Parallel processing of physical and lexical auditory information in humans.

We usually process auditory information at the physical level (e.g., pitch or side of presentation) and the formal level (e.g., semantic or syntax) simultaneously. However, most physiological studies investigate either the former or the latter level of processing. In this experiment, words belonging to three lexical categories (nouns, verbs, and adverbs) were randomly presented to the right or left ear. Participants were required to count all nouns presented on the relevant side. All side-relevant stimuli elicited two negativities between 100-250 and 450-650 ms post-stimulus. Nouns yielded more positive potential amplitudes between 200 and 700 ms than verbs and adverbs. This effect was observed for words presented in both relevant and irrelevant ear, i.e., it was independent of whether nouns were targets or not. After 600 ms post-stimulus, a P300-like wave was recorded to target nouns only. This component was maximal at left temporal sites. The data contradict the hierarchic processing hypothesis (first side selection, then target selection) and indicate parallel processing of physical and lexical information. Implications for the issue of language specificity of brain potentials are discussed.