RT Journal Article SR Electronic T1 Elapsed Decision Time Affects the Weighting of Prior Probability in a Perceptual Decision Task JF The Journal of Neuroscience JO J. Neurosci. FD Society for Neuroscience SP 6339 OP 6352 DO 10.1523/JNEUROSCI.5613-10.2011 VO 31 IS 17 A1 Timothy D. Hanks A1 Mark E. Mazurek A1 Roozbeh Kiani A1 Elisabeth Hopp A1 Michael N. Shadlen YR 2011 UL http://www.jneurosci.org/content/31/17/6339.abstract AB 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 on the reliability of the evidence gathered up to that 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 performed a 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. We found that prior probability was incorporated into the decision process as a dynamic bias signal that increases as a function of decision time. This bias signal depends on the speed–accuracy setting of human subjects, and it is reflected in the firing rates of neurons in the lateral intraparietal area (LIP) of rhesus monkeys performing this task.