You know the expression, "I feel your pain?" Some people actually can.

Boston-based neurologist Dr. Joel Salinas experiences a medical condition known as mirror-touch synesthesia, so rare that some in the medical profession doubt its existence.

PHOTOS NOT TO MISS: 11 beautiful examples of art inspired by science

Synesthesia is a condition that affects 1-2 percent of the population. It’s when the stimulation of one sense produces a sensation in a different sense. People with synesthesia may feel numbers, hear colors, or taste shapes.

A common form of synesthesia, called grapheme-color synesthesia, involves seeing monochromatic letters, digits and words in unique colors. Not all synesthetes see the same colors corresponding to the same numbers — for one, the number three may be green; for another, it’s distinctly blue.

Joel Salinas' profile photoSome might call mirror-touch synesthesia an illness. Others might call it a veritable superpower, allowing Salinas (pictured at right) and others like him to readily feel exactly what others are feeling. Salinas recently explained to Pacific Standard magazine that in high school, he would see someone else getting a hug or getting slapped and feel both sensations against his body. What makes him unique is that as a doctor, he's able to use his condition to benefit others, though it wasn't always so easy. In the article, he discusses his struggles as a first-year resident in the emergency room treating gunshot victims while trying to maintain his composure.

Indeed, many hospital patients are asked to describe their pain on a scale from 1 to 10 and doctors and nurses can only use visual cues such as a grimace to make their best guess as to what the patient might be going through. Salinas can feel the pain he sees. From the article in Pacific Standard magazine:

"Empathy itself is another quality that modern doctors are said to lack in sufficient doses. Here, too, Salinas' mirror-touch synesthesia gives him advantages, particularly his heightened facility for reading people's facial expressions and emotional states. He says he usually zeroes in on a person's mouth, more than any other body part, when reading emotions."

The first documented case of mirror-touch synesthesia was uncovered by London-based cognitive neuroscientist Sarah Jayne Blakemore during a talk about synesthesia, when a 41-year-old approached her about her own ability to feel touch when she saw others being touched, and up until that point, didn't realize that her ability wasn't the norm. She published her findings in a 2005 edition of Brain, including that synesthetes had a higher level of activity in the somatosensory cortex than non-synesthetes.

Since then, a British duo of psychologists, Michael Banissy of Goldsmiths, University of London, and Jamie Ward of the University of Sussex, have been studying anyone they can find who exhibits the symptoms of mirror-touch synesthesia. Indeed, their research, published in Nature Neuroscience in 2007, found that synesthetes scored significantly higher than non-synesthetes on a questionnaire designed to measure empathy. Understanding mirror-touch synesthesia may help us understand what helps some people grasp empathy better than others. It may even help in research relating to autism, which is thought to correlate to a low level of understanding of other people’s emotions.

Still, the exact mechanisms of mirror-touch synesthesia and other kinds of synesthesia, remain to be understood. Until then, Salinas has found a way to channel his synesthesia to help others, all while experiencing a constant barrage of sensory stimulation. Writes Erica Hayasaki in the Pacific Standard piece, "It's as if Salinas feels everything and nothing at the same time. By necessity, he keeps himself remarkably detached from all the errant sensations that tug at his attention. He would probably lose his sense of self if he didn’t."

This doctor really can feel your pain
Mirror-touch synesthesia gives Joel Salinas unique insight into his patients.