Abstract: Problem size, split, parity and correctness of answer affect mental arithmetic problem-solving. Behavioral and cognitive neuroscience researches on mental arithmetic showed that there are different strategy selection in problem size effect, split effect, and parity effect. Moreover, these three factors can hardly be distinct from each other. Split and parity even have contradictive effect. These caused confusions in past researches. And also, few researches noticed the mix of the factors. How do people make their choice when these factors are mixed? One hypothesis is that split has priority compare to parity. Moreover, the electrophysiological feathers of parity effect remained unknown. There were also some problems of the presentation mode in past researches. The present study tried to give an answer with a new presentation mode.
High-density event-related potentials (Neuroscan) of 15 college students were recorded. The stimuli consisted of mathematic equations between 0 and 60. Each trial began with a fixation cross shown for 500 ms. 500ms later, the answer shown for 250ms. Another 500ms later, the equations appeared and remained on the screen for 3000 ms followed by the varied ITI of 800-1200ms. There were 6 blocks, which consisted of 110 trials. All the trials and blocks were randomized on-line. Repeated measures ANOVA was used for data analyze.
An arithmetic N400 was present for small incorrect problems. The parity congruent problems caused more significant N400 than parity incongruent problems in small split and small problems. A late positive wave was present and manipulated by split and parity for large problems. And the parity congruent problems elicited lower late positive slow wave than parity incongruent problems in small split and large problems. No significant difference emerged between parity congruent and incongruent problems in large split and large problems.
Problem size, split and parity are the important variables in mental arithmetic research. When processing with these factors, split has priority over parity both in small problems and large problems. Moreover, in small problem processing, the split and parity have effects on retrieval; while in large problem processing, the effect of parity appears in the condition that split information isn’t enough to produce a direct judgment.

Key words: strategy selection in mental arithmetic, problem size effect, split effect, parity effect, event- related potentials