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Juergen Kornmeier, Bach Michael; Evidence for early visual processing in perceptual disambiguation of ambiguous figures. Journal of Vision 2004;4(8):249. doi: https://doi.org/10.1167/4.8.249.
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© ARVO (1962-2015); The Authors (2016-present)
Background. The percept of ambiguous figures (e.g. the Necker cube) changes spontaneously even while the figure stays unchanged. Event related potentials (ERPs) can trace the time course of the underlying neural processes. In case of endogenous events, like perceptual reversals, this technique suffers from the high temporal jitter of subjects' reactions when used as time reference to average EEG data. However, in discontinuous presentation endogenous reversals often occur at stimulus-onset, thus stimulus-onset provides an improved time reference for averaging. Using this technique we had found ERP correlates to the perceptual reversal, beginning with a surprisingly early component (120 ms after onset), which occurred occipital only. We here present experiments where the off-times were reduced down to 40 ms, thus mimicking eye blinks in continuous presentation. Methods. An ambiguous “Necker lattice” appeared repeatedly for 800 ms after breaks of 40 or 400 ms. In addition, unambiguous lattices (with depth cues) were presented, with same or reversed orientation. Our subjects indicated whether they perceived a “reversal” at stimulus-onset. EEG was recorded from 13 channels in 5 subjects; the difference traces between reported “reversal” and “stability” were analyzed. Results. Highly significant (p<0.001) occipital and parietal ERP negativities were seen for 400 ms breaks as correlates of the endogenous reversal. Reducing the breaks to 40 ms increased the negativites by factor five. An additional early positivity at 120 ms occurred for Necker lattices (p<0.01) but not in case of unambiguous lattices. We interpret the earliest occipital signal as a direct correlate of the neural processes underlying perceptual reversals. Based on the time structure of the identified ERP components, we estimate that the disambiguation of the ambiguous figures is initiated very early in the visual cortex, around 300 ms before perceptual awareness is established.
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