(CNN) -- Some people check their appearance in any mirror, window or computer screen they can find, but not out of vanity. It's because they hate the way they look so much.

An obsession with one or more bodily features, normally in the face, indicates a condition called body dysmorphic disorder. This mental illness, which leads some patients to seek multiple plastic surgeries, is not uncommon; 1 to 2 percent of the population is thought to have it. But most people who have the disorder don't get a diagnosis; they just think they are ugly, said Dr. Jamie Feusner psychiatrist at the University of California, Los Angeles.

Some celebrities have BDD, but few have been public about it, Feusner said. He and colleagues have treated celebrities at UCLA, but cannot disclose which ones.

"Everyone else would consider them attractive -- they don't consider themselves attractive," he said.

It turns out that people with this condition have abnormal brain function when it comes to looking at pictures of their own faces, according to a new study led by Feusner and published in the Archives of General Psychiatry.

When viewing themselves in photographs, patients with BDD underutilize parts of the brain used in seeing the face's overall shape and size, he said.

"If you just see the pieces of your face, and not seeing how they fit into the whole, then it's going to look distorted," he said. "That's how we interpret the findings."

Researchers used functional magnetic resonance imaging to look at the brains of participants as they viewed photographs of their own faces and familiar faces. Subjects viewed pictures that were high spatial resolution (showing details such as skin blemishes), and low spatial resolution (showing the general shape of the face), and unaltered.

The findings showed that when the BDD patients viewed normal and low-resolution photos of themselves, they had abnormal brain activity in visual processing systems. Also, the part of the brain that helps guide behavior and maintain emotional flexibility, the frontostriatal systems, had unusual activation patterns.

"What we think may be happening is that they may be seeing details, perhaps even seeing details normally, but that they're not able to contextualize it," he said.

Although the sample size is small -- 17 patients with BDD and 16 healthy people participated -- for a functional imaging study a 32-person subject group is reasonable, experts say.

The study is "groundbreaking" in its demonstration that patients with BDD are too focused on the details aren't able to see the whole picture when they see themselves, said Sabine Wilhelm, director of the BDD Clinic and Research Unit at Massachusetts General Hospital, who was not involved in the study. This can be observed anecdotally in treatment, but this study confirms it at the level of brain processing, she said.

Feusner's previous research found that when BDD patients viewed the faces of other people -- not their own faces -- the brain's left hemisphere, associated with categorization and details, was more more active. Healthy individuals used the right hemisphere, the part of brain that processes more holistically.

It is too early to directly apply the findings to treatment, but one possibility is that patients with BDD could be retrained to see their own faces, he said.

Treatments available today for BDD include medications called selective serotonin reuptake inhibitors, also used for depression and anxiety disorders, and cognitive behavioral therapy, Wilhelm said. She also sees implications for treatment in the study's findings.

"What we need to do in cognitive behavioral therapy treatment development is to really enhance efforts at teaching patients how to see the big picture," she said.

Many people with BDD will try to have what they perceive as flaws -- pimples, scars, birthmarks, or other features -- surgically or dermatologically removed. But plastic surgery almost never fixes their unhappiness about their appearance: 81 percent are dissatisfied with results of cosmetic treatments, Wilhelm said.

"The problem is really on the inside, it's not what they really look like," Wilhelm said.

Besides seeking surgeries, other symptoms include frequent camouflage with makeup, compulsive picking of skin, and asking for reassurance from others, she said. People with BDD may also check their reflection to the point of endangering themselves while driving while looking at themselves in the rearview mirror, she said.

The likelihood of attempting suicide or being hospitalized is high among BDD patients. Not enough research has been done on the subject to determine whether BDD is the results of genes, environment, or some combination thereof. It is unclear which of these pathways led to the brain abnormalities found in the study, Feusner said.

Some brain patterns seen in BDD are also associated with obsessive compulsive disorder, although BDD is probably not a form of it, Feusner said. People with OCD tend to recognize that their fears and behaviors are excessive, whereas BDD patients are more delusional and are convinced that the ugliness they perceive in themselves in real.

BDD is also different than eating disorders because it is purely about an imagined appearance and not eating habits, Wilhelm said.

But Feusner does suspect a connection between BDD and anorexia, and his next project is a brain imaging study comparing patients with the two conditions. If you are interested in participating in this study, contact Feusner's lab.