Event-related brain potentials uncover activation dynamics in the lexicon of multiplication facts

The present study was designed to ascertain whether the cortical dynamics underlying multiplication fact retrieval can be modulated as a function of arithmetic relatedness. To this end, event-related brain potentials (ERPs) were recorded from 22 participants engaged in a number-matching task. The task was to decide whether a single probe number had been shown as part of a previously presented pair of cue numbers. Probes in the non-matching condition (50% of total trials) were either the product of the cue numbers (strong cue-probe association), the multiple of either cue numbers (weak cue-probe association), or an arithmetically neutral number with respect to the cue numbers (no cue-probe association). Behavioral data showed a clear interference effect (LeFevre interference), in that performance in the non-matching condition was significantly better when the probe number was arithmetically neutral compared to when it was arithmetically related to the cue digits either strongly (i.e., through the product) or weakly (i.e., through its neighbor in the multiplication table). LeFevre interference was not statistically different in magnitude for the two arithmetically related conditions. In contrast, ERPs allowed us to dissociate cortical processing dynamics for these latter conditions: whereas initially (250-350 ms) both product and neighbor probes evoked a relatively more pronounced positivity compared to neutral probes, in a later interval (350-450 ms, N400-like component), neighbors critically diverged from products, showing relatively more negative values, similar to those of neutral probes. The observed dissociation in ERP measures is interpreted as evidence of activation spreading in the network of multiplication facts, with the short-lasting response elicited by neighbors likely reflecting activation dissipating over time because of a weaker association to the cue digits.