Neural coding of sound intensity and loudness in the human auditory system

Abstract

Inter-individual differences in loudness sensation of 45 young normal-hearing participants were employed to investigate how and at what stage of the auditory pathway perceived loudness, the perceptual correlate of sound intensity, is transformed into neural activation. Loudness sensation was assessed by categorical loudness scaling, a psychoacoustical scaling procedure, whereas neural activation in the auditory cortex, inferior colliculi, and medial geniculate bodies was investigated with functional magnetic resonance imaging (fMRI). We observed an almost linear increase of perceived loudness and percent signal change from baseline (PSC) in all examined stages of the upper auditory pathway. Across individuals, the slope of the underlying growth function for perceived loudness was significantly correlated with the slope of the growth function for the PSC in the auditory cortex, but not in subcortical structures. In conclusion, the fMRI correlate of neural activity in the auditory cortex as measured by the blood oxygen level-dependent effect appears to be more a linear reflection of subjective loudness sensation rather than a display of physical sound pressure level, as measured using a sound-level meter.

FIG. 1

Typical results of loudness scaling for two different hearing environments for one participant and its polynomial approximation of third degree. During the psychophysical measurements inside the MRI Scanner room, a 2.7-s gradient noise burst (TR = 15 s) preceded every 5-s pink-noise stimulus.

FIG. 2

A, B Probability map of the auditory masks to derive the percent signal change. An individual auditory mask consisted of those voxels that showed a significant (p < 0.05, FDR) sound-induced activation for any sound pressure level.

FIG. 3

General response characteristics of fMRI parameters derived from statistical parametric maps for 45 participants. A The group’s mean volume of activation within the predefined regions of interest as a function of sound pressure level on a logarithmic scale (p < 0.05, FDR). B Group’s mean percent signal change relative to baseline within individual auditory masks as a function of sound pressure level.

FIG. 4

Percent signal change relative to baseline plotted as a function of perceived categorical loudness for a fixed presentation level of 80 dB SPL.

FIG. 5

Distribution of the signal-to-noise ratio within subcortical and cortical regions of interest employed for the quality assessment of the PSC measurements in IC and HG.