(b) Analyze the cumulative impact of specific word choices on meaning. Reread paragraph 4, paying close attention to how the narrator describes his
impression of the house and its surroundings. What does the author's choice of
the phrases “somewhat childish experiment,” “my superstition,” “a fancy so
ridiculous,” and “so worked upon my imagination” reveal about the narrator's
point of view about his impression of the house and its surroundings?
- Fall of the house of usher

This question is incomplete, here´s the complete question.

Analyze the cumulative impact of specific word choices on meaning.

Reread paragraph 4, paying close attention to how the narrator describes his  impression of the house and its surroundings. What does the author's choice of  the phrases “somewhat childish experiment,” “my superstition,” “a fancy so  ridiculous,” and “so worked upon my imagination” reveal about the narrator's  point of view about his impression of the house and its surroundings?

- Fall of the house of usher

I have said that the sole effect of my somewhat childish experiment--that of looking down within the tarn--had been to deepen the first singular impression. There can be no doubt that the consciousness of the rapid increase of my superstition--for why should I not so term it?--served mainly to accelerate the increase itself. Such, I have long known, is the paradoxical law of all sentiments having terror as a basis. And it might have been for this reason only, that, when I again uplifted my eyes to the house itself, from its image in the pool, there grew in my mind a strange fancy--a fancy so ridiculous, indeed, that I but mention it to show the vivid force of the sensations which oppressed me. I had so worked upon my imagination as really to believe that about the whole mansion and domain there hung an atmosphere peculiar to themselves and their immediate vicinity--an atmosphere which had no affinity with the air of heaven, but which had reeked up from the decayed trees, and the grey wall, and the silent tarn--a pestilent and mystic vapour, dull, sluggish, faintly discernible, and leaden-hued.

The first five paragraphs of Poe´s short story build the gothic mood through the narrator´s description of the eerie atmosphere in the Usher family mansion.

Explanation:

The selection of the narrator´s phrases shows he´s trying to use reason to understand his uneasiness around the house (as an experiment, a superstition or part of his imagination) which foreshadows scary or paranormal events.