Quotes from "Mind Time: The Temporal Factor in Consciousness" by Benjamin Libet, 2004. Here he summarizes his main experimental findings in terms of the ``time-on'' theory:
Conscious and unconscious mental functions differ most importantly in the presence of awareness for the former and the absence of awareness in the later. We found that the brain requires substantial time (about 0.5 sec) to ``produce'' awareness of a sensory signal. while unconscious functions appear to require much less time (100 msec or so). What was the brain doing during the shorter periods of activations that did not last long enough to produce awareness? Far from being silent, the brain exhibited recordable neuronal responses that resembled those that went on to finally become adequate for awareness. These shorter-lasting trains of nerve cell responses could not produce awareness. But, we asked, could they provide a mechanism for an unconscious detection of a sensory signal? That question led us to propose a time-on theory for explaining the transition between brain activities required for unconscious mental functions and those required for conscious functions.
The time-on theory has two simple components:
(1) To produce a conscious sensory experience (in other words, with awareness), appropriate brain activities must proceed for a minimum duration of about 500 msec (when the event is near threshold). ...
(2) ... when these same brain activities have durations shorter than those required for awareness, they could nevertheless be involved in producing an unconscious mental function, without awareness. An unconscious function might then be transformed into a conscious one simply by increasing the duration (time-on) of the appropriate brain activities. We realized that time-on was probably not the only factor in the transition between unconscious and conscious, but we saw it as a controlling factor.
... what is that makes some time-ons long enough for awareness and most of the others not long enough? We don't have a full answer to that. However, there is good reason to believe that focusing attention on a given sensory signal may be an agent for making the sensory response a conscious one. We don't yet know what brain mechanism ``decides'' to focus attention on one signal and not on others. But there is evidence that the attention mechanism could ``light up'' or activate some areas of cerebral cortex; such an increase in excitability level of those areas might facilitate their lengthening the duration of their nerve cell response to achieve the time-on for awareness. ---pp. 101-102, Mind Time