Spontaneous alpha-band oscillations bias subjective contrast perception

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 cortical excitability have been linked to fluctuations in the amplitude of pre-stimulus alpha oscillations ({approx}8-13 Hz), which are in turn are associated with fluctuations in subjects tendency to report the detection of a stimulus. It is currently unknown whether alpha oscillations bias post-perceptual decision making, or even bias subjective perception itself. To answer this question, we used a contrast discrimination task in which subjects reported which of two gratings - one in each hemifield - was perceived as having a stronger contrast. Our EEG analysis showed that subjective contrast was reduced for the stimulus in the hemifield represented in the hemisphere with relatively stronger pre-stimulus alpha amplitude, reflecting reduced cortical excitability. Furthermore, the strength of this spontaneous hemispheric lateralization was strongly correlated with the magnitude of individual subjects biases, suggesting that the spontaneous patterns of alpha lateralization play a role in explaining the intersubject variability in contrast perception. These results indicate that spontaneous fluctuations in cortical excitability, indicted by patterns of pre-stimulus alpha amplitude, affect perceptual decisions by altering the phenomenological perception of the visual world. Significance StatementOur moment to moment perception of the world is shaped by the features of the environment surrounding us, as much as by the constantly evolving states that characterize our brain activity. Previous research showed how the ongoing electrical activity of the brain can influence whether a stimulus has accessed conscious perception. However, evidence is currently missing on whether these electrical brain states can be associated to the subjective experience of a sensory input. Here we show that local changes in patterns of electrical brain activity preceding visual stimulation can bias our phenomenological perception. Importantly, we show that the strength of these variations can help explaining the great inter-individual variability in how we perceive the visual environment surrounding us.