Hierarchical models of auditory processing often posit that optimal stimuli will increase in bandwidth and decrease in modulation rate as one ascends the auditory neuraxis. Here we tested how bandwidth and modulation rate interact at several loci along the human central auditory pathway using fMRI in a cardiac-gated sparse acquisition design. Participants listened passively to both narrowband and broadband carriers (1/4- or 4-octave pink noise) that were jittered about a mean sinusoidal amplitude modulation (sAM) rate of 0, 3, 29, or 57 Hz. The jittering was introduced to minimize stimulus-specific adaptation. The results revealed a clear difference between spectral bandwidth and temporal modulation rate: sensitivity to bandwidth (broadband > narrowband) decreased from subcortical structures to non-primary auditory cortex, while sensitivity to slow modulation rates was largest in non-primary auditory cortex and largely absent in subcortical structures. Further, there was no parametric interaction between bandwidth and modulation rate. These results challenge simple hierarchical models in that broadband stimuli evoked stronger responses in primary auditory cortex (and subcortical structures) rather than non-primary cortex. Furthermore, the strong preference for slow modulation rates in non-primary cortex demonstrates the compelling global sensitivity of auditory cortex to modulation rates that are dominant in the principal signals we process, e.g. speech.
- auditory system
- temporal modulation rate
- spectral bandwidth
- hierarchical organization
- Copyright © 2011, Journal of Neurophysiology