Proceedings of the Annual Meeting of the Cognitive Science Society

The judgments that people make are not independent –initial decisions can bias later perception. This has beenshown in tasks in which participants first decide whetherthe direction of moving dots is to one side or the otherof a reference line: their subsequent estimates are biasedaway from this reference line. This interesting bias has beenexplained in past work as either a consequence of weightingsensory neurons, or as a consequence of participants adjustingtheir estimate to match their decision. We propose anew explanation: that people sequentially sample evidenceto make their decision, and reuse these samples to maketheir estimate (i.e., amortised inference). Because optimalstopping leads to samples that strongly favor one or anotherdecision alternative, the subsequent estimates are also biasedaway from the reference line. We introduce a sequentialsampling model for posterior samples that does not assumeconstant thresholds, and provide evidence for our explanationin a new experiment that generalizes the perceptual bias to anew domain.