in two groups . The normal-hearing group (NH1) included 16 subjects, 10 men and 6 women, with. HTL
J Am Acad Audiol 10 : 198-210 (1999)
Auditory Temporal and Spectral Resolution in Normal and Impaired Hearing Birgitta Larsby* Stig Arlinger*
Abstract Temporal, spectral, and combined temporal-spectral resolution of hearing was assessed by recording masked hearing thresholds . The masker was an octave band noise. Spectral resolution was assessed by introducing a spectral gap of half an octave bandwidth in the masker. A 50-msec gap assessed temporal resolution . The spectral and temporal gaps were used separately or simultaneously . Normal-hearing and hearing-impaired subjects participated . For each masking condition, the subjects were tested at masker levels 50, 60, 70, and 80 dB SPL and at test-tone frequencies 0.5, 1, 2, and 4 kHz. Normal-hearing subjects showed reduced masking with spectral and temporal gaps . The combination of spectral and temporal gap reduced masking further. The release of masking was dependent upon the masker level. Hearing-impaired subjects showed less release of masking than normal-hearing subjects. The degree of hearing impairment was inversely related to release of masking. Reliability of the test procedure was assessed . Key Words: Combined temporal-spectral resolution, hearing, masker level, normal hearing, release of masking, sensorineural hearing impairment, spectral resolution, temporal resolution Abbreviations: HI =hearing impaired, MCL = most comfortable level, NH =normal hearing, OAD = obscure auditory dysfunction, RoM = release of masking
M
any studies have reported that measures of frequency and time resolution
deteriorate in cases of sensorineural hearing impairment (Arlinger and Dryselius, 1990 ; Schorn and Zwicker, 1990 ; Peters et al, 1998 ; for reviews, see Humes, 1983 ; Tyler, 1986). There is also evidence that temporal and spectral resolution can vary substantially among subjects with the same absolute hearing threshold level (Festen and Plomp, 1983). It has been suggested (Moore, 1985) that the impaired spectral and temporal resolution in cochlear hearing impairment are a major source of the difficulties experienced by these patients in attempting to understand speech, particularly in noisy situations. The fact that the first sign of a hearing
*Division of Technical Audiology, Department of Neuroscience and Locomotion, Linkoping University, Linkoping, Sweden Reprint requests : Birgitta Larsby, Department of Neuroscience and Locomotion, Division of Technical Audiology, University Hospital, S-581 85 Linkoping, Sweden
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impairment is often the report of difficulty in understanding speech in noise could indicate that the spectral and temporal resolution are impaired at an early stage in the development of a hearing impairment before any major effect is seen in peripheral sensitivity as measured in the pure-tone audiogram (HTLs within normal range) . Smoorenburg (1992) reported that a noise-induced hearing impairment of only 10 to 15 dB HL or more had a significant effect on the speech recognition threshold in noise. Also, among people with a normal pure-tone audiogram one can find persons with pronounced difficulties in understanding speech in noisy environments (Abel et al, 1989 ; Middelweerd et al, 1990). In patients with obscure auditory dysfunction (OAD), a self-reported auditory disability with no audiometric abnormality, Saunders and Haggard (1992) have reported that frequency resolution is one parameter that can distinguish between a normal and an OAD group of subjects . A test of auditory resolution might help to explain some of the differences in hearing per-
Auditory Resolution/Larsby and Arlinger
formance, both for normal-hearing (NH) and hearing-impaired (HI) subjects, that cannot be explained by loss in peripheral sensitivity (Glas-
poral resolution vary with frequency as well as with degree of hearing loss at that particular frequency. A test of auditory resolution should thus
ral resolution in a simple test procedure can
dependency for HI subjects is less well documented . A test of auditory resolution should also, if being complete, include evaluation over
berg and Moore, 1989) . There are many studies that report on a variety of methods to study auditory resolution . Most investigations have assessed either temporal or spectral resolution . A few have assessed both temporal and spectral resolution . It is possible that a study of combined spectral-tempo-
include evaluation at different frequencies . In NH subjects, both temporal and spectral resolution are known to vary with level of the masker (Saunders and Haggard, 1992 ; Cox and Alexander, 1993 ; Lindblad et al, 1993) . The level
give additional information about the interaction between these factors . Problems in one dimension might, for example, be compensated for in
a wide range of hearing levels . Thus, the following aims were formulated .
the other dimension . In a previous study (Larsby and Arlinger, 1998), we presented a method by which temporal, spectral, and combined spectral-temporal resolution could be quantified by recording masked hearing threshold levels of a pulsed pure tone . By introducing a spectral gap in the noise, a measure of spectral resolution was obtained . A temporal gap gave a measure of temporal resolution and a combined spectraltemporal gap yielded a combined resolution factor. The resolution was expressed as release of masking (i .e ., the difference in masked HTLs between the test without and with a noise gap) . The effect of gap sizes and age on release of masking from the different gaps was studied on NH subjects in two age groups . In the previous study, we found that release of masking increased with increasing gap size and that combining a spectral gap with a temporal gap gave more release of masking than that obtained by using each gap separately. An effect of age was seen, with more release of masking in the young NH group than in the old . The masking noise was a broadband noise with a spectrum having a slope of 3 dB/octave (pink noise) . When extending the use of this method to HI subjects, the wide-band noise with equal power per octave may be less useful since most hearing impairments involve a sloping hearing loss . This may result in widely different masking effects at different frequencies . To avoid this problem when investigating HI subjects, the earlier method has been modified by replacing the wide-band pink noise with an octave band noise as masker.
Auditory resolution parameters similar to those used in this study have been shown to vary with frequency for NH subjects (Jesteadt et al, 1982 ; Larsby and Arlinger, 1998). Whether this holds also for HI subjects is not yet known. Most HI subjects have a frequency-dependent hearing loss, and it is likely that the spectral and tem-
The aims of the present study were as follows: 1. To present a modified method to study spectral, temporal, and combined spectral-temporal resolution of hearing using an octave band masking noise at different frequencies and at different masking conditions . 2. To evaluate whether release of masking at different masking conditions depends on degree of hearing impairment, level of the masking noise, and test frequency. 3. To study if the release of masking is related to the hearing threshold level. 4. To determine test-retest accuracy for release of masking. MATERIAL AND METHOD Subjects A total of 59 subjects participated in the study.
The main experiment involved 32 persons in two groups . The normal-hearing group (NH1) included 16 subjects, 10 men and 6 women, with HTL
