Occipital midline EEG and subjective rating of task difficulty as indices of mental task strain.

Three measures of mental task strain, namely performance level, occipital midline beta-2 (Oz beta 2) amplitude and subjective rating of task difficulty, were taken in 34 healthy male students aged 19-24 years as they performed either a paced or a self-paced calculating task for 4 h. The performance levels were so constant throughout the work periods that they were of no use in evaluating mental task strain. The Oz beta 2 amplitudes increased significantly with the execution of both tasks, and a significant increase took place during the self-paced task work periods. From this, a critical value of Oz beta 2 amplitude for a heavy task strain was estimated to be about 3.5 microV (the difference between the values at work and at rest). The subjective ratings of task difficulty rose linearly with work time in both tasks. The subjective task strain attributable to work time was found to exceed the critical level long before the objective measure. In addition, other results suggest that the Oz beta 2 activity attributable to task execution is related to the capacity for achieving the task.