dB Sensation Level Research Articles

Stimulus-focused attention speeds up auditory processing

Stimulus-focused attention enhances the processing of auditory stimuli, which is indicated by enhanced neural activity. In situations where fast responses are required, attention may not only serve as a means to gain more information about the relevant stimulus, but it may provide a processing speed gain as well. In two experiments we investigated whether attentional focusing decreased the latency of the auditory N1 event related potential. In Experiment 1 slowly emerging, soft (20dB sensation level) sounds were presented in two conditions, in which participants performed a sound-detection task or watched a silent movie and ignored the sounds. N1 latency was shorter in the sound-detection task in comparison to the ignore condition. In Experiment 2 we investigated whether the attentional N1 latency-decrease was caused by a frequency-specific attentional preparation or not. To this end, tone sequences were presented with a single tone frequency or with four different frequencies. N1 latency was shorter in the sound-detection task in comparison to the ignore condition regardless the number of frequencies. These results suggest that stimulus-focused attention increases stimulus processing speed by generally increasing sensory gain.

Garlic-Supplemented Diet Attenuates Gentamicin-Induced Ototoxicity: An Experimental Study

Gentamicin sulfate is a potent aminoglycoside antibiotic associated with serious side effects, including ototoxicity. Garlic, with its intrinsic antioxidant activity, may prove beneficial in prevention of ototoxicity. In this study, we investigated the effect of a 4% garlic-supplemented diet on the ototoxicity induced by gentamicin in rats by using brain stem evoked response audiometry. Eighteen male Wistar rats with an intact Preyer's reflex and an initial weight of 220 to 260 g were randomly assigned to a group with gentamicin injection and garlic supplementation, a group with gentamicin injection without garlic supplementation, or a control group (6 rats each group). Gentamicin was given by intraperitoneal injection at 120 mg/kg body weight once daily for 16 days. The garlic-supplemented diet was prepared by adding pulverized whole garlic cloves to standard chow in a 4% proportion. After 21 days, hearing thresholds were evaluated by use of brain stem evoked response audiometry at 10 kHz. The mean (+/- SD) amplitudes of the auditory thresholds (sensation level) measured by use of brain stem evoked response audiometry for the group with garlic supplementation, the group without garlic, and the control group were 43.3 +/- 8.16, 78.0 +/- 4.47, and 16.7 +/- 5.16 dB sensation level, respectively. The differences were statistically significant (p < 0.05). A garlic-supplemented diet seems to attenuate aminoglycoside-induced hearing loss.

Assessment of Linearity of Bone-Conducted Ultrasound Transmission in the Human Head

We investigated the linearity of bone-conducted ultrasound transmission through a living human head. The frequency response functions and the coherence functions of head vibration at different signal levels were examined. Vibrations in the heads of six subjects were measured using a small accelerometer in the ear canal under bone-conducted tone stimulation from audible high frequencies to ultrasonic frequencies. Stepped sine signals from 10 to 45 kHz in 0.5 kHz steps at two different signal levels were used. The signal levels were set to the levels corresponding to 5 and 10 dB sensation level (SL) at 30 kHz in each subject. The result suggests that the head vibration increases almost linearly across an audible to ultrasonic frequency range and there was no significant difference in nonlinear behavior between the audible and ultrasonic sound transmissions.

Assessment of Linearity of Bone-Conducted Ultrasound Transmission in the Human Head

We investigated the linearity of bone-conducted ultrasound transmission through a living human head. The frequency response functions and the coherence functions of head vibration at different signal levels were examined. Vibrations in the heads of six subjects were measured using a small accelerometer in the ear canal under bone-conducted tone stimulation from audible high frequencies to ultrasonic frequencies. Stepped sine signals from 10 to 45 kHz in 0.5 kHz steps at two different signal levels were used. The signal levels were set to the levels corresponding to 5 and 10 dB sensation level (SL) at 30 kHz in each subject. The result suggests that the head vibration increases almost linearly across an audible to ultrasonic frequency range and there was no significant difference in nonlinear behavior between the audible and ultrasonic sound transmissions.

