Auditory fMRI of Sound Intensity and Loudness for Unilateral Stimulation

Abstract

We report a systematic exploration of the interrelation of sound intensity, ear of entry, individual loudness judgments, and brain activity across hemispheres, using auditory functional magnetic resonance imaging (fMRI). The stimuli employed were 4 kHz-bandpass filtered noise stimuli, presented monaurally to each ear at levels from 37 to 97 dB SPL. One diotic condition and a silence condition were included as control conditions. Normal hearing listeners completed a categorical loudness scaling procedure with similar stimuli before auditory fMRI was performed. The relationship between brain activity, as inferred from blood oxygenation level dependent (BOLD) contrasts, and both sound intensity and loudness estimates were analyzed by means of linear mixed effects models for various anatomically defined regions of interest in the ascending auditory pathway and in the cortex. The results indicate distinct functional differences between midbrain and cortical areas as well as between specific regions within auditory cortex, suggesting a systematic hierarchy in terms of lateralization and the representation of sensory stimulation and perception.

Representation of Instantaneous and Short-Term Loudness in the Human Cortex

“…(2) Of those transformations taking place in the cortex, where do these transformations take place? Some authors have put the first question differently, by asking: “at what stage or stages along the auditory pathway is sound intensity transformed into its perceptual correlate (i.e., loudness)?” (Behler and Uppenkamp, 2016 ). In our view, this question is not meaningful, since sound intensity is never directly represented in the auditory pathway.…”

Neuroimaging correlates of hearing loss, tinnitus, and hyperacusis

“…Our findings are consistent with these reports and indicate an increase of activation for both the frequencies affected and unaffected by the loss of auditory sensitivity, i.e., hearing loss. The relation between increased functional MRI response amplitudes and increased perceived loudness is well established (Hall et al, 2001;Harms and Melcher, 2002;Sigalovsky and Melcher, 2006;Langers et al, 2007;Behler and Uppenkamp, 2016). In line with patient reports that soft to moderately-intense stimuli are perceived as incredibly loud, the increased auditory cortex activation in hyperacusis presumably reflects the increase in perceived loudness.…”

“…In line with this, in normal-hearing listeners, increases in loudness result in increased midbrain activation (Harms and Melcher, 2002;Sigalovsky and Melcher, 2006), even when sound energy is constant. For the primary auditory cortex, it has been established that with increased loudness, increased activation is observed in both normal-hearing and hearing-impaired participants (Hall et al, 2001;Langers et al, 2007;Behler and Uppenkamp, 2016).…”

“…Consequently, the measured BOLD-response and the derived contrast between baseline and sound-evoked activation may be reduced (figure 6.1, A & B). However, there is evidence that in the human auditory cortex, sound intensities up to 96 dB SPL (Hart et al, 2002), 97 dB SPL (Behler and Uppenkamp, 2016), and up to 100 dB SPL (Langers et al, 2007) do not result in a saturation effect in participants with normal hearing and hearing loss. In our study, in only one participant, the stimulation at 8 kHz exceeded 100 dB SPL (i.e., 107 dB SPL).…”

“…The dB HL scale is used to describe hearing thresholds that are depicted with an audiogram. The perceptual correlate of sound intensity is the loudness of a sound, which is additionally affected by the bandwidth, frequency, and temporal characteristics of a sound (Fletcher and Munson, 1933;Behler and Uppenkamp, 2016). The subjective loudness of pure tones can be expressed with a phon scale, as is illustrated in figure 1.1.…”