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•The classic P3 tracks the evolution of an internal category-specific decision variable in humans.•The amplitude of the P3 evoked by competing evidence items predicts choice on a single-trial level.•The decision variable is modulated by an internal time-dependant urgency signal.•This urgency signal was stronger in conditions with lower rates of evidence.•Primacy and recency effects can be accounted for by context-dependant urgency modulations. 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 this study, we identify a neural correlate that tracks how asymmetries between competing alternatives evolve over the course of a decision. 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 ambiguous evidence. We found that the classic P3 event-related potential evoked by sequential evidence items tracked decision-making processes and predicted participants’ categorical choices on a single trial level, both when evidence was strong and when it was ambiguous. The P3 amplitudes in response to evidence supporting the eventually selected option increased over trial time as decisions evolved, being maximally different from the P3 amplitudes evoked by competing evidence at the time of decision. Computational modelling showed that both the neural dynamics and behavioural primacy and recency effects can be explained by a combination of (a) competition between mutually inhibiting accumulators for the two categorical choice outcomes, and (b) a context-dependant urgency signal. In conditions where evidence was presented at a low rate, urgency increased faster than in conditions when evidence was very frequent. We also found that the readiness potential, a classic marker of endogenously initiated actions, was observed preceding movements in all conditions - even when those were strongly driven by external evidence.

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 participants' 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. Competing Interest Statement The authors have declared no competing interest.