It has been well established that specific anatomical areas of the superior temporal and inferior parietal cortex, including primary auditory cortex, adjacent auditory association cortex, such as Wernicke's area, and the planum temporale, play an important role in processing speech. The exact relationship between phonological processing of verbal information and the auditory structure has not yet been determined. Our long-term research plan is to seek evidence concerning the location and organization of the cortical regions that transform the acoustic representations of speech into phonological representations. The theoretical premise of our studies is the hypothesis that these transformations involve a spatial remapping of tonotopic representations into phonological representations within auditory cortices. As the first step towards our overall objective, this proposal will address three basic aspects of auditory processing in humans, using functional Magnetic Resonance Imaging (fMRI): 1) Initial experiments will use simple auditory stimuli (pure-tones) that vary along the frequency axis to define the tonotopic organization of human auditory cortex. In particular, we will seek evidence of additional tonotopic representations outside the primary auditory cortex, and will evaluate the spatial location and boundaries of these multiple tonotopic areas. 2) In contrast to pure tones, spatial activation patterns across cortex elicited by band-passed noise [BPN] bursts and frequency modulated [FM] sweeps, which are features common in human speech sounds, will be compared. This will allow a systematic investigation of how these cortical areas participate in the hierarchical processing of auditory signals. 3) Finally, using CV syllables and scrambled CV syllables that are spectrally identical, we will seek to delineate the acoustic features of speech processing and phonological aspects of auditory representations. The empirical bases for this project are our preliminary fMRI studies of auditory signal processing using various stimuli approaching human speech sounds in spectral and temporal aspects. Our studies demonstrated frequency-specific activation within multiple areas in human auditory cortex. We have shown that BPN and FM stimuli lead to more activation in higher order auditory processing areas than pure tones, and that secondary auditory cortical fields exhibit stronger activation in response to stimuli having phonetic properties of speech in contrast to spectrally identical stimuli lacking the temporal structure of speech sounds.
