## 1 Introduction
### New Challenges in the Evaluation of Executive Functions
#### 1.2.1 Virtual Reality  
VR allows for the development of realistic spatial and temporal scenarios, situations, objects, and stimuli that reproduce conditions of everyday life, allowing an ecologically valid evaluation of EFs [25; 26; 27; 28], as well as reliability and psychometric validity [29; 30; 31]. In other words, VR appears to be an appropriate tool for the assessment of EFs because it provides the chance to perform many everyday tasks in ecologically valid, secure, and controlled environments [26]. Thus, clinicians can directly observe their patients in an everyday setting (e.g., supermarkets, kitchens, or work office), while performing several executive tasks [32,33]. Moreover, the use of virtual environments in neuropsychological assessments guarantees better control of the perceptual environment, a more precise presentation of the stimulus, greater applicability, user-friendly interfaces, and the acquisition of data and analysis of performance in real time [27,28,31,34,35,36,37]. Several authors studied ED in PD through VR-based instruments that required performance of real-life tasks (e.g., a shopping task) in virtual environments that reproduced a real supermarket [13,38,39,40,41]. Among the EFs, planning represents an essential component of cognitive PD alteration. From the early stages of PD, planning deficits have been found and probably reflect front-triatal circuit degeneration [42]. [PERSON] and colleagues developed a virtual supermarket to evaluate visuospatial and temporal aspects of planning using a 3D environment [13,38]. In this VR-based tool, clinicians evaluated the strategic choices and planning abilities of individuals with PD in buying a specific number of products. Data showed the absence of a significant difference between the PD group and the control group for the number of correct actions. On the contrary, the increased distance and duration and the inefficient trajectory observed in the PD group suggested a gradual decrease in planning processes in PD (slowness of information processing) and inefficient use of contextual elements. In other words, these results underlined the spatial and temporal aspects of planning deficits in PD. Moreover, the inefficient trajectory could also suggest a dysfunction in the switching mechanism necessary to process, in parallel, large amounts of information. Interestingly, VR allowed for the description of alterations in planning by testing \"pure\" mental sequences without the interference of possible motor disability. In later years, several authors used the virtual version of the Multiple Errand Test (VMET) to evaluate executive disorders in PD patients ecologically. The VMET allows for the assessment of patients' abilities in formulating and checking a list of goals to effectively respond to environmental requests to achieve a series of tasks (e.g., buy a specific product, or ask the examiner information about a product to be purchased). In a preliminary study, [PERSON] and colleagues showed the presence of many deficits in planning, which is the strategy that allows the correct execution of the task in problem-solving and set-shifting [43]. Specifically, individuals with PD use few strategies, and show more perseveration and difficulties in set-shifting and sustained attention. The following study conducted by [PERSON] and colleagues also showed impairments in decision making in individuals with PD [44]. Patients showed strategies full of errors, suggesting impulsive decision making. These data can support the hypothesis that behavioural disturbance (in some individuals with PD) might precede cognitive dysfunctions in PD. Similar findings were found by [PERSON] and colleagues that compared PD with normal cognition (PD-NC) and a control group [40]. Patients made more errors in the tasks of the VMET and demonstrated reduced use of effective strategies to complete the tasks, like careful planning before starting a subtask or using the map for navigating the virtual supermarket.