Michael J. Fox got a bright and early start to his career, packing more hits into his 20s than most stars do in a lifetime.

After getting his big break on the show Family Ties and landing the iconic role of Marty McFly in Back to the Future, it seemed the actor would have his pick of projects in Hollywood. But at 29 years old, the movie star learned he had Parkinson’s disease, a diagnosis that doctors said would allow him at most 10 more years in the acting world. Despite their dire prediction, he kept acting successfully until retiring in 2020—all while helping to raise millions for Parkinson’s research.

In a recent interview with comedian Mike Birbiglia, Fox opened up about never-before-shared aspects of his condition, including one heartbreaking symptom he has that’s common among Parkinson’s patients. Read on for his most recent update.

Since going public with his condition in 1998, seven years after his diagnosis, Fox has been open about his Parkinson’s symptoms, which include tremor, speech difficulty, and muscle rigidity. While speaking with Birbiglia, he added another surprising symptom to that list: complete loss of his sense of smell. Though he no longer has this particular sense, he still enjoys remembering the smells of his childhood, he says. “I remember the smell of pine, just after Christmas, in this apartment building I lived in.

It had balconies, fire escapes, and everyone would put the trees out there for New Year’s before they got picked up, because you couldn’t put them on the road. And the whole place smelled like pine. It smelled like a pine forest,” Fox reminisced.

Tremor is considered one of the most characteristic signs of Parkinson’s, but few people realize that olfactory changes are even more common. While tremor occurs in roughly 70 percent of patients with the neurodegenerative disease, a 2011 study published in the journal Parkinson’s Disease found that over 96 percent of Parkinson’s patients experience significant changes to their sense of smell. This sometimes begins years or even decades before other symptoms arise. However, it rarely leads to a diagnosis until it is accompanied by other, more obvious symptoms.

In recent years, Fox developed another symptom of Parkinson’s, which he says impacted his decision to step away from acting: he had difficulty memorizing scripts. “I just had this blank, I couldn’t remember the lines,” he recalled of a challenging moment on set of Good Fight, a spinoff of Good Wife. In his early days of acting, Fox says he could recite pages of dialogue under tremendous studio pressure to get the shot—”not a trickle of sweat on my brow,” he recalled. Still, struggling to find the words for a few lines of dialogue came as a shock, but not a cause for panic. “I didn’t freak out. I just went, “Well that’s that. Moving on. A key element of this process is memorizing lines, and I can’t do it,” he said. “So, I go to the beach.”

The recent changes in his short term memory have pushed the former actor to find new creative outlets. Though he can no longer physically write or type—her dictated his last book to an assistant,. “My short-term memory is shot,” Fox recently told People. “My guitar playing is no good. My sketching is no good anymore, my dancing never was good and acting is getting tougher to do. So it’s down to writing,” he said. “Luckily, I really enjoy it.”