Perceptual decisions are biased towards previous stimuli, a phenomenon
known as serial dependence. However, the mechanisms of this bias are
unknown. For example, it is debated whether serial dependence reflects a
perceptual bias or interference of prior stimuli in working memory. We
addressed this question by testing whether visual working memory load
modulates serial dependence. We presented sequences of Gabors appearing at
the same or different locations. At the beginning of each sequence, we
presented a display with two lines. Participants reproduced the orientation
of each of the three Gabors while holding the orientation of the lines in
memory. In a control condition, participants reproduced only the
orientation of each Gabor, ignoring the lines. Working memory load caused a
repulsive bias in the adjustment responses when the previous Gabor was
presented at a different location. In the control condition, Gabors at
different locations caused no bias at all. When presented at the same
location, however, the bias was attractive for small orientation
differences (<45°) and repulsive for larger ones (>45°), resembling typical
serial dependence. Under load, this pattern was qualitatively more
pronounced. Our results suggest that working memory load may interact with,
rather than prevent, serial dependence by making perceptual decisions more
vulnerable to biases. [This research was supported by funding from the
Swiss National Science Foundation (grant no. 415 PZ00P1_179988 to DP) and
the Swiss Government Excellence Scholarship awarded by Federal Commission
for Scholarships for Foreign Students FCS (NT). The funders had no role in
the study design, data collection and analysis.]