I imagine that what happens if you literally think of nothing would be like having a general anesthetic.. the time would probably just disappear... dunno
Take a heavy dose of ketamine, it pretty much divides your brain from your body and puts you down a little wormhole where senses are completely disconnected. Kind of like being under general anaesthesia whilst still awake. Very odd and unpleasant.
More on the thread... didn't Hume state (albeit in a more empiricist way than Descartes) in An Enquiry Concerning Human Understanding that the brain and its faculties are essentially combinatorial? We correlate and amalgamate the abstract with the experiential. To think of 'nothing' would be like reverse-engineering the human brain.