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  • Kornmeier, J., Wörner, R., & Bach, M. (2016). Can I trust in what I see? EEG evidence for a cognitive evaluation of perceptual constructs. Psychophysiology, 53(10), 1507–1523. https://doi.org/10.1111/psyp.12702

    Environmental information available to our senses is incomplete and to varying degrees ambiguous. It has to be disambiguated in order to construct stable and reliable percepts. Ambiguous figures are artificial examples where perception is maximally unstable and alternates between possible interpretations. Tiny low-level changes can disambiguate an ambiguous figure and thus stabilize percepts. The present study compares ERPs evoked by ambiguous stimuli and disambiguated stimulus variants across three visual categories: geometry (Necker cube), motion (stroboscopic alternative motion stimulus, SAM) and semantics (Boring's old/young woman). We found that (a) disambiguated stimulus variants cause stable percepts and evoke two huge positive ERP excursions (Cohen's effect sizes 1-2), (b) the amplitudes of these ERP effects are inversely related to the degree of stimulus ambiguity, and (c) this pattern of results is consistent across all three tested visual categories. This generality across visual categories points to mechanisms at a very abstract (cognitive) level of processing. We discuss our results in the context of a high-level Bayesian inference unit that evaluates the reliability of perceptual processing results, given a priori incomplete, ambiguous sensory information. The ERP components may reflect the outcome of this reliability estimation.

  • Kornmeier, J., Pfäffle, M., & Bach, M. (2011). Necker cube: stimulus-related (low-level) and percept-related (high-level) EEG signatures early in occipital cortex. Journal of Vision, 11(9), 12. https://doi.org/10.1167/11.9.12

    During observation of an ambiguous Necker cube, our percept changes spontaneously although the external stimulus does not. An EEG paradigm allowing time-resolved EEG measurement during endogenous perceptual reversals recently revealed a chain of ERP correlates beginning with an early occipital positivity at around 130 ms (Reversal Positivity, "RP"). In order to better understand the functional role of this RP, we investigated its relation to the P100, which is spatiotemporally close, typically occurring 100 ms after onset of a visual stimulus at occipital electrodes. We compared the relation of the ERP amplitudes to varying sizes of ambiguous Necker cubes. The main results are: (1) The P100 amplitude increases monotonically with stimulus size but is independent of the participants' percept. (2) The RP, in contrast, is percept-related and largely unaffected by stimulus size. (3) A similar pattern to RP was found for reaction times: They depend on the percept but not on stimulus size. We speculate that the P100 reflects processing of elementary visual features, while the RP is related to a processing conflict during 3D interpretation that precedes a reversal. The present results indicate that low-level visual processing (related to stimulus size) and (relative) high-level processing (related to perceptual reversal) occur in close spatial and temporal vicinity.

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