Neural basis of concurrent deliberation toward a choice and degree of confidence

Decision confidence plays a key role in flexible behavior, but exactly how and when it arises in the brain remains unclear. Theoretical accounts suggest that confidence can be inferred from the same evidence accumulation process that governs choice and response time (RT), implying that a provisional confidence assessment could be updated in parallel with decision formation. We tested this using a novel RT task in nonhuman primates that measures choice and confidence with a single eye movement on every trial. Monkey behavior was well fit by a 2D bounded accumulator model instantiating parallel processing of evidence, rejecting a serial model in which choice is resolved first followed by post-decision accumulation for confidence. Neural activity in area LIP reflected concurrent accumulation, exhibiting within-trial dynamics consistent with parallel updating at near zero time lag, and significant covariation in choice and confidence signals across the population. The results demonstrate that monkeys concurrently process a single stream of evidence to arrive at a choice and level of confidence, and illuminate a candidate neural mechanism for this ability.