Evoked potential correlates of semantic meaning - A brain mapping study

According to the 'semantic differential technique' the affective meaning of words can be quantified in statistically defined, independent dimensions where every word is uniquely located on the three dimensions evaluation ('good-bad'), potency ('strong-weak'), and activity ('active-passive'). Two experiments were performed on a total of 52 adults: first, 162 nouns were rated by 30 subjects. All words had a comparable number of letters and frequency of occurrence in the German language. A factor analysis followed by varimax rotation on the ratings yielded three semantic dimensions, and for each dimension up to 20 words were selected which scored highly positive or highly negative on one of the three dimensions, and had small scores on the others. This resulted in six semantic word classes which were then used in electrophysiological experiments performed on another group of 22 healthy right-handed adults. Stimuli were presented sequentially on a computer monitor in a randomized order, and the EEG was recorded in 30 channels and continuously stored on hard disk. A checkerboard reversal stimulus was used in a control condition. Evoked potentials were computed off-line for each semantic class. Comparison of the factor structure revealed highly similar semantic dimensions and classification of all words used. in the electrophysiological data, specific brain activity occurred that was related to semantic processing. These components, however, showed distinctive differences to brain activity elicited by contrast reversing checkerboard patterns as was evident from significant differences in component latency, amplitude, and scalp topography. Significant differences in scalp topography, latency and field strength between semantic word classes were not restricted to late 'cognitive' components, but brain activity at small latencies was affected by semantic meaning of the stimuli. Our data show how visually evoked brain activity is modulated by the meaning of the stimuli at early processing stages without reflecting hemispheric differences. (C) 1998 Elsevier Science B.V.