Hemispheric specialization in mental arithmetic: Insights from functional transcranial Doppler Sonography

Understanding hemispheric specialization in arithmetic offers insights into the neural basis of numerical cognition. Previous research using functional transcranial Doppler Sonography (fTCD) suggested a left-lateralized pattern for multiplication and bilateral activation for subtraction, but these findings were limited by small samples and uncontrolled task difficulty. The present study aimed to replicate and extend these results using a substantially larger sample. Participants completed multiplication and subtraction tasks of varying difficulty while cerebral blood flow velocity (CBFV) in the middle cerebral arteries was recorded with fTCD. Results confirmed significant left-hemispheric activation for multiplication but not subtraction. Increased task difficulty also enhanced left-lateralization across both operations. These findings align with the Triple Code Model, which links multiplication to left-lateralized verbal memory retrieval and subtraction to bilateral magnitude processing. The increase in left-lateralization for complex tasks may reflect additional engagement of verbal working memory, such as subvocal rehearsal. Hemispheric asymmetries for both operations were positively correlated, suggesting shared neural resources. Overall, our study validates fTCD as a reliable method for assessing arithmetic lateralization and highlights the importance of task difficulty and individual variability. These findings advance understanding of arithmetic processing by demonstrating operation- and difficulty-dependent specialization and the role of domain-general resources.