A new measure for determining the degree of abnormality of psychophysical tuning curves in hearing-impaired subjects.

Psychophysical tuning curves were determined in hearing-impaired and normal-hearing subjects. Frequency selectivity, determined from the tuning curves of the hearing-impaired, decreased with increasing hearing impairment. As this may be attributed to the subjects' hearing impairment as well as to the inevitably higher test (probe)-tone level, we also collected psychophysical tuning curves from normal-hearing subjects as a function of test-tone level. Their frequency selectivity also decreased with test-tone level, but to a lesser degree. This indicates a real decrease in frequency selectivity in hearing-impaired subjects. Two measures were used to describe frequency selectivity, the d1oct and the newly introduced AALD. The latter is the average absolute difference in masker levels between the individual's psychophysical tuning curve and the average normal tuning curve, obtained at the same test-tone level. The AALD turned out to be a more sensitive tool for measuring pathological frequency selectivity than the d1oct.     

Tuning of the auditory brainstem OFF responses is complementary to tuning of the auditory brainstem ON response

Continuous masking studies show a complementary pattern of effects on the auditory brainstem responses (ABRs) which are generated by the onset and by the offset of a midfrequency tone. The masking profiles of the two responses are almost opposite with a probe stimulus frequency of 32 kHz (16-32 kHz is the midfrequency region for the CBA/J mouse). The Offset and Onset ABR tuning curves (TCs) also reveal very different properties at the midfrequencies of 16, 20, 24 and 32 kHz. The Offset TC is exquisitely sensitive to masking by very low intensity stimuli at a narrow range of frequencies which are lower than the probe stimulus frequency. Continuous masking produces a well-tuned low frequency tip to the Offset TC. For Offset TCs generated in response to midfrequency tones, the Q+10 dB of this tip averages 8.3. Masking at this low frequency tip of the Offset TC has no observable effect on the Onset ABR. The Offset ABR is also sensitive to masking by a narrow range of frequencies which are higher than the probe stimulus frequency. This occurs at an intensity which also has no observable effect on the Onset ABR. The Q+10 dB of this high frequency tip averages 9.2. The average frequencies where these Offset TC tips occur fit the cubic difference formula (2f1-f2), which describes a distortion product of two-tone suppression. At low probe stimulus frequencies, there is only a high frequency Offset TC tip; at high stimulus frequencies, only a low frequency tip. The high frequency tip has a higher threshold and appears more susceptible to metabolic disturbance. The Offset ABR TC also has a peak which corresponds to the probe stimulus frequency. Continuous masking with the stimulus frequency produces nonmonotonic enhancement of the Offset ABR, while it simultaneously reduces the magnitude of the Onset ABR. The tuning of this Offset TC peak (measured as Q-10 dB) is almost always much sharper than the corresponding Onset TC tip in the same mouse. These values for midfrequency stimuli average 6.2 for the Onset, and 13.6 for the Offset TCs. This fine tuning of the Offset TC at the probe stimulus frequency occurs at SPLs from 50 to more than 90 dB.     

Role of probe tone parameters in frequency selectivity for normal listeners

The objective of this research was a determination of the role of probe tone parameters in frequency selectivity. Frequency selectivity is the ability of the auditory system to analyze a complex sound into its frequency components, and is typically evaluated by measuring the psychophysical tuning curve. Generally this entails documenting the effect of varying the center frequency of a narrowband noise masker for a given pure tone probe, 10 dB above threshold. In the present study, a new approach was taken. A family of tuning curves were generated for a series of tone probes which differed in frequency, duration and intensity, but which remained constant in energy. The results for three normal-hearing subjects indicated that the critical parameter for the shape of the tuning curve was the energy of the probe. Decreasing the energy below values generally tested resulted in aberrant shapes across subjects, which bore some resemblance to those documented in hearing-impaired listeners. Temporal summation was suggested as the unifying principle.     

Frequency selectivity in normal-hearing listeners using a sweep, frequency-modulated masker: a preliminary report

It was the objective of this preliminary investigation to determine whether psychoacoustic tuning curves could be obtained on naive listeners utilizing a sweep, frequency-modulated masker. Psychoacoustic tuning curves were obtained on 10 normal-hearing young adult listeners. A simultaneous masking paradigm was utilized. The probe tone was a 2 kHz signal and the masker was a sweep, frequency-modulated tone between and including 750 Hz and 4 kHz. A mean Q value of 4.93 with a standard deviation of 0.85 was obtained from the subjects. Clinical implications and needs for further research are discussed.     