Growth of suppression in humans based on distortion-product otoacoustic emission measurements.

Distortion-product otoacoustic emissions (DPOAEs) were used to describe suppression growth in normal-hearing humans. Data were collected at eight f(2) frequencies ranging from 0.5 to 8 kHz for L(2) levels ranging from 10 to 60 dB sensation level. For each f(2) and L(2) combination, suppression was measured for nine or eleven suppressor frequencies (f(3)) whose levels varied from -20 to 85 dB sound pressure level (SPL). Suppression grew nearly linearly when f(3) ≈ f(2), grew more rapidly for f(3) < f(2), and grew more slowly for f(3) > f(2). These results are consistent with physiological and mechanical data from lower animals, as well as previous DPOAE data from humans, although no previous DPOAE study has described suppression growth for as wide a range of frequencies and levels. These trends were evident for all f(2) and L(2) combinations; however, some exceptions were noted. Specifically, suppression growth rate was less steep as a function of f(3) for f(2) frequencies ≤ 1 kHz. Thus, despite the qualitative similarities across frequency, there were quantitative differences related to f(2), suggesting that there may be subtle differences in suppression for frequencies above 1 kHz compared to frequencies below 1 kHz.

Distortion-product otoacoustic emission suppression tuning curves in humans.

Distortion-product otoacoustic emission (DPOAE) suppression data as a function of suppressor level (L(3)) for f(2) frequencies from 0.5 to 8 kHz and L(2) levels from 10 to 60 dB sensation level were used to construct suppression tuning curves (STCs). DPOAE levels in the presence of suppressors were converted into decrement versus L(3) functions, and the L(3) levels resulting in 3 dB decrements were derived by transformed linear regression. These L(3) levels were plotted as a function of f(3) to construct STCs. When f(3) is represented on an octave scale, STCs were similar in shape across f(2) frequency. These STCs were analyzed to provide estimates of gain (tip-to-tail difference) and tuning (Q(ERB)). Both gain and tuning decreased as L(2) increased, regardless of f(2), but the trend with f(2) was not monotonic. A roughly linear relation was observed between gain and tuning at each frequency, such that gain increased by 4-16 dB (mean ≈ 5 dB) for every unit increase in Q(ERB), although the pattern varied with frequency. These findings suggest consistent nonlinear processing across a wide frequency range in humans, although the nonlinear operation range is frequency dependent.

Mechanisms underlying the detection of frequency modulation

Frequency modulation detection limens (FMDLs) were measured for carrier frequencies (f(c)) of 1000, 4000, and 6000 Hz, using modulation frequencies (f(m)) of 2 and 10 Hz and levels of 20 and 60 dB sensation level (SL), both with and without random amplitude modulation (AM), applied in all intervals of a forced-choice trial. The AM was intended to disrupt excitation-pattern cues. At 60 dB SL, the deleterious effect of the AM was smaller for f(m) = 2 than for f(m) = 10 Hz for f(c) = 1000 and 4000 Hz, respectively, while for f(c) = 6000 Hz the deleterious effect was large and similar for the two values of f(m). This is consistent with the idea that, for f(c) below about 5000 Hz and f(m) = 2 Hz, frequency modulation can be detected via changes in phase locking over time. However, at 20 dB SL, the deleterious effect of the added AM for f(c) = 1000 and 4000 Hz was similar for the two values of f(m), while for f(c) = 6000 Hz, the deleterious effect of the AM was greater for f(m) = 10 than for f(m) = 2 Hz. It is suggested that, at low SLs, the auditory filters become relatively sharp and phase locking weakens, so that excitation-pattern cues influence FMDLs even for low f(c) and low f(m).

