I think that the author is trying to convey that the disease has only made them more paranoid. It makes them think about the old mans eye constantly. This choice of words helps the author put the point across that the man is most likely mad. Yes paranoia sharpens the senses and makes people more aware but the author is still trying to say that the man is mad because of a disease.
Sorry if this doesn’t help much, I don’t know where you are in the story but this is what I learned this year in my English class. I hope this helped!
