You could replace "brain" with the word "mind". This is a synonym for "brain" but it refers to the element of a person that enables them to be aware of the world and their experiences, to think, and to feel; the faculty of consciousness and thought, or a person's intellect, whereas "brain" could refer to the physical brain. Using the word "mind" would help you to understand the analogy, as Emily Dickinson is referring not to the physical state of the brain, but the intellectual state of the brain, often called the mind.