Perceptual interactions in the loudness of combined auditory and vibrotactile stimuli

The loudness of auditory (A), tactile (T), and auditory-tactile (A+T) stimuli was measured at supra-threshold levels. Auditory stimuli were pure tones presented binaurally through headphones; tactile stimuli were sinusoids delivered through a single-channel vibrator to the left middle fingertip. All stimuli were presented together with a broadband auditory noise. The A and T stimuli were presented at levels that were matched in loudness to that of the 200-Hz auditory tone at 25 dB sensation level. The 200-Hz auditory tone was then matched in loudness to various combinations of auditory and tactile stimuli (A+T), and purely auditory stimuli (A+A). The results indicate that the matched intensity of the 200-Hz auditory tone is less when the A+T and A+A stimuli are close together in frequency than when they are separated by an octave or more. This suggests that A+T integration may operate in a manner similar to that found in auditory critical band studies, further supporting a strong frequency relationship between the auditory and somatosensory systems.

Investigating possible mechanisms behind the effect of threshold fine structure on amplitude modulation perception

Detection thresholds for sinusoidal amplitude modulation at low levels are higher (worse) when the carrier of the signal falls in a region of high pure-tone sensitivity (a minimum of the fine structure of the threshold in quiet) than when it falls at a fine-structure maximum. This study explores possible mechanisms behind this phenomenon by measuring modulation detection thresholds as a function of modulation frequency (experiment 1) and of carrier level for tonal carriers (experiment 2) and for 32-Hz wide noise carriers (experiment 3). The carriers could either fall at a fine-structure minimum, a fine-structure maximum, or in a region without fine structure. Modulation frequencies varied between 8 Hz and one fine-structure cycle, and carrier levels varied between 7.5 and 37.5 dB sensation levels. A large part of the results can be explained by assuming a reduction in effective modulation depth by spontaneous otoacoustic emissions-or more generally cochlear resonances-that synchronize to the carrier at fine-structure minima. Beating between cochlear resonances and the stimulus ("monaural diplacusis") may hamper the detection task, but this cannot account for the whole effect.

Behavioral evidence for off-frequency compression at 4 kHz

Most behavioral techniques for estimating the basilar-membrane response rely on the assumption that the response to an off-frequency masker is linear at the signal place. To test this assumption, we modified the additivity of forward masking technique. The signal was a 4-kHz, 10-ms pure tone, presented at 10 dB sensation level. The maskers were 500-Hz-wide bands of noise centered on 1.5, 2, or 4 kHz. Masker 1 had a duration of 200 ms and was followed immediately by masker 2 with a duration of 20 ms. The masker 2-signal silent interval ranged from 10 to 60 ms. For each interval, the masker level required to mask the signal was determined, for each masker presented individually, and for the two maskers combined. In the combined case, the levels of the two maskers were set to be roughly equally effective, and adaptively varied together. Combining two equally effective maskers should produce a 3-dB reduction in masker level at threshold if the system is linear. A reduction greater than this indicates compression. Although compression was greatest for the 4-kHz maskers, compression was also observed at 1.5 and 2 kHz, suggesting that the assumption of a linear off-frequency response may not valid.

The Contributions of Audibility and Cognitive Factors to the Benefit Provided by Amplified Speech to Older Adults

In this review of recent studies from our laboratory at Indiana University, it is argued that audibility is the primary contributor to the speech-understanding difficulties of older adults in unaided listening, but that other factors, especially cognitive factors, emerge when the role of audibility has been minimized. The advantages and disadvantages of three basic approaches used in our laboratory to minimize the role of audibility are examined. The first of these made use of clinical fits of personal amplification devices, but generally failed to make the aided speech stimuli sufficiently audible for the listeners. As a result, hearing loss remained the predominant predictor of performance. The second approach made use of raised and spectrally shaped stimuli with identical shaping applied for all listeners. The third approach used spectrally shaped speech that ensured audibility (at least 10 dB sensation level) of the stimuli up to at least 4000 Hz for each individual listener. With few exceptions, the importance of cognitive factors was revealed once the speech stimuli were made sufficiently audible.

