Decisions are often based on a combination of new evidence with prior knowledge of the probable best choice. Optimal combination requires knowledge about the reliability of evidence, but in many realistic situations, this is unknown. Here we propose and test a novel theory: the brain exploits elapsed time during decision formation to combine sensory evidence with prior probability. Elapsed time is useful because (1) decisions that linger tend to arise from less reliable evidence, and (2) the expected accuracy at a given decision time depends onthe reliability of the evidence gatheredup tothat point. These regularities allow the brain to combine prior information with sensory evidence by weighting the latter in accordance with reliability. To test this theory, we manipulated the prior probability of the rewarded choice while subjects performeda reaction-time discrimination of motion direction using a range of stimulus reliabilities that varied from trial to trial. The theory explains the effect of prior probability on choice and reaction time over a wide range of stimulus strengths.Wefoundthat priorprobability wasincorporated intothedecisionprocessasadynamic biassignal that increasesasafunction ofdecisiontime. Thisbiassignal dependsonthespeed-accuracysettingofhumansubjects, anditisreflectedinthefiringratesofneurons in the lateral intraparietal area (LIP) of rhesus monkeys performing this task.
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