How does imagination overcome reason in "The Fall of the House of Usher" and create fear?

Isolated for years inside the Usher house, Roderick's imagination overcomes reason as his seclusion creates a confining and unnatural environment. This bizarre environment engenders his imaginative and hallucinatory attribution of strange occurrences to that of preternatural causes. 

After his twin sister Madeline dies, Roderick is left with his only personal friend from his school days, the narrator. With a terrible sense of isolation now, the crack of the house, like the crack between the living...

Isolated for years inside the Usher house, Roderick's imagination overcomes reason as his seclusion creates a confining and unnatural environment. This bizarre environment engenders his imaginative and hallucinatory attribution of strange occurrences to that of preternatural causes. 

After his twin sister Madeline dies, Roderick is left with his only personal friend from his school days, the narrator. With a terrible sense of isolation now, the crack of the house, like the crack between the living and the dead twins, seems to widen. In fact, Roderick's troubled mind seems to create a pathetic fallacy--the attributing of human feelings and actions to inanimate things--with the decaying mansion and family home. Where there was once a "similitude between brother and sister," now there seems a "similitude" between Usher and his environment. For instance, one night he knocks on the door of his friend (the narrator) and appears to have "restrained hysteria in his whole demeanor." This behavior matches that of what occurs outside the mansion; Roderick opens one of the casements [windows] and exposes a tempestuous night that is

...wildly singular in its terror and beauty. A whirlwind had apparently collected its force...and there were frequent and violent alterations in the direction of the wind.

The narrator closes the casement and urges his disturbed friend not to subject himself to the fury of Nature. In his effort to quiet his friend, the narrator reads aloud to Usher about a medieval knight; however, the narrator now believes that he hears "the very cracking and ripping sound" which Sir Launcelot had so particularly described in the book that he reads aloud. Further, as he continues to read, the narrator is disturbed by a real sound that matches the description of the sound of Launcelot's shield which fell from a wall at his feet. Unnerved by this sound, the narrator jumps up; however, Roderick Usher shudders. He tells his friend that he has also heard this sound. But, with his acute sense of hearing, he has identified it as the sound of Madeline making feeble movements in her casket. Usher tells his friend that rather than being the 

"...breaking of the hermit's door, and the death cry of the dragon, and the clangor of the shield!"

It is, instead

"...the rending of her coffin, and the grating of the iron hinges...and her struggles within the coppered archway of the vault!"

As he has listened to the story along with the accompanying noises in the house, Usher has become increasingly distraught. He insists that Madeline is standing right outside the door. When the door is opened and Madeline does, indeed, stand there in bloody robes, she utters a moaning cry and falls upon her brother as he, too, dies from his shock and fright. In terror, also, the narrator flees.