It was the kind of utterance that makes professional transcribers question their career choice:

... there is no collusion between certainly myself and my campaign, but I can always speak for myself and the Russians, zero.

When President Trump offered that response to a question at a press conference last week, it was the latest example of his tortured syntax, mid-thought changes of subject, and apparent trouble formulating complete sentences, let alone a coherent paragraph, in unscripted speech.

He was not always so linguistically challenged.

STAT reviewed decades of Trumps on-air interviews and compared them to Q&A sessions since his inauguration. The differences are striking and unmistakable.

Research has shown that changes in speaking style can result from cognitive decline. STAT therefore asked experts in neurolinguistics and cognitive assessment, as well as psychologists and psychiatrists, to compare Trumps speech from decades ago to that in 2017; they all agreed there had been a deterioration, and some said it could reflect changes in the health of Trumps brain.

In interviews Trump gave in the 1980s and 1990s (with Tom Brokaw, David Letterman, Oprah Winfrey, Charlie Rose, and others), he spoke articulately, used sophisticated vocabulary, inserted dependent clauses into his sentences without losing his train of thought, and strung together sentences into a polished paragraph, which and this is no mean feat would have scanned just fine in print. This was so even when reporters asked tough questions about, for instance, his divorce, his brush with bankruptcy, and why he doesnt build housing for working-class Americans.

Trump fluently peppered his answers with words and phrases such as subsided, inclination, discredited, sparring session, and a certain innate intelligence. He tossed off well-turned sentences such as, It could have been a contentious route, and, These are the only casinos in the United States that are so rated. He even offered thoughtful, articulate aphorisms: If you get into whats missing, you dont appreciate what you have, and, Adversity is a very funny thing.

Now, Trumps vocabulary is simpler. He repeats himself over and over, and lurches from one subject to an unrelated one, as in this answer during an interview with the Associated Press last month:

People want the border wall. My base definitely wants the border wall, my base really wants it youve been to many of the rallies. OK, the thing they want more than anything is the wall. My base, which is a big base; I think my base is 45 percent. You know, its funny. The Democrats , they have a big advantage in the Electoral College. Big, big, big advantage. ... The Electoral College is very difficult for a Republican to win, and I will tell you, the people want to see it. They want to see the wall.

For decades, studies have found that deterioration in the fluency, complexity, and vocabulary level of spontaneous speech can indicate slipping brain function due to normal aging or neurodegenerative disease. STAT and the experts therefore considered only unscripted utterances, not planned speeches and statements, since only the former tap the neural networks that offer a window into brain function.

The experts noted clear changes from Trumps unscripted answers 30 years ago to those in 2017, in some cases stark enough to raise questions about his brain health. They noted, however, that the same sort of linguistic decline can also reflect stress, frustration, anger, or just plain fatigue.

Ben Michaelis, a psychologist in New York City, performed cognitive assessments at the behest of the New York Supreme Court and criminal courts and taught the technique at a hospital and university. There are clearly some changes in Trump as a speaker since the 1980s, said Michaelis, who does not support Trump, including a clear reduction in linguistic sophistication over time, with simpler word choices and sentence structure. ... In fairness to Trump, hes 70, so some decline in his cognitive functioning over time would be expected.

Some sentences, or partial sentences, would, if written, make a second-grade teacher despair. Well do some questions, unless you have enough questions, Trump told a February press conference . And last week, he told NBCs Lester Holt, When I did this now I said, I probably, maybe will confuse people, maybe Ill expand that, you know, lengthen the time because it should be over with, in my opinion, should have been over with a long time ago.

Other sentences are missing words. Again, from the AP: If they dont treat fairly, I am terminating NAFTA, and, I dont support or unsupport leaving out a me in the first and an it (or more specific noun) in the second. Other sentences simply dont track: From the time I took office til now, you know, its a very exact thing. Its not like generalities.

There are numerous contrasting examples from decades ago, including this with sophisticated grammar and syntax, and a coherent paragraph-length chain of thought from a 1992 Charlie Rose interview: Ross Perot, he made some monumental mistakes. Had he not dropped out of the election, had he not made the gaffes about the watch dogs and the guard dogs, if he didnt have three or four bad days and they were real bad days he could have conceivably won this crazy election.

The change in linguistic facility could be strategic; maybe Trump thinks his supporters like to hear him speak simply and with more passion than proper syntax. He may be using it as a strategy to appeal to certain types of people, said Michaelis. But linguistic decline is also obvious in two interviews with David Letterman, in 1988 and 2013, presumably with much the same kind of audience. In the first, Trump threw around words such as aesthetically and precarious, and used long, complex sentences. In the second, he used simpler speech patterns, few polysyllabic words, and noticeably more fillers such as uh and I mean.

The reason linguistic and cognitive decline often go hand in hand, studies show, is that fluency reflects the performance of the brains prefrontal cortex, the seat of higher-order cognitive functions such as working memory, judgment, understanding, and planning, as well as the temporal lobe, which searches for and retrieves the right words from memory. Neurologists therefore use tests of verbal fluency, and especially how it has changed over time, to assess cognitive status.

Those tests ask, for instance, how many words beginning with W a patient can list, and how many breeds of dogs he can name, rather than have patients speak spontaneously. The latter is too hard to score, said neuropsychologist Sterling Johnson, of the University of Wisconsin, who studies brain function in Alzheimers disease. But everyday speech is definitely a way of measuring cognitive decline. If people are noticing [a change in Trumps language agility], thats meaningful.

Although neither Johnson nor other experts STAT consulted said the apparent loss of linguistic fluency was unambiguous evidence of mental decline, most thought something was going on.

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

No one observing Trump from afar, though, can tell whether thats an indication of dementia, of normal cognitive decline that many people experience as they age, or whether its 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 dont 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 hes under, or to annoyance that things arent 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.

Northwestern University psychology professor Dan McAdams, a critic of Trump who has inferred his psychological makeup from his public behavior, said any cognitive decline in the president might reflect normal aging and not dementia. Research shows that virtually nobody is as sharp at age 70 as they were at age 40, he said. A wide range of cognitive functions, including verbal fluency, begin to decline long before we hit retirement age. So, no surprise here.

Researchers have used neurolinguistics analysis of past presidents to detect, retrospectively, early Alzheimers disease. In a famous 2015 study , scientists at Arizona State University evaluated how Presidents Ronald Reagan and George H.W. Bush spoke at their news conferences. Reagans speech was riddled with indefinite nouns (something, anything), low imageability verbs (have, go, get), incomplete sentences, limited vocabulary, simple grammar, and fillers (well, basically, um, ah, so) all characteristic of cognitive problems. That suggested Reagans brain was slipping just a few years into his 1981-1989 tenure; that decline continued. He was diagnosed with Alzheimers disease in 1994. Bush showed no linguistic deterioration; he remained mentally sharp throughout his 1989-1993 tenure and beyond.

Sharon Begley answered reader questions about this article on Facebook. Read the conversation here .