[Two semantic brain systems for rapid and slow differentiation of the abstract and concrete words].

In the previous studies, semantic processing was repeatedly found to be reflected in the late (300-800 ms) components of event-related potentials (ERP). However, a human may catch the meaning of the words much earlier. In this work we investigated brain mechanisms of semantic analysis of abstract and concrete written words in four experimental conditions: (1) simple reading (a subjects had to read words to him/herself); (2) simple classification (a subjects had to classify presented words into abstract and concrete categories); (3) cued and (4) uncued selective classification conditions (a subject had to classify only the words typed in a particular color). In the simple reading condition, early (40-100 ms) differences between the ERPs to concrete and abstract words were found mostly in the right frontal areas. During simple classification, the early differences between the ERPs were observed in more dorsal areas. Differences in the late ERP components (450-700 ms) in the left frontal area were characteristic of all cases of explicitly classified words. The findings suggest the existence of two semantic systems: a rapid implicit system which involves the right frontal cortex and can be suppressed by task demands, and a slow explicit system of word classification involving, predominantly, the left frontal area.