Temporal relationship between speech auditory brainstem responses and phonemes envelopes

Auditory brainstem responses evoked by synthetic phonemes (speech-ABR) were recorded on normal hearing subjects stimulated monaurally on the right ear at 60 dB sensation level. The speech-ABR consists of an onset response (OR) followed by the frequency following response (FFR), which results of the summation of phase locking underlying activities in the auditory cells of the brainstem nuclei. The FFR mimicked accurately the envelope of the stimulus, defined as the absolute value of the stimulus filtered at the cutoff frequency of the speech ABR spectrum. However, the FFR was more shifted relative to the vowel than the OR did relative to the onset of the stimulus, which is close to the cochlear delay usually reported. Little interindividual discrepancies were found. Delay values give information about the sites of generation of the speech-ABR. This method may help to objectivate the temporal encoding of phonemes in the brainstem. Hence the potential assessing of speech intelligibility, as temporal speech envelopes retain perfect intelligibility [R. Drullman, J. Acoust. Soc. Am. 98, 1796–1798 (1995)]. [Work supported by CNRS.]

The effect of interaural differences of time on speech intelligibility in simulated electric-acoustic hearing

Individuals with residual hearing restricted to frequencies below about 500–750 Hz have recently been implanted with a relatively short electrode array designed to preserve as much of the residual hearing as possible. Those who also have some low-frequency hearing in the non-implanted ear have the potential to benefit from interaural time differences (ITD). The goal of the current experiments was to determine whether such benefit exists in simulations of implant listening and how it is influenced by sensation level (SL) in the low frequencies. Presentation was via headphones. Target and distractor speech were low-pass filtered at 500 Hz. The target was presented diotically and the distractor was presented with a 0-s (spatially centered image) or 600-s (image to the right) ITD. The target and distractor were also processed by a four-channel vocoder and presented to the left ear only. The SL of the low-frequency speech was varied both symmetrically and asymmetrically. In the symmetric conditions (15–60 dB SL), intelligibility improved by as much as 40% when the target and distractor were spatially separated. In the asymmetric conditions (60 dB in one ear, 20–50 dB in the other), approximately 15%–20% improvement was observed in all conditions. [Work supported by NIH and NOHR.]

Frequency and amplitude discrimination along the kinesthetic-cutaneous continuum in the presence of masking stimuli

Frequency and amplitude discrimination thresholds along the kinesthetic to cutaneous continuum were evaluated on the left index fingerpad using a multifinger tactual display. Target stimuli were presented either in isolation (no-masker condition) or in the presence of masking stimuli (one- or two-masker conditions). Six reference target signals in the frequency range 2-300 Hz (two each from low-, medium-, and high-frequency regions) and at an amplitude of either 20 or 35 dB sensation levels (SL) were used. In the no-masker condition, the range of frequency Weber fraction was 0.13-0.38 and 0.14-0.28, and the range of amplitude discrimination threshold was 1.82-2.98 dB and 1.65-2.71 dB, at 20 and 35 dB SL, respectively. In the masking conditions, average frequency Weber fractions rose to 0.60 and 0.46, and average amplitude thresholds rose to 3.63 and 3.72 dB, at 20 and 35 dB SL, respectively. In general, thresholds were largest in the two-masker condition and lowest in the no-masker condition. Although the frequency and amplitude thresholds generally increased in the presence of masking stimuli, there was some indication of channel independence for low- and high-frequency target stimuli. The implications of the results for tactual communication of speech are discussed.

Alteration of Frequency Range for Binaural Beats in Acute Low-Tone Hearing Loss

The effect of acute low-tone sensorineural hearing loss (ALHL) on the interaural frequency difference (IFD) required for perception of binaural beats (BBs) was investigated in 12 patients with unilateral ALHL and 7 patients in whom ALHL had lessened. A continuous pure tone of 30 dB sensation level at 250 Hz was presented to the contralateral, normal-hearing ear. The presence of BBs was determined by a subjective yes-no procedure as the frequency of a loudness-balanced test tone was gradually adjusted around 250 Hz in the affected ear. The frequency range in which no BBs were perceived (FRNB) was significantly wider in the patients with ALHL than in the controls, and FRNBs became narrower in the recovered ALHL group. Specifically, detection of slow BBs with a small IFD was impaired in this limited (10 s) observation period. The significant correlation between the hearing level at 250 Hz and FRNBs suggests that FRNBs represent the degree of cochlear damage caused by ALHL.

