DIFFERENT EVENT-RELATED PATTERNS OF GAMMA-BAND POWER IN BRAIN WAVES OF FAST-READING AND SLOW-READING SUBJECTS

Fast- and slow-reacting subjects exhibit different patterns of gamma-b and electroencephalogram (EEG) activity when responding as quickly as possible to auditory stimuli. This result appears to confirm long stan ding speculations of Wundt that fast- and slow-reacting subjects produ ce speeded reactions in different ways and demonstrates that analysis of event-related changes in the amplitude of EEG activity recorded fro m the human scalp can reveal information about event-related brain pro cesses unavailable using event-related potential measures. Time-varyin g spectral power in a selected (35- to 43-Hz) gamma frequency band was averaged across trials in two experimental conditions: passive listen ing and speeded reacting to binaural clicks, forming 40-Hz event-relat ed spectral responses. Factor analysis of between-subject event-relate d spectral response differences split subjects into two near equal gro ups composed of faster- and slower-reacting subjects. In faster-reacti ng subjects, 40-Hz power peaked near 200 ms and 400 ms poststimulus in the react condition, whereas in slower-reacting subjects, 40-Hz power just before stimulus delivery was larger in the react condition. Thes e group differences were preserved in separate averages of relatively long and short reaction-time epochs for each group. gamma-band (20-60 Hz)-filtered event related potential response averages did not differ between the two groups or conditions. Because of this and because gamm a-band power in the auditory event-related potential is small compared with the EEG, the observed event-related spectral response features m ust represent gamma-band EEG activity reliably induced by, but not pha se-locked to, experimental stimuli or events.