Mental movements without magnitude? A study of spatial biases in symbolic arithmetic

McCrink (McCrink, Dehaene, & Dehaene-Lambertz (2007). Moving along the number line: Operational momentum in nonsymbolic arithmetic. Perception and Psychophysics, 69(8), 1324-1333) documented an "Operational Momentum" (OM) effect - overestimation of addition and underestimation of subtraction outcomes in non-symbolic (dot pattern) arithmetic. We investigated whether OM also occurs with Arabic number symbols. Participants pointed to number locations (1-9) on a visually given number line after computing them from addition or subtraction problems. Pointing was biased leftward after subtracting and rightward after adding, especially when the second operand was zero. The findings generalize OM to the spatial domain and to symbolic number processing. Alternative interpretations of our results are discussed.