Motion sickness is the body's response to conflicting messages about motion that are sent to the brain. The conflict is between what the eyes see and what the body senses.

What is going on in the body?

Information about motion and how the body is moving is obtained through the eyes and two places within the inner ear. This information is then sent to the brain so that the body can respond by coordinating muscles and maintaining balance. Information from these parts may indicate that a person is walking, doing hand stands, or even cartwheels. Other information is about the surface under a person or the area surrounding them. For instance, a person on top of a surfboard may not be moving but the surfboard and the water under the person is moving. Signals about the movement of the water and the surfboard are sent to the brain. These signals give information that helps the person to balance on the surfboard. When the brain receives one message from the eyes and another message from the inner ears, a person may feel the symptoms of motion sickness.

Different factors, such as speed, may cause motion sickness. Slow movement up and down while accelerating forward is more likely to stimulate motion sickness rather than fast movement up and down. For instance, more people feel motion sickness while riding a camel than riding a horse. Motion sickness is often reported while traveling aboard ships, but not while windsurfing.

Conflicting information regarding motion can be seen when a person is traveling on a ship looking out at the waves in the ocean. The inner ear perceives the up and down motion of the body riding on the ship, while the eyes perceive the random forward movement of the waves.

Another type of conflicting message occurs when the inner ear perceives movement while the eyes detect no movement. This can occur while standing or sitting still inside a moving ship or vehicle with no windows to visually explain the movement. It can also happen when a person is trying to read when traveling in a car. The inner ear perceives the movement of the car but the eyes detect no movement.

What are the causes and risks of the condition?

While there are usually no real risks associated with motion sickness, prolonged nausea and vomiting without eating or drinking can lead to dehydration and weakness. In this weakened condition a person may injure himself or herself due to a fall or accident.