Clashing memories in working memory modulate serial dependence.

Serial dependence describes how prior and current stimuli interact, producing attractive biases toward previously seen inputs, particularly under uncertainty. Previous studies varied external stimulus noise; here we manipulated the quality of internal representations through an intermediate task during the retention interval. Participants viewed oriented Gabor patches and reproduced their orientation after a delay, during which they either performed a control task or judged circle size, line length/orientation, or tone frequency. These manipulations allowed for a detailed assessment of the role of working memory load and inter-item competition on the fidelity of the encoded Gabor orientation and subsequently, on the strength of serial dependence. Tasks requiring additional encoding increased serial dependence, even when the task was auditory. Increased working memory load is known to alter the quality and accessibility of maintained representations, which may enhance the influence of prior stimuli on current stimulus perception. However, the similarity of the feature dimension did not further increase serial dependence, though it did increase response variability. These findings offer novel evidence that increased working memory load, not just stimulus noise, enhances biases towards past input, shaping serial dependence through internal uncertainty.