The brain functional microstate immediately before each of about 3000 identical tone stimuli was classified using extracted reference-free descriptors (locations of maximal and minimal potential) of the landscape of the brain's momentary electric field, in 8 volunteers. Six prestimulus microstate map classes occurred more than 30 times in each subject, and were clustered into two map class types (totals of 242 and 283 cases, respectively, on the average per subject). Event-related potential (ERP) map series were averaged for each subject and prestimulus map class. Map descriptors were extracted from the ERP maps at times of maximal Global Field Power during the component time windows N100, P200 and P330. Discriminant functions were estimated; for the maps of N100 and P330, the discriminant scores differed significantly between the maps associated with the two prestimulus map class types (paired t-tests, df = 7, p = .014 and p = .005, respectively). The dominant axis of the poststimulus class type II ERP maps deviated clockwise from that of the type I ERP maps in all components. We conclude that subtle changes in the brain's spontaneous momentary functional microstate (as classified by spatial descriptors of a single map) influence event-related information processing by the brain, following common rules over subjects.