| BackgroundMental arithmetic means arithmetic operation within the brain without external apparatus.Its mechanism may involve use the knowledge of quantity,order and rules. One of the classic results observed in symbolic comparison studies is the distance effect,which refers to the reaction time(RT) decreases while the difference between stimuli increases.The mental arithmetic has a effect of problem-size related to the distance effect,such as(12+27=?) more difficult than(2+3=?).The simple addition question may be in the form of the production task(X+Y =?) or the confirmed task (X+Y=Z,true or false?).Most mental arithmetic studies mainly use the number materials.Because of most adults have skilled in number mental arithmetic,so they can retrieve the answer from memories rather than calculate.It's difficult to use the number materials to reveal the real neural mechanisms of mental arithmetic with adults.The alphabetical mental arithmetic is not familiar with adult(the alphabet addition may be E+2=? or E+2=H,true or false?) is more suitable for study mental arithmetic with adults when they use the counting strategy.We apply alphabetical mental arithmetic to separate two types of mental arithmetic processing mechanism, retrieval and counting,when participants performed alphabetical order task use retrieval strategy and performed alphabetical counting task use counting strategy. Aims to isolate and reveal the neural mechanisms of two types of mental arithmetic strategies,we record event-related potential(ERP). MethodsSixteen students from the Southern Medical University were recruited, right-handed,aged from 22 to 28,with normal or corrected vision and without neurological and psychiatric disorders.Uppercase letters and three numbers(1,2,3) were selected as stimulate material. The gray reference number present in the middle and blue letters were present on both sides.The distance between letters is equal to the middle reference number or number added 1,a pairs of letters were ascending or descending order.Each participant performed two kinds of tasks.(1) The order task:one had to judge the order relation between the two letters:ascending or descending order.(2) The counting task:one had to judge if the distance between the letters(distance between adjacent two-letters as one unit) equal the reference number:true or false? The stimulus materials of both tasks were the same.The executive order of both tasks was arranged as:the order task, the ascending alphabetical counting task and the descending alphabetical counting task.RTs,correction rate(%) and EEG of 19 channels of 10-20 system were recorded simultaneously.EEG segments with artifact were rejected.The analysis time of ERP was from 100 ms before and 1000 ms after stimuli onset.A two-factor repeated-measures factorial design was adopted with the task factor (order,counting) and the distance factor(1,2,and 3).RTs and correction rate were analyzed with SPSS13.0.The analysis result of ERP was presented in the form of statistical parametric mapping(SPM) of F-value and t-value of two-sides with significance levelα=0.05.ResultThe reaction time of the counting task was significantly slower than that of the order task and the correction rate of the former was higher than the latter's. Behavioral performance of the counting task and the distance between letters increased simultaneously.Spatiotemporal patterns of ERP of both tasks were similar in early period(before 300 ms):activated frontal-parietal regions.But later(after 350 ms) the right prefrontal and the left hemisphere were significantly activated under the order task and the bilateral frontal and parietal-temporal-occipital network were specially activated under the counting task.The spatiotemporal patterns SPM(F) suggested the interaction effect of task×distance were significantly at the right frontal-parietal-temporal and central areas(860～1000 ms);the main effect of task was significantly at the right frontal, left occipital(100～300 ms) and bilateral frontal-parietal-occipital-temporal areas (350～1000 ms).The main effect of distance was significantly at bilateral frontal and left occipital(220～280 ms).The simple effect of the order task were significantly at left frontal(240～300 ms),right frontal-parietal(860～900 ms) and those of the counting task occurred in right frontal,left occipital(180～280 ms),right parietal-temporal(760～900 ms) respectively.Further,spatiotemporal patterns of SPM(t) suggested that no areas were activated increased as the distance between two letters increased under order task,while the left occipital and right parietal-temporal regions activated increased as distance between letters increased except the reference value 2 and 3.ConclusionThe more activation in the right frontal and left hemisphere for the order task suggested the right hemisphere retrieval and utilize knowledge of alphabetical order which stored in the left hemisphere;The frontoparietal network were significant activated in the counting task,suggesting that the executive mechanism of working memory involved in and the left occipital and right parietal-temporal(at 860～890 ms) reflects the problem-size effect.The participants use the retrieval strategy to perform the order task and use counting strategy to fulfill the counting task.
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