moving visual scene is a strong cue in the real world, and the visual system in a simulator uses computer graphics to model the real scene. Proprioceptors Proprioceptors are receptors located in muscles, tendons, joints and gut, which send signals to the brain in proportion to accelerations to which the body is subject. An example of a "popular" proprioceptor mentioned by aircraft pilots, is the feeling in the "seat of the pants". Proprioceptors respond to stimuli generated by muscle movement and muscle tension. Signals generated by exteroceptors and proprioceptors are carried by sensory neurons or nerves and are called electrochemical signals. When a neuron receives such a signal, it sends it on to an adjacent neuron through a bridge called a synapse. A synapse "sparks" the impulse between neurons through electrical and chemical means. These sensory signals are processed by the brain in response to signals that travel along motor nerves. Motor neurons, with their special fibres, carry these signals to muscles, which are instructed to either contract or relax. These are all sensors of acceleration, and do not respond when a constant speed or velocity is reached. At constant speed, visual cues give cues of motion until another acceleration takes place and the body's motion sensors once more send signals to the brain. In simulator motion platforms, after an initial acceleration is produced, the platform is re-set to a neutral position at a rate below human motion threshold so that the subject does not detect the so-called "wash out" phase of simulator motion cueing. The motion system is then ready to make the next acceleration which will be detected by the subject, as in the real world. This so-called "acceleration onset cueing" is an important aspect in simulators with motion platforms and models the way humans feel motions in the real world. Vestibular system The vestibular system is the balance and equilibrium system of the body that includes the left and right vestibular organs of the "inner ear". It consists of three semicircular canals, or tubes, arranged at right angles to one another. Each canal is lined with hairs connected to nerve endings and is partially filled with fluid. When the head experiences acceleration the fluid moves within the canals, causing the hair follicles to move from their initial orientation. In turn the nerve endings signal to the brain, which interprets them as acceleration in the three rotations pitch,