Common Neural Choice Signals reflect Accumulated Evidence, not Confidence

While much research has investigated decision confidence, its neural underpinnings remain unclear. Although several studies have shown that centro-parietal EEG signals correlate with the reported level of confidence, according to recent computational work these signals reflect evidence which feeds into the computation of confidence, but they do not themselves track confidence. To test this prediction, we capitalized on our recent finding that a causal manipulation of prior beliefs selectively affects confidence, while leaving objective task performance unaffected. Using EEG recordings, we tested whether neural signals known to correlate with confidence (CPP and Pe) reflect evidence accumulation or confidence. Behaviorally, we replicated the finding that manipulating prior beliefs causally and selectively affected confidence without changing objective task performance. The EEG data showed a monotonic relation between the reported level of confidence and both CPP and Pe amplitudes. Importantly, this finding is compatible both with the theory that these signals track confidence as well as with the alternative theory that they track accumulated evidence. Critically, both neural signals were insensitive to the influence of prior beliefs on confidence, showing that they reflect the accumulated evidence that is used by the system to compute confidence, instead of directly reflecting confidence. Likewise, oscillatory activity in alpha and beta band was insensitive to the influence of prior beliefs on confidence. Decoding analyses revealed that the brain does hold shared representations for prior beliefs and confidence, and we identified a frontal signal that is sensitive to both confidence and prior beliefs. These findings are in line with recent views suggesting that the CPP and the Pe reflect evidence accumulation, which is combined with prior beliefs to form a confidence judgment.