Abstract
What we see at any moment is strongly biased by what we saw in the past. Previous studies have shown that the perception of current stimulus is pulled toward previously seen stimuli, due to serial dependence (Fischer & Whitney, 2014; Liberman et al., 2014). We previously reported that the brain oscillation at theta-to-alpha band just before a face appears predicts how much the face appears pulled toward the face in the previous trial (Murai et al., VSS 2018). In the present study, we further investigated the temporal tuning of such an oscillatory modulation of serial dependence. In the experiment, a test face was briefly presented for 500 ms, and then subjects adjusted a response face to match its identity to the test face, with EEG signals recorded concurrently throughout the experiment. To examine the relationship between brain oscillation and serial dependence, we sorted trials based on the oscillation phase of various oscillation frequencies at various timings relative to the time of stimulus onset, and calculated the magnitude of serial dependence separately for trials with different oscillation phases. We found that the prestimulus alpha phase is correlated with the serial dependence for the upcoming face, and such a correlation between alpha phase and serial dependence was observed about every 300-400 ms before the stimulus appeared. Permutation tests confirmed the significance of this slow periodic fluctuation. This periodic modulation of serial dependence by alpha phase can be traced back to the time of previous stimulus onset, up to 10 seconds before the current stimulus. Our results suggest that the alpha oscillation is involved in an active temporal smoothing of visual perception within a wider temporal window, not only simply modulating attention or periodic sampling of incoming sensory input.