Neural coding of sound intensity and loudness in the human auditory cortex
Röhl M, Uppenkamp S
J Assoc Res Otolaryngol (2012); 13(3), 369-379
Neuronal processing of sound intensity and loudness in the human auditory cortex is still discussed controversially. In a large study with 45 normal hearing listeners, auditory fMRI was used to investigate whether the neural activity in the auditory cortex is more related to physical measures like sound intensity or perceptual quantities like individual loudness ratings. The functional dependence of activation size and activation magnitude, as measured by the number of activated voxels and voxel intensity, was examined. Individual loudness sensitivity was assessed by a categorical loudness scaling procedure inside the MRI scanner. The dependence of neural activation magnitudes on sound intensity and loudness was analyzed, using continuous pink noise at sound pressure levels up to a value just below the individual categorical rating of "very loud". The blood-oxygen level dependent (BOLD) signal grows almost linearly with sound intensity and linearly with categorical loudness in the auditory cortex over the whole range of presentation levels used. BOLD signal strength discriminates significantly between different categorical loudness sensations even at a fixed sound pressure level. Therefore, the neural activity in auditory cortex appears to be a direct linear reflection of subjective loudness sensation, rather than a display of sound pressure level.