Max Væhrens1, Rodrigo Ordoñez2
1Acoustics and Audio Technology, Aalborg University, Aalborg, Denmark
2Department of Electronic Systems, Aalborg University, Aalborg, Denmark

Loss of frequency selectivity can affect a person’s ability to understand speech in noisy environments. Therefore, being able to assess frequency resolution can be an important tool in the treatment of hearing disorders. In the present study two indirect measures of frequency selectivity are compared, sensitivity to spectrotemporal modulation (STM) and optimal ratio for distortion product oto-acoustic emissions (DPOAE).  Changes in STM modulation threshold between normal and impaired listeners have been correlated with speech intelligibility in noise and may represent deficiencies related to frequency selectivity and temporal fine structures in impaired hearing. Varying the ratio between the primary frequencies in a DPOAE measurement, an optimal primary ratio for the specific measurement parameters representing the highest DPOAE level can be obtained. The optimal DPOAE ratio decreases with increasing frequency similarly to the equivalent rectangular bandwidth for auditory filters. Changes in the optimal ratio or the lack of a clear maximum may indicate loss of frequency tuning in the basilar membrane (BM). STM sensitivity threshold and optimal DPOAE ratios were measured at test frequencies of 1 and 4 kHz on eleven volunteers with hearing considered normal. The results show no systematic relationships between the two measures for individual subjects. Based on the DPOAE results two groups were identified. Five subjects show well-defined optimal ratios and high DPOAE levels at both tested frequencies, and six subjects show low DPOAE levels at 4 kHz and no clear optimal ratio at 1 kHz. Comparison of the average STM sensitivity thresholds for these two groups only shows a statistical difference with the 4 kHz stimulus with a spectral modulation of 2 cycles per octave.