One element linking themes in Romanticism and Gothic literature is the power of the imagination. In more benevolent works, the imagination is praised for its ability to link the thinker/writer with nature in a profound or transcendental sense. There is a connection between the individual and nature that is metaphorically or literally spiritual. On the other hand, in darker Gothic and Romantic works, writers such as these (Hawthorne and Poe) illustrate the dark power of the imagination and, quite literally in these two stories, the power of the imagination in taking over an individual's mind.
One way to conceptualize this is that the individual is overpowered by his own imagination. But it is more accurate to say that, because of some personality flaw, he allows his imagination to take control. In both stories, the narrator's imagination is intoxicating - to the point that he becomes addicted to it (like a drug, alcohol, etc.). In "The Haunted Mind," the narrator tries to remain in that space between sleep and consciousness because of the perceptive possibilities it opens up. He tries to embrace pleasant perceptions but sometimes the dark impressions move in and take over. He is an experimenter in this middle state between dreams and reality. The narrator in "The Black Cat" is much more deplorable and morally flawed. He allows his dark thoughts to dominate his actions and thinking. This begins with his association of black cats with witches, a harmless association/myth. The cat does nothing wrong to him; therefore, we can conclude that his increasing disregard for the cat, his wife, and morality in general occurs solely in his mind.