The interaction of task-relevant and task-irrelevant stimulus features in the number/size congruency paradigm: An ERP study

We studied whether task-relevant numerical information and task-irrelevant physical size information interact during perceptual and/or response processing in the number/size congruency paradigm (NSCP). Participants decided which of two simultaneously presented numbers was larger numerically. The physical size of numbers delivered neutral, congruent, or incongruent information with numerical magnitude. Both stimulus- and response-locked event-related brain potentials (ERPs) were analyzed. The lateralized readiness potential (LRP) was used for indexing motor preparation. Similar early facilitation and interference effects appeared in the amplitude of ERPs between 150 and 250 ms after stimulus presentation, focused over parieto-occipital electrode-sites. We conclude that these effects reflected a similar process in both facilitation and interference, related to a general increase of processing load and/or conflict detection. Further, we have replicated our former findings demonstrating late facilitation and interference effects between 300 and 430 ms. These effects may be related to the conflict monitoring and response-selection activity of the anterior cingulate cortex, or may be related to higher level contextual analysis. Our findings suggest that facilitation and interference effects appear at multiple levels of stimulus and response processing. We have also demonstrated ERP amplitude effects as a function of numerical difference between the to-be-compared numbers both in stimulus- and response-locked ERPs.