Answer:
His sense of hearing is heightened
He is nervous
Explanation: Poe clearly writes that he is "...nervous-very, very dreadfully nervous...", and "The disease had sharpened my senses...'.
And another added possible answer could be 'He is mentally unstable'! Poe doesn't exactly tell us that he is mentally unstable, but by context clues, we can conclude that he is most definitely unstable. The question asks what he 'explicitly' tells us so it is likely not part of the answer, but please think whether or not you should include this choice in your answer, I hope you choose right!