Matched spatial filters for analysis of late ERP components during conflict resolution

In this event-related brain potentials (ERPs) study, we investigated the resolution of conflict by using the modified version of size-congruity comparison, a Stroop-like task in which numerical value and physical size were varied independently under task-relevant and -irrelevant conditions. The electrophysiological results demonstrated that the incongruent condition elicited greater parietal-distributed P300 than the incongruent condition for both numerical and physical comparison tasks. We further investigate the consistency of the congruency responses over all scalp electrodes to take into account the distribution of activity across the sensor array.

Matched-spatial filtering was applied over the time interval between 300 and 700ms. This technique is used to create spatial filters based on the difference between two conditions. By projecting the trial onto the corresponding spatial filters, surrogate single trials are created in which multi-sensor activity is reduced to one time series which exposes experimental effects. If the congruency effect appears in two or more non-contiguous regions, with potentially opposite signs this technique allow us to retrieve the full scale effect. Moreover we don’t have to intentionally select a “region of interest” treating equally all its channels even though some may carry much more signal than others.

Analysis of variance shows significant changes over spatially filtered signals in incongruent and congruent conditions. Three time intervals were analyzed further, and signals were superimposed in each time window to exclude influence of previous ERP. No significant difference was found for physical task after 500ms while numerical task showed more sustained differences between “congruent” and “incongruent” over time, maybe reflecting a prolonged cognitive workload. Reaction times portray a similar pattern.

Overall, we presented a method that reduces multi-variate time series data to a single time course by projecting the data onto a single vector that is chosen so as to reveal a given experimental effect. We found that spatially filtered signals retain the characteristics previously described in literature over single channels ERP “congruent” vs “incongruent” between 300 and 650ms. A longer-lasting difference demonstrated in the numerical task than in the physical task is compatible with the distinctive cognitive workload for resolving task relevant conflicts.