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. 2012 Apr;107(8):2042-56.
doi: 10.1152/jn.00308.2011. Epub 2012 Feb 1.

Sensitivity to temporal modulation rate and spectral bandwidth in the human auditory system: fMRI evidence

Tobias Overath  1 , Yue Zhang, Dan H Sanes, David Poeppel
Affiliations

Sensitivity to temporal modulation rate and spectral bandwidth in the human auditory system: fMRI evidence

Tobias Overath et al. J Neurophysiol. 2012 Apr.

Abstract

Hierarchical models of auditory processing often posit that optimal stimuli, i.e., those eliciting a maximal neural response, 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 functional MRI in a cardiac-gated, sparse acquisition design. Participants listened passively to both narrowband (NB) and broadband (BB) carriers (1/4- or 4-octave pink noise), which were jittered about a mean sinusoidal amplitude modulation 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 (BB > NB) decreased from subcortical structures to nonprimary auditory cortex, whereas sensitivity to slow modulation rates was largest in nonprimary auditory cortex and largely absent in subcortical structures. Furthermore, there was no parametric interaction between bandwidth and modulation rate. These results challenge simple hierarchical models, in that BB stimuli evoked stronger responses in primary auditory cortex (and subcortical structures) rather than nonprimary cortex. Furthermore, the strong preference for slow modulation rates in nonprimary cortex demonstrates the compelling global sensitivity of auditory cortex to modulation rates that are dominant in the principal signals that we process, e.g., speech.

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Figures

Fig. 1.
Fig. 1.
A, Top: 3-s excerpt of a broadband (BB) stimulus with regular 3-Hz sinusoidal amplitude modulation (sAM). Bottom: 3-s excerpt of a BB stimulus with jittered mean modulation rate of 3 Hz sAM. Note the temporally jittered (variable) 1/2-period lengths of the individual modulations (cf. the dashed, vertical lines). Regular and jittered sAM stimuli were used in Study 1; in Study 2, only temporally jittered stimuli were used. B: normalized bar histogram distributions of the temporally jittered sAM stimuli with mean rates of 3, 29, and 57 Hz.
Fig. 2.
Fig. 2.
Activation-strength analysis in Study 1. A: cortical regions of interest (ROIs). The differently shaded gray bars plot contrast estimates [±90% confidence interval (CI)] of the 6 experimental conditions in the 30% probability map ROIs in bilateral Te1.0 (purple), Te1.1 (cyan), Te1.2 (orange), and planum temporale (PT; red). Sin, sinusoid; NB, narrowband; BB, broadband. B: subcortical ROIs. The differently shaded gray bars plot contrast estimates (±90% CI) of the 6 experimental conditions in the functionally defined ROIs in bilateral inferior colliculus (IC; green) and medial geniculate body (MGB; magenta).
Fig. 3.
Fig. 3.
Activation-extent analyses in Study 1. A: the bar plots display the overall mean activation extent (±SE) for the 6 experimental conditions in left and right auditory cortex, respectively. The overlay image in the middle depicts a visual combination of overall activation-extent and activation-strength analyses. Color code: white (Sin and NB and BB); green (Sin only); blue (NB only); red (BB only); black (bandwidth increase: Sin < NB < BB). Note that in areas coded in green (Sin only), blue (NB only), and red (BB only), the activation strengths among Sin, NB, and BB are not statistically different from each other; only the areas in black reveal a statistically stronger response with increasing spectral bandwidth. Results are superimposed on a tilted (pitch: −0.5) axial section of the 4 participants' average structural scans. B: mean percentage activation extent (±SE) in the cortical ROIs Te1.0 (purple), Te1.1 (green), Te1.2 (red), and PT (cyan) in the 6 experimental conditions.
Fig. 4.
Fig. 4.
Activation-strength analysis in Study 2. A: cortical ROIs. The differently shaded gray bars plot contrast estimates (±90% CI) of the 8 experimental conditions in the 30% probability map ROIs in bilateral Te1.0 (purple), Te1.1 (cyan), Te1.2 (orange), and PT (red). B: subcortical ROIs. The differently shaded gray bars plot contrast estimates (±90% CI) of the 8 experimental conditions in the functionally defined ROIs in bilateral IC (green) and MGB (magenta).
Fig. 5.
Fig. 5.
Activation-extent analyses in Study 2. A: the bar plots display the overall mean activation extent (±SE) for the 8 experimental conditions in left and right auditory cortex, respectively. The overlay image in the middle depicts a visual combination of overall activation-extent and activation-strength analyses. Color code: white (NB and BB); blue (NB only); red (BB only); black (BB > NB). Note that in areas coded in blue (NB only) and red (BB only), the activation strengths between NB and BB are not statistically different from each other; only the areas in black reveal a statistically stronger response to BB than to NB sounds. Results are superimposed on a tilted (pitch: −0.5) axial section of participants' average structural scans. B: mean percentage activation extent (±SE) in the cortical ROIs Te1.0 (purple), Te1.1 (green), Te1.2 (red), and PT (cyan) in the 8 experimental conditions.
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