What effect does the first sentence of "The Tell-Tale Heart" have on the reader?

"TRUE! nervous, very, very dreadfully nervous I had been and am; but why WILL you say that I am mad?"

By starting the story with the narrator asking the audience to judge his sanity, Poe is creating an atmosphere of mystery and suspense. Immediately the reader begins looking for signs of true insanity. The result is a short story that is more psychologically oriented than externally oriented. As the narrator continues to defend his sanity by showing the rational thoughts and actions he had in dealing with the gray eye, the reader is made more and more aware of the mental disease plaguing the narrator. While there is much external conflict, the focus remains on the mental state of the narrator, and to the driving force behind the action. Through this focus, Poe is able to create a story that is horrific both in the physical action and in the mental implications, appealing to multiple audiences.