[Inziladun]

Along the lines of what Legate said, I would note also that the entrance of the "wraith-world" wasn't noticeable to Frodo until he had his experience wearing the One in the presence of the Ringwraiths on Weathertop. This despite the fact that he had worn it in the Shire while hiding from one of them. I would agree then that Frodo's wound had something to do with his noticing his differences of perception while wearing the One, and that Sam's similar observation was due to his proximity to Mt. Doom.