When the medical journal The Lancet asked Matthieu Zellweger to photograph any psychiatric condition that intrigued him, he thought of a close friend who has been living with bipolar disorder. He knew how his friend lamented that it was an “invisible handicap” that you couldn’t just snap out of, as some well-meaning but frustrated people would suggest.

But Mr. Zellweger also recognized something in his friend that led him to propose a photo essay on bipolar disorder.

“I’ve been around him quite a bit,” Mr. Zellweger said. “And the one thing that definitely surprised me is that — let’s face it — very intelligent people are overrepresented among bipolar people. A lot of them are very lucid about their disease. They have thought about how it impacts their lives. It was very stimulating to talk with them.”