Residual hearing capacity of severely hearing-impaired subjects.

Speech reception functions (speech reception threshold and maximum discrimination score for phonemes) and auditory functions (hearing threshold, difference limens for intensity and frequency, temporal modulation threshold function, critical ratio and temporal integration) have been investigated in a group of severely hearing-impaired subjects (64 ears, median Fletcher Index of 80 dB). The results were separated into low-, middle- and high-frequency regions. The investigations were intended to describe and quantify the residual hearing capacities and the correlations between the different functions. It was found that the speech functions, the difference limen for frequency, the critical ratio and the time constant of temporal integration deteriorate gradually with increasing hearing loss. These functions show a relatively high mutual correlation in the low-frequency region and considerable scatter in the high-frequency part. The sensitivity for modulations and the difference limen for intensity were least affected. The results revealed that the residual capacity can be described by means of two independent factors: frequency discrimination at the higher frequencies and a decrease in processing efficiency of the system. The latter is dominated by the middle- and low-frequency hearing elements.