The M-Webster dictionary gives the following definitions for threshold
“…a physical marker, like the plank, stone or piece of timber that lies under a door. The end or boundary, the place or point of entering or beginning; the point at which a physiological or psychological effect begins to be produced. A level, point, or value above which something is true or will take place and below which it is not or will not.”
The point of interest for me is that portion of that definition that speaks of the “end or boundary, the point of entering or beginning,” that ambiguous point that is both beginning and end, true and not true, real and unreal – and one which can be considered physically, psychologically or symbolically.
I think perhaps Nancy, with his poetic play on words, intimates the possibilities of the word – threshold – and places our involvement with the image in these multiple realms of experience, for (I have found) Nancy never leaves a word with one bound meaning, but enjoys the layered and shifting sense that each word can invoke.