Periodic appearance of theta rhythm in the frontal midline area during performance of a mental task.

The distinct theta rhythm in the frontal midline area during performance of a mental task has been called Fm theta. In the present study, periodicity in the appearance of Fm theta was investigated. Thirty male university students were requested to perform the Uchida-Kraepelin test (a test of continuous arithmetic addition) for 5 min continuously after a 2 min practice and 5 min rest. EEGs were recorded monopolarly from disc electrodes placed on Fz and Cz during the whole procedure. A 5 min distribution diagram of appearance of Fm theta was defined according to our criteria. Fm theta appeared in 19 out of 30 subjects. The total appearance time of Fm theta in each subject showed no correlation either to the amount of performed task or to the number of errors. In the subjects who showed a relatively small amount of Fm theta, it tended to appear mainly at 6 points in the 5 min distribution diagram: about 20, 60, 100, 200, 240 and 290 sec after the beginning of calculation. In contrast, Fm theta was almost uniformly distributed in those who showed a larger amount of Fm theta. These results suggest that appearance of Fm theta shows individual differences and there might exist some periodicity of an attention mechanism in certain subjects.