Brainstem, whole-nerve AP and single-fiber suppression in the gerbil: normative data

Suppression of gerbil brainstem responses (BSRs) and whole-nerve AP responses was studied by means of a forward masking procedure in which a tone-burst probe was preceded by a narrow-band masker. The effectiveness of the masker in reducing the brainstem response to the probe can be diminished by presenting a tone burst simultaneously with the masker. By varying the frequency and intensity of the third stimulus, BSR suppression areas can be determined. These flank the tails and high-frequency sides of BSR tuning curves in a manner similar to the suppression areas of AP tuning curves. The shapes and sizes of the BSR and AP suppression areas vary greatly across probe frequencies and animals. However, the lower boundaries of suppression areas associated with the tails of the tuning curves occur at similar absolute levels regardless of probe frequency or tuning curve shape. The BSR and AP suppression areas are in some respects similar to areas of two-tone rate suppression in single auditory nerve fibers of the gerbil.     

Frequency selectivity in patients with acoustic neuroma

Frequency selectivity was compared in subjects with hearing loss due to acoustic neuroma and cochlear pathology, and normal listeners. A particular interest was the role of probe tone parameters on the shape of the tuning curve. Psychophysical tuning curves (PTCs) were measured for each of two equal energy 2000-Hz probe tones (10 dB SL/300 msec and 17 dB SL/60 msec), using simultaneous 1/3-octave narrowband noise maskers centered at 1, 1.25, 1.6, 2.5, 3.15, and 4 kHz. The results showed that the critical masker levels obtained for impaired listeners were significantly greater than those from normal subjects. The slope of the low-frequency limb of the PTC was steeper for normal compared to hearing-impaired listeners but there was no difference due to site of lesion. In all three groups, the critical masker levels obtained with the short probe were significantly greater than those for the long probe, negating the hypothesis that equal energy probes would yield the same outcomes. Tuning in listeners with hearing loss was highly correlated with audiometric threshold but not with tumor size, width of the internal auditory canal, or tumor location within the cerebellopontine angle. The main conclusion was that cochlear and retrocochlear hearing loss are similar with respect to their effect on frequency selectivity.     

A comparison of AP and ABR tuning curves in the guinea pig

AP and ABR tuning curves were measured using a forward masking paradigm in guinea pigs with chronically implanted electrodes. Measurements were made before exposure to wide-band noise and at several intervals after exposure. The noise exposure was sufficient to produce temporary threshold shifts up to 60 dB lasting several days. Results showed similar reductions in Q10, tip-to-tail ratio and slope of simultaneously recorded AP and ABR tuning curves as a function of threshold shift following noise exposure. Tuning curves based on changes in response latency are presented as well as tuning curves based on changes in response amplitude. AP and ABR latency tuning curves showed similar form and changes with hearing loss as amplitude tuning curves. The similarities between AP and ABR tuning characteristics provide evidence that the ABR is sensitive enough to peripheral changes to be useful as a tool to study auditory frequency selectivity. The similarities between amplitude and latency tuning characteristics suggest that information regarding frequency selectivity of the auditory system can be obtained using response latency as well as response amplitude.     

Offset tuning curves generated by simultaneous masking are more finely tuned than those generated by forward masking

The rapid ending of a tone produces an evoked potential which has different properties than that which is produced by the sudden onset of a tone. At the level of the round window, the offset N1 N2 follows the ending of the cochlear microphonic (CM) by approximately the same amount of time as does the onset N1 N2 to the onset of the CM. Both onset and offset responses are abolished with cochlear lesion. Continuous masking was used to generate tuning curves (TCs) from the NI-PI component of the evoked potential recorded from the round window of the gerbil. Those evoked potentials generated in response to the tone onset were complementary in appearance to those generated in response to the tone offset. TCs generated by continuous masking of the NI-PII component of the auditory brainstem response (ABR) of the gerbil show the same pattern. When it is generated by simultaneous masking, the midfrequency offset TC in the gerbil and mouse is W-shaped. It has two well tuned tips which occur at frequencies below and above that of the probe stimulus used to generate the TC. It also has an even better tuned peak occurring at or slightly above the probe stimulus frequency, which becomes sharper as the masker sound pressure level (SPL) is increased from 50 to over 80 dB. Because the midfrequency onset response is approximately 40 dB lower than the midfrequency offset response, probe stimuli for onset TCs are generally set at lower SPLs. When the onset probe stimulus is set to the same level as that of the offset probe, the Q10 dB of the offset TC may be up to 10 times the value of the Q10 dB of the onset TC. The offset TC generated in the CBA/J mouse by forward masking is quite different from that produced by simultaneous masking. Both forward and simultaneous conditions utilized a 40 ms duration tone to mask the PI-NI component of offset and onset ABRs of the mouse which were evoked by a 10 ms duration, 32 kHz tone, presented at an interstimulus interval of 160 ms. Forward masking (when compared with simultaneous masking) resulted in a more sharply tuned onset TC. But the offset TC was much less sharply tuned in the forward masking condition. This suggests that the offset response may reflect functions which are involved with fine tuning at moderate to high intensities in the presence of simultaneous sounds of similar spectral characteristics.(ABSTRACT TRUNCATED AT 400 WORDS)     

