Why Decisions Bias Perception: An Amortised Sequential Sampling Account

Abstract

The judgments that people make are not independent - initial decisions can bias later perception. This has been shown in tasks in which participants first decide whether the direction of moving dots is to one side or the other of a reference line: their subsequent estimates are biased away from this reference line. This interesting bias has been explained in past work as either a consequence of weighting sensory neurons, or as a consequence of participants adjusting their estimate to match their decision. We propose a new explanation: that people sequentially sample evidence to make their decision, and reuse these samples to make their estimate (i.e., amortised inference). Because optimal stopping leads to samples that strongly favor one or another decision alternative, the subsequent estimates are also biased away from the reference line. We introduce a sequential sampling model for posterior samples that does not assume constant thresholds, and provide evidence for our explanation in a new experiment that generalizes the perceptual bias to a new domain.