A joint alpha power-phase dynamic shapes visual sensitivity

Perceptual experience is influenced by alpha-band oscillations (8-14 Hz) that dominate parietal and sensory cortices. However, there is uncertainty around the perceptual mechanisms that are affected by oscillatory power and phase. Previous work has linked power and phase to behaviour separately despite their theorised common effects via pulsed-inhibition, potentially misrepresenting or conflating their effects. Here we recorded brain activity using electroencephalography to investigate how alpha oscillations affect the psychometric function over visual contrast in both detection and discrimination tasks. We found that prestimulus power and phase predicted the strength of subsequent evoked neural responses and behavioural accuracy. We then combined power and phase into a joint model of pulsed-inhibition and estimated its effects within a signal detection model of behaviour. The model revealed response gain modulation of visual sensitivity in both tasks, and perceptual bias modulation in detection. Critically, oscillatory power suppressed visual sensitivity more strongly than phase, suggesting a sustained effect of alpha oscillations that is not accounted for by the pulsed-inhibition hypothesis. We conclude that alpha-band activity shapes visual perception by divisively suppressing sensory evidence and baseline sensory noise, with joint power-phase modelling revealing asymmetric contributions to visual sensitivity.