Serial dependence in vision: Merely encoding the previous-trial target is not enough

The reported perception of a visual stimulus on one trial can be biased by the stimulus that was presented on the previous trial. In the present study we asked whether encoding the previous-trial stimulus is sufficient to produce this serial dependence effect, or whether the effect also depends on postencoding processes. To distinguish between these possibilities, we designed a task in which participants reported either the color or the direction of a set of colored moving dots on each trial. The to-be-reported dimension was indicated by a postcue after stimulus offset, so participants were required to encode both features of every stimulus. We assessed serial dependence for motion perception as a function of which feature dimension had been reported on the previous trial. In Experiment 1, we found a serial dependence effect for motion only when participants had reported the direction of motion on the previous trial, and not when they had encoded the direction of motion but reported the color of the stimulus. Experiment 2 confirmed that this pattern of results was not driven by the difficulty of the color task. When we used the same response modality for both motion and color reports in Experiment 3, we found significant serial dependence effects following both color-report and motion-report trials, but the effect was significantly weaker following color-report trials. Together, these findings indicate that postperceptual processes play a critical role in serial dependence and that the mere encoding of the previous-trial target is not sufficient to produce the serial dependence effect.