We investigated both stimulus-locked and response-locked activity because some cognitive processes are more tightly time-locked to stimulus onset and others to behavioral responses (Gerson et al., 2005; Makeig et al., 2004). We extracted 1000 ms stimulus-locked and response-locked epochs (with baseline removal on the 200 ms prior to stimulus) from both the 43-channel gradient-free dataset and the 34-electrode re-referenced dataset. By visual inspection we discarded trials containing motion or blink artifacts, evidenced by sudden high-amplitude deflections, and also those with incorrect responses, identically for both datasets. This left approximately 95% of the target trials remaining. Because the paradigm does not require a behavioral response to standard stimuli, we randomly assigned reaction times (RTs) to standard trials chosen from the probability distribution of target trial RTs (Goldman et al., 2009). This provided a baseline activity for response-locked single-trial EEG analysis, and it did not affect interpretation of results since we only viewed results derived from the variability within the target class.
