EEG CORRELATES OF MENTAL-IMAGERY - AN AMPLITUDE AND COHERENCE STUDY

In 20 subjects of both gender we examined the question whether electrophysiological correlates of mental imagery can be detected using amplitude/coherence analysis. 40 concrete words denoting objects were presented to the subjects both visually and acoustically for 2 s each, with the only instruction of attentively perceiving the stimulus. The careful interview afterwards revealed that in some subjects hearing or reading a word had provoked prompt, automatic and detailed mental images of the object, whereas in others not. This differentiation (visualizers - non visualizers) was used to make a comparison between EEG changes elicited by visualization of a word and EEG changes elicited by perceiving the word without visualizing it. Since a subject with visual imagery while reading the words not necessarily had visual imagery while hearing them, both groups are independent in this respect; thus differences in the EEG changes of visualizers versus non-visualizers for one task (visually) need not necessarily be the same in the other (acoustically). If we nevertheless would find repeating differences, this would strengthen the evidence that they reflect the process of visualization. The EEG was recorded according to the 10/20 system with respect to averaged signals of both ear lobes. Amplitude and coherence were computed for the frequency ranges theta to beta 3; significant de- and increases between each task (visually, acoustically) and a resting condition with eyes opened were mapped. Comparing the maps ''acoustically - eyes opened'' and ''visually - eyes opened'' reveals different EEG changes for the two modalities; dividing each task into two subgroups of ''visualizers'' and ''nonvisualizers'' reveals differences between the groups for each of the tasks. We here only report about the differences between these two subgroups: 1.) During reading as well as during listening to words the group of subjects in which the words had elicited mental images in the mean shows higher frontal coherence increases than the control group (fig. 2). 2.) A statistical group comparison reveals significantly higher beta-amplitude in the occipital region in the group of visualizers; this equally applies for both tasks (visually, acoustically) (fig.4. 3). beta 1-coherence in the group of visualizers 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 findings, demonstrating that mental visualization is partly evolved in visual areas and partly in frontal, association areas.