Human auditory neuroimaging of intensity and loudness.

The physical intensity of a sound, usually expressed in dB on a logarithmic ratio scale, can easily be measured using technical equipment. Loudness is the perceptual correlate of sound intensity, and is usually determined by means of some sort of psychophysical scaling procedure. The interrelation of sound intensity and perceived loudness is still a matter of debate, and the physiological correlate of loudness perception in the human auditory pathway is not completely understood. Various studies indicate that the activation in human auditory cortex is more a representation of loudness sensation rather than of physical sound pressure level. This raises the questions (1), at what stage or stages in the ascending auditory pathway is the transformation of the physical stimulus into its perceptual correlate completed, and (2), to what extent other factors affecting individual loudness judgements might modulate the brain activation as registered by auditory neuroimaging. An overview is given about recent studies on the effects of sound intensity, duration, bandwidth and individual hearing status on the activation in the human auditory system, as measured by various approaches in auditory neuroimaging. This article is part of a Special Issue entitled Human Auditory Neuroimaging.

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

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.

Spectral loudness summation takes place in the primary auditory cortex.

Auditory functional magnetic resonance imaging (fMRI) was used to assess neural activation in the human auditory brainstem (AB) and cortex (AC) as a function of bandwidth (BW). We recorded brain activation of 22 normal hearing listeners induced by band pass filtered pink noise stimuli with equal sound pressure level of 70 dB SPL. Tested bandwidths were 50, 500, 1,500, 3,000, 6,000, and 8,000 Hz. The center frequency was 4,000 Hz. Categorical loudness scaling had been performed in a silent booth with all of these stimuli. Loudness as a function of bandwidth followed a concave-shaped curve which reflected the influence of spectral loudness summation (SLS) for higher BW and the influence of large amplitude fluctuations for very low BW, which itself could be explained by peak-listening. While neural activation of the AB, as measured by the percent signal change from baseline (PSC), was tuned to the physical BW of the stimuli in a straight linear fashion, the trend of perceived loudness as a function of BW was reflected in several aspects by corresponding neural activation in the primary auditory cortex (PAC). Finally, from the absolute differences of the PSC between PAC and AB, gains in perceived loudness associated with SLS and the effect of large amplitude fluctuations could be predicted with an accuracy of 1-2 dB for the whole group of participants.

An auditory fMRI correlate of impulsivity.

Impulsivity and serotonergic neurotransmission have previously been shown to be linked to the intensity dependence of auditory evoked potentials. The present study investigates whether impulsivity in normal healthy subjects has a similar influence on the neuronal correlates of the coding of sound intensity using functional magnetic resonance imaging (fMRI). Forty-four participants completed Cloninger's Tridimensional Personality Questionnaire (TPQ). The dependence of fMRI activation on sound intensity was examined using continuous pink noise with varying intensity as acoustic stimuli. Imaging data were analyzed for the volume of activation sensitive to sound intensity. Impulsivity has a significant effect on the volume of activation sensitive to sound intensity. Persons with high impulsivity scores on the TPQ scale show approximately twice the volume of activation when compared with persons with low impulsivity scores. The neuronal correlate of impulsivity as revealed by fMRI gives strong evidence of a link between impulsive behavior and neural activity evoked by auditory stimulation. This link may prove useful for measuring central serotonergic neurotransmission in a clinical setting.