The way we perceive motion in our body and surroundings is driven by how our brain interprets signals from our sensory systems, including sight, sound, and touch. If a motion simulation causes motion sickness, it’s often because our sensory systems aren’t fully tricked by the simulation. The design of the simulator plays a role, but the sensitivity and complexity of human senses are crucial factors.
Special sensory receptors, or sensory “pads,” translate stimuli into sensory signals. External receptors (exteroceptors) respond to outside stimuli like light, sound, pressure, and temperature. Internal receptors (enteroceptors) respond to stimuli within blood vessels.
Postural stability is maintained through vestibular reflexes acting on the neck and limbs, controlled by three sensory inputs:
Proprioceptors are located in muscles, tendons, joints, and the inner ear, sending signals to the brain about the body’s position. For instance, race car drivers often refer to the “seat of the pants” feeling, which comes from proprioceptors. These sensors respond to muscle movement and tension, sending electrochemical signals through neurons to the brain and spinal cord. The brain then sends motor signals to muscles, instructing them to contract or relax.
When accelerating away from a traffic light, you feel yourself pushed back into the seat. This sensation is transmitted to your brain via proprioceptors. Upon braking, different proprioceptors activate, causing the brain to signal muscles to stop you from sliding forward.
However, once a constant speed is reached, these sensors stop reacting, and the brain relies on visual cues until another movement occurs. In motion simulation, this can lead to a “washout” where the system returns to a neutral position without the occupant realizing it, essential for maintaining realistic sensations.
The vestibular system in the inner ear includes three semicircular canals arranged at right angles, detecting movement in three planes: forward/backward, left/right, and up/down. Fluid in the canals displaces hair follicles during acceleration, which the brain interprets as movement.
However, with sustained acceleration, the fluid displacement ceases to affect the hair follicles, and the brain perceives the acceleration as stopped. This threshold allows the washout movement in simulators, as slow, gradual motion below the threshold won’t be sensed by the vestibular system.
Our ears are critical in motion simulation, providing the brain with information about the craft’s position, velocity, and attitude. The motion must synchronize with the visual output to prevent motion sickness. For example, when rolling the control left, the visual display must show the craft rolling left, and the body must feel this turn through proprioceptors and the vestibular system. If not synchronized, motion sickness can occur.
Understanding how human physiology processes and responds to motion cues is essential for designing effective motion simulators. By aligning the design with the sensory systems, particularly proprioceptors, the vestibular system, and visual inputs, we can create more realistic and comfortable simulations, minimizing the risk of motion sickness.
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