Abstract
Perceptual decisions depend both on the features of the incoming stimulus and on the ongoing brain activity at the moment the stimulus is received. Specifically, trial-to-trial fluctuations in the power of prestimulus alpha oscillations (~8-13 Hz) are associated with fluctuations in sensory detection bias, i.e. the tendency to report stimulus absence or presence. It is currently unknown whether prestimulus alpha oscillations affect only such overt reports, or even modulate our subjective perception. To fill this gap, we used a contrast discrimination task in which subjects reported which of two gratings – one in each hemifield – was perceived as having stronger contrast. Our EEG analysis showed that subjective contrast was reduced for the stimulus in the hemifield represented in the hemisphere with relatively stronger prestimulus alpha power. This effect of alpha lateralization is consistent with previous studies showing that strong spontaneous alpha power suppresses neuronal and sensory excitability. Moreover, by applying a Common Spatial Pattern analysis on the prestimulus window (500 ms before stimulus onset) in the alpha range, we were able to decode the reported subjective contrast independent of the stimuli’s objective contrast difference. Interestingly, a classifier trained to decode the hemifield with stronger subjective contrast based on spontaneous alpha lateralization could not decode attended hemifield based on cue-induced alpha lateralization and vice versa. This lack of generalization suggests that spontaneous and cue-induced lateralization are different phenomena.