The present experimental design resembled that of our previous study (Ulrich et al., 2014). Major changes, however, applied to the length and number of blocks to meet the requirements imposed by the experimental realization as an fMRI block design to measure the blood oxygen level-dependent (BOLD) signal. Subjects performed visually presented mental arithmetic tasks. On each trial, a math expression appeared in black font above an on-screen keyboard in the center of the screen. Stimuli were presented on a white background on MRI compatible video goggles (VisuaStim Digital, Resonance Technology Inc., Northridge, CA, USA) at a resolution of 800 × 600 pixels. The on-screen keyboard was controlled by an MRI compatible trackball (NAtA TECHNOLOGIES, Coquitlam, Canada). Participants were asked to sum the presented two or more numbers in their mind and to enter the result as accurately and as fast as possible using the on-screen keyboard. After a period of 10 s per each math calculation, or when participants had submitted the result, there was a break of 1 s during which the string “xxx + x” appeared on the screen. Calculations and intermitting breaks were presented until the block length of 30 s had elapsed.
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