 The experiment day will start at 7.30 a.m. with the test subjects equipped with heart rate recording device. After take-off and after work will be permitted, the 2 subjects will go to the experimental area where they will be seated. They will be equipped with one of the HMD optical- or video-see through HM. During a parabola the subjects perform the experimentation task: symbolic input of five letters onto virtual keyboard depending on the experimental task during a parabola. 