P物质抗体对豚鼠耳蜗核下丘核团内听觉诱发电位调谐曲线的影响

探索Ｐ物质在听觉脑干中枢中对声信号的频率分析作用。方法采用短音刺激，短纯音前掩蔽法和豚鼠耳蜗核、正丘、核团内电极，记录ＣＮ及ＩＣ核团内听觉诱发电位。观察核团内注射微量Ｐ物质抗体或对照注射等量兔血清后ＣＮ和ＩＣ核团内听觉诱发电位调谐曲线的变化。     

Digital filtering and spectral analysis of the low intensity ABR

Spectral analysis along with zero and standard-phase shift digital filtering were performed on evoked potentials recorded from 12 normal hearing subjects. The results indicated a progressive shifting of the mean spectral content of the ABR toward the low frequencies as the stimulus intensity was lowered. Despite this, the effects of zero-phase shift high-pass digital filtering at 100 Hz (36 dB/oct) did not significantly differ between waveforms elicited by a 75 dB nHL, 55 dB nHL, and 35 dB nHL stimulus. The major response frequency of the ABR is related to the distance between the peak (IV/V) and the following major trough (approximates one-half the response period). In waveforms where the major trough occurred before 10 msec, the use of 100 Hz, 36 dB/oct, zero-phase shift high-pass filters produced only a small reduction in response amplitude, even at low stimulus intensity levels. Waveforms which had a major trough (Na1) between 10 to 15 msec were reduced in amplitude by 100 Hz, 36 dB/oct, zero-phase shift high-pass filters (the longer period of the response energy in these waveforms corresponds to a lower energy frequency). However, this trough has a latency that prevents it from being recorded on a 10 msec time base or defined as an ABR. Based on these results, the use of zero-phase shift high-pass filters with a high-pass cutoff frequency that is equal to or less than the resolution of the time base (1/time base) appears to be a desirable method of reducing muscle artifact and other electrical contamination of the ABR.(ABSTRACT TRUNCATED AT 250 WORDS)     

High-level psychophysical tuning curves: forward masking in normal-hearing and hearing-impaired listeners.

Forward-masked psychophysical tuning curves (PTCs) were obtained for 1000-Hz probe tones at multiple probe levels from one ear of 26 normal-hearing listeners and from 24 ears of 21 hearing-impaired listeners with cochlear hearing loss. Comparisons between normal-hearing and hearing-impaired PTCs were made at equivalent masker levels near the tips of PTCs. Comparisons were also made of PTC characteristics obtained by fitting each PTC with three straight-line segments using least-squares fitting procedures. Abnormal frequency resolution was revealed only as abnormal downward spread of masking. The low-frequency slopes of PTCs from hearing-impaired listeners were not different from those of normal-hearing listeners. That is, hearing-impaired listeners did not demonstrate abnormal upward spread of masking when equivalent masker levels were compared. Ten hearing-impaired ears demonstrated abnormally broad PTCs, due exclusively to reduced high-frequency slopes in their PTCs. This abnormal downward spread of masking was observed only in listeners with hearing losses greater than 40 dB HL. From these results, it would appear that some, but not all, cochlear hearing losses greater than 40 dB HL influence the sharp tuning capabilities usually associated with outer hair cell function.     

Growth of masking in sensorineural hearing loss.

