EEG Asymmetry During Covert Mental Activity and its Relationship with Hypnotizability

Abstract

Parieto-occipital EEG was recorded bilaterally while 20 high and 20 low hypnotizable Ss performed, in the eyes-closed condition, 2 covert right-hemisphere tasks (visual long-term memory and fantasy) and 2 covert left-hemisphere tasks (multiplication and verbal long-term memory). Ss were not, however, hypnotized during any aspect of the psychophysiological testing. After each task, Ss rated orally their degree of involvement in the tasks. The integrated amplitude alpha, the alpha density, and the alpha ratio as a measure of hemispheric asymmetry, were evaluated. Finally, the proportion of relatively greater right activation periods during right-hemisphere tasks minus the analogous proportion during left-hemisphere tasks was used as index of hemispheric specificity. The high hypnotizable Ss showed significantly higher alpha amplitude than the low hypnotizables; the alpha amplitude was correlated with hypnotizability, while the alpha density was not. The alpha amplitude of the right hemisphere during right-hemisphere tasks was significantly lower than the same amplitude during left-hemisphere tasks, while no significant differences related to task-type were detected in the left hemisphere. The pattern of task-effect on alpha ratio scores was the same as that on alpha amplitudes. Verbal and multiplication ratings were related to the alpha ratio, imaginative-visual memory ratings were not. No differences in hemispheric specificity between high and low hypnotizable Ss were found to exist, and no relationship between hypnotizability and hemispheric specificity was observed.