(NewsNation) — John Fetterman‘s difficulty with answering questions during a debate Tuesday night was a clear indication he struggles with expressing and elaborating on his thoughts, according to a board-certified neurologist. But it’s hard to determine how much, if at all, Fetterman’s comprehension might be impaired.

Fetterman’s health has become a central focus of the race between him and Mehmet Oz, the Republican and celebrity TV personality. Fetterman suffered a stroke in May.

Analyzing his performance during the debate, Dr. Huma Sheikh said it’s likely that Fetterman suffers from Broca’s aphasia, in which spontaneous speech is diminished and there is a loss of grammatical structure. Sheikh pointed to instances in which Fetterman said words that are similar to the ones he was actually trying to use, such as “bicartisan” instead of bipartisan.

“For him, his major issue is production of language,” Sheikh said. “He had difficulty with fluency.”

Sheikh emphasized, however, that Fetterman’s difficulty in articulating words does not necessarily indicate a decline in cognitive function.

“It’s hard to tell how much his comprehension is impaired because that requires different testing,” Sheikh said.

Fetterman has said he struggles with auditory processing as a result of his stroke and asked for the use of a closed-captioning system during the debate. In a statement after the debate, his campaign team said he delivered a “strong debate performance” and that the captions were “filled with errors.”

He rejected calls to release his full medical records, instead pointing to a letter from his doctor indicating he’s fit to serve in office.

Fetterman continues to undergo speech therapy, though Sheikh said many stroke patients see the most improvement three to six months after the stroke.

“There can be some improvement over the next year, but I think he’s about six months out now, so I think if he was doing therapy right after his stroke, this is probably where he has improved to the most,” Sheikh said. “There might be some (more improvement) and it’s hard to tell because there are other factors … that will play a role in how well he does.”