Our earlier evidence had indicated that a substantial duration of appropriate cerebral activity (up to about 0.5 s) is required for the production of a conscious sensory experience; this means the sensory world is experienced delayed with respect to real time. Subjective timing of the experience can be retroactively referred to the time of the earliest signal arriving at the cortex. Our 'time-on' theory states that the transition from an unconscious to a conscious mental function is determined, at least in part, by an increase in the duration of appropriate neural activities. Our experimental finding that conscious intention to act appears only after a delay of about 350 ms from the onset of specific cerebral activity that precedes a voluntary act provided indirect evidence for the theory. In a direct experimental test a signal (stimulus to somatosensory thalamus) was correctly detected in a forced-choice test even when the stimulus duration was too short to produce any awareness of the signal; to go from correct detection with no awareness to detection with awareness required an additional 400 ms of the repetitive identical neural volleys ascending to sensory cortex. 'Time-on' theory has important implications for a variety of unconscious-conscious interactions.