A direct neural signature of serial dependence in working memory

Abstract

Our perception is biased towards sensory input from the recent past. While this “serial dependence” has been well established in behavior, a direct neural correlate of the phenomenon has not been determined. Applying multivariate analyses to magnetoencephalographic data during a visual working memory task, we found an attraction of neural representations to previous stimuli, which was consistent with behavioral responses. This is the first evidence for a direct neural signature of serial dependence. In addition, the present task, together with the high temporal resolution of magnetoencephalography, allowed us to identify the timing of serial dependence. In line with current theoretical models, we found serial dependence to emerge at late, post-encoding stages of information processing in working memory.Serial dependence describes the phenomenon that current object representations are attracted to previously encoded and reported representations. While attractive biases have been observed reliably and across various modalities in behavioral reports, a direct neural correlate has not been established. Previous studies have either shown a reactivation of past information without observing a neural signal related to the bias of the current information, or a repulsive distortion of current neural representations contrasting the behavioral bias. The present study recorded neural signals with magnetoencephalography during a working memory task to identify neural correlates of serial dependence. Participants encoded and memorized two sequentially presented motion directions per trial, one of which was later retro-cued for report. Multivariate analyses provided reliable reconstructions of both motion directions. Importantly, the reconstructed directions in the current trial were attractively shifted towards the target direction of the previous trial. This neural bias mirrored the behavioral attractive bias, thus reflecting a direct neural signature of serial dependence. The use of a retro-cue task in combination with magnetoencephalography allowed us to determine that this neural bias emerged at later, post-encoding time points. This timing suggests that serial dependence in working memory affects memorized information during read-out and reactivation processes that happen after the initial encoding. Taken together, we identified a direct neural signature of serial dependence, which occurs during later processing stages of working memory representations.