Problem size effect and processing strategies in mental arithmetic.

We investigated the event-related brain potentials elicited by arithmetical operations whose solution requires direct memory retrieval or non-retrieval strategies. The problem size effect--the increment in reaction time for arithmetical problems with large operands--appears to be due to the selective use of non-retrieval procedures, and studies with event-related potentials have demonstrated an amplitude modulation of a late positive slow wave (range between 400-800 ms) related to the problem size effect. Two arithmetic operations (additions and subtractions) and three levels of problem size (adding or subtracting 2, 4 or 6) were used. We found an amplitude modulation of the late positive slow wave in subtractions, where non-retrieval procedures are mainly used. This amplitude modulation was not evident in additions, where direct retrieval strategies are believed to be used. Our results suggest that the problem size effect is related to non-retrieval procedures of calculation.