Agog

there are, not infrequently, episodes, particularly on falling asleep
(hypnagogic imagery) and waking up (hypnapompic), when the imagery,
being of a rapid, phantasmogoric and involuntary character, defies
perception, presenting a kaleidoscopic field, not unlike a
screen-saver on television, in which no distinct object can be
discerned.[1]

Agog

Terry terror is in the room
coalesced against the gloom
in greenish light, with tadpole white
forms a spectre out of night,
a lady in a shroud it seems:
arrived from neither sleep nor dream,
She offers
no words of consolation

just hovers

in a corner space

between the bed and door.

the vast majority of philosophical and scientific investigations of
the topic focus upon visual mental imagery. It is often assumed (e.g.,
by Aristotle: On the Soul III.3 428a) that, like humans, many types of
animals are capable of experiencing mental images. However, owing to
the fundamentally subjective nature of the phenomenon, there is little
evidence either for or against this view.