The purpose of the present study was to measure the growth of masking in both normal-hearing and sensorineural hearing-impaired subjects. The masker was a narrow band of noise centered at 1,000 Hz, and masked thresholds for signal frequencies both above and below the masker frequency were obtained for a range of masker levels. For signal frequencies above the masker frequency, the slopes of the growth of masking functions were greater than 1 dB/dB for the normal-hearing group, while for the hearing-impaired subjects the slopes were less than those of the normal subjects, and in many cases slopes were less than 1. The slope of masking was inversely related to the threshold at the signal frequency. These data support the concept that a loss of nonlinearity at the signal place is responsible for the slower growth of masking in hearing-impaired subjects for signal frequencies greater than the masker. In addition, the slower-than-normal growth of masking of the hearing-impaired subjects suggests that some hearing aid signal-processing strategies which provide greater amounts of high-frequency emphasis at high input levels may not be appropriate.     

Patterns of residual masking

‘Residual masking’ was measured in a tonal forward masking paradigm. In one experiment, psychophysical tuning curves and masking patterns were obtained at several frequencies and levels for a fixed masker—probe time delay. In a second experiment, tuning curves and masking patterns were measured as masker-probe time delay was varied. Our results indicate that tuning curves and masking patterns are sharpest at low levels, high frequencies and brief masker-probe time delays. In addition, we observed that masked probe threshold returned to the level of unmasked probe threshold at approximately the same post-masker time regardless of masker level or the probe-to-masker frequency relationship. These findings suggest that frequency, level and time delay all affect the degree of frequency selectivity observed with these measures.     

Masking of short probe sounds by tone bursts with a sweeping frequency

The maske threshold of short ($\text{? 40}\text{ms}$) probe tone or a short one-third octave probe noise appears to increase if the frequency of a tonal masker is swept. Frequency sweeps were exponential (octaves/s) and unidirectional. Probe sounds were presented in the time center of the masker at the center frequency of the masker. The sweep speed, S, appeared to be an important parameter; masker duration was much less important. For 10-ms tonal probes, in-phase with the masker in their common time center, 100-ms maskers, and upward sweeps, increase of the masked threshold appeared to be maximal at a sweep speed of 30 oct/s and the masked threshold was 21 dB higher than the masked threshold found for the stationary masker (S = 0 oct/s). Above 30 oct/s the masked threshold decreased. For downward sweeps masking was maximal at S = 20 oct/s and the threshold was 15 dB higher. Sweeping upward the increase in masking was 12 dB for both noise probes and tonal probes with phase differing by 90° from the masker in their common time center. The results are inconsistent with current models of masking: sweeping the frequency the masked threshold increases whereas the energy within the critical band at the probe frequency decreases.     

Masking of tinnitus and central masking

In the first experiment reported here, for subjects with sensorineural hearing loss and tinnitus, the masking of tinnitus is primarily dependent on the masker intensity; masking is nearly independent of masker frequency. In the second experiment reported here, for subjects with normal hearing, the central masking of a continuous tone (used to stimulate the tinnitus) is primarily dependent on the intensity of a contralateral masker; masking is nearly independent of masker frequency. Implications of the flat tuning curves on the design of tinnitus maskers and one possible interpretation of the similarity of tinnitus masking and central masking are discussed.     

High-level psychophysical tuning curves: simultaneous masking by pure tones and 100-Hz-wide noise bands

Simultaneous-masked psychophysical tuning curves were obtained from normal-hearing listeners using low-level (20-25 dB SPL) probe tones in quiet and high-level (60 dB SPL) probe tones, both in quiet and in the presence of a broad-band background noise. The background noise was introduced to eliminate combination tones or combination bands and other off-frequency listening cues that exist at high levels. Tuning curves were obtained using pure-tone maskers and 100-Hz-wide narrow-band noise maskers for probe tones at 1000 and 4000 Hz. High-level tuning curves for pure-tone maskers demonstrated large discontinuities or "notches" on the low-frequency sides of the tuning curves. Broad-band background noise eliminated those notches, indicating that the notches were due to the detection of off-frequency listening cues at combination-tone frequencies. High-level tuning curves for 100-Hz-wide narrow-band maskers also demonstrated notches on the low-frequency sides. Those notches were eliminated with broad-band background noise, which indicates that combination bands strongly influenced the shapes of high-level tuning curves obtained with narrow-band maskers. The influence of combination bands was dependent upon test frequency. At 1000 Hz, combination bands had very little influence on the shapes of high-level tuning curves. At 4000 Hz, where the masker bandwidth was substantially less than the critical bandwidth, combination bands strongly affected the low-frequency sides of the tuning curves. In 2 subjects tested at a probe frequency of 2000 Hz with 100-Hz-wide masking bands, combination bands also influenced the low-frequency sides of high-level tuning curves. The presence of combination-tone or combination-band cues essentially steepened the low-frequency slopes of tuning curves, resulting in sharper estimates of tuning. Comparisons of tuning curves obtained with pure-tone maskers and narrow-band maskers, in the same listeners, revealed that pure-tone maskers were more effective than narrow-band maskers when the masker frequencies were in the tail region of the tuning curve. The results of these experiments support the notion that tuning in the normal auditory system broadens notably with stimulus level, once off-frequency listening cues such as combination tones or combination bands are eliminated. The low-level simultaneously masked tuning curve demonstrates a sharp bandpass tuning characteristic, whereas the high-level simultaneously masked tuning curve in background noise demonstrates a broad low-pass tuning characteristic.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interaction between excitation and inhibition affects frequency tuning curve,response size and latency of neurons in the auditory cortex of the big brown bat,Eptesicus fuscus

Neurons in the auditory cortex (AC) receive convergent excitatory and inhibitory inputs from the lower auditory nuclei. Interaction between these two opposing inputs shapes different response properties of AC neurons. In this study, we examined how this interaction might affect the frequency tuning curves (FTCs), number of impulses and latency of AC neurons in the big brown bat, Eptesicus fuscus, using a probe (excitatory tone) and a masker (inhibitory tone) under different stimulation conditions. Excitatory FTCs of AC neurons were either V-shaped, closed (i.e. upper threshold) or double-peaked. Inhibitory FTCs were obtained either at both flanks or only at the low or high flank of excitatory FTCs. Application of bicuculline, an antagonist for gamma-aminobutyric acid A receptors, produced expansion of excitatory FTCs into predrug inhibitory FTCs. Inhibition of probe-elicited responses occurred when a masker was presented at certain intertone intervals. Maximal inhibition typically took place when a masker was presented within 4 ms prior to the probe. During maximal inhibition, a neuron had the minimal number of impulses and the longest response latency. Inhibition became stronger with increasing masker intensity but became weaker with increasing intertone interval. Biological significance of these data is discussed.     

Quantifying psychoacoustic tuning curves for clinical use

Psychoacoustic tuning curves (PTCs) were obtained in a number of normal-hearing and hearing-impaired subjects using a simultaneous tone-on-tone masking technique. PTCs at three different centre frequencies (500, 1,000 and 2,000 Hz) were studied. In order to estimate the degree of frequency selectivity of the inner ear, an algorithm for quantification of PTCs was constructed. The method of linear regression was used to calculate the slopes of the linearized curves. Three different parameters of the curve, the low and high frequency slope and the height of the curve, were calculated using computer programs which allowed automatic evaluation of the data. The values of these parameters concerning normal and hearing-impaired subjects are presented in terms of scores and percentages of normal values. The algorithm for quantification of PTCs presented in this paper may in clinical work serve as a method of evaluating auditory frequency selectivity, particularly for very abnormal PTCs.     

Frequency selectivity of the middle latency response

A forward masking paradigm was used to assess the frequency selectivity of the middle latency response (MLR). Tuning curves of the MLR were obtained in unanesthetized gerbils. Changes in the amplitudes of MLR waves A, B, and C with latency values of 10 to 13 ms, 14 to 17 ms, and 20 to 25 ms, respectively, were analyzed as a function of masker frequency and intensity. Tuning curves of the MLR were also compared to tuning curves of the auditory brainstem response (ABR), which was recorded simultaneously with the MLR. The MLR and ABR differed in their response to forward masking. The MLR was reduced in amplitude or eliminated by masker stimuli that had minimal or no effect on the ABR. Forward masking often caused variable and non-monotonic changes in the amplitude of the MLR. Tuning curves of the MLR indicate that the MLR is less frequency selective than the ABR. The MLR is an electrophysiological measure of auditory function central to the auditory brainstem. Therefore, it may provide information concerning central components of normal and pathological auditory function. However, because of the variability of MLR amplitudes with forward masking, tuning curves of the MLR are difficult to obtain and are not efficient for routine measurements of frequency selectivity.     

Recovery of psychophysical tuning curves following noise exposure

Psychophysical tuning curves (PTCs) at 2 kHz and auditory thresholds (2 kHz and 4 kHz) were obtained from 18 normal-hearing listeners before and after exposure to a 5-min 110 dB SPL white noise. PTCs were quantified on five dimensions (Q10 tip, Q10 probe, d1oct, tip level and tip frequency). PTCs revealed continued cochlear effects beyond the time when TTS at 2 kHz demonstrated complete recovery.