The effect that this response has on the speaker is that the speaker’s mental state shifts more and more toward madness.
The raven is constantly repeating the same word - nevermore. In the beginning, the narrator just thought it was a weird coincidence that the bird should say that one single word, but as time passes by, the weight of that word becomes unbearable as the narrator realizes that this word can be applied to all of his questions. So as time goes by, the narrator becomes more and more depressed about not being allowed to see his beloved and thus he falls into madness.