Attractive serial dependence arises during decision-making

Recall of stimuli is biased by stimulus history, variously manifested as an attractive bias toward or repulsive bias from previous stimuli (i.e., serial dependence). It is unclear when attractive versus repulsive biases arise and if they share neural mechanisms. A recent model of attractive serial dependence proposes a two-stage process in which adaptation causes a repulsive bias during encoding that is later counteracted by an attractive bias at the decision-making stage in a Bayesian-inference-like manner. Neural evidence exists for a repulsive bias at encoding, but evidence for the attractive bias during the response period has been more elusive. We recently (Hajonides et al., J Neurosci 43:2730-40, 2023) showed in a working memory task that while different stimuli in trial history exerted different (attractive or repulsive) serial biases on behavioral reports, during encoding the neural representation of the current item was always repulsively biased. Here, we assessed whether this discrepancy between neural and behavioral effects is resolved during subsequent decision-making. Multivariate decoding of human magnetoencephalography data during working memory recall showed a neural distinction between attractive and repulsive biases that is consistent with the two-stage model: an attractive neural bias was found in recall period. And stimuli that created a repulsive bias on behavior led to an early repulsive neural bias that is likely to have already been incorporated during the encoding phase. The neural attractive bias late in the trial was replicated in an independent electroencephalogram experiment. Our results suggest that attractive (but not repulsive) serial dependence arises during decision-making, and that priors that influence post-perceptual decision-making are updated by the previous trial's target, but not by other stimuli.