An autocorrelation model with place dependence to account for the effect of harmonic number on fundamental frequency discrimination

Fundamental frequency (f0) difference limens (DLs) were measured as a function of f0 for sine- and random-phase harmonic complexes, bandpass filtered with 3-dB cutoff frequencies of 2.5 and 3.5 kHz (low region) or 5 and 7 kHz (high region), and presented at an average 15 dB sensation level (approximately 48 dB SPL) per component in a wideband background noise. Fundamental frequencies ranged from 50 to 300 Hz and 100 to 600 Hz in the low and high spectral regions, respectively. In each spectral region, f0 DLs improved dramatically with increasing f0 as approximately the tenth harmonic appeared in the passband. Generally, f0 DLs for complexes with similar harmonic numbers were similar in the two spectral regions. The dependence of f0 discrimination on harmonic number presents a significant challenge to autocorrelation (AC) models of pitch, in which predictions generally depend more on spectral region than harmonic number. A modification involving a "lag window"is proposed and tested, restricting the AC representation to a limited range of lags relative to each channel's characteristic frequency. This modified unitary pitch model was able to account for the dependence of f0 DLs on harmonic number, although this correct behavior was not based on peripheral harmonic resolvability.

Informational masking in hearing-impaired and normal-hearing listeners: Sensation level and decision weights

Informational masking (IM) refers to elevations in signal threshold caused by masker uncertainty. The purpose of this study was to investigate two factors expected to influence IM in hearing-impaired listeners. Masked thresholds for a 2000-Hz signal in the presence of simultaneous multitone maskers were measured in 16 normal-hearing (NH) and 9 hearing-impaired (HI) listeners. The maskers were 70 dB SPL average total power and were comprised of fixed-frequency components between 522 and 8346 Hz that were separated from each other by at least 1/3 oct and from the signal by at least 2/3 octs. Masker uncertainty was manipulated by randomly presenting each masker component with probability p = 0.1,0.2,...,0.9, or 1.0 across different trial blocks. Energetic masking was estimated as the amount of masking for p = 1.0, where masker uncertainty was minimum. IM was estimated as the amount of masking in excess of energetic masking. Decision weights were estimated by a regression of the listener's yes/no responses against the presence or absence of the signal and masker components. The decision weights and sensation levels (SLs) of the stimulus components were incorporated as factors in a model that predicts individual differences in IM based on the level variance (in dB) at the output of independent auditory filters [Lutfi, J. Acoust. Soc. Am. 94, 748-758 (1993)]. The results showed much individual variability in IM for the NH listeners (over 40 dB), but little IM for most HI listeners. When masker components were presented to a group of NH listeners at SLs similar to the HI listeners, IM was also similar to the HI listeners. IM was also similar for both groups when the level per masker component was 10 dB SL. These results suggest that reduced masker SLs for HI listeners decrease IM by effectively reducing masker variance. Weighting efficiencies, computed by comparing each listener's pattern of weights to that of an ideal analytic listener, were a good predictor of individual differences in IM among the NH listeners. For the HI listeners weighting efficiency and IM were unrelated because of the large variation in masker SLs among individual listeners, the small variance in IM, and perhaps because broadened auditory filters in some listeners increased the covariance in auditory filter outputs.

Testing the concept of softness imperception: loudness near threshold for hearing-impaired ears.

Buus and Florentine [J. Assoc. Res. Otolaryngol. 3, 120-139 (2002)] have proposed that loudness recruitment in cases of cochlear hearing loss is caused partly by an abnormally large loudness at absolute threshold. This has been called "softness imperception." To evaluate this idea, loudness-matching functions were obtained using tones at very low sensation levels. For subjects with asymmetrical hearing loss, matches were obtained for a single frequency across ears. For subjects with sloping hearing loss, matches were obtained between tones at two frequencies, one where the absolute threshold was nearly normal and one where there was a moderate hearing loss. Loudness matching was possible for sensation levels (SLs) as low as 2 dB. When the fixed tone was presented at a very low SL in an ear (or at a frequency) where there was hearing impairment, it was matched by a tone with approximately the same SL in an ear (or at a frequency) where hearing was normal (e.g., 2 dB SL matched 2 dB SL). This relationship held for SLs up to 4-10 dB, depending on the subject. These results are not consistent with the concept of softness imperception.

