To Pharrell Williams, the verses of his years-long hit "Happy" have always been yellow and red. Or reddish. "[W]ell ... it's not red, it's more like orange, but then it's a little pink, a little rainbow-y, because of the minor chords or whatever," Williams told the Dinner Party Download.

Williams has an auditory-specific form of a neurological condition called synesthesia. It's a condition in which one's sensory perceptions are involuntarily unified. Seeing colors and patterns based off music is one of the most common varieties, also called chromaesthesia. But other synesthetes can taste music, or, see colors associated with certain numbers or days of the week.

Though it was once thought of as a disorder, it is common to some of the most brilliant musicians. The list of artists who have it is long, including Billy Joel, Kanye West, Lady Gaga, Dev Hynes, Frank Ocean and Mary J. Blige. This is what we know about the mysterious inside of the synesthete's mind.

Pharrell Williams/YouTube

Disorder or eccentricity? For years, synesthesia has suffered the stigma of being a mental "disorder." Throughout the years, many synesthetes remained silent about their gifts. In some artists' cases, they feared it would invalidate their actual musical talents.

"People were very afraid to admit they had it because they didn't want people to think that this special gift was the sole basis for their talent," Carol Steen, co-founder of the American Synesthesia Association, told Pitchfork. "They'd think, 'If I tell people that I have this gift, maybe they'll think that all the practicing I've done doesn't mean anything.'"

But the sting of that stigma has lessened recently, replaced by a fascination. Indie pop crooner Dev Hynes has been very open about his synesthesia. Last November, he gave a TED talk describing his process of writing music using his synesthetic connections. His tone colors are markedly different from those Pharrell describes, which points to a fundamental truth about synesthesia: Every artist's color associations and experiences with the condition are different. Several artists, like Melissa McCracken, even put what they see onto canvas.

Early research. Scientists largely declined to study synesthesia until around the 1980s, when MRIs suggested there was a legitimate neurological basis for the feeling. Early research suggested that synesthesia was simply a case of crossed mental wires. The auditory cortex, where music and sound are first processed, is close to the occipital lobe, where the brain distinguishes color and shape. Simultaneously triggered senses are examples of the brain making excess neurological connections or failing to prune existing connections. In fact, the brains of infant primates show evidence that their senses are all a hyperconnective blend until a few weeks or months after birth. By that logic, we all may be synesthetes until we grow out of it.

Neonatal theory has been criticized in recent years, though, as has all research that frames synesthesia as a neurological disorder. Researchers out of the University of London and the University of Oxford argued that the development of adult synesthesia is "better explained by their being learned" than by erroneously pruned connections.

Learning to see sound. In March 2015, neurologists Jean-Michel Hupé and Michel Dojat studied the brain scans of synesthetes and found no evidence of any structural differences in their brains. "If none of the proposed structural or functional differences [claimed to exist in synesthesia are] confirmed," they write, according to Discover, "this would speak against synesthesia being a neurological condition. But, then, what could be the nature of synesthesia?"

They proposed that synesthesia may be learned and arise from childhood memories. A 2014 study performed by Olympia Colizoli of University of Amsterdam supports this hypothesis to a certain extent. Colizoli trained a group of participants to associate colors with specific letters to simulate a grapheme-color synesthesia (one of the most common forms) by having participants read passages where specific letters were colored. Another 2014 study from the University of Sussex reviewed a 1944 study in which researchers successfully taught listeners to create sound-color synesthesia connections.

How exactly children learn sound-color synesthesia, and why some children retain the ability and some don't, remains a mystery. But science is getting closer to finding an answer, and if it does nail down the process, it could prove to be a game changer for music education. In a February 2015 article for the Psychologist, Jack Dutton looked at research that found that people with chromaesthesia are more likely to engage with creative pursuits and play instruments, which would explain the long list of artists with that specific neurological profile.

A visual representation of a synesthetic response to"Goodbye Blue Sky" by Pink Floyd Commissioned by David Coalburn/Tumblr

"These findings imply that if scientists are able to figure out a way to teach people chromaesthesia, it may enhance how well people learn to read and compose music," Dutton wrote.

It's a fascinating prospect. One day, any child might be able to grow up see Pharrell-like rainbows in music, if they so choose. Whether it's considered a disorder, a blessing or a choice, synesthesia is one of the most strange and magnificent things the human mind can do.