Comparison of psychoacoustics and speech perception in individuals with auditory neuropathy and normal subjects

The main result of hearing impairment is reduction of speech perception. Patient with auditory neuropathy can hear but they can not understand. Their difficulties have been traced to timing related deficits, revealing the importance of the neural encoding of timing cues for understanding speech.

In the present study psychoacoustic perception (minimal noticeable difference in intensity, frequency and time), and word discrimination ability of young patients with auditory neuropathy was compared with normal controls.

This cross-sectional analytical study was performed on 16 young right-handed people with auditory neuropathy, (mean age 20±5.30 years), and on 28 subjects with normal hearing, (mean age 22±3.80 years). The minimal noticeable difference in sound intensity, frequency and time were measured by conducting "intensity discrimination'' and "gap detection" tests at supra-threshold levels and "frequency discrimination" test at different octave band frequencies in both groups. The word discrimination score was measured in silence and in three different signals to noise. Results of tests were compared between two groups.

Significant difference was shown between patients with auditory neuropathy and control subjects in gap detection (at all intensity levels), frequency discrimination (in all octave band frequencies) and intensity discrimination tests at all intensity levels, (p<0/003). The word discrimination score was decreased in the presence of noise in three different signals to noise ratios of zero, +5 and +10 dB in both groups, and there was a significant difference between results of word discrimination test in patients and controls in both silence and different levels of noise.

The results showed that people with auditory neuropathy have a deficit in temporal processing and poor frequency discrimination. Severe impairment in temporal processing is probably the major factor contributing to poor speech recognition in people with this disorder. This data not only demonstrates the importance of neural synchrony for auditory perception, but also provides guidance for better diagnosis and treatment of auditory neuropathy.