Gap detection in Mongolian gerbils ( Meriones unguiculatus)

Hearing thresholds for white-noise stimuli and temporal gap-detection thresholds in six Mongolian gerbils ( Meriones unguiculatus) were determined in a GO/NOGO procedure using the method of constant stimuli. Gerbils were offspring of animals collected in the field and only bred in captivity for four generations or less. Hearing thresholds for 800 ms bursts of white noise ranged between −0.8 and 6.3 dB SPL. The median minimum-detectable gap centered in an 800 ms burst of white noise presented at 50 dB SPL was 2.1 ms. At levels of 40 dB SPL and above, gap-detection thresholds were independent of the sound-pressure level of the stimulus. At 30 dB SPL and below, the minimum-detectable gap increased with decreasing sound-pressure level. Near 5 dB sensation level, gap-detection thresholds ranged from 13.6 to 29.4 ms. The median threshold for the detection of gaps inserted 5 ms after the onset of an 800 ms burst of white noise of 50 dB SPL was 2.3 ms. The individuals’ detection thresholds varied between 2.3 and 3.6 ms for stimuli in which the relative sound-pressure levels of the noise before and after the gap differed by up to 20 dB. The data found in the Mongolian gerbil match results from other mammal and bird species.

Auditory cortical response patterns to multiple rhythms of AM sound.

Multiple amplitude modulation (AM) of a tone results in a time series with a complex structure of envelope fluctuations. In this study the cortical responses elicited by such an auditory stimulus were investigated. It was hypothesized that the auditory cortex responds simultaneously with different patterns of activity corresponding to the different rhythm of periodical stimulus fluctuations. The relations of these response patterns to either steady-state responses (SSR) in the 40 Hz range or the transient slow auditory evoked responses were investigated. Auditory evoked magnetic fields were recorded from the left temporal cortex of nine healthy humans in response to 250 Hz tones that were amplitude modulated with the sum of two sinusoids of 38 and 40 Hz. The stimulus was presented continuously to the subject's right ear at the intensity of 70 dB sensation level. In addition, the responses to 250 Hz tone-burst stimuli with duration of 20, 50, 100, and 200 msec, respectively, were recorded. The tone-bursts were presented at an inter-stimulus interval (ISI) of 0.5 sec, which was equivalent to the slowest rhythm of the multiple-AM stimulus. Magnetic source analysis was applied to all elicited waveforms. From all subjects SSR at 38 and 40 Hz were clearly recognizable and were superimposed by a periodical response pattern corresponding to 2 Hz stimulus envelope fluctuations. A positive peak with latency of 70 msec was the mostly pronounced response to tone-burst stimulation at the ISI of 0.5 sec and resembles the low-frequency response pattern to the AM stimulus. The N1-P2 complex showed an almost complete suppression at 0.5 sec ISI. In contrast, it was the most prominent evoked response activity when using a longer ISI of 3 sec. Source locations in overlapping areas were estimated for the SSR in the 40 Hz range, the prominent peak of low-frequency response to the AM stimulus, and the P1 peak at tone-burst stimulation, respectively. These were separated from the N1 source location by about 0.5 cm in anterior and 0.5 cm in medial direction. As a result of multiple AM the stimulus signal envelope fluctuates with frequencies of 2 Hz and around 40 Hz. The auditory cortex responds to this stimulus with different response patterns. Both modulation signals at 38 and 40 Hz were almost linearly represented as steady-state responses. The evoked response patterns to stimulus fluctuations at low-frequency rhythm resemble the slow cortical auditory evoked responses. The 2 Hz stimulus rhythm is perceived as loudness fluctuation, and 40 Hz modulation is perceived as a rough sound. It is assumed that the different cortical representations correspond to both types of perception of the complex AM sound.