**1. Introduction**

A chronic and degenerative condition without a widely available curative treatment, Parkinson's disease (PD) is characterized by a complex constellation of motor and non-motor symptoms [1,2]. Preferential treatment targets motor symptoms and consists in substitutive dopaminergic therapy. Yet, when disabling symptoms persist despite optimal medical treatment, patients may be proposed to undergo deep brain stimulation (DBS), a stereotactic neurosurgical procedure during which electrodes are implanted into strategic deep nuclei of the brain [3]. DBS results in rapid motor improvement, in some cases immediately observable after the stimulation is switched on [4]. In addition, DBS allows neurologists to drastically decrease patients' dopaminergic medication, leading to a positive impact on drug-induced motor and non-motor symptoms [5,6]. Beneficial e ffects on motor symptoms and medication decrease for patients treated with DBS over those only treated with medication had large size e ffects with a Cohen's *d* approaching 1.35 in both cases [7].

As illustrated by the therapeutic objectives of DBS, there is a medical focus on motor symptoms in PD that underscores the under-appreciation of health professionals and researchers regarding non-motor symptoms [8]. This factual situation is nevertheless paradoxical as, in the long run, patients consider non-motor symptoms as more disabling than motor symptoms [9]. In this regard, it should be mentioned that recent research points out the importance to consider non-motor symptoms along with motor symptoms in the development of future treatments such as closed-loop DBS [10]. Among the numerous non-motor symptoms of PD identified in the literature, some are related to mental health such as depression, apathy, anxiety, or hallucinations [1,8]. Although a leading treatment like DBS provides moderate improvement (*d* ≈ 0.30) on variables related to mental health and quality of life [7], it is unsurprising that a range of non-motor symptoms remain in the patients' clinical picture even after a medically successful DBS.

The paradox of patients experiencing a medically successful DBS, objectively assessed in measuring the pre/post-operative di fference of motor functioning, but nevertheless, a psychosocially unsuccessful DBS characterized by dissatisfaction regarding the everyday life was pictured as a 'burden of health' by Burkhard et al. [11]. Progressively, the concept of social or psychosocial adjustment has emerged as a pivotal theme in the post-DBS rehabilitation of PD patients [12,13]. More recently, a study has provided empirical data supporting the beneficial e ffects of designing specific rehabilitation programs based on psychosocial adjustment [14]. In this context, researchers have suggested that the burden of normality, a model of psychosocial adjustment initially developed for epilepsy surgery [15,16], could relevantly describe the ins and outs of psychosocial adjustment di fficulties throughout the DBS process provided that characteristics inherent to PD (e.g., symptoms continuing to develop post-surgery) are taken into account [17–19]. Among the various di fficulties listed in the burden of normality regarding the post-surgical adjustment process, patients may encounter psychological changes and notably feelings of self-transformation [16]. In PD, patients have indeed reported identity-related complains following DBS surgery, such as feelings of strangeness, loss of body control and dehumanization [12,20–23], which were associated with life adjustment di fficulties. As patients undergoing DBS for PD expect improvement of their motor symptoms, unexpected side e ffects of brain surgery leading to psychological changes would constitute, in addition to adjustment problems, an ethical issue: Indeed, such involuntary changes would possibly induce more harm than benefits and therefore be experienced negatively because they are undesired [24–26]. In a recent literature review, Gilbert et al. [27] have nevertheless pointed out the lack of empirical data supporting changes after DBS in variables that could labelled 'psychological' such as personality, self-identity, agency, authenticity and autonomy. This publication has given rise to a scientific debate that is still ongoing [28,29]. Among similar psychological types of variables, neither illness representations nor coping strategies have been widely studied in the context of DBS, notably for PD.

Both illness representations and coping strategies are psychological constructs pivotal to understand the dynamics that are in play in the patients' subjective perception of their clinical condition. The common sense model [30,31] indeed posits that internal or external stimuli (e.g., appearance of clinical symptoms) generate cognitive and emotional representations of what is associated with a potential danger (e.g., an illness). These representations are addressed with coping strategies, whose e fficiency in the symptom/illness managemen<sup>t</sup> is appraised, leading to possible changes to better adapt the situation. Thus, patients' subjective representations of their illness are closely intertwined with the way they attempt to cope with the stress related to their clinical condition, as both illness representations and coping strategies are likely to be modified depending on the perceived outcome on health [30–32]. From this perspective, one may wonder whether DBS could constitute a life experience disruptive enough to elicit psychological changes in patients; such kinds of changes would besides have a potential impact on psychosocial adjustment as illness representations and coping strategies have been associated to a variety of adaptive and maladaptive outcomes in terms of well-being and social functioning [33].

To our knowledge, no research has been published on the illness representations of patients undergoing DBS for PD. Nevertheless, based on the common sense model, one might expect that DBS would bring in changes in patients' feelings of personal control over their symptoms. Indeed, in advanced PD, patients look to gain a certain degree of control over their symptoms, including motor and non-motor fluctuations, by managing themselves medication intake [12,22,34–37]. Yet, DBS requires active and regular neurologist intervention to adjust stimulation parameters to PD continuous development, which might lead patients to experience feelings of increased dependence on external intervention in their postsurgical disease management. In line with this, one may wonder whether DBS would lead patients to consider PD as more cyclical with regard to the regular search for stimulation adjustment. On the other hand, it is likely that PD would remain identified as such and associated with severe consequences despite the motor improvement brought in by DBS, as the diseased continues to develop and is frequently associated with psychosocial symptoms [20,23,38].

In contrast, a limited number of studies have longitudinally addressed coping strategies before and after surgery, yielding contradictory findings. While some found that patients use more frequently instrumental coping strategies (i.e., task-oriented responses, such as looking for information or for efficient treatment) before than after surgery [14,39,40], others noted that coping strategies were not employed di fferently over this period [41]. When observed, changes in coping strategies were associated with patients' situation regarding their illness. Those about to undergo surgery or who could possibly be treated by DBS in the future sought out further information on this procedure, hence a more frequent recourse to instrumental strategies. Yet, patients already operated appeared to be concerned by new life issues and, accordingly, adjusted their ways of coping; they nevertheless kept using more instrumental strategies than those not selected for DBS, who employed predominantly emotional coping strategies (e.g., avoidance, emotional preoccupation) [39,40].

In light of the above, the present study aimed to address, at least partially, the current lack of empirical data regarding potential changes in the psychological experience of patients undergoing DBS highlighted by Gilbert et al. [27]. This study focused on two specific aspects, namely illness representations and coping strategies, in patients treated with DBS for idiopathic PD through an 18-month longitudinal investigation. Taking limitations of our methodological design into account, we decided to preferentially test hypotheses underlying potential pre/post-DBS changes. Thus, based on the existing literature, we assumed that patients should consider PD after surgery as more cyclical and less controllable in comparison with the pre-DBS period (Hypothesis 1). Second, as all patients included in the study knew that they were about to undergo surgery, they should have resorted to instrumental strategies more frequently before DBS than after (Hypothesis 2).

### **2. Materials and Methods**
