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The Journal of Neuroscience, July 8, 2009, 29(27):8715-8725; doi:10.1523/JNEUROSCI.0962-09.2009

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Behavioral/Systems/Cognitive
How Ongoing Fluctuations in Human Visual Cortex Predict Perceptual Awareness: Baseline Shift versus Decision Bias

Valentin Wyart^1,2,3 and Catherine Tallon-Baudry^1,2,3,4

¹Université Pierre et Marie Curie–Paris 06, Unité Mixte de Recherche (UMR) 7225, S-975, Centre de Recherche de l'Institut Cerveau-Moelle (CRICM), ²CNRS (Centre National de la Recherche Scientifique), UMR 7225, CRICM, ³Inserm (Institut National de la Santé et de la Recherche Médicale), UMR-S 975, CRICM, and ⁴CENIR (Centre de Neuroimagerie de Recherche), Hôpital de la Pitié-Salpêtrière, Paris F-75013, France

Correspondence should be addressed to Valentin Wyart, Centre de Recherche de l'Institut Cerveau-Moelle, LENA (Laboratoire de Neurosciences Cognitive & Imagerie Cérébrale), Bâtiment Médecine Physique et Réadaptation, 47 Boulevard de l'Hôpital, Paris F-75013, France. Email: valentin.wyart{at}upmc.fr

Visual perception fluctuates across repeated presentations of the same near-threshold stimulus. These perceptual fluctuations have often been attributed to baseline shifts—i.e., ongoing modulations of neuronal activity in visual areas—driven by top-down attention. Using magnetoencephalography, we directly tested whether ongoing attentional modulations could fully account for the perceptual impact of prestimulus activity on a subsequent seen–unseen decision. We found that prestimulus gamma-band fluctuations in lateral occipital areas (LO) predicted visual awareness, but did not reflect the focus of spatial attention. Moreover, these prestimulus signals influenced the decision outcome independently from the strength of the following visual response, suggesting that baseline shifts alone could not explain their perceptual impact. Using a straightforward decision-making model based on the accumulation of sensory evidence over time, we show that prestimulus gamma-band fluctuations in LO behave as a decision bias at stimulus onset, irrespectively of subsequent stimulus processing. In contrast, spatial attention suppressed prestimulus alpha-band signals in the same region, and produced a sustained baseline shift that also predicted the outcome of the seen–unseen decision. Together, our results indicate that prestimulus fluctuations in visual areas can influence the conscious detection of an upcoming stimulus via two distinct mechanisms: an attention-driven baseline shift in the alpha range, and a decision bias in the gamma range.

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Received Feb. 26, 2009; revised May 19, 2009; accepted May 30, 2009.

Correspondence should be addressed to Valentin Wyart, Centre de Recherche de l'Institut Cerveau-Moelle, LENA (Laboratoire de Neurosciences Cognitive & Imagerie Cérébrale), Bâtiment Médecine Physique et Réadaptation, 47 Boulevard de l'Hôpital, Paris F-75013, France. Email: valentin.wyart{at}upmc.fr

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