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A Waikiki reader writes, "On a boat, or with this inner ear infection I have, my stomach is in revolt. But I'm never seasick when I'm in the water. Why don't I get sick when I'm bodysurfing, snorkeling or diving? And why don't seals, whales and dolphins get seasick?"

Good questions. No one knows the exact cause of this miserable malady, but medical researchers do agree that seasickness has to do with sensory confusion in the brain.

Humans, and most animals, are equipped with position sensors that continually provide our brains with information about movement. These sensors are our eyes; structures in our joints, tendons, and muscles; and fluid-filled balance organs in our inner ears. When working together properly, these sensors keep us from falling down.

If, however, a person has an infection in the inner ear like my reader, the sensors don't agree. Her eyes and muscles, for example, tell her that her body is not moving but her inner ear, disrupted by the illness, falsely registers movement.

A similar disagreement happens in a heaving boat. Our eyes send the message to our brains that our bodies are moving all over the place while our muscles and inner ears sense we are sitting still.

That's why closing your eyes or staring at the horizon sometimes helps relieve seasickness.

Being in and under the water is another story.

When our bodies are immersed in water, such as during snorkeling or body surfing, all motion sensors agree about what's going on. The same goes for scuba divers, who also have the advantage of descending beneath the bumpy surface to calm waters.

Marine mammals don't get seasick for the same reason we humans don't while we're in or under the water: All the motion sensors concur. Also, unlike us land animals, marine animals' sensory systems evolved to cope with an environment that's always moving.

Most of us know all too well that when our brain gets mixed messages about motion, it tells our stomach to throw up. Why this happens isn't clear, but it's nearly universal: Approximately 90 percent of humans get seasick, as well as most cats, dogs and other land mammals. Why some people are more susceptible than others to this illness is unknown.

One theory about why such a large percentage of people and terrestrial animals have this vomiting reaction to motion is that it's an evolutionary adaptation against poisoning.

Many toxins disrupt nerve communication and thus produce sensory confusion. Vomiting eliminates the offending toxins.

The good news about seasickness is that you can't die from it. Eventually, the brain sorts out conflicting sensory information and the illness ends. The amount of time this takes varies for each individual depending upon the person, the boat and the weather.

The best cure for seasickness is prevention, because once it hits, it's hard to reverse. Take medication before you get on the boat and once under way, don't do anything you know makes you sick, such as going below deck.

Seasick remedies range from medicine to meditation. The only way to know which works best for you is to try them one at a time.

Both NASA and the Navy are interested in motion sickness, and researchers from both groups continue to study it. The wide variations and exceptions in both getting seasick and the effectiveness of its cures show there's still a lot to be learned about this bugaboo of boating.