Electrocochleography used as a clinical hearing test in difficult-to-test children.

Thirty difficult-to-test children have been tested with transtympanic ECoG. When possible, informal hearing tests and/or free-field testing were performed. Children in whom no action potential (AP) could be recorded were submitted to conventional radiography of the inner ear, and vestibular tests. The correlation between free-field test thresholds and AP "thresholds" was good, especially in subjects with relatively good hearing. Response amplitude increased, and latency decreased, with increasing frequency of the stimulus implying that different parts of the basilar membrane are stimulated according to the frequency of the stimulus. Input-output curves of response amplitude and latency were plotted, and three different types were distinguished. ECoG can contribute to the evaluation of peripheral hearing in difficult-to-test children, and vestibular tests should always be performed on a child with suspected deafness or sensorineural hearing loss. Conventional radiography of the inner ear, however, seems to be of little value.

Discrimination of frequency ramps in subjects with cochlear hearing loss.

The auditory sensitivity for detecting linear frequency sweeps of a continuous pure tone has been studied in ten young subjects with cochlear hearing loss. The mean thresholds were elevated by a factor of 2.8 as compared with a normal group over the whole range of ramp durations studied (10-500 msec). The results show that this elevation is most likely caused mainly by the cochlear lesion per se, other possible factors having only a minor effect. No clear correlations could be found between thresholds for frequency change and results of other pure tone audiometric tests. Such tests thus cannot predict a subject's frequency discrimination.

Thresholds for linear frequency ramps of a continuous pure tone.

The human auditory sensitivity in detecting linear frequency ramps of a continuous pure tone has been studied. It is shown that for short ramp durations (less than 200 msec) discrimination depends on the difference between base and plateau frequencies, the mean threshold being about 3 Hz at 1 kHz. For longer ramp durations (greater than 200 msec), discrimination was found to be based on detection of the actual frequency sweep. No significant difference was found between thresholds for upward and downward sweeps. Expressed in Hz, the threshold for frequency change was approximately constant for base frequencies up to 1 kHz, above which it increased, reaching approximately 14 Hz at 4 kHz. There was no significant difference in the threshold for frequency change from 40 to 80 dB HL but at 20 dB HL the threshold was significantly higher than at 40 dB HL. Intra-individual variation in thresholds was found to be smaller than inter-individual variation. The results are discussed in relation to previous frequency discrimination data, where either tone pulse pairs, continuous frequency modulation or frequency ramps were used as stimuli.

Slow evoked cortical responses to linear frequency ramps in subjects with cochlear lesions.

Slow evoked cortical potentials in response to linear frequency ramps of a continuous pure tone with a 1 kHz base frequency have been recorded from ten relatively young subjects with hearing loss of cochlear origin. At small frequency ramps, the N1-latency of their responses to the three ramp durations studied (20, 100, 500 msec) was significantly longer than those of a group with normal hearing. As the ramps are made larger, the difference between the latencies of the pathological group and the normal group becomes smaller; above a certain rate of frequency change, the latency of N1 becomes smaller in the pathological cases than in the normal group. This crossover occurs at frequency change rates around 1-3 kHz/sec. It is concluded that recording of evoked cortical responses to frequency ramps may provide an additional tool in the differential diagnosis of hearing disorders.

Reliability in warble-tone sound field audiometry.

Test-retest reliability of sound field audiometry has been evaluated by means of repeated determinations of hearing threshold levels for frequency-modulated tones on normal-hearing and hearing-impaired subjects. Two modulation frequencies, 5 and 20 Hz, and two frequency deviations, 4 and 25%, were used, with centre frequencies in the range from 250 to 8,000 Hz. No significant influence on the standard deviations for test-retest differences was found for any of the three parameters studied: modulation frequency, frequency deviation, and normal versus impaired hearing in the listeners. The standard deviation values obtained were smaller than those previously obtained for regular pure-tone threshold audiometry using headphones. They indicate a test-retest reliability allowing functional gain measurements of hearing aids on the real ear to be performed at least as accurately as insertion gain measurements at frequencies above 2 kHz. In the hearing-impaired listeners, some influence was found from modulation frequency and frequency deviation on the sound field hearing threshold levels obtained.

The influence of attack time and release time on speech intelligibility. A study of the effects of AGC on normal hearing and hearing impaired subjects.

Automatic gain control may be used in hearing aids intended for different types and degrees of hearing loss; to average the levels, to reduce the dynamics of speech or to set the maximum output level of the hearing aid without adding nonlinear distortion. Our study concerns hearing impaired listeners with recruitment. A group of normal hearing subjects was used as a control group. The compression was introduced over a laboratory unit. The compression ratio was set to 30:5 dB. A fixed ratio between the attack time and the release time, tr = 200 x ta, was used. The values used varied from 0.05 to 5 ms for ta and 10 - 1000 ms for tr. The speech material was nonsense syllables of the CVC type presented with S/N = 60 dB and S/N = 5 dB. With S/N = 5 dB a significant deterioration of the discrimination of the nonsense syllables was found at time constant combination ta = 5 ms and tr = 1000 ms. No statistically significant difference in discrimination was found between the different combinations of attack and release times for the S/N = 60 dB condition neither for the normal hearing nor for the hearing impaired group.

Thresholds for linear amplitude change of a continuous pure tone.

The human auditory sensitivity in detecting linear amplitude change of a continuous pure tone has been studied in normal-hearing subjects. It is shown that for short glide durations (less than 100 ms) the duration of the following plateau exerts a significant influence on the DLI. The average DLI at 1 kHz and 60 dB HL was found to be of the order of 0.8 dB when the intensity glide had a duration of 10 ms and was followed by a much longer plateau. For longer glide durations (greater than or equal to 200 ms) the DLI increased significantly as compared with shorter durations. There was no significant difference between increasing and decreasing intensity change. Significantly larger DLIs were found at 250 and 500 Hz than at 1, 2 and 4 kHz. The sound level was found to have a significant influence on the DLI. At low levels of 40 dB HL, and lower, the increase in DLI for detecting sound levels is highly significant. A falling exponential function offers a mathematical description of the relationship with good fit. It is concluded that an integrating mechanism with an integration time of approx. 200 ms could explain the auditory ability to detect linear amplitude glides of a continuous tone. The results are discussed in relation to previous intensity discrimination data, where pulse pairs, continuous intensity modulation or intensity glides were used as stimuli.

Slow evoked cortical responses to linear frequency ramps of a continuous pure tone.

Slow evoked cortical potentials from ten young normal-hearing subjects have been recorded as responses to linear frequency ramps of a continuous pure tone. Frequency changes from 10 to 500 Hz were studied; the rate of frequency change was varied from 0.02 to 50 kHz/s while the duration of the change was varied from 10 to 500 ms. The rate of frequency change was shown to have the greatest bearing on the responses except for frequency ramp durations below 50 ms and frequency changes below 50 Hz. The base frequencies (250-4000 Hz) and sound levels (20-80 dB HL) exerted an influence on the evoked responses that was qualitatively similar to their influence on behavioral thresholds. The direction of the frequency sweep had no significant influence on the evoked responses. A functional model is proposed in which the time derivate of the signal frequency is integrated with an adaptable integration time that is controlled by the rate of the frequency change.