Authors: Kumar Seluakumaran1*; Wan Nur Asyiqin Rasidi1
1Department of Physiology, Faculty of Medicine, University of Malaya
Background Early sensorineural hearing loss (SNHL) is typically attributed to cochlear outer hair cell (OHC) dysfunction, but other deficits including afferent nerve fiber (ANF) degeneration could be present. Along with a shift in hearing thresholds, these changes may also affect the sharp frequency tuning of the auditory system (frequency selectivity or FS). This study assessed whether a simplified FS measure could be used in clinical subjects with normal-hearing (NH) and mild SNHL to distinguish those with normal from abnormal FS, as they could indicate separate underlying conditions.
Method The FS measure was taken as the difference between two masked thresholds, one in a narrowband noise and another in a notched-noise masker with a similar spectrum level (40 dB SPL). FS data were obtained at five center frequencies in 51 subjects and plotted relative to the mean normative values available from a separate study to generate an ‘FS audiogram’ plot. Measurements were obtained using consumer-grade equipment as the goal was to develop an affordable FS testing method.
Results Sixteen subjects had NH (≤ 15 dB HL) while 27 subjects had slight-to-moderate SNHL (15-40 dB HL), mainly at the higher test frequencies. As expected, FS loss (> 2 SD of the normative mean) occurred mostly in SNHL subjects at the frequencies corresponding to their hearing loss. However, not all subjects with thresholds >15 dB HL had FS loss. The reverse was also true, where some NH subjects had abnormal FS.
Conclusion FS loss is expected to occur in SNHL subjects with OHC dysfunction as both cochlear FS and hearing sensitivity are dependent on the OHC micromechanical activity. However, normal FS found in a group of SNHL subjects suggests that the FS measure may not be always sensitive to mild OHC damage. On the other hand, FS loss in NH subjects could be attributed to selective loss of sharply-tuned low spontaneous rate ANFs as reported in hidden hearing loss animal models.