Evidence accumulation under uncertainty - a neural marker of emerging choice and urgency

To interact meaningfully with its environment, an agent must integrate external information with its own internal states. However, information about the environment is often noisy. In our task participants had to monitor a stream of discrete visual stimuli over time and decide whether or not to act, on the basis of either strong or weak evidence. We found that the classic P3 event-related potential evoked by sequential evidence items tracked decision-making processes and encoded participants9 choice, both when evidence was strong and when it was weak. We also found that the readiness potential, a classic marker of self-paced actions, was observed preceding all actions - even when those were strongly driven by external evidence. Computational modelling showed that both neural dynamics and behavioural results can be explained by a combination of (a) competition between mutually inhibiting accumulators for the two categorical choice outcomes, and (b) a context-dependent urgency signal.