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Bruno Breitmeyer, Maximilian Bruchmann; Dissociating Electrophysiological Correlates of Luminance and Brightness Using Metacontrast Masking . Journal of Vision 2016;16(12):1152. doi: 10.1167/16.12.1152.
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Physical stimulus luminance and perceived brightness are typically highly correlated. It is therefore a methodological challenge to dissociate neural processes that scale with a stimulus's luminance from those that scale with its phenomenally perceived brightness. With a metacontrast masking paradigm we exploited the fact that metacontrast masks varying in luminance can differentially modulate the association between a target's luminance and its perceived brightness. Target contrasts were 6.25, 12.5, 25, 50 and 100%. Subjective ratings assessed the perceived brightness of each target when its visibility was suppressed by a weak, 12.5%-contrast mask and when it was suppressed by a strong 100%-contrast mask. The difference between the brightness ratings obtained with the two masks was computed for each of the five target contrasts and were then correlated with corresponding differences between the post-target EEG waveforms. We found that low-level occipital activity originating 80 ms and spreading in feedforward manner along the ventral and dorsal visual stream until 120 ms after target onset scaled with stimulus luminance. In contrast, first neural correlates of brightness (NCBs) emerged from 120 to 150 ms after target onset, predominantly in the right superior parietal lobe. From 160 to 180 ms NCBs emerged in the left temporal lobe and spread via feedback towards primary visual areas, from where they in turn spread to parietal and temporal areas again. The results suggest that the first feedforward sweep is defined by physical stimulus features, but that already after 120 ms recurrent activity involving parietal attention- and ventral object recognition-related areas scales with the perceptual outcome, rather than the physical input. No neural activity originating in frontal areas correlated with either the luminance or the perceived brightness of the target, suggesting that frontal activation is not necessary for stimuli to register in phenomenal awareness.
Meeting abstract presented at VSS 2016
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