What Defines a Work as "Yours"?

Here's a question.

I had entered The Writing Show's Halloween story contest. I had written, over the course of four or five days, a 5,000 word story (okay, it was closer to 6,000 at the time, but it had to be pared down for requirement reasons).

As you can tell from reading blog entries my "first drafts" are entirely in need of editing. After I wrote the story I had help from my wife in cutting sections and seeing what areas she thought could use rewording, rework, or clarification as well as grammar and punctuation alterations.

I had some very cursory feedback from the sponsor of the contest; it sounded like my entry, in her opinion, was decent. The contest goes on until the end of August so there's plenty of time for others to far outshine me but just having someone say that it seemed like a good short story to read was a nice boost to my confidence.

One thought nagged me, though. She never saw what I actually wrote. What she got was probably 95% mine, but the edits? Grammar and punctuation that was corrected? Some impetus to rewrite or cut paragraphs? Those were my wife. My story was cut from near 6,000 words to around 4,800 or so.

Part of me tells my inner voice that it's ridiculous. The story was mine. Plot. Majority of descriptions, theme, etc...I came up with all that in this case. But I still have that nagging thought of, "At what point is the story no longer yours?"

Is there a point where the editing is so different that your original writing is just too different to be really yours, but instead a definite collaboration?

Maybe that's a question for others more experienced in the field of authorship. Right now it's an intellectual curiosity for me. Anyone out there have some ideas?