Function Of Sensation Level Research Articles

Discrimination of envelope statistics reveals evidence of sub-clinical hearing damage in a noise-exposed population with ‘normal’ hearing thresholds

Noise exposure can affect the functioning of inner hair cells (IHC) within the cochlea, leading to a ‘noisy’ representation of the properties of sounds even when absolute thresholds are normal. This may lead to a reduced ability to discriminate sounds based on their statistical properties, especially for narrowband sounds presented at low levels, since such sounds excite only a small proportion of the IHCs. This idea was tested by requiring subjects to discriminate a Gaussian noise from a ‘low-noise noise’ with minimal envelope fluctuations. The noises were centred at 2, 3, or 4kHz. The stimulus duration required for threshold was measured as a function of sensation level (SL) for two groups of normal-hearing subjects, one rarely exposed to high-noise events (control group, C) and one frequently exposed to high-noise events (experimental group, X). For group X, threshold consistently rose for SLs below about 20dB, while for group C the threshold did not consistently rise at low SLs (although it did for a minority of subjects). The worsening in performance with decreasing level was significantly greater for group X than for group C.

Reaction Time to 1- and 4-kHz Tones as a Function of Sensation Level in Listeners with Normal Hearing

Measures of reaction time (RT) near threshold have been used to indicate whether listeners with hearing losses of primarily cochlear origin experience greater loudness at elevated thresholds than at normal thresholds. These measurements have been based on the assumption that RTs near threshold are not affected by stimulus frequency in the 1- to 4-kHz range. The present study tests this hypothesis. To gain an understanding of how RT is affected by frequency, RTs to 1- and 4-kHz tones were measured in 16 listeners with normal hearing across a wide range of sensation levels (SLs). Statistical analyses indicate that RTs are affected by frequency in some listeners. This effect is most common at low SLs but is also present at higher SLs. Learning effects could not account for the observed differences between RTs at the two frequencies. Although reaction time-especially at low levels-is affected by stimulus frequency in some listeners, the effect is not large enough to account for the differences in RTs measured in all listeners with impaired hearing in other studies.

Discrimination of changes in the spectral shape of two-tone complexes

Discrimination experiments were performed for changes in the amplitude of two-tone complexes. Thresholds were measured, among other things, as a function of bandwidth and center frequency. Usually, the overall intensity was roved in each and every presentation to prevent subjects from using loudness cues. The results show that changes in the spectral shape of broadband two-tone complexes can be perceived with and without a roving intensity level. For narrow-band signals, attempts have been made to explain the results in terms of the EWAIF model [Feth, Percept. Psychophys. 15, 375-378 (1974)]. This model could account for the data for signal bandwidths of 1 semitone or less. Modified EWAIF models did not yield better predictions. Thresholds for a change in the spectral shape were measured as a function of sensation level. The "near-miss" to Weber's law was not observed. This result is consistent with excitation pattern models.

Auditory duration discrimination in primates

Auditory duration DLs at 2 kHz were measured in Old World monkeys (Macaca, Cercopithecus) and humans using a repeating standard AX procedure and positive reinforcement operant conditioning techniques. For a 200-ms standard tone, monkey DLs (45–62 ms) were about 2–3 times larger than human (15–27 ms), Weber fractions (ΔT/T) for all species were smallest at standard durations of 200 and 400 ms and increased with decreasing standard duration. DLs for all species did not vary as a function of sensation level from 30 to 60 dB, but were elevated slightly at 20 dB. Monkeys experienced difficulty in discriminating duration decrements, in contrast to humans. Results are related to other comparative psychoacoustic data and vocal communication. [Work supported by NSF and the Guggenheim Foundation.]

Intensity difference limens at high frequencies.

Thresholds for amplitude modulation detection were obtained from four subjects at frequencies of 2, 4, 6, 8, and 10 kHz for sensation levels of 15, 30, 45, and 60 dB and modulation rates of 2, 4, and 8 Hz. High-frequency difference limens calculated from amplitude modulation thresholds were found to change nonmonotonically as a function of sensation level, independent of modulation rate. This nonmonotonic relation stemmed mainly from a gradual reduction of the difference limen at the lowest sensation level with increasing frequency. Difference limens for pulsed tone discrimination were also measured in two of the subjects at 2, 6, and 10 kHz and sensation levels of 15, 30, 45, and 60 dB. The relation between intensity discrimination and sensation level was similar to that found for amplitude modulation detection. These findings are interpreted as indicating that the nonmonotonic relation between sensation level and intensity resolution is a general characteristic of stimulus processing at higher frequencies.

The effects of high‐frequency masking bands on midfrequency frequency discrimination

To evaluate the contribution of high‐frequency hearing during frequency discrimination at lower frequencies, an adaptive 4‐IFC pure‐tone frequency discrimination task was performed by two highly practiced normal‐hearing listeners at signal levels from 10 to 80 dB SPL, with and without high‐frequency band‐pass masking noises. The spectra of the high‐frequency masking noises ranged from 1.8 to 2.1 kHz, from 2.8 to 3.1 kHz, and from 4 to 8 kHz. The DLF‐intensity functions, describing DLFs as a function of sensation level, that were obtained in the presence of the high‐frequency masking noises were essentially the same as the DLF‐intensity functions obtained without masking noises. DLF‐intensity functions obtained from one of the two listeners were in close agreement with the results reported by Wier et al. [J. Acoust. Soc. Am. 61, 178–184 (1977)]. The functions from the other listener were similar in form, but exhibited consistently larger DLFs at all sensation levels. No practice effects were found throughout the investigation. [This research supported by NIH research grant NS12125.]

Loudness Summation with Binaural Hearing Aids

Hearing-impaired persons who had been fitted with binaural hearing aids performed a loudness matching task under binaural and monaural aided conditions. Loudness summation as a function of presentation level was determined for sensation levels of 10 dB, 20 dB, 30 dB and 40 dB. The results indicated a binaural advantage comparable to that which has been reported for normal-hearing persons. The extent of the advantage ranged from approximately 3 dB to 5 dB and tended to increase as a function of sensation level. This finding adds support to the view that the advantages of binaural hearing enjoyed by normal-hearing persons may also be available to the hearing-impaired while using binaural hearing aids.

Frequency Discrimination in Noise

Frequency DLs were obtained near masked threshold (1–15 dB SL) at 250, 1000, and 4000 Hz. Three spectrum levels (31.5, 51.5, and 71.5 dB) were used. An adaptive sequential procedure based on 4IFC was used in estimating ΔF. Also, two signal modes were used. In one, the pulsed tone presented during the “signal interval” was frequency modulated. In the other, the pulsed tone presented during the “signal interval” was a steady, but different, frequency. Characteristic improvement was seen at all frequencies when ΔF was plotted as a function of sensation level. At 4000 Hz, however, a significant “noise-level” effect was noted, in which discrimination at the 51.5-dB spectrum level (band level of 86 dB) was poorer than that for 31.5 and 71.5 dB. This “noise-level” effect was essentially the same for both modes of signal presentation. While this effect is difficult to explain in psychophysical terms, the literature on physiological acoustics offers some promise of accounting for it.

Frequency discrimination near masked threshold

Frequency DLs (Δf) at 1000 Hz were obtained in quiet and under masking conditions similar to those used in pitch-shift experiments, narrow-band noise at levels of 60, 80, and 100 dB SPL and tones at 15 dB SL or less. The Δfs were obtained by means of a tracking task in which the S controlled the input voltage to a frequency modulator. Characteristic improvement was seen when Δf was plotted as a function of sensation level. However, noise level itself was a significant factor, with more intense noise resulting in larger Δfs for tones of equal sensation level re masked threshold. This departure from previous findings is attributed to the signal and noise levels used, although the possibility exists that it is due to the use of modulated tones.

Frequency Discrimination in Noise

Frequency DL's at 1000 Hz were obtained in quiet and at three noise levels—60, 80, and 100 dB SPL. Sensation levels of 4, 6, and 10 dB SL were used at each noise level. The DL's were obtained by means of a tracking task in which the subject controlled the input voltage to a frequency modulator. Characteristic improvement is seen when frequency DL's are plotted as a function of sensation level. The curve for the DL's for 60-dB SPL noise runs parallel to the curve for quiet, with the noise DL's slightly greater. The curves for the DL's for the 80- and 100-dB SPL noises are less steep, and the average DL's are greater than the DL's for quiet by ratios of 1.5:1 and 2:1, respectively. [This research was supported by research grants from the National Institute of Neurological Diseases and Blindness, U. S. Public Health Service.]

Some Parameters of Contralateral Masking

A masking tone presented to one ear produces a threshold shift in the contralateral ear within a narrow frequency range surrounding the frequency of the masker. This is true when sound transmission across the head is effectively eliminated. The effect has been studied as a function of sensation level and duration of the masking tone for various frequency differences between the masking and the masked tones. Interaural sound transmission was minimized by using insert phones. In view of the large number of parameter values, von Békésy's tracking method has been found suitable for the investigation. The obtained data are interpreted in terms of neural excitation and inhibition patterns.

Just Noticeable Differences in Dichotic Phase

The just noticeable difference in dichotic phase, as a function of sensation level and of frequency, has been determined on a number of listeners with normal hearing. The test tones were transmitted by earphones, and the phase difference between the ears was varied by means of an electronic phase shifter. The psychophysical method used combined paired comparisons and forced choice. The first tone pulse of each pair presented was kept at a constant phase difference at which the subject localized the sound source as equidistant from his ears. The dichotic phase difference of the second pulse was varied irregularly (“randomly”). The results show that the sensitivity to dichotic phase difference is highest (2° of phase) at medium sensation levels, and that the jnd increases with positive acceleration as the sound frequency increases. Around 1300 cps the jnd becomes so great that it cannot be measured. The dichotic time difference calculated from the measured jnd in phase has a minimum near 800 cps.

Just Noticeable Dichotic Phase Difference

The just noticeable dichotic phase difference, as a function of sensation level and of sound frequency, has been determined on a number of listeners with normal hearing. The test tones were transmitted by earphones and the phase difference between the ears was varied by means of an electronic phase shifter. The psychophysical method used combined paired comparisons with forced choice. The first tone pulse of each pair presented was kept at a constant phase difference at which the subject localized the sound source as equidistant from his ears. The dichotic phase difference of the second pulse was varied irregularly (“randomly”). The results show that the sensitivity to dichotic phase difference is highest (2° of phase) at medium sensation levels and that the jnd increases with positive acceleration as the sound frequency increases. Around 1300 cps the jnd becomes so great that it cannot be measured. The dichotic time difference calculated from the measured jnd of phase has a minimum around 800 cps.