On hemispheric differences in evoked potentials to speech stimuli.

Eight subjects listened to lists of speech sounds (pa or ba) or pure tones (250 or 600 c/sec). Within each list one of the sounds (the "frequent") occurred more often than the other (the "target") in a ratio of approximately 4:1. Subjects were required to count the targets in each list; concurrently, evoked responses to both targets and frequents were being separately averaged from electrodes at vertex at symmetrical left and right parietal locations. The evoked responses show the expected sequence of deflections at all three electrode sites, including large P3 waves (about 350 msec latency) to the target stimuli. However, the left and right hemispheric responses to speech or tones, either frequent or target, were strikingly similar, both to the eye and by statistical tests intended to reveal differences between them.