Auditory Cortex Activation by Infrasonic and Low-frequency Sound of Equalized Individual Loudness

Regular paper

Robert Kühler

Physikalisch-Technische Bundesanstalt

Wednesday 3 june, 2015, 14:00 - 14:20

0.9 Athens (118)

Categorical loudness scaling as standardized in ISO 16832 (2006) provides an easy and fast procedure to determine the loudness over the whole dynamic range of the auditory system. Within this method the loudness is rated by the subject on a scale with named categories, such as soft, moderate, loud, etc. In clinical audiology this method is widely used to assess the amount of recruitment in hearing impaired listeners. We extended this method to infrasonic and low frequencies in order to investigate the loudness perception for such frequencies. 30 otologically normal subjects performed the categorical loudness scaling procedure for monaural stimulation with pure tones between 8 and 125 Hz. Stimuli were presented by means of a newly designed insert-earphone sound source for infrasonic frequencies. The loudness functions showed a significant decrease of the dynamic range towards lower frequencies. To investigate the hearing mechanism for infrasonic frequencies more extensively, brain responses were measured by means of Magnetoencephalography (MEG). We investigated the variation of the M100 brain response stimulated by pure tones with decreasing frequencies from 250 Hz down to 8 Hz for 16 otologically normal subjects. The stimuli were ramped sine tones of equal individually perceived loudness determined by the categorical loudness scaling method described above. Magnetic brain responses were recorded by a Yokogawa MEG-system and averaged to obtain the M100 response. Subsequently, the positions of the underlying magnetic field generators were estimated offline, using a dipole fit routine. Stable responses were measured for stimulation frequencies between 20 and 250 Hz. At 8 and 12 Hz the results are difficult to interpret. This paper will present equal loudness contours derived from the loudness scaling data and objective magnetic brain responses for infrasonic and low frequencies.

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