The phase of ongoing EEG oscillations predicts visual perception

The state of the cortex at the moment information about a stimulus reaches it can affect how this information will be processed. In particular, neuronal oscillations that induce large variations in local electrical fields could influence certain aspects of the neuronal response. While the amplitude of ongoing oscillatory activity is known to correlate with perceptual performance, the influence of oscillatory phase on perception remains unknown. Since phase varies on a much faster time scale than the more sluggish amplitude fluctuations, phase effects are more likely to reveal the fine-grained neural mechanisms underlying perception. We presented brief flashes of light at the individual contrast threshold while EEG was recorded. Even though the stimulus on each trial was identical, subjects detected approximately half of the flashes (hits) and entirely missed the other half (misses). Phase distributions across trials were compared between hits and misses. We found that shortly before stimulus onset, each of the two distributions exhibited significant phase concentration, but at different phase angles. This effect was strongest in the theta and alpha frequency bands at frontal midline electrodes. In this time-frequency range, oscillatory phase accounted for at least 16% of variability in detection performance and allowed the prediction of performance at the single trial level. This finding indicates that the detection threshold fluctuates over time along with the phase of ongoing EEG activity, supporting the notion that ongoing oscillations shape our perception, possibly by providing a temporal reference frame for neural codes that rely on precise spike timing.