Strange screen during start up sequence

Hi all,

For about a month I have been seeing this strange pixelated screen during the start up sequence on my MBP running 10.8.5. Have searched for a similar screen shot / issue but haven't seen anything like it.

It changes back to the normal log in screen and the computer seems to be functioning normally. I have been backing up every day in case this is some sign of the apocalypse. What could this be a symptom of, and what can I do to fix it?

TIA!

- Stew