Does the min-counting strategy for simple addition become automatized in educated adults? A behavioural and ERP study of the size congruency effect.

Recent studies have proposed that the sum-counting strategy for simple addition (i.e., count up of the summed value of the two operands one by one) used at early age becomes automatized in adults, challenging the long held view that skilled adults solve simple addition problems by fact retrieval. As arithmetic skill develops, however, the sum-counting strategy usually is replaced by a more advanced and efficient min-counting strategy (i.e., start counting at the value of the larger addend and count up by ones equal to the smaller or "min" addend). Thus, one would expect the min strategy, rather than the sum strategy, to become automatized if we assume automatic counting procedures exist. The present study sought evidence of the min-strategy in adults by investigating the size congruency effect (SCE) through behavioural and event related brain potential (ERP) experiments. The SCE is observed in number comparison tasks (e.g., identify the larger of two numbers), where RT is slower when the physical and numerical size of the numbers are incongruent compared to when they are congruent. The min-counting strategy inherently requires a number comparison stage, because the min and max number must be determined before the counting begins. Experiment 1 tested 72 participants on addition and number comparison tasks. The results showed a robust behavioural SCE for number comparison but not for simple addition. Experiment 2 tested 20 participants with a large number of addition and number comparison problems and recorded ERP. The behavioural results replicated the findings of Experiment 1. The ERP results revealed brain signatures in line with previous studies and the current behavioural findings. No SCE indicated the absence of a number comparison stage for addition; thus, the present findings ruled out the possibility of a fast min-counting strategy, or more generally a min strategy, for adults' simple addition.