Increased vitamin plasma levels in Swedish military personnel treated with nutrients prior to automatic weapon training.

Noise-induced hearing loss (NIHL) is a significant clinical, social, and economic issue. The development of novel therapeutic agents to reduce NIHL will potentially benefit multiple very large noise-exposed populations. Oxidative stress has been identified as a significant contributor to noise-induced sensory cell death and NIHL, and several antioxidant strategies have now been suggested for potential translation to human subjects. One such strategy is a combination of beta-carotene, vitamins C and E, and magnesium, which has shown promise for protection against NIHL in rodent models, and is being evaluated in a series of international human clinical trials using temporary (military gunfire, audio player use) and permanent (stamping factory, military airbase) threshold shift models (NCT00808470). The noise exposures used in the recently completed Swedish military gunfire study described in this report did not, on average, result in measurable changes in auditory function using conventional pure-tone thresholds and distortion product otoacoustic emission (DPOAE) amplitudes as metrics. However, analysis of the plasma samples confirmed significant elevations in the bloodstream 2 hours after oral consumption of active clinical supplies, indicating the dose is realistic. The plasma outcomes are encouraging, but clinical acceptance of any novel therapeutic critically depends on demonstration that the agent reduces noise-induced threshold shift in randomized, placebo-controlled, prospective human clinical trials. Although this noise insult did not induce hearing loss, the trial design and study protocol can be applied to other populations exposed to different noise insults.

Two-state compression of spectral tilt: individual differences and psychoacoustical limitations to the benefit from compression.

A psychoacoustic rationale was developed for a hearing aid design in which compression of spectral tilt was incorporated without any instantaneous nonlinear distortion. This involved switching between a 'flat' and a 'rising' frequency response; the switching was done slowly to avoid audible transients and was controlled by feedback derived from comparison of output levels in low- and high-frequency channels, approximating voiced/unvoiced detection. The effect of this switching process was to narrow the distribution of spectral tilt values compared with the input. Asynchrony between the switching and the triggering speech structures was avoided by also delaying the signal path. Unfortunately, hearing-impaired listeners performed more poorly on the switching system than on either of the control 'flat' or 'rising' frequency-responses. An explanation is offered (on the basis of growing evidence from perceptual experiments) of the perceptual importance of temporal envelope contours within individual frequency bands. It was possible, in part, to predict individuals' results in the switching condition from age and audiometric or psychoacoustic characteristics. The results suggest a modification to the switching design, and they point to an intrinsic limit to the ability of all hearing aids of the compression type to enhance intelligibility.

Relations between speech intelligibility and psychoacoustical modulation transfer function (PMTF).

The modulation transfer function, MTF, has proved to be a powerful measure for predicting speech intelligibility in speech transmission channels. We extended it to include the ear, by measuring the psycho-acoustical MTF, i.e. the PMTF. Tone thresholds of 11 normal-hearing and 20 hearing-impaired subjects were measured in presence of unmodulated and intensity modulated noise. Octave frequencies from 500 to 4,000 Hz were used. The noise was octave filtered around the frequency of the probe tone. Six modulation frequencies from 1 to 50 Hz were used. From these results the PMTFs were calculated, as well as the corresponding psycho-acoustical speech transmission indices, i.e. the PSTIs. The subjects' speech discrimination scores in quiet and speech reception thresholds in noise were also measured. A correlation coefficient of 0.85 between the speech discrimination score in quiet and the PSTI was obtained. For the speech reception threshold in noise and the PSTI, the correlation was 0.71. The first of these two figures is promising, but our method needs some improvement, as it gave some problems due to fatigue effects.

Psychoacoustical and audiometric prediction of auditory disability for different frequency responses at listener-adjusted presentation levels.

Audiometric prediction of word identification scores has typically used one fixed presentation level for all subjects in the sample, with presentation in quiet and a wide range of hearing impairment among the listeners; under such conditions it is hardly surprising that moderate to good predictions are found. To see if prediction is possible under clinically relevant conditions, that is, on a homogeneous clinical sample of new hearing-aid candidates and to listener-adjusted levels, as would obtain in use of a hearing aid. In addition to audiometric variables, we employed a clinical approximation to the psychoacoustic tuning curve. We tested speech identification (FAAF) performance both with a 'rising'(+9 dB/octave) and with a 'flat' frequency response. Prediction of performance in the 'flat' condition was only good when a full set of audiometric frequencies entered the multiple-regression formula, each with its own weighting. Audiometric prediction for the 'rising' frequency response was particularly poor. Thus, the fairly good predictability from thresholds found traditionally for word identification scores or other disability measures appears to be a special case, depending partly on the wide range of hearing levels employed. Within our clinical sample the predictive power of formulae based on the mean of all thresholds or of mid-frequency thresholds alone (as used in compensation schemes) or on a priori combinations of thresholds (such as slopes) was generally poor. However, a three-parameter model taking account separately of low (0.25 kHz) and high-frequency (greater than 2.0 kHz) thresholds was effective. This and other audiometric descriptions were valuably supplemented by a psychoacoustic measure of frequency resolution at 2 kHz. In particular, such supplementation here allowed a satisfactory level of prediction to be achieved for speech heard with a +9 dB/octave frequency response, which the audiogram alone did not. The limitations of the prediction paradigm are discussed and several conceptual and statistical problems not previously emphasised in the audiological literature are illustrated in relation to the data.

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.

Frequency discrimination for bands of noise.

Recoding of speech in order to improve discrimination for profoundly deaf people has been tried in various ways. One of these is frequency transposition of voiceless consonants. Some authors report positive and very promising results. Others, however, have not found any significant difference when using control groups with normal amplification. One reason might be a poor frequency discrimination. Is it possible to establish what minimum hearing capacity the listener must show for a successful use of frequency transposition? Using noise bands with different Q factors and varying the levels, the thresholds as minimum midfrequency shift have been measured. Great individual variations have been shown. There is a significant difference between (1) threshold values at different Q factors, (2) threshold values at different frequencies, and (3) threshold values for increasing and decreasing frequency, position of deltaf. There is a significant interaction between (1) frequency and position of deltaf, (2) Q factor and position of deltaf, and (3) between Q factor and frequency. There is also a significant triple interaction between Q factor, frequency, and position of deltaf.