Temporal Modulation Transfer Function Through Analog and Digital Hearing Aids

Abstract

Temporal modulation transfer function (TMTF) is the representation of temporal resolution of the auditory system. Natural speech has amplitude variations in its envelope which needs to be properly decoded. Pathologies of cochlea as well as auditory nerve are reported to degrade the temporal resolution, in turn contributing to the poor speech perception observed in these individuals. Most of these individuals are rehabilitated through hearing aids for their reduced hearing sensitivity. Although there is a strong scientific evidence for improved hearing sensitivity with the hearing aids, just this may not be sufficient for perceiving all the cues of speech. An ideal aid should also efficiently enhance the temporal resolution which will result in better speech perception. Hence, the objectives of the study was to compare the TMTF between analog and digital hearing aids in normal hearing individuals as well as individuals with sensorineural hearing loss.A total of 21 adults were included in the study out of which 11 (20 ears) were individuals with sensorineural hearing loss (SNHL) and 10 (20 ears) were with normal hearing sensitivity. TMTF was estimated for 5 modulation frequencies; 8, 16, 32, 64, and 128 Hz, without and with hearing aids (analog and digital). The results showed reduced modulation sensitivity with the increase in modulation frequency in both the target groups and in all the experimental conditions. There was a significant difference between the TMTF of normal hearing individuals and individuals with SNHL and across 3 experimental conditions. The deficits in temporal resolution are attributed to the damage to the OHCs that leads to reduced frequency selectivity and broadening of the frequency tuning curves. The current hearing aids (analog and digital) distort the envelope of signal further adding to the already existing inherent distortions in the temporal resolutions in individuals with SNHL. Such distortions are higher in analog hearing aids. Hence, it is concluded that hearing aids with present technology does not fulfill all the requirements to enhance speech intelligibility to its best in individual with SNHL.