John Montgomery, a psychologist in New York City and adjunct professor at New York University, said “it’s hard to say definitively without rigorous testing” of Trump’s speaking patterns, “but I think it’s pretty safe to say that Trump has had significant cognitive decline over the years.”

No one observing Trump from afar, though, can tell whether that’s “an indication of dementia, of normal cognitive decline that many people experience as they age, or whether it’s due to other factors” such as stress and emotional upheaval, said Montgomery, who is not a Trump supporter.

Even a Trump supporter saw and heard striking differences between interviews from the 1980s and 1990s and those of 2017, however. “I can see what people are responding to,” said Dr. Robert Pyles, a psychiatrist in suburban Boston. He heard “a difference in tone and pace. … What I did not detect was any gaps in mentation or meaning. I don’t see any clear evidence of neurological or cognitive dysfunction.”

Johnson cautioned that language can deteriorate for other reasons. “His language difficulties could be due to the immense pressure he’s under, or to annoyance that things aren’t going right and that there are all these scandals,” he said. “It could also be due to a neurodegenerative disease or the normal cognitive decline that comes with aging.” Trump will be 71 next month.