The role of the dorsolateral prefrontal cortex in random number generation: A study with positron emission tomography

Random number generation (RNG) engages a number of executive processes. We used positron emission tomography (PET) to measure regional cerebral blood flow (rCBF) in six volunteers who performed RNG and a control counting (COU NT) task at six rates paced by a tone. This provided a systematic variation of difficulty of RNG. Relative to COUNT, RNG was associated with significa nt activation of the left dorsolateral prefrontal cortex (DLPFC), the anter ior cingulate, the superior parietal cortex bilaterally, the right inferior frontal cortex, and the left and right cerebellar hemispheres. Faster rate s of RNG were associated with a significant decrease in regional cerebral b lood how (rCBF) in the left and right DLPFC and the right superior parietal cortex. rCBF in the left DLPFC was significantly and negatively associated with count score 1, a measure of habitual counting during RNG. These resul ts are discussed in relation to the network modulation model of RNG; develo ped on the basis of our previous studies using transcranial magnetic stimul ation and dual task paradigms. This suggests that during RNG;, suppression of habitual counting is achieved through the modulatory (inhibitory) influe nce of the left DLPFC over a number associative network distributed in the superior temporal cortex. At faster rates of RNG the synchronization demand s of paced RNG result in the breakdown of this modulatory influence, which is evident from decreased rCBF in the left DLPFC and increased habitual cou nting at faster rates. (C) 2000 Academic Press.