Brain Areas Activated during Electric Microstates of Mental Imagery versus Abstract Thinking

Microstates of EEG and ERP data are time epochs of quasi-stable map landscapes of momentary potential distribution, lasting <100 ms in duration. In study no. 1, during spontaneous mentation, subjects were prompted to report „what just went through your mind“. Two classes of reports were distinguished, mental (visual) imagery and abstract thought (e.g., „house“ vs. „theory“). The last EEG microstate before prompting was identified as imagery- or abstract-related. In study no. 2, while reading visually imaginable („house“) and abstract („theory“) nouns at 1/s, subjects had to repeat the last word if it was followed by a question mark. The ERP microstate 286–354 ms post-stimulus had significantly different potential maps for the two word classes. LORETA tomographic images were computed from the EEG and ERP potential maps. In both studies, maximum activation was for visual imagery right-posterior, for abstract thought left-anterior. Common across subjects for both studies, visual imagery-associated activity was right inferior-temporal in Brodmann areas 20, 36 and 37, for abstract thought left orbito-frontal in Brodmann areas 38 and 47. In both studies, subjects had to remember the last spontaneous mentation or presented word, but did not know that imagery vs. abstract mentation was studied. In sum, the identified brain areas were activated automatically without task to imagine or to formulate, and identical areas were active regardless of whether mentations were triggered by internal, spontaneous events (sudden ideas) or by external, imposed events (word display). The EEG and ERP microstates seem to be valid candidates for „atoms of thought“.