Individual differences in the obligatory activation of addition facts

In two experiments, we found evidence for individual differences in the obligatory activation of addition facts. Subjects were required to verify the presence of a target digit (e.g., 4) in a previously presented pair (e.g., 5+4). Subjects rejected targets that formed the sum of the initial pair (e.g., 5+4 and 9) more slowly than they rejected unrelated targets (e.g., 5+4 and 7). This interference of the sum was largest for subjects who were relatively skilled at multi-digit arithmetic. Less skilled subjects did not show statistically significant effects of obligatory activation. In comparison with less skilled subjects, skilled subjects showed differential interference on plus-one (e.g., 3+1) and standard (e.g., 2+3) problems when the plus sign was presented, and on ties (e.g., 2 2) when the plus sign was omitted. These results suggest that network models of arithmetic fact retrieval are appropriate for skilled subjects, but that alternative models need to be considered for less skilled individuals.