What is the meaning of "I felt a Funeral, in my Brain"?

This is a curious poem that can often be hard to interpret
in terms of its meaning. However, it appears that this poem traces the various stages of
some kind of mental breakdown, using the imagery of a funeral to describe those
stages.

The lyric begins by establishing the basic
proposition of the poem: the speaker has had some kind of mental experience which is
likened to the events of a funeral - a psychological death becomes merged with a
physical death:

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I felt a Funeral, in my
Bran,

And Mourners to and
fro

Kept treading - treading - till it
seemd

That Sense was breaking through
-

In the speaker's mind, the
mourners seem to be walking up and down, perhaps paying their last respects to the dead
person lying in the coffin. They are then seated and the funeral service begins. Once
this stage of the service is complete, the coffin is lifted and taken to
the graveyard while the church bell tolls. The coffin is placed on planks set
over the grave and then one of the planks breaks and the coffin drops down into the
grave:

And
then a Plank in Reason, broke,

And I dropped down, and down
-

And hit a World, at every
plunge,

And Finished knowing - then
-

Critics argue that this
final stanza suggests the mind's final plunge into the abyss of despair or
depression.

Although we are never told precisely what
triggered these effects, it is clear that the funeral imagery serve to give concrete
form to abstract feelings. The much-debated last stanza does obviously indicate some
final descent, but some critics argue that this represents the complete loss of all
thought, understanding and knowledge. Others argue that after breaking through the
bounds of reason the speaker finishes by retaining or learning something, as indicated
by the open dash. Either way, this poem traces some form of mental breakdown through
using the stages of the funeral ritual to chart the gradual disintegration of the
speaker's senses.