*2.3. Instruments*

The revised version of the Illness Perception Questionnaire (IPQ-R) [42] assesses attributes of illness representations based on the common sense model of Leventhal et al. [30,31]. This instrument proposes a self-evaluation of the main attributes defined by the common sense model, that is the identity of the illness (i.e., the observed symptoms, which may form a label—typically, the name of a disease), the possible causes of the illness, the timeline defining an acute, chronic or cyclic perception of the illness course, the consequences of the illness on daily living, the representations of the controllability or curability of the illness, the perception of the illness as a more or less coherent entity, and the emotional impact of illness representations. For the specific need of this study, we focused on two types of illness representations, namely cyclical timeline and curability/controllability.

The IPQ-R proposes scales to measure separately each of these attributes. Timeline is assessed with one scale whereas curability/controllability is evaluated with two scales (i.e., personal control and treatment control). These scales are designed with Likert-type items proposing five response possibilities (score range per item: 1–5). Test-retest reliability for the two illness representations considered in this study (i.e., cyclical timeline, and personal control and treatment control) showed mixed stability at 3 weeks (correlations from 0.46 to 0.72) and 6 months (correlations from 0.35 to 0.57) [42]. A version of the scale in French language is available on its o fficial website (http://www.uib.no/ipq/).

The Brief COPE [43] is a questionnaire assessing 14 coping strategies with two items for each, leading to a total of 28 items. Coping strategies are evaluated using four-point Likert scales. The score range for each coping strategy is 2–8, with high scores indicating frequent use of the strategy in question. In order to investigate the use of instrumental coping strategies before and after DBS (Hypothesis 2), we created a composite variable by adding the scores of three subscales of the Brief COPE, namely active coping (e.g., item 7: "I've been taking action to try to make the situation better"), use of instrumental support (e.g., item 10: "I've been getting help and advice from other people"), and planning (e.g., item 25: "I've been thinking hard about what steps to take"). This composite variable had a score range of 6–24, with higher scores suggesting greater use of instrumental coping strategies. Like the IPQ-R, internal reliability of the Brief COPE scales was mixed with correlations ranging from 0.50 to 0.90 [43].
