A Common Neural Signal of Evidence Accumulation for Perceptual and Mnemonic Decisions

Humans frequently make decisions based on sensory input from the external environment or information retrieved from memory. The centro-parietal positivity (CPP), an event-related EEG potential, has recently been identified as a neural correlate of sensory evidence accumulation during perceptual decision-making tasks. However, it remains unclear whether this component also reflects evidence accumulation during decisions based on long-term memory. The present study investigated whether the CPP serves as a domain-general signal of evidence accumulation across both perception and semantic memory-based decisions. We designed a visual discrimination task in which participants discriminated between the luminance of two alphanumeric strings across three levels of difficulty defined by the luminance difference. In a semantic memory task, participants discriminated between the populations of two different U.S. states, binned into three difficulty levels using US census data. After each decision, participants rated their confidence on a scale from 1 (very low) to 4 (very high). We observed a CPP component in both tasks, the slope of which scaled with the sensory or mnemonic evidence in both stimulus- and response-locked analyses. Furthermore, these CPP slopes were sensitive to reaction times (RT) and confidence in both tasks. Finally, the peak height of the CPP just prior to the response, indicative of ones decision boundary, was strongly correlated across tasks, suggesting that a common threshold was applied to the abstract evidence quantity being accumulated. Our results indicate that the CPP can be used to track the unfolding dynamics underlying decisions made both on the basis of external sensory evidence and internal mnemonic evidence.