# Postural Control Disturbances Induced by Virtual Reality in Stroke Patients
## 4 Discussion  
This study aimed to compare the impact of static and dynamic virtual environments (i.e., visual flow modulation including a crowd of walking avatars) on postural control in SP with HA. CoP velocity was significantly greater for SP than for HA in the dynamic virtual environment, and Vrcost was greater in SP than HA in the dynamic virtual environment of dense and narrow conditions, showing that the difficulty of VE more influenced the postural control in SP compared with HA.  
While we found that the postural control of SP and HA was not differently altered in the SVE condition, it was more perturbed in SP than in HA under DVE conditions.  
Figure 2: CoP velocity (**A**) and Vrcost (**B**) between HA (healthy adults) and SP (stroke patients). CoP: Center of Pressure; Vrcost: Virtual Reality cost; D: dense; N: Narrow; S: sparse; W: Wide.  
In a recent study, [PERSON] et al. [14] reported that the percentage of postural control deficit in a virtual environment increased with age and amplitude of the moving scene. In addition, it has been shown that postural control of SP is affected by visual rather than proprioceptive or vestibular perturbation [5]. Many authors have suggested that in comparison with young adults, older adults and SP are not able to adapt their postural behavior to the external perturbations [15; 18; 19]. In our study, challenging VE led to more pronounced postural perturbation in SP than in HA, which is consistent with their strong reliance on visual information [5]. Furthermore, in contrast to other studies evaluating the effect of VR on postural control [12; 13; 15], we proposed a multiple complex task with various optical flow induced by changing the density and avoidance space of avatars in a crowded environment. All in all, we suggest that immersive and various ecological virtual environment conditions can be used in SP virtual reality rehabilitation programs with progressive challenging situations.  
In addition, it has been reported that attentional demand increases in SP compared to HA, especially when conditions become more challenging [20]. In a near-infrared spectrometry study, [PERSON] et al. [21] showed that attentional cost, illustrated by prefrontal cortex activation modulation, rose in older adults under challenging balance conditions. This process may be exacerbated in SP and could be partially related to postural control alteration in a challenging VR environment. VR permits a standardized and reproducible environment that can be used to present ecological situations [11] and can provide an additional assessment by observing SP behavior in daily living activities.  
While the VR environment provided clear postural impairment in SP, our study was not free of limitations. While the sample size of SP was small, our study showed that among 22 SPs, only five met our inclusion criteria. This limited enrolment is related to the fact that screening was performed in routine practice and we chose to recruit patients with high postural capabilities (PASS: 33.6 [32:36]) to test a wide range of VR conditions. Regardless of these limitations, our pilot study provides new insights that should be considered in future randomized controlled trials. Furthermore, investigation of the underlying mechanisms of postural control alteration in VR for both SP and HA needs to be undertaken to improve the VR rehabilitation program.