One observation supporting this theory is that many sufferers immediately feel better when they enter the kind of moving environment that touched off the disorder in the first place — a boat, car or plane. “When I get in the car, I feel better,” Mrs. Josselyn said, “but when I get back out, it gets worse again.” According to Dr. Hain’s theory, this is because their internal models are once again consistent with what is actually happening around them.

Image Credit... Yuko Shimizu

But not every doctor who has worked with mal de débarquement patients agrees with Dr. Hain and Dr. Oman’s take on the condition. Carlos Gordon, a neurologist at Meir Hospital in Kfar Saba, Israel, is not convinced there is a direct connection between long cruises and the persistent symptoms typically associated with the disease. “There’s no proof that exposure to the abnormal environment is the cause,” Dr. Gordon said. “There are many patients that have the same symptoms following head trauma or a viral infection without previous exposure to sailing.”

He pointed out that since there is no laboratory examination to confirm the presence or absence of the condition — sufferers typically perform normally on standard tests of balance and coordination — there is no clear-cut reason to believe the disease has a defined physical cause.

Robert Baloh, a neurologist at the University of California, Los Angeles, hopes to resolve this dispute by taking a closer look at how the patients’ minds actually work. He is seeking approval to perform M.R.I. studies that would compare the neural activity of mal de débarquement patients with that of normal subjects and patients who are “in remission,” experiencing temporary relief from their symptoms.

“The condition may be related to the brain areas that sense movement, the temporal and parietal lobes,” Dr. Baloh said. “We want to see if there are differences in the way people process visual motion.”

Zohar Nachum, a neurologist at the Israel Naval Medical Institute in Haifa, has performed one of the only studies to date that lends insight into specific abnormalities in patients’ brain activity. Dr. Nachum recruited 17 subjects who were prone to short-term bouts of mal de débarquement and 17 others who were not, sent them on a short sailing voyage, and performed neurological tests on them before and after the trip.

The sailors susceptible to mal de débarquement used less input from their vestibular and visual systems to maintain balance, and also had delayed motor responses to unexpected perturbations, such as when a boat rolls over a breaker. These findings suggest that sufferers’ brains may not rely enough on certain types of changing information from the environment for orientation.