A New Dynamic Virtual Stimulation Protocol to Evoke M-VEP and Linear Vection during Orthostatic Posture Control

The dynamic virtual stimulation (DS) was employed to investigate postural control response using the motion- related visual evoked potential (M-VEP) and vection. The multi-channel electroencephalogram (EEG) and the center-ofpressure (COP) displacement signals were acquired simultaneously during stabilometric test with DS scenes interspersed by 10 s of static scene (SS). The trials from 29 healthy volunteers were performed in orthostatic position on a force platform observing a virtual scene (1.72 × 1.16 m) projected 1 m ahead and centered at the vision line (visual angle: θl= 81.4° and θv = 60.2°). The scenario was moved randomly in forward (DSF) or backward direction (DSB), during 250 ms (velocity: 2 m/s). For each DS, the luminance varied 2 cd/m2. Such dynamic effect was employed to generate an optical flow as a tunnel pattern. For each subject, the M-VEP was estimated by coherent averaging up to 50 artifact-free EEG epochs, synchronized by the onset of DS. Similar procedure was applied for the COP signal. The running t-test (α = 0.05) suggested that there is difference between DSF and DSB M-VEPs 600 ms after DS onset, with an increasing time delay from occipital to central derivations. Moreover, the DS induced COP displacements in the same direction of the exhibiting scene (p < 0.001). Therefore, vection is dependent on the DS direction, mainly during DSF, for which the linear vection varies from 72% to 90%. These findings suggest that the cognitive, planning and motor processing to control balance depends on the DS direction and hence indicate the potential applications of this dynamic virtual protocol in postural control studies.