Regular articleNeural correlates underlying mental calculation in abacus experts: a functional magnetic resonance imaging study
Introduction
To perform complex calculations, most people have relied on physical devices such as pencils and papers, mechanical calculators, slide rules, and more recently digital computers. One such device, gradually downplayed in the computer age, is an abacus, or “soroban” in Japanese. Interestingly, however, abacus experts not only manipulate the tool skillfully in its physical form but also gain the ability to mentally calculate extraordinarily large numbers—often more than 10 digits—with unusual speed and accuracy (Stigler, 1984). To achieve this level of skill, abacus players initially practice to simulate abacus operations in mind with actual finger movements, as if they could push imagined abacus beads (Fig. 1A). However, they eventually cease to use overt finger movements because it is believed that finger movements rather slow down calculation speed at higher levels of skill.
Mental arithmetic requires integration of multiple cognitive functions including number recognition, retrieval of arithmetic facts, temporary storage of intermediate results, and manipulation of mental representations. Much effort has been devoted to clarify cognitive mechanisms and underlying neural correlates of number recognition and arithmetic operations. Linguistic processing is suggested to play an essential role in exact mental calculation in adults on one hand (Dehaene et al., 1999); it is likely that visuospatial processing is also important, especially in the developmental stage, for acquiring mathematical concepts on the other (Simon, 1999). Recently, a neuroimaging study on a nonabacus calculation prodigy has indicated that expertise uses different brain regions from those for nonexperts (Pesenti et al., 2001).
The neural correlates of the calculation strategy employed by abacus experts remain unknown, although psychological studies have shown that visual strategy underlies this unusual calculation ability Hatano et al 1977, Hatano and Osawa 1983, Stigler 1984, Hatta and Ikeda 1988. In the present study, we investigated brain activity during mental calculation in six top-level Japanese abacus experts, using functional magnetic resonance imaging (fMRI). Based on previous psychological evidence Hatano et al 1977, Hatano and Osawa 1983, Stigler 1984, Hatta and Ikeda 1988, we hypothesized that the mental calculation of abacus experts might primarily depend on brain areas involved in visual, visuospatial, and visuomotor imagination rather than those involved in linguistic information processing.
To test this hypothesis, we used mental-operation tasks in which subjects serially and mentally updated mental representations according to external stimuli Hanakawa et al 2002, Sawamoto et al 2002. The mental-operation tasks had advantages for the study of mental calculation in abacus experts. First, no motor response was required during task periods. We considered this important because motor responses for behavioral reports would affect the activity of brain areas underlying visuomotor imagery. Previous neuroimaging studies on visuomotor imagery showed that the underlying neural correlates substantially overlapped with brain areas for high-order motor control Grezes and Decety 2001, Hanakawa et al. Second, the mental-operation tasks can investigate different mental representations (e.g., numeral, spatial, or verbal representations) by changing a rule associating a given stimulus with a requisite mental operation. Third, accuracy of task performance was appraisable after each task period. This process provided a behavioral observation, which was necessary for assessing experimental confounders such as task difficulty.
Section snippets
Subjects
The abacus-expert group consisted of one man and five women, ranging between 24–30 (mean ± SD = 28 ± 2.8) years old. All had been qualified as Japanese abacus masters with the highest degree (10th level) for both mental calculation and actual abacus operations. All had experienced greater than 17 years of almost daily abacus training for the purpose of competition. They were all right-handed with a laterality index (LI) of 0.8–1, except for one ambidextrous woman (LI = −0.2) (Oldfield, 1971).
Task performance
For the numeral mental-operation task, accuracy was statistically not different between the experts during six-digit number addition (87% correct) and the nonexperts during single-digit number addition (77% correct, P = 0.36 by U test). However, the abacus experts during the numeral task for the other number sizes (100% correct for both single- and three-digit numbers) demonstrated significantly superior performance in task accuracy over the nonexperts (P = 0.01). Accuracy was comparable
General discussion
The present study, for the first time to our knowledge, reported the neural correlates during mental calculation in abacus experts. The most significant finding was that the left posterior superior parietal cortex/precuneus, where controls also revealed some calculation-related activity, showed much enhanced activity during mental arithmetic of abacus experts. Additionally, there was no significant between-group difference in activity of this area for the two control tasks. We tested
Acknowledgements
We cordially thank the members of Hatano Shuzan-kyojo and Inokuma Shuzan-kan for their kind support. Dr. Seven P. Wise provided critical comments on an early version of the manuscript. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas (Advanced Brain Science) from the Japan Ministry of Education, Science, Sports, and Culture to H.S. (12210012) and to M.H. (15016113), and an NINDS Intramural Competitive Fellowship to T.H.
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