EEG CORRELATES OF MENTAL-IMAGERY - AN AMP LITUDE AND COHERENCE STUDY

In 20 subjects of both gender we examined the question whether electro physiological correlates of mental imagery can be detected using ampli tude/coherence analysis. 40 concrete words denoting objects were prese nted to the subjects both visually and acoustically for 2 s each, with the only instruction of attentively perceiving the stimulus. The care ful interview afterwards revealed that in some subjects hearing or rea ding a word had provoked prompt, automatic and detailed mental images of the object, whereas in others not. This differentiation (visualizer s - non visualizers) was used to make a comparison between EEG changes elicited by visualization of a word and EEG changes elicited by perce iving the word without visualizing it. Since a subject with visual ima gery while reading the words not necessarily had visual imagery while hearing them, both groups are independent in this respect; thus differ ences in the EEG changes of visualizers versus non-visualizers for one task (visually) need not necessarily be the same in the other (acoust ically). If we nevertheless would find repeating differences, this wou ld strengthen the evidence that they reflect the process of visualizat ion. The EEG was recorded according to the 10/20 system with respect t o averaged signals of both ear lobes. Amplitude and coherence were com puted for the frequency ranges theta to beta 3; significant de- and in creases between each task (visually, acoustically) and a resting condi tion with eyes opened were mapped. Comparing the maps ''acoustically - eyes opened'' and ''visually - eyes opened'' reveals different EEG ch anges for the two modalities; dividing each task into two subgroups of ''visualizers'' and ''nonvisualizers'' reveals differences between th e groups for each of the tasks. We here only report about the differen ces between these two subgroups: 1.) During reading as well as during listening to words the group of subjects in which the words had elicit ed mental images in the mean shows higher frontal coherence increases than the control group (fig. 2). 2.) A statistical group comparison re veals significantly higher beta-amplitude in the occipital region in t he group of visualizers; this equally applies for both tasks (visually , acoustically) (fig.4. 3). beta 1-coherence in the group of visualize rs is significantly lower in the occipital region in both tasks (fig. 3). These results show that mental imagery is correlated with frontal as well as with occipital EEG changes and therefore support other find ings, demonstrating that mental visualization is partly evolved in vis ual areas and partly in frontal, association areas.