Transient lowering of mental activity during performance of a mental task.

To objectively detect the lowering in mental activity during mental tasks, twenty-five male students were given the calculating tasks of adding or subtracting two 2-digit numbers at three grades of task load for 1 hr each. At any task load there was little variation in the percentage of correct responses in successive 5-min periods. There was no difference among the three task loads in the mood states of stress or arousal rated at the end of the task. Among the six bands in eight leads the amplitude of the beta-2 band in the Oz-A1 lead (Oz beta 2) showed the greatest fall corresponding to a drowsy state in one subject. It was judged from this pattern that mental activity might decrease sporadically in half of the subjects, irrespective of the task load. The mean Oz beta 2 amplitudes for all the task loads decreased with work time.