roll, or yaw. The vestibular system generates reflexes to maintain perceptual and postural stability, compared to the other senses of vision, touch and sound, the vestibular input is registered quickly by the brain, whereas visual changes such as perspective and horizon movement, follow shortly after. Therefore, in a simulator it is essential that visual cues are not perceived before motion cues, the reverse of the real-world situation, or so-called "simulator sickness" can occur. Second, if the head experiences sustained accelerations on the order of 10 – 20 seconds, the hair follicles return to the “zero” or vertical position and the brain interprets this as the acceleration ceasing. Additionally, there is a lower acceleration threshold of about 2 degrees per second that the brain cannot perceive. In other words, slow motion below the threshold will not generate vestibular cues. As discussed in the preceding “Proprioceptors” section, this allows simulator motion to be "washed out" below threshold, ready for the next acceleration to be produced by the motion platform and sensed by the subject of the simulator. Visual inputs The human eye is an important source of information in motion simulation where a high resolution picture is available such as by day in good visibility. The eye relays information to the brain about the craft's position, velocity, and attitude relative to the ground. As a result, it is essential for realistic simulation that cues from a motion platform (if fitted) works in synchronization to the external visual scene. As discussed above, in the real world motion cues are processed by the brain before visual changes, and this must be followed in a simulator or dizziness and even nausea can occur in some people, so called "simulator sickness". For example, if the occupant commands the vehicle to roll to the left, the visual displays must also roll by the same magnitude and at the same rate. Simultaneously, the cab tilts the occupant to imitate the motion. The occupant's proprioceptors and vestibular system sense this motion. The motion and change in the visual inputs must align well enough such that any discrepancy is below the occupant's threshold to detect the differences in motion. In order to be an effective training or entertainment device, the cues the brain receives by each of the body's sensory inputs must agree. Simulator operations It is physically impossible with most existing systems to correctly simulate large-scale motion in the