Electrophysiological correlates of response time outliers: Outlier related potentials

In response time (RT) research, RTs which largely deviate from the RT distribution are considered "outliers". Outliers are typically excluded from RT analysis building upon the implicit assumption that cognitive processing is distorted in outlier trials. The present study aims to test this assumption by comparing cognitive processing indexed by event-related potentials (ERP) of trials with outliers and valid trials in two different tasks. To this end, we compared stimulus- and response-locked ERPs for outliers identified by nine different methods with valid trials, using cluster-based permutation tests. Consistently across outlier exclusion methods and tasks, the late positive complex (P3) associated with response-related processes was reduced in outliers. Analyses of response-locked ERPs related this P3 attenuation to a slower and temporally more extended increase of the P3, possibly indexing reduced evidence accumulation speed in outliers. P3 peak amplitude in response-locked ERPs was similar between outliers and valid trials, suggesting that the absolute amount of evidence required for a response remained comparable. Furthermore, in addition to these more general ERP correlates of outliers, the contingent negative variation (CNV) ERP component was reduced in outliers as a function of preparatory demands of the task. Hence, electrophysiological correlates, and thus cognitive processing, are altered in outliers compared to valid trials. In order to avoid distortion of observed ERP differences between conditions, the RT outlier distribution should be considered for the analysis of ERPs in combined ERP and RT studies.