Visually powerful scenes like the daring car chase in the movie "The French Connection" can make people sick.

The eyes are telling the brain that the viewer is inside the car -- cruising at the same speed as Gene Hackman's car. Yet the inner ear -- which senses acceleration -- knows the person is seated in a chair in a movie theater.

Such contradictory information, or "sensory mismatch," received by the brain can produce motion sickness -- an abrupt headache, followed by dizziness, clammy skin and nausea. "When the brain receives different signals, it doesn't know how to deal with the dilemma and produces motion sickness," said Dr. Michael M. Cohen, chief of the neurology service at the Philadelphia Veterans Administration Medical Center.

Motion sickness is a problem faced not only by travelers on ocean voyages or airplanes, but also by car passengers and subway commuters. A simple thing like reading on the subway can cause contradictory signals to be sent to the brain. "When you fixate on a book, as far as your visual system is concerned, you are not moving at all," Cohen said.

The balance system -- made up of the eyes, inner ears and sensory receptors in joints and muscles -- is responsible for keeping the body oriented in space.

The central nervous system in the brain receives information on the body's position from the eyes. It learns acceleration, whether the body is falling down or spinning around, from the inner ear. Sensory receptors in joints and muscles tell what part of the body is moving or touching another object.

The brain analyzes the information and compares it to previous experiences learned at a younger age when infants and toddlers explore how their body relates to space.

When the brain receives conflicting information, it sends out commands that stimulate the balance system and the vegas nerve, which controls heart rate, blood pressure and digestive functions, said Dr. Aina Julianna Gulya, associate professor of surgery at George Washington University Medical Center. "You get the nausea and vomiting with the stimulation of the stomach."

Some people have a more sensitive balance system than others. "Women tend to be more susceptible than men to all forms of motion sickness," said Cohen in his book, "Dr. Cohen's Healthy Sailor Book." Children below age 2 are almost totally immune. The risk then increases until adolescence, when it begins to decline slowly. The elderly are fairly immune."

Motion sickness can last 24 hours or longer, but eventually the brain deals with it, Cohen said. In some people, the brain is never able to adjust and the person could be sick for the whole trip.

Ocean voyages can be particularly difficult because there are three basic types of motion on a boat -- the roll from side to side, the pitch back and forth, and the yaw -- rotation of the boat on its axis. Admiral Horatio Nelson, an 18th-century British naval hero, would get violently seasick for the first three days of a voyage, George Washington University Medical Center's Gulya said. "He toughed it out for three days and then was fine."

Today, there are methods to help combat the problem. Antihistamines such as Antivert or Dramamine (available over the counter) are commonly used. They suppress the inner ear balance center activity so you get less conflicting information going into the brain and the symptoms are alleviated, Gulya said. They can cause drowsiness.

A new approach is a patch containing scopolamine, a drug that has a direct effect on the brain and inner ear balance organs. One advantage is the patch can be worn, usually behind the ear, for up to three days. The medicine is absorbed through the skin.

"The patch is slow releasing and you don't have to remember to take pills." said Dr. Lawrence D'Angelo, clinical associate professor of medicine at Georgetown University Medical Center. "It's of benefit particularly for people who are going on a trip where they are perpetually in motion."

The side effects are dryness of the mouth and eyes, blurred vision and drowsiness. One problem with wearing the patch is for people who find they are highly sensitive to the drug. "Even if you take the patch off," Gulya said, "you still have some of the drug left in the area and it will continue to be dispensed into the system."

There are some things you can do to feel better. Georgetown's D'Angelo recommends: Getting some fresh air if you start to feel ill. If you are sitting in the back seat of a car, move to the front where you can see. On a boat, go up on deck and establish a horizon line. Practice slow, deep breathing.

Motion sickness is a problem not only for motor vehicle passengers but also for operators. Studies in naval aviation show that the body's balance system can play all sorts of tricks on you, George Washington University's Gulya said.

"The graveyard spiral," where the aircraft keeps revolving in circles, can happen because of the balance system's response to continued acceleration. Instead of recognizing the spiral motion, the balance system restores itself to a normal state and the pilot thinks nothing is wrong.

If a pilot suddenly turns his head and looks back in the cockpit during landing, said Gulya, his ears get a totally different set of information, which tells him the plane's nose is moving up. The pilot responds by trying to put the nose down, and crashes the plane into the runway.

Because of the balance system, "a key to learning how to fly," said Gulya, is to look at your instruments and sometimes ignore what your instincts are telling you."More Information

The American Academy of Otolaryngology-Head and Neck Surgery Inc. offers a free pamphlet on "Dizziness & Motion Sickness." Write: 1101 Vermont Ave. NW, Suite 302, Washington, D.C. 20005.