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Spectral analysis of human evoked potentials Davis, Alan Edwin


The purposes of this investigation were: (1) to apply spectral or frequency analysis to visual and auditory evoked potentials (EPs) recorded from the scalp; (2) to meaningfully relate the resulting frequency-domain descriptions to physiological factors such as speech dominance, stimulus modality and cortical area; (3) to determine if simple flash and click stimuli could generate left-to-right hemispheric differences, and if so; (4) to study various aspects of these differences, such as their relationship to handedness, speech dominance and stimulus modality. It was hypothesized that spectral analysis might detect EP differences that were not observable by measures of peak amplitudes and latencies, and that asymmetric responses could be generated by simple stimuli. Reviewed evidence suggested that verbal stimuli, such as speech or letters, were processed within the left, speech-dominant hemisphere, while non-verbal stimuli, such as geometric patterns or melodies, were processed within the right, speech non-dominant hemisphere. The results showed that EP amplitudes were largest over the specific projection cortex of the stimulated modality, but hemispheric differences of amplitudes were not related to the known speech dominance of epileptic patients. In contrast, coherence or similarity of form between pairs of EPs was related to speech dominance, and was greater over the speech-dominant hemisphere for click stimuli, and over the speech non-dominant hemisphere for flash stimuli. These results suggested that the amplitude of responses represented a bilateral cortical response to the sensory stimulus, while the lateralized, coherent spread represented perceptual processing or extraction of meaning from that stimulus.

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