = 0.717;
p = 0.410, η2 = 0.046). Further interrogation of which parameter was significantly modulated by OCD status using a one-way ANOVA showed that only non-critical stop reaction time was statistically significantly altered (F (1, 17) = 4.859; p = 0.042, η2 = 0.011).
Measure Measure description
Critical direction Critical Non-critical
Go RT to go stimulus in the critical direction 501.64 (77.31) 494.65 (76.48)
p(inhibit) % correct inhibition 62.39 (18.20) 61.32 (19.64)
Stop Respond RT on failure to stop trials 419.00 (77.14) 461.22 (95.44) Go error % of go discrimination errors 1.14 (1.94) 1.18 (1.85) Stop signal delay Delay between go and stop signals 190.26 (38.59) 185.00 (41.51) SSRT Calculated time taken to abort response 334.98 (89.63) 332.30 (87.00) Non-critical direction
Go RT to go stimulus in the non-critical direction 429.72 (64.95) 457.60 (103.73) Other variables
Response delay effect (Critical go) - (Non-critical go) RT 71.93 (58.48) 37.05 (17.67) Right hand rule
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Table 5.2: Behavioural measurements from the Conditional stop-signal task performed in patients with Tourette syndrome and tic disorders.
The table shows the behavioural measures from the CSST. As two blocks were performed, each block’s results are presented. Measures are accompanied by SD in brackets. Reaction times are given in milliseconds.
5.3.2.2 Patients employ a similar decision-making strategy to healthy controls when stopping might be required
The DMAT toolbox was again used to quantify the variables changed during stopping between critical and non-critical go trials. We found that patients increased their decision boundary (t = 4.393, p < 0.001, d = 1.40) and decreased their non-decision time (t = 2.695, p = 0.015, d = -0.92) in the face of potential stopping. Drift rates were not significantly modulated between stopping conditions (t = 0.606, p = 0.552, d = 0.162). Note that both healthy control subjects and patients were shown to heighten their boundary separation when stopping might be required. However, control subjects were shown to decrease their drift rate when stopping might be required, although this finding should be interpreted with caution due to reasons outlined in Chapter 4. Curiously, non-decision time was significantly reduced in patients, which may be a mathematical compensation for the higher drift rate during critical than non-critical go trials; it has been shown that fixing or changing in one parameter can lead to changes in another, yielding similar behavioural predictions (214–216).
As OCD and ADHD might influence the strategy used to slow down, we performed a mixed ANOVA using the DDM parameter as the dependent variable and OCD and ADHD statuses as main factors. We found no significant effects of OCD (F (1, 15) = 0.169; p = 0.687, η2 = 0.011) or ADHD (F (1, 15) = 0.007; p = 0.933, η2 = 0.275) status on the DDM parameter used during the CSST.
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Figure 5.2: Drift-diffusion model parameters for the Conditional stop-signal task. Estimated DDM parameters are shown for individual subjects, for boundary separation, non-decision time and drift rate, for right-handed, critical and non-critical go trials. Black stars represent mean parameter estimation, whilst error bars reflect SEM.
5.3.2.3 Evolution of corticospinal excitability in the conditional stop-signal task in patients with Tourette syndrome and tic disorders
We assessed how CSE would evolve between trials when stopping might be required (critical trials) against those where stopping was not (non-critical trials) by plotting CSE in a stimulus-locked manner. Baseline CSE as measured by MEP amplitude was the same for critical and non-critical go trials (t = 1.047, p = 0.309, d = 0.244). We found that CSE became statistically significantly greater than that at baseline later for critical go trials (300ms: t = 2.941, p = 0.009, d = 0.992) than non-critical go trials (250 ms: t = 2.931, p
= 0.009, d = 0.881). These results appeared to show a slower rise to threshold when stopping might be required. If this was the case, then this should also be observed when
Critical Non-critical
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the reaction times are controlled for between conditions. We therefore performed a response-locked analysis. A two-way repeated measures ANOVA with main factors CONDITION and TIME showed that motor execution was equivocal between stopping conditions: CONDITION (F (1, 14) = 1.335; p = 0.267, η2 = 0.087), TIME (F (3, 42) = 46.31; p < 0.001, η2 = 0.768), CONDITION*TIME interaction (F (3, 42) = 0.944;
p = 0.428, η2 = 0.063). This showed that movement execution occurs the same, regardless of the stopping requirements of the go trial. We therefore concluded that movement did not occur in these patients due to a slower rise to threshold and that the findings from the stimulus-locked analysis were solely due to differences in the reaction time difference between conditions.
Figure 5.3: Evolution of corticospinal excitability in the Conditional stop-signal task. Left: MEPs are plotted against the time from cue presentation for go trials in the non-critical direction (blue circles) and non-critical trials (red squares). Right: Corticospinal excitability is plotted in 50 ms time bins determined by the time between TMS and
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response, such that smaller values represent data points closer to responses. Error bars represent mean±SEM.
These results together show that the putative mechanisms of response preparation and response execution are retained within patients with TS and tic disorders, and that these processes are still independent. We next aimed to assess whether motor preparation and execution were significantly altered relative to a population of healthy control subjects, without tics or TS. To do this, we compared data from this patient group with that outlined in Chapter 4.