Given the fact that many of Emily Dickinson's poems are characterized more by ambiguity than anything else, it is possible, perhaps likely, that the subject of ED's poem "I felt a funeral in my Brain" (ca. 1861) is her fear of the loss of reason.
The funeral metaphor may indicate that Dickinson feels--as her hold on reason becomes tenuous--that her loss of reason called for a formal funeral ceremony, and then, beginning in Stanza 3, the mourners essentially buried her reason:
And then I heard them lift a Box/And creak across my Soul/With those same Boots of lead, again,/Then Space--began to toll. . . . (ll.19-22)
It is as if the mourners--actually Dickinson's own consciousness--take her reason from the church to the burying ground. She explicitly acknowledges the departure of her mourners and her incredible loneliness in the third stanza when she says
And I, in Silence, some Strange Race/Wrecked, solitary, here--
Whatever hold on reason she has at the poem's beginning, that hold seems to slip away by the fourth stanza.
Using the image of a coffin being let down into the grave, Dickinson's reason appears to be lost:
And then a Plank in Reason, broke,/And I dropped down, and down -/And hit a World, at every plunge,/And Finished knowing. . . . (ll.17-20)
At this point, one has to conclude that Dickinson believes she no longer has the faculty of reason--she is, in effect, "finished knowing." But, as is also characteristic of Dickinson, the last word in the poem adds an important ambiguity: "And Finished knowing--then--. Clearly, the addition of the word then may indicate that reason is not quite dead. For Dickinson, hope is always the "thing with feathers," and the last line here indicates that the finality of reason's demise might not actually be final.