Serial dependence during saccades is mediated by alpha rhythms

Serial dependence, the assimilative bias towards previous stimuli in a sequence, is usually taken as a demonstration of the action of predictive perceptual processes. In natural viewing, saccadic eye movements cause sequential sampling of the world, where predictive serial dependence-like processes are fundamental, a probable essential for visual continuity. Recent theory and evidence suggest that the neural signaling of predictions may be mediated by alpha rhythms. To study the role of alpha rhythms, we measured serial dependence for stimuli at the time of saccades while monitoring EEG from 32 scalp electrodes. While saccading to a target, participants judged the orientation of a brief (17 ms) Gabor patch (±35º, ±45º or ±55º), presented at screen center at random delays after flashing the saccadic target. Trials were classed as congruent or incongruent, depending on whether the orientation of the previous trial was in the same or different quadrant as the current trial. The psychophysical reproduction of orientation showed clear biases towards the previous stimuli, and the biases oscillated at 11 Hz (alpha frequency), synchronized to saccadic onset. EEG traces (synchronized to saccadic onset) also oscillated reliably in the alpha range (8-14 Hz), clearly visible in the artifact-free pre-saccadic period for post-saccadic stimuli. The oscillation was stronger in congruent than incongruent trials. Subtracting incongruent from congruent trials cancelled the saccade artifact, revealing alpha oscillations over the entire period. Importantly, the oscillations in orientation bias in congruent trials correlated with that of the EEG profiles, over the entire scalp. The center of the cross-correlation envelope varied with position on the scalp, suggesting variability in transmission times. Our results show that serial dependence is encoded in saccade-synchronized alpha oscillations, which may play a key role in communication of visual predictions and trans-saccadic continuity.