Bistable perception alternates between internal and external modes of sensory processing.

Perceptual history can exert pronounced effects on the contents of conscious experience: when confronted with completely ambiguous stimuli, perception does not waver at random between diverging stimulus interpretations but sticks with recent percepts for prolonged intervals. Here, we investigated the relevance of perceptual history in situations more similar to everyday experience, where sensory stimuli are usually not completely ambiguous. Using partially ambiguous visual stimuli, we found that the balance between past and present is not stable over time but slowly fluctuates between two opposing modes. For time periods of up to several minutes, perception was either largely determined by perceptual history or driven predominantly by disambiguating sensory evidence. Computational modeling suggested that the construction of unambiguous conscious experiences is modulated by slow fluctuations between internally and externally oriented modes of sensory processing.