Mental activity and the e.e.g.: Task and workload related effects

The e.e.g. was recorded from the left and right parietal and temporal regions of the scalps of 23 normal subjects while they were mentally at rest and then while they were executing tasks intended to engage, firstly, their dominant cerebral hemispheres, and secondly then their nondominant hemispheres. In each of the alpha and gamma frequency bands, 43 features of the e.e.g. expressed in terms of average power densities, coefficients of power density variation, coherencies and phases were calculated for each of the subjects. A method of separating the features which were related to the nature of the mental task from those which were related only to the mental workload represented by each of the tasks is presented. Of the features considered, many were found to be workload related, but only five, namely a power density variance, a right to left power ratio, an anterior to posterior phase angle and two coherency ratios, showed task-dependent behaviour. In this latter group, only two features point to the lateralisation of verbal and spatial cognitive processes to the dominant and nondominant cerebral hemispheres. The problem of the confounding of cerebral and mygenic effects and possible bias in the results owing to this effect is discussed.