Do not choose a topic if you cannot provide concrete examples for the body of the essay. As the saying goes, if you re going to talk the talk, you better walk the walk. If you plan on writing something technical for an application, make sure you can back up your interest in a topic and not merely throw around big scientific words. Unless you convince the reader that you actually have the life experiences to back up your interest in neurobiology, the reader will assume that you are trying to impress him or her with shallow tactics. Also, be sure that you can write to admissions officers and that you are not writing over their heads.
Many people just list off their accomplishments and never relate it to the theme of the question.
The entire essay must be interesting, considering admissions officers will probably spend only a few minutes reading each essay. Some students are so concerned about making the correct impression that they edit out anything that would help their essay stand out. They submit a safe essay that is, in reality, sterile, monotonous, and deadly boring. Most topics are in fact overdone, and this is not necessarily a bad thing, but a unique and convincing answer to a classic topic can pay off big. Furthermore, when applying to a competitive program that might be out of your reach, taking a risk in the essay may help your chances by standing out.