Sounds in time: Cerebral organization of auditory processing in the temporal domain.

摘要

We have described a set of experiments examining temporal organization in the auditory system on several timescales, using several stimulus types, to gain a broad picture of the representation of temporal features of sound in human auditory cortex. Experiment one examined the common claim of left hemisphere specialization for speech based on temporal factors (Schwartz & Tallal, 1980, Zatorre et al., 2002), and how these patterns could be modulated by task demands. Subjects listened to words with or without rapid formant transitions, and performed either a semantic or a phonemic task. We found that in the context of whole-word natural language processing, the presence of rapid temporal change does not result in expected left-lateralized effects. When participants attended to sound analysis, stimulus effects became more likely, but were found in both hemispheres. Our results stress the importance of task demands in evaluating theories of auditory perception.;Experiment two used a novel stimulus in fMRI, repeated frozen noise, to examine periodotopic organization in auditory cortex for periods of 25, 50, 100, 200 and 400 ms. We found that primary auditory structures were sensitive to all periodicities, with each condition activating distinct additional areas in the temporal plane, and the most anterior areas preferentially sensitive to the longest periods. Despite the use of stimuli within timescales described elsewhere as preferentially driving hemispheric lateralization patterns (Poeppel, 2003), no effects of hemisphere were evident, suggesting that temporal hierarchical organization within hemisphere may be a more dominant structure than that across hemispheres.;In experiment three, we used a modified auditory oddball paradigm to find regions of auditory cortex that code for sequence processing for sequences of differing lengths (1, 3 or 6 tones). We found regions sensitive to coding stimulus deviance throughout the temporal lobes, bilaterally. Additionally, downstream areas (i.e. anterior temporal and sensorimotor regions) were increasingly recruited with increasing sequence length. Different areas were active for the three and six-tone conditions, with the six-tone sequence activating the most anterior regions. These spatial patterns, along with those from experiment two, suggest the possibility of a length-based organization for sound in the human temporal lobe.