Introduction: Prolonged viewing of ambiguous stimuli (eg. the Necker Cube) induces spontaneous changes in perception. Previous EEG studies investigating perceptual reversals used subjects’ response as time reference for averaging. This, however, entails marked latency jitter. We introduced a new paradigm as follows: Spontaneous reversals were entrained to specific time instances by onset/offset presentation. Further a comparison condition was added, containing depth-shaded, non-ambiguous stimuli, which reversed externally.
Methods. Subjects viewed a “Necker lattice” as ambiguous stimulus for spontaneous reversal (Exp. 2) and a nonambiguous version for externally induced reversal (Exp. 1). Stimuli were presented intermittently and subjects indicated during the inter-stimulus-intervals (ISI) whether they had perceived a reversal compared to the preceding orientation.
Results. (1) Externally induced reversals: The ERP-difference-traces (reversing vs. non-reversing condition) showed a series of three components beginning with a negativity at occipital and parietal locations, 200 ms after onset. (2) Spontaneous reversal: The difference traces showed a series of four components, beginning with a positivity restricted to the occipital locations 100 ms after onset. The subsequent three components had the same succession, polarities and scalp locations as those in (1) but consistently appeard about 60 ms later.
Conclusions. Disambiguation of the visual input seems to be accomplished by structures early in the visual stream, indicated by the occipital positivity at 100 ms. In case of ambiguous stimuli, the “normal” perceptual processes, as taking place with unambigous stimuli, seem to be delayed by the preceding neural disambiugation activity.