Intermittent ambiguous stimuli: implicit memory causes periodic perceptual alternations.

When viewing a stimulus that has multiple plausible real-world interpretations, perception alternates between these interpretations every few seconds. Alternations can be halted by intermittently removing the stimulus from view. The same interpretation dominates over many successive presentations, and perception stabilizes. Here we study perception during long sessions of such intermittent presentation. We demonstrate that, rather than causing truly stable perception, intermittent presentation gives rise to a perceptual alternation cycle with its own characteristics and dependencies, different from those during continuous presentation. Alternations during intermittent viewing typically occur once every few minutes--much less frequently than the seconds-scale alternations during continuous viewing. Strikingly, alternations during intermittent viewing occur at fairly regular intervals, making for a surprisingly periodic alternation cycle. The duration of this cycle becomes longer as the blank duration between presentations is increased, reaching dozens of minutes in some cases. We interpret our findings in terms of a mathematical model that describes a neural network with competition between alternative interpretations. Network sensitivities depend on prior dominance, thus providing a memory for past perception. Slow changes in sensitivity produce both perceptual stabilization and the regular but infrequent alternations, meaning that the same memory traces are responsible for both. This model provides a good description of psychophysical findings, and offers several indications regarding their neural basis.