Function of «это» in «это у меня воображение разыгралось»

Question

Все думаю, это у меня воображение разыгралось или нет...

{instead of}: Все думаю, это у меня воображение разыгравшееся или нет...

This is part of an IM I received from my girlfriend, and I think it essentially means:

Still wondering if it wasn't just my imagination playing tricks on me. Or...

The thing is that I'd expected to see "разыгравшееся" used instead, at least more in keeping with the structure of the English phrasing. In which case, I could see the function of "это" as the subject: "it's my imagination having played tricks on me".

In the sentence she used, on the other hand, I'm not sure how to parse and interpret the function of "это", as the subject is apparently "воображение". A relevant post: What is the function of "это" in "Его зовут X, это он впустил ..."?

Answer 1

In this type of usage, like in your other question, “это” is kind of outside its sentence and instead of coupling words, it couples sentences.

Девушка услышала странный голос. Это у неё воображение разыгралось, наверное.

This can be turned into the definition pattern (“X — [это] Y”):

Странный голос — это игра воображения девушки.

Similarly,

— Кто там?
— Это я пришёл.

which, if you want to apply usual sentence parsing rules, should be rephrased along the lines of

Звонок в твою дверь — это следствие моего визита.

Answer 2

Technically you can use past participle разыгравшееся, however in this case for the sentence to sound more idiomatically у меня needs to be replaced with моё (as in English) and order of words changed

Все думаю, это моё разыгравшееся воображение или нет?

Yet, this very phrasing, albeit grammatically sound, in itself isn't very common and not 100% identical in meaning to the other version, because it equates the work of your imagination to that certain thing rather than signifies its being the result of the imagination's work, like the other version does.

Answer 3

Everything you've said about это is correct, but there's some important point missing. The use of это construction may add emphasis to actual subject of the sentence. See the following example:

Without это:

Все думаю, у меня воображение разыгралось или нет. I can't stop thinking, is my imagination playing tricks on me or not?

With это (emphasised by intonation):

Все думаю, это у меня воображение разыгралось или нет. I can't stop thinking, is this my imagination playing tricks on me or not? ( Maybe it was somebody else, who actually got imagination issues )

With это (emphasised by intonation):

Все думаю, это у меня воображение разыгралось. I can't stop thinking, all this was just my imagination playing tricks on me. ( Everything what happened was my imagination )

Answer 4

Всё думаю, это у меня воображение разыгралось или нет...

It's about current situation. Like, if something just has happened, and your imagination "playing tricks on you" right now.

But

Всё думаю, это у меня воображение разыгравшееся или нет...

Could have the same meaning. But it's more about your imagination in general, like "your imagination always plays tricks on you"