Spontaneous EEG Activity and Biases in Perception of Supra-Threshold Stimuli Andrey R. Nikolaev, Sergei Gepshtein, and Cees van Leeuwen Abstract Human perception of oriented visual stimuli is biased: some orientations are seen more often than others. We studied how the orientation bias is represented in the electrical brain activity that preceded presentation of ambiguous supra- threshold visual stimuli. We examined scalp EEG over the parieto-occipital regions during 1 sec before stimulus presentation. The alpha activity of pre-stimulus EEG was associated with the orientation bias: the preference for vertical orientation in most observers corresponded to low pre-stimulus alpha power. The results indicate that the orientation bias is encoded in intrinsic properties of ongoing cortical dynamics, forming spontaneous orientation-specific patterns of activity. Keywords EEG • Spontaneous alpha activity • Perceptual organization • Perceptual bias 1 Introduction The perception of a stable and continuous world is mediated by neural mechanisms that are adept at resolving ambiguities of stimulation. One factor that helps to resolve the ambiguities is expectation of stimuli from prior experience in similar perceptual situations. Perception can therefore be viewed as a competition of two A.R. Nikolaev () RIKEN Brain Science Institute (BSI), 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan e-mail: nikolaev@brain.riken.jp S. Gepshtein RIKEN Brain Science Institute (BSI), 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan Salk Institute for Biological Studies, La Jolla, San Diego, CA, USA C. van Leeuwen RIKEN Brain Science Institute (BSI), 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan Y. Yamaguchi (ed.), Advances in Cognitive Neurodynamics (III), DOI 10.1007/978-94-007-4792-0 39, © Springer ScienceCBusiness Media Dordrecht 2013 289