AN ELECTROENCEPHALOGRAPHIC (EEG) STUDY OF HEMISPHERIC SPECIALIZATION BASED UPON AVERAGE EVOKED RESPONSE METHODS AND POWER SPECTRAL DENSITY METHODS OF SIGNAL ANALYSIS

Eighteen right-handed subjects (Ss) between 19 and 33 years of age participated in this EEG study of hemispheric specialization. Ss performed a battery of tasks in three phases which required a range of cognitive activity. The Phase I task involved passive reception of multiple presentations of four auditory stimuli (a pure tone, the vowel /a/, the stop consonant /b/, and a frequency and amplitude modulated tone sweep). Signal averaging was used to derive average evoked responses (AERs) to 28 presentations of each Phase I stimulus at four electrode locations (F(,3), F(,4); P(,3), P(,4); reference: linked ears). In Phase II, Ss performed two auditory attention tasks. In one task the stimuli were six different tones; in the other, they were the six stop consonant speech sounds. In both tasks Ss were instructed to attend to the stimuli and compare the sound following the appearance of a signal light to the one which preceded it and indicate by pressing a hand-held microswitch whether the sounds were the same or different. Five comparison trials were presented during each task; no EEG analysis was performed during the comparison trials. AERs to 34 stimulus presentations in each task were derived at the four electrode sites. In addition, frequency aspects of the EEG during Phase II tasks were examined by computing power spectra. All stimuli in Phases I and II were generated by computer, were randomly ordered, 300 milliseconds in duration, with an interstimulus interval randomized between 2, 3, and 4 seconds. Phase III was the most cognitively demanding Phase wherein Ss performed verbal, baseline, and spatial tasks. The verbal and spatial tasks were performed twice with an alternate form of each task used for the repetition. The baseline task was performed prior to each cognitive task. Items for the verbal task were nouns of low imagery and concreteness value while the spatial task involved mental rotation of geometric shapes. The baseline task elicited sensory and motor activity similar to that required by the verbal and spatial tasks, but did not require problem solving. Power spectra were computed during each of the Phase III tasks. Measures of the AER and power spectra data from the three phases were statistically analyzed one at a time using univariate analysis of variance with an Epsilon correction. These analyses showed many task and lead main effects in all three phases of the study, indicating that the Ss did have different EEG responses to the various stimuli and tasks. In Phase III there was a task by lead interaction in the alpha band (8-13 Hz) analysis of parietal activity in which the right parietal power was greater than the left parietal power during the verbal task. There was a difference in the same direction and of the same magnitude during the baseline task. During the spatial task the relative difference between right and left parietal leads was at its maximum, but its direction was counter to the view of the relation between the EEG and hemispheric specialization wherein one would predict relative alpha suppression in the right hemisphere during the performance of a spatial task. The results of this experiment are not consistent with the notion that the EEG reflects hemispheric specialization