Spectrotemporal features of the auditory cortex: the activation in response to dynamic ripples

Functional MRI was used to investigate the characteristics of the human cerebral response to dynamic ripples. Dynamic ripples are sound stimuli containing regular spectrotemporal modulations, which are of major importance in speech processing; however, in contrast to speech, dynamic ripples can be characterized fully by a limited number of parameters. Extensive activation consisting of multiple separate regions was found bilaterally in the auditory cortex, particularly along the Heschl's gyri. This agrees with the presence of a structural cortical subdivision into functional fields. The level and the extent of activation were measured and correlated highly (R(2) = 0.97). Both measures depended strongly on the spectral density, temporal frequency, and amplitude of the modulations and matched the perceptual discernibility of the spectrotemporal modulations. The largest responses occurred for parameter values near the optimal human sensitivity. The drift direction of the modulations did not influence the activation. No quantitative differences were found between the two hemispheres. Average brain activation levels proved to be separable with regard to the spectral density and temporal frequency of the modulations. Topographic mappings of the modulation density and frequency onto the cortical surface were shown, approximately in posterolateral-to-anteromedial and lateral-to-medial directions, respectively. Posterolateral regions were most sensitive to spectrotemporal features at a scale similar to phonemes. Anteromedial regions, however, were also relatively sensitive to smaller scale acoustic features. This spatially dependent sensitivity suggests a functional topographic and hierarchical organization of the auditory cortex.