[Temporal summation in masking and speech recognition]

Temporal summation in masking has been measured simultaneously with a resolution of the masker's spectral structure to find psychoacoustic characteristics for estimation of speech intelligibility, to detect the manifestation of peripheral processes in auditory perception in humans. For this, detection thresholds of a test signal with different durations were determined. The test signal was pulse with a Gaussian envelope and a sine-wave carrier. It was presented simultaneously with a noise masker. The minimal pulse duration was inversely proportional to width of the critical bands of hearing, formed at the pulse's center frequency. The maximal pulse duration always was 50 ms. We adopted pulses with duration of 1-10 ms as a model of consonants and pulses with duration of 20-50 ms as a model of vowels. The band pass noises with rippled structure of the amplitude spectrum of two types were used as maskers. The central frequency of one masker coincided with a spectral hump and the central frequency of the other--with the spectral failure. If the pulses and maskers central frequencies were equal, the first and second maskers were called on- and - off- maskers. If the auditory system could discriminate the rippled structure of the masker's spectra, the difference in the detection thresholds of the pulses, which was presented with each of the maskers, was not equal to zero. The difference in the detection thresholds allows us to estimate resolution of the masker's spectra, i.e. frequency selectivity. Changes in the pulse's and masker's central frequencies allow us to evaluate the hearing ability in certain frequency domain. Changes in the masker's levels allow us to find influence of nonlinear dynamic properties of cochlea on temporal summation and frequency selectivity. This paper presents the results of measurements of temporal summation in masking, obtained in two frequency domains 2 and 4 kHz, in 4 subjects with normal hearing and in 1 subject with age-related hearing loss, who complained about deterioration of speech intelligibility. It has been found an increasing temporal summation and an improving the resolution of the rippled structure of the amplitude spectra for the maskers with average levels. We believe, the reasons could be found in the properties of the peripheral pulse coding, such as (1) the stabilization zones of excitation of the basilar.membrane, the range of the characteristic frequencies and the number of excited auditory nerve fibers and the number ofspikes, generated by fibers, due to the nonlinearity of the dynamic properties of the cochlea, (2) increasing synchronization of a reaction of the excited fibers and shortening in time of this reaction, due to thefiber's